Advancements in Drainage Consolidation Technology for Marine Soft Soil Improvement: A Review

Abstract

Marine soft soils are characterized by high compressibility, low strength, and low permeability, which often result in excessive settlement and stability problems. Drainage consolidation methods are widely regarded as effective solutions for improving such soils. This review summarizes recent progress from four perspectives: optimization of traditional techniques, combined applications of multiple methods, development of emerging innovative approaches, and advances in drainage element materials and structures. Traditional methods such as surcharge and vacuum preloading have been refined through innovations in loading schemes, drainage improvements, and design approaches, while hybrid combinations with electroosmosis, thermal treatment, and dynamic loading have further enhanced their efficiency and applicability. In parallel, novel techniques such as siphon drainage, aerosol-assisted consolidation, and osmosis-based drainage show promise for sustainable applications. Furthermore, biodegradable and multifunctional drainage elements provide new directions for environmentally friendly and efficient soft soil improvement. Looking ahead, drainage consolidation technology is expected to move toward greener, low-carbon, and intelligent solutions. This review offers a comprehensive reference for engineering practice and a useful basis for guiding future research in marine soft soil improvement.

1. Introduction

Coastal areas possess advantages such as convenient transportation, humid climate, and abundant natural resources, making them favorable for the concentration of population and economic activities [1]. The rapid increase in population and booming economic growth are often accompanied by strong land demand. Land resource scarcity is a crucial factor limiting further urban development. Developing unused low-lying areas and offshore regions is an effective solution to address the shortage of land resources [2]. Numerous international land reclamation projects have been implemented, such as the Tianjin Lingang Economic Zone, Shanghai Yangshan Port, Hong Kong International Airport, Dubai Palm Island, Kansai International Airport in Japan, the Fisherman Island expansion project in Australia, and the Zuiderzee Works and Delta Works in the Netherlands [3,4,5]. These coastal, riverine, and lacustrine areas are widely distributed with soft soils. Soft soil possesses advantages such as large reserves and low acquisition costs, enabling its resource utilization as filling material for land reclamation [6,7]. However, due to its high compressibility, high water content, low permeability, and low strength, the bearing capacity of soft ground formed by reclamation is extremely poor [8,9]. Various infrastructures built on soft ground are prone to settlement and differential settlement, which affect their normal operation [10]. Therefore, soft ground must be treated before construction to prevent deformation hazards [11,12].

Among various ground improvement methods, the drainage consolidation method is widely regarded as one of the most cost-effective and environmentally friendly techniques for addressing large-area soft soil settlement and stability issues [13,14]. In 1923, K. Terzaghi proposed the theory of soil consolidation and the principle of effective stress. These concepts linked soil permeability, deformation, and strength, and together established the theoretical foundation for drainage consolidation methods. Currently, the drainage consolidation theory used in engineering mainly applies to saturated soft ground. For this type of saturated soft soil drainage consolidation method, a practical axisymmetric consolidation calculation theory has been developed. This is based on the differential equation of axisymmetric consolidation established by Barron in 1948 [15]:

$${\displaystyle \frac{\partial u}{\partial t}}=C_{\mathrm{v}}{\displaystyle \frac{{\partial }^{2}u}{\partial z^2}}+C_{\mathrm{h}}\left({\displaystyle \frac{{\partial }^{2}u}{\partial r^2}}+{\displaystyle \frac{1}{r}}{\displaystyle \frac{\partial u}{\partial r}}\right)$$

(1)

where u represents the excess pore pressure; t is time; C_v is the vertical consolidation coefficient; C_h is the radial consolidation coefficient; z is the vertical coordinate; and r is the radial coordinate. This theory includes seepage in both the radial and vertical directions.

The drainage consolidation methods generally include a drainage system and a loading (or unloading) system. As shown in Figure 1, drainage systems are installed inside and outside the soft ground, while loading (or unloading) systems are applied to the soil to promote pore water discharge. With the reduction in void ratio and the increase in effective stress, soil consolidation occurs simultaneously with the enhancement of soil strength. The drainage system usually consists of vertical drainage elements (e.g., sand drains, prefabricated vertical drains) and horizontal drainage elements (e.g., sand blankets), which provide more drainage channels and shorten drainage paths. The loading (or unloading) system typically includes static load, dynamic load, groundwater drawdown, and vacuum, which create potential energy differences in groundwater to induce seepage.

Drainage consolidation method has undergone decades of research and achieved significant advancements, providing technical support for numerous soft soil improvement projects [16,17]. In recent years, the challenges of soft soil improvement projects have increased, with stricter settlement control requirements and heightened energy and environmental concerns. The limitations of conventional drainage consolidation method have gradually become evident. To meet the new requirements of soft ground improvement in the modern era, the development of novel, green, low-carbon, efficient, and intelligent soft ground improvement methods is of great importance. In this study, we use key terms that are central to sustainable engineering practices. Green refers to methods and technologies that minimize environmental impact, particularly in terms of resource consumption, pollution, and ecological damage, often associated with sustainability. Low Carbon refers to practices and technologies that reduce greenhouse gas emissions, especially carbon dioxide, by using cleaner energy sources or minimizing reliance on fossil fuels. Efficient processes are those that achieve the desired outcome while minimizing resource use, including not only speed but also overall effectiveness and results. Finally, Intelligent technologies involve the use of automation, data analysis, or AI to optimize system performance, enhance decision-making, and enable real-time adjustments.

This paper systematically summarizes the latest progress in drainage consolidation methods and drainage elements, and further explores the future development directions. This study aims to advance the field of soft soil improvement by focusing on innovative drainage consolidation technologies that address the challenges of settlement and stability in coastal areas. By examining both traditional and emerging methods, it provides a comprehensive overview of their environmental, energy, and performance benefits. The research highlights the importance of integrating sustainable, low-carbon, and intelligent solutions to enhance the efficiency and applicability of soil improvement techniques. Ultimately, this study serves as a critical reference for future developments in the field, offering insights into greener, more efficient practices for land reclamation and infrastructure projects.

2. Traditional Techniques and Their Optimization Progress

2.1. Surcharge Preloading Method

2.1.1. Technical Principle and Optimization Directions

The surcharge preloading method is one of the oldest techniques for soft soil improvement. It involves placing a certain height of sand or other materials on the surface of the soft ground, utilizing the additional stress induced by the surcharge to improve the soft soil. Its fundamental principle follows Terzaghi’s consolidation theory. The applied load increases the total stress within the soil. At this stage, the applied surface load is primarily borne by the pore water in the soft soil, causing pore water pressure to rise and exceed that within the sand drains. Under this pressure difference, pore water in the soft soil seeps toward and accumulates within the drainage elements, and is subsequently discharged outward through them, ultimately achieving soil consolidation [18]. As presented in Figure 2, traditional surcharge preloading usually incorporates a horizontal drainage layer and vertical drainage elements to increase drainage channels and shorten drainage distances, thereby accelerating consolidation.

The surcharge preloading method is simple, and its optimization can be achieved from two aspects: drainage conditions and loading methods. Improving drainage conditions includes shortening drainage paths, increasing the number of drainage channels, and maintaining the drainage capacity of drainage elements. Conventional surcharge materials often incur significant economic costs and environmental impacts, thus offering considerable potential for optimization.

2.1.2. Optimization Advances

Traditional surcharge materials mainly consist of soil and aggregates. Wang et al. (2018) [19] integrated embankment construction with surcharge preloading, thereby reducing surcharge fill consumption. In recent years, the “water bag surcharge”, substituting soil with water, has gained attention [20]. As soft ground improvement areas are typically located in coastal or riverine regions with abundant water sources, water can be drawn locally. The filling and draining of water bags allow rapid construction and removal. Furthermore, water bags can be reused, significantly reducing the use of aggregates, as well as labor and costs. Liao et al. (2025) [21] employed soil bags filled with in situ soil as surcharge layers (200–250 kPa load) and examined the consolidation characteristics of soft ground improvement by soil bags. This innovation supports reversible recovery of temporary platforms (e.g., offshore wind farms), advancing surcharge preloading toward greater efficiency and sustainability.

Optimization of drainage elements is crucial, and this paper provides a systematic summary of innovative drainage elements in Section 4. During surcharge processes, fine particles easily clog drainage channels, resulting in a reduction in drainage efficiency. Flocculants have been introduced into surcharge preloading. Flocculants alter the balance between electrostatic repulsion and van der Waals attraction among particles. As a result, fine particles aggregate into larger flocs. This process increases the void ratio and permeability, reduces the risk of clogging, and ultimately accelerates the consolidation of high-water-content slurry. Han et al. (2023) [22] used an anionic polyacrylamide (APAM) flocculant to accelerate drainage, improve the secondary consolidation coefficient, and explore the optimal dosage. This study promotes efficient treatment of low-permeability slurry and shortens preloading time.

Spross and Larsson (2021) [23] proposed a probabilistic observational method for the design of surcharge preloading, incorporating uncertainty. This method evaluates surcharge loading through Monte Carlo simulation, ensuring that design standards (e.g., degree of consolidation) are met with acceptable probability. Compared to deterministic methods, this represents a shift in traditional techniques toward risk management and greener improvements, reducing excessive surcharges, lowering carbon emissions, and minimizing material consumption. These developments reflect the evolution of surcharge preloading from empirical to intelligent (probabilistic models), green (recycled materials), and efficient (flocculation optimization).

2.2. Vacuum Preloading Method

2.2.1. Technical Principle and Optimization Directions

As illustrated in Figure 3, the vacuum preloading system consists of prefabricated vertical drains (PVDs), horizontal pipes, vacuum pumps, sand blankets, and sealing membranes [24]. Vacuum preloading generates vacuum pressure through the vacuum pump, which is transmitted through the PVDs to the deeper soft soil layers. Due to the vacuum pressure within the PVDs, the internal water pressure is lower than that of the pore water pressure in the soft soil, leading to the seepage of pore water into the PVDs until the pressures are balanced [25]. Field tests have shown that vacuum preloading and surcharge preloading have similar effects in terms of depth of action and soil strengthening [26].

The key to vacuum preloading lies in improving the transmission of the vacuum pressure to all locations within the soft soil. A larger hydraulic gradient must be formed to accelerate drainage. Therefore, optimization can be approached in two aspects: reducing energy losses in the vacuum system and increasing pore water pressure to enhance the hydraulic gradient. Consequently, optimization directions include:

• Improving sealing materials and methods to reduce air leakage.
• Optimizing drainage channel designs to reduce vacuum transmission losses.
• Reducing the negative impact of soft soil consolidation on vacuum pressure transmission.
• Adding external measures to increase pore water pressure within the soft soil.

From a green and low-carbon perspective, optimizing the operation of vacuum pumps, reducing the use of sand blankets, and sealing membranes are key areas to focus on.

2.2.2. Optimization Advances

As vacuum preloading has been widely applied globally, several new techniques have emerged based on traditional vacuum preloading. These include membrane-less vacuum preloading, pressure-variable vacuum preloading, flocculation combined with vacuum preloading, air-boosted vacuum preloading, and horizontal drainage vacuum preloading. Below is a brief introduction to these typical new vacuum preloading methods.

(1)

Membrane-less Vacuum Preloading.

In engineering practice, the sealing state of the treated area is usually achieved by laying sealing membranes. However, the application of sealing membranes in vacuum preloading presents several drawbacks [28]. These include environmental pollution, a limited reinforcement area, difficulties in installation on ultra-soft soils, vulnerability to damage, and high maintenance costs. Membrane-less vacuum preloading eliminates the sealing membrane based on the traditional direct drainage (no sand) vacuum preloading, directly connecting the PVDs to the vacuum pipes through horizontal vacuum tubes. Liang et al. (2013) [29] proposed the footed-in filter tubes vacuum preloading, which connects the PVDs with vacuum pipes, avoiding the use of traditional membranes. Field tests showed that this method is feasible. Subsequently, Liang et al. (2015) [30] extended this technology, focusing on optimizing mud covering layers as an alternative to sealing, avoiding crack leakage, and improving vacuum transmission efficiency (up to 80–90 kPa). Sun et al. (2017) [31] used special connectors and flexible horizontal pipes to directly connect the vacuum pipes to the PVDs, then covered the horizontal pipes with a clay layer, further reducing vacuum leakage. Membrane-less vacuum preloading technology has been applied in soft soil reinforcement projects in several countries [27,32], including China, Singapore, and Mexico.

(2)

Flocculation Combined with Vacuum Preloading.

This technique improves the drainage and consolidation properties of dredged slurry by adding chemical conditioners (such as FeCl₃, lime, or polyacrylamide). In recent years, progress has been made in terms of green, low-carbon, and efficient aspects. Early studies focused on optimizing a single conditioner. For example, Lin et al. (2014) [33] verified through vacuum consolidation tests that FeCl₃ conditioning can significantly increase the coefficient of consolidation of sludge. In field tests, the sludge volume was reduced by 47.5%, the water content decreased from 860% to 140–450%, and the shear strength reached 10 kPa. Wang et al. (2017) [34] proposed combining vacuum preloading with lime treatment. In this method, Ca(OH)₂ was added to induce cation exchange and flocculation, thereby determining the optimum lime modification value. The treatment reduced the risk of PVD clogging and improved soil permeability and shear strength, particularly in deep layers. As a result, the consolidation rate was significantly enhanced. Lei et al. (2019) [35] introduced anionic polyacrylamide (APAM) for sludge conditioning. Filtration tests were used to determine the optimum dosage of 30 mg/L. With this treatment, the consolidation time was shortened by 30–50%, pore water dissipation was accelerated, and shear strength increased by 30–55%. Overall, the method achieved efficient drainage and low-carbon treatment. Wang et al. (2019) [36] developed a FeCl₃-APAM composite flocculant (ratio 1:5), and laboratory tests showed that drainage rates were accelerated, the water content decreased from 140% to 50%, and heavy metal stabilization exceeded 88%. Song et al. (2024) [37] applied prefabricated horizontal drainage boards (PHDs) together with APAM in the staged filling slurry vacuum consolidation process. The results showed that flocculation significantly enhanced the initial consolidation rate and increased slurry strength by a factor of 1–2. Based on these findings, the authors suggested arranging alternating layers of PHDs to achieve more efficient drainage.

(3)

Pressure-variable Vacuum Preloading.

Pressure-variable vacuum preloading optimizes traditional vacuum preloading by adopting graded, intermittent, or alternating methods. Its primary goals are to alleviate clogging of PVDs, reduce uneven settlement and vacuum decay, and ultimately improve consolidation efficiency while supporting greener practices. Early research concentrated on step loading. Yuan et al. (2012) [38] revealed through laboratory tests that step vacuum preloading optimized the pore size distribution of dredged soil, thereby reducing PVD clogging and improving consolidation uniformity. Building on this, Xu et al. (2016) [39] showed that applying small vacuum gradients further minimized filter membrane clogging and permeability decay, enhancing drainage efficiency. In the same line of inquiry, Liu et al. (2017) [40] demonstrated that a 20 kPa increment per step was optimal, yielding more uniform water content and strength after consolidation. Extending the step-loading concept, Wang et al. (2018) [41] introduced a two-stage approach: the first stage stabilized shallow soil using half of the PVDs, while the second stage reinforced deeper layers with all PVDs. This method significantly improved settlement, pore water pressure dissipation, and shear strength while reducing clogging. Similarly, Li et al. (2020) [42] validated multi-stage vacuum loading in high-clay-content dredged soil, showing that staged loading removed weakly bound water, achieved uniform permeability coefficients, and limited fine particle migration. A comprehensive study by Wang et al. (2021) [43] confirmed that a 20 kPa gradient effectively reduced surface differential settlement and improved microstructure, reinforcing its status as the optimal loading parameter.

As research advanced, attention shifted from stepwise improvements to more dynamic mechanisms. Lei et al. (2017) [44] demonstrated that intermittent vacuum preloading increased settlement by 1.2 times and accelerated pore-water pressure dissipation compared with conventional methods. Building on this intermittent concept, Liu et al. (2018) [45] proposed synchronous and alternate vacuum preloading, which achieved a 27.9% increase in drainage volume and improved pore size distribution. More recently, Lei et al. (2023) [46] developed alternating-radial-drain vacuum preloading, further enhancing drainage, settlement, and pore pressure dissipation rates by 8.68%, 8.84%, and 40.02%, respectively. Moving toward energy efficiency, Wang et al. (2024) [47] introduced cyclic vacuum preloading, where pumps were switched periodically, saving 80% of energy while maintaining comparable consolidation performance.

(4)

Air-boosted Vacuum Preloading.

Air-boosted vacuum preloading enhances consolidation by injecting compressed air to generate an additional pressure gradient. Early research focused on equipment development and feasibility verification. Wang et al. (2016) [48] integrated air-boosted vacuum tubes with PVDs, achieving higher drainage efficiency and settlement in field tests, while Lei et al. (2017) [44] introduced an intermittent vented system that increased settlement by about 1.8 times and alleviated PVD clogging. Subsequent studies emphasized parameter optimization and mechanistic analysis. Anda et al. (2020) [49] identified 60% consolidation as the optimal injection timing to avoid vacuum loss or delayed fracture formation. Lei et al. (2021) [50] compared PVD air-boosted vacuum preloading (PAVP) and tube air-boosted vacuum preloading (TAVP). The model tests demonstrated that PAVP achieved higher drainage volume, settlement, and shear strength while lowering water content significantly. Scanning electron microscopy and mercury intrusion porosimetry verified that PAVP more effectively alleviated drainage board clogging and densified the microstructure. More recent advances include novel equipment designs, such as flexible membrane airbags for combined air–vacuum loading (Wu et al., 2022) [51], and numerical modeling frameworks for systematic sensitivity analyses of injection pressure, spacing, and cycling regimes (Feng et al., 2022) [52]. Lei and Feng (2024) [53] systematically summarized the logic of parameter selection for air-boosted vacuum preloading, including air injection pressure range, initiation timing, spacing between injection/extraction components, number of cycles, and duration. They emphasized that optimized parameter selection can reduce energy consumption, improve consolidation efficiency, and promote green applications.

(5)

Horizontal Drainage Vacuum Preloading.

The low stiffness of PVDs and their connecting pipes makes them susceptible to bending deformation under continuous soil compression, which reduces vacuum transfer efficiency [54]. During consolidation, however, PHDs (defined in Section 2.2.2) do not undergo significant bending, and thus exhibit higher vacuum transfer efficiency than PVDs [55]. PHDs can be installed prior to hydraulic filling, eliminating pre-treatment steps and enabling simultaneous filling and consolidation. Shinsha and Kumagai (2014) [56] proposed vacuum preloading with horizontal drains, and field tests showed that the volume of landfill slurry was reduced by approximately 10%. The results confirmed that the PHD was far more efficient than the PVD when treating relatively thin slurry deposits. Yin et al. (2022) [57] proposed a staged consolidation approach using multi-layer PHDs with vacuum loading, and model tests demonstrated that combined drainage could significantly reduce water content and increase soil strength. Once a surface crust formed, the process could then transition to the PVD stage, achieving an efficient dredged fill consolidation. Subsequently, Yang et al. [58] investigated the drainage efficiency and consolidation uniformity of double-layer PHD vacuum preloading at near-engineering scale. Results showed that cross-arrangement and multi-layer settings enhanced consolidation rates, while PHDs exhibited less bending and clogging. From an engineering design perspective, Song et al. (2024) [59] systematically evaluated key parameters, including the number of layers, spacing, and vacuum levels. Based on field trials, they conducted sensitivity analyses and provided corresponding design recommendations. Building on this, Pan et al. (2025) [60] focused on optimizing the spacing of grid PHDs in combination with PVDs. Their study demonstrated how drainage dimensions and spacing influence both consolidation degree and construction efficiency, thereby establishing a basis for parametric design. In 2025, Song et al. [61] further incorporated grid-shaped PHD–PVD combinations into staged dredged fill consolidation frameworks and verified accelerated consolidation effects through model testing.

However, when PHDs are used alone, the vacuum consolidation process remains relatively time-consuming. In addition, soil deformation may cause displacement of the PHDs, leading to difficulties in quality control and related issues. To address this issue, Chu et al. (2021) [62] proposed horizontal-drain-enhanced geotextile sheets (HDeGs). In this method, PHDs are connected with horizontal geotextiles, reducing positional uncertainty and enhancing overall drainage capacity. Chen et al. (2023) [63] further proposed combining the HDeGs approach with vacuum preloading. Large-scale model tests validated its efficiency, showing shortened project duration and reduced material consumption. Compared with conventional PHDs, HDeGs improved drainage paths and achieved more uniform consolidation.

2.3. Electroosmotic Drainage

2.3.1. Technical Principle and Optimization Directions

Electroosmotic drainage [64,65] is a consolidation method that uses an electric field to drive pore water in soil toward the cathode, and it is suitable for reinforcing soft soils with high water content and low permeability. The principle is that soil particles carry negative surface charges. These charges attract cations in the diffuse double layer, which migrate together with water molecules toward the cathode through electroosmotic flow. The technique can significantly shorten the consolidation period and is particularly effective for marine soft clays and dredged slurries. However, electroosmotic drainage suffers from problems such as electrode corrosion, high energy consumption, and non-uniform consolidation. The major optimization directions can be summarized into three categories:

• Improving electrode materials, electrode types, and electrode configurations to reduce energy consumption and corrosion;
• Optimizing power supply modes and applied voltages to improve drainage uniformity and energy efficiency;
• Developing electro-chemo-bio synergistic consolidation, in which chemical agents or microorganisms are introduced to alter inter-particle forces and optimize soil microstructure.

2.3.2. Optimization Advances

Early studies employed conventional metal electrodes (e.g., iron, copper, aluminum), but these faced corrosion and high energy consumption issues. Substantial research has since focused on developing innovative electrode materials with improved durability and energy efficiency. For instance, Zang et al. (2018) [66] compared electrokinetic geosynthetics (EKG) and iron electrodes in contaminated soil treatment. Their results showed that although the contact resistance of EKG cathodes was 56% higher, the anode resistance was 60% lower, and the unit energy consumption per pollutant was 1.895 kJ/g, thereby optimizing electrochemical reactions. Building on this direction, Liu et al. (2019) [67] evaluated electrokinetic vertical drains (EVDs) for soft clay consolidation. They found that conductive polymer EVDs outperformed traditional PVDs by reducing water content and accelerating consolidation, although electrode material still influenced electrochemical reactions. Similarly, Ling et al. (2021) [68] designed novel EKG electrodes (carbon fiber fabric + drainage pipe, see Figure 4) for subgrade electro-dewatering. Their tests demonstrated strong corrosion resistance and low surface contact resistance, making the electrodes suitable for long-term subgrade improvement. In a related study, Mahalleh et al. (2021) [69] tested graphite and stainless-steel electrodes in high-plasticity clay. After 28 days, stainless-steel electrodes yielded greater drainage and strength improvement. SEM analysis revealed particle aggregation and iron-based precipitation, while chemical analysis confirmed calcium ion migration. Recently, Jin et al. (2024) [70] compared steel, copper, aluminum, and composite carbon fiber (CCF) electrodes for electro-osmotic consolidation in marine clay. They identified 1 V/cm as an optimal potential gradient for CCF electrodes. This gradient yields higher discharge rates, greater and more uniform soil strength, and lower corrosion than metal electrodes, while balancing energy consumption and consolidation efficiency. Laboratory tests, which evaluated various gradients, confirmed that 1 V/cm maximizes water discharge and boosts soil bearing capacity (6.3–12 times initial values). It outperforms lower gradients (e.g., 0.15 V/cm), which provide insufficient driving force, and higher ones (e.g., 3 V/cm), which increase corrosion and energy use. Moreover, recent advancements in data-driven modeling, particularly artificial neural network (ANN) approaches, present a promising avenue for predicting electrode corrosion in electro-osmotic drainage systems. A study by Ahmad et al. (2025) [71] utilized ANN models to forecast corrosion rates of steel reinforcement in clay-dominated soils, achieving a high correlation coefficient (R = 0.998) by analyzing parameters such as sodium chloride concentration, inhibitor dosage, and exposure duration. These results indicate that analogous data-driven methodologies could be adapted to refine electrode material selection and optimize operational parameters in electro-osmotic consolidation, potentially mitigating corrosion-related energy losses and improving the long-term efficacy of drainage systems in marine soft soil environments.

Power supply optimization in electroosmotic consolidation has been investigated from three main perspectives. First, improvements in supply modes such as polarity reversal, intermittent supply, and cyclic supply were introduced to enhance efficiency. Fu et al. (2018) [72] compared intermittent current and polarity reversal in Wenzhou dredged clay. Intermittent supply achieved greater drainage and lower energy consumption, while polarity reversal improved uniformity and significantly enhanced consolidation efficiency. Sun et al. (2022) [73] further proposed cyclic progressive electroosmosis (CPE) using EKG, which not only enhanced drainage by filling cracks but also increased the bearing capacity of fine-grained soils and alleviated non-uniform reinforcement. Second, research has focused on voltage regulation. Fu et al. (2019) [74] demonstrated that applying high-voltage gradients improved electromigration, electroosmosis, and electrophoresis in Wenzhou clay slurry, thereby accelerating the treatment of high-water-content soils. However, careful control of electrolysis was required to avoid uneven hydrogen/oxygen distribution. Sadeghian et al. (2022) [75] further examined voltage and pH effects on electrode and pile durability, confirming stainless steel as the most reliable material. They also found that drainage gains plateaued beyond 35 V, while corrosion risks increased, emphasizing the importance of controlled voltage for low-carbon improvements. Finally, large-scale and intelligent systems have been explored. Zhuang (2021) [76] conducted an 800 m² field test, integrating EKG with intelligent DC power supplies and rolling polling programs. This reduced current demand to one-third, with energy consumption below 1 kWh/m³. The design was optimized using energy gradient theory, demonstrating the feasibility of scaling up electroosmotic consolidation.

In the field of electro-chemo-bio synergistic consolidation, research has progressed along two primary directions: chemical enhancement and biological reinforcement. On the chemical side, Wang et al. (2024) [77] investigated the combined use of nano-Fe₃O₄, APAM, and chemical solutions. Their results showed improved conductivity and optimized pore structures. In a related effort, Ge et al. (2024) [78] developed a coupled chemical-osmotic model that incorporated nonlinear parameters, aiming to optimize electroosmotic consolidation under complex chemical conditions. Turning to biological approaches, Tian et al. (2021) [79] applied MICP to marine clay and demonstrated improved settlement uniformity, with the coefficient of variation reduced by 53.2%. This improvement was attributed to CaCO₃ precipitation and biofilm formation, which mitigated uneven reinforcement. Extending this line of research, Nabizadeh et al. (2025) [80] combined electrokinetic techniques with bacterial injection. They compared simultaneous and sequential injection modes of Sporosarcina pasteurii and cementation solution. The simultaneous injection mode yielded significantly higher unconfined compressive strength and shear strength compared with pure electroosmosis, underscoring its superiority.

2.4. Vibration-Drainage Consolidation Method (VDCM)

2.4.1. Technical Principle and Optimization Directions

The VDCM refers to a soil improvement technique that combines dynamic loading (e.g., tamping or vibration) with drainage. Its principle involves applying periodic dynamic loads to the foundation through mechanical vibration or impact, generating instantaneous excess pore water pressure in the soil, which subsequently accelerates drainage and consolidation. A typical example is the dynamic compaction method, where repeated hammer impacts on the ground surface induce high pore pressure in the soil and promote rapid water discharge toward nearby drainage channels. Once the excess pore pressure dissipates, the soil reconsolidates with improved density.

Traditional VDCMs have primarily relied on impact loading, whereas recent studies have shifted the focus toward vibration loading. Optimization directions include adjusting vibration frequency, vibration location, and confining pressure to achieve enhanced reinforcement effects. Moreover, the development of green and intelligent equipment can contribute to more efficient and low-carbon ground improvement.

2.4.2. Optimization Advances

Early studies examined how vibration loading influences the drainage behavior of soft soils. Miao et al. (2019) [81] demonstrated through laboratory tests that both confining pressure (20–100 kPa) and vibration frequency (0–5 Hz) significantly affect drainage. Drainage volume decreased linearly with increasing confining pressure, while the maximum drainage occurred at 1 Hz due to resonance effects. Yin et al. (2020) [82] confirmed that the dynamic deformation of soft soil treated with VDCM is strongly frequency-dependent. They observed a logarithmic decrease in axial strain rate with the number of cycles. Moreover, when the cyclic loading frequency approached the soil’s natural frequency, strain levels were reduced and structural stability improved. Dai et al. (2021) [83] explored microstructural changes under dynamic loading using SEM and image analysis. They found that pores became denser: small pores increased, large pores decreased, and pore distribution showed greater orientation. Structural weakening was more pronounced at higher consolidation pressure, stress ratio, and overconsolidation ratio. Building on these findings, Zhang (2024) [84] introduced the vibration-boosting drainage consolidation method (see Figure 5), which integrates surcharge loading with vibration. Laboratory results showed a drainage rate of 0.53 mL/min, outperforming conventional methods. The water content decreased to as low as 43.1%, achieving the highest consolidation effectiveness. In parallel, Southeast University developed a pneumatic vibrating-rod compaction technique. This approach requires no filler, reaches reinforcement depths of up to 25 m, shortens construction time by 50%, reduces costs by 40%, and can also be applied in liquefaction foundation assessments [85].

2.5. Dewatering Preloading Method

2.5.1. Technical Principle and Optimization Directions

The dewatering preloading method consolidates foundation soils by lowering the groundwater table. It is most often applied as a temporary reinforcement technique in excavation pits and soft ground treatment. The basic principle is to install dewatering wells around or within the site. Vacuum or submersible pumps are then used to lower the groundwater level to the required depth. This process increases the effective stress, lowers pore water pressure, and induces soil consolidation. Effective consolidation in soft soils requires a substantial reduction in pore water pressure across a wide area. Conventional gravity well-point systems in clayey soils are limited by slow seepage, which causes localized flow around the well bore. Optimization strategies include three main directions. First, the use of high-efficiency vacuum pumps can reduce energy consumption and promote low-carbon operation. Second, overcoming pumping depth limitations increases hydraulic gradients and improves drainage efficiency. Third, intelligent monitoring systems can dynamically adjust pumping parameters during operation.

2.5.2. Optimization Advances

Research on dewatering preloading has evolved from conventional light well-point systems to more advanced vacuum tube-wells and super well points (SWP), reflecting a continuous pursuit of greater consolidation depth, efficiency, and sustainability. For shallow reinforcement, Zhang et al. (2014) [86] demonstrated through field trials that vacuum light well-points significantly improved the strength of coastal soft soils. After 75 days of pumping, cone penetration resistance increased from 200 kPa to 300 kPa, while shear strength rose by 30–50 kPa. These improvements highlight the method’s effectiveness for near-surface consolidation. For deeper deposits, subsequent studies explored tube-well systems. Vu et al. (2016) [87] confirmed through laboratory simulations that vacuum tube-wells maintain a vacuum more effectively than conventional PVD-based systems, thereby promoting deeper consolidation. Similarly, Zeng et al. (2021) [88] applied vacuum tube-well dewatering in the Yangtze River floodplain. By optimizing well layouts, they achieved consolidation efficiency surpassing that of standard vacuum preloading, underscoring the potential of this approach for thick soft soil layers. To improve energy efficiency and system durability, Heng et al. (2017) [89] introduced the SWP technique, integrating vacuum with pumping and adopting optimized well-pipe designs such as double-layer filters. This not only reduced energy consumption but also enhanced drainage efficiency, making it particularly attractive for large-scale reclamation projects. Koh et al. (2022) [90] further verified the benefits of SWP through unit-cell tests. Their results showed higher average degrees of consolidation compared with traditional vacuum preloading, confirming the synergy between controlled water levels and vacuum pumping.

2.6. Thermal Consolidation Method

2.6.1. Technical Principle and Optimization Directions

By heating or calcining soil, heat conduction occurs within the soil matrix and pore fluid. This process reduces the viscous resistance of pore water, increases the soil’s permeability coefficient, and alters the stress field, thereby facilitating dissipation of excess pore water pressure and volume compression, which ultimately achieves ground improvement. In recent years, this principle has been extended to the treatment of soft soils in the form of thermal consolidation technology. A typical approach combines PVDs with heating devices, such as embedding closed-loop heating pipes (circulating warm water) or electric heating elements within PVDs. This raises the temperature of the soil surrounding the drains, thereby inducing drainage and consolidation. The thermal consolidation method mainly involves the sources, utilization, and transfer of heat. Accordingly, optimization can be pursued in three directions: adopting green energy sources (e.g., solar energy, industrial waste heat); optimizing heating schemes; and developing high-efficiency heated drainage boards.

2.6.2. Optimization Advances

Research on thermal consolidation remains at the experimental stage, with large-scale engineering applications yet to be achieved. Current studies can be grouped into three main directions: experimental validation, mechanism and modeling, and renewable-energy applications.

Experimental validation has provided the foundation. Lei et al. (2020) [91] conducted temperature-controlled triaxial tests on hydraulic fill soils and observed a three-stage pattern of thermal consolidation deformation: rapid increase, gradual increase, and stabilization. Both drained and undrained shear strengths improved significantly, demonstrating the strengthening effect of heating. Houhou et al. (2021) [92] used mercury intrusion porosimetry (MIP) and CT scanning to analyze illite and kaolinite under heating–cooling cycles. They found that illite macropores collapsed, producing thermal shrinkage that intensified under higher effective stress, while kaolinite showed weaker responses. This confirmed that microstructural rearrangement is the core mechanism. Similarly, Cao et al. (2022) [93] simulated geothermal conditions and showed that pore pressure and settlement rebounded with rising temperature, and cumulative effects grew with cycling, though settlement gradually stabilized.

Mechanistic studies and predictive models have further advanced understanding. Many scholars [94,95] employed thermal triaxial tests to evaluate strength improvement, attributing gains to thermal creep and shifts in the critical state line. They also proposed predictive models incorporating temperature and overconsolidation ratio, offering practical guidance for optimized design. Additional experimental insights into thermal behavior enhance these models by exploring soil responses under varying conditions. Recent studies on cyclic and static thermal loads in saturated and dry sands have identified a thermal charging effect under cyclic conditions and the formation of convection cells in saturated environments, which improve heat dissipation [96]. These findings provide a basis for refining predictive frameworks, particularly by accounting for dynamic thermal effects and moisture migration, which could optimize the design of thermal-assisted drainage systems for marine soft soils.

More recent work has explored renewable energy applications. Deng et al. (2022) [97] developed solar-heated vertical well drainage tests. Heating the soil by 15 °C increased the consolidation coefficient by 1.54 times, shortened the time to 90% consolidation by 34%, and raised settlement by 12%, indicating the potential for low-carbon reinforcement. Xia et al. (2023) [98] combined solar heating experiments with computational analysis under multistage loading. Results showed sinusoidal responses: pore pressure rose during daytime and dissipated at night, while settlement rates decreased in the day and increased at night. These findings reveal that renewable energy can effectively drive thermal consolidation, although its cyclic nature introduces additional complexity.

2.7. Combined Applications of Multiple Techniques

In recent years, soft soil improvement has shifted from single-method approaches to integrated applications of multiple techniques. This transition stems from the complementary mechanisms of different methods and the limitations of individual approaches in efficiency, depth, and energy consumption. By combining vacuum preloading with surcharge, electroosmosis, thermal consolidation, or dynamic consolidation, researchers have achieved substantial improvements in reinforcement effectiveness while advancing green, low-carbon, and efficient solutions.

Among these methods, the vacuum–surcharge combination is the most widely applied. The surcharge provides additional loading, while the vacuum reduces differential settlement. Together, they coordinate to accelerate consolidation. Recent studies further showed that such method can both shorten construction periods and significantly enhance deep bearing capacity [99,100], while also demonstrating good economic feasibility and stability in engineering practice [101].

Vacuum–electroosmosis combinations are particularly effective for rapid reinforcement of low-permeability clays. Because electroosmosis is independent of soil permeability, it compensates for the limitations of vacuum preloading in deep and far-field consolidation. At the same time, vacuum action reduces the energy consumption of electroosmosis and improves drainage uniformity [102]. More recent studies have introduced intermittent electroosmosis to lower energy demand and mitigate electrode corrosion [103]. Optimization of electrode configurations has further improved overall efficiency [104,105]. Additives have been shown to enhance soil conductivity and drainage [106], while electrokinetic geosynthetics (EKG) offer new opportunities for optimizing electric field distribution [107]. A notable development is the triple combination of vacuum, electroosmosis, and surcharge, which has achieved faster consolidation and greater strength gains in field trials [108]. Building on this, the exploration of electro–magnetic coupling has deepened mechanistic understanding and pointed toward multi-field synergy [109]. Collectively, these studies indicate that the technology is moving from conventional electroosmosis toward low-energy, multi-field-coupled, and environmentally sustainable models.

Vacuum–dynamic consolidation methods also show promise. Dynamic compaction or vibration can rapidly improve soil permeability, but insufficient drainage may cause the “pudding soil” phenomenon. Combining vacuum well-point dewatering with dynamic consolidation addresses this limitation. Lowering the groundwater table and water content before tamping, followed by accelerated dissipation of excess pore pressure after tamping, enables layered reinforcement and deeper consolidation [100]. Although effective in shortening construction time and increasing reinforcement depth, challenges remain in reducing energy consumption and simplifying equipment.

Vacuum–thermal treatment combinations are another emerging direction. In vacuum preloading, soil permeability near drains often decreases due to disturbance, limiting efficiency. Moderate heating can counter this effect by enhancing permeability, accelerating excess pore pressure dissipation, and improving consolidation [110]. More recent findings show that heating also promotes organic matter decomposition and improves water migration pathways [111]. Within the context of sustainable development, using industrial waste heat or solar energy as low-carbon heat sources represents a promising research avenue.

In summary, the combined application of multiple techniques has progressed from empirical practice to mechanism-driven and multi-field synergistic approaches. Research priorities have also shifted. Instead of focusing solely on consolidation rate and bearing capacity, studies now emphasize energy efficiency and environmental sustainability. Measures such as intermittent power supply, renewable energy utilization, and dynamic regulation are helping to increase efficiency while aligning with green, low-carbon objectives. Looking ahead, multi-technology combinations are expected to provide greater adaptability and reliability under complex geological conditions. They are also likely to play a vital role in the sustainable development of soft soil improvement.

3. Emerging Drainage Consolidation Methods

3.1. Siphon Drainage Method

3.1.1. Technical Overview

Siphon drainage (see Figure 6) is a method that relies on liquid pressure differences to drive flow. It has been widely used in roof drainage, slope drainage, reservoir dredging, farmland irrigation, and wetland flood control [112,113,114]. In contrast, traditional drainage consolidation methods—such as vacuum preloading and well-point dewatering—require a continuous external energy supply to remain effective. This dependence results in high energy consumption and maintenance costs. Siphon drainage, however, stands out for its green, low-carbon, and energy-free operation. In recent years, it has been introduced as a promising new approach for soft soil improvement.

As illustrated in Figure 7, a siphon drainage system is composed of permeable pipes, siphon pipes, a water collection well, and submersible pumps. The system operates by maintaining the water level in the collection well below the designated control waterline. Under siphon action, water from the permeable pipes is drawn into the well, which stabilizes the water level in the pipes at approximately 10 m depth. Groundwater within the soft soil then flows into the permeable pipes under the influence of gravity. As the groundwater level continues to decline, the pore water pressure across the soft soil stratum decreases accordingly.

3.1.2. Research Status

Early siphon drainage systems were often interrupted during operation, which limited their application in engineering practice. The main causes of interruption were excessive lift height and intermittent discharge, both of which led to air accumulation at the pipe crest [116,117]. Sun et al. [118,119] demonstrated through experiments and theoretical analysis that reducing pipe diameter can generate a stable slug flow. This flow expels trapped air and enables siphon drainage to operate effectively over the long term, particularly when pipe diameters are reduced to 4–5 mm.

In 2017, siphon drainage was first applied to soft soil consolidation [120]. Experimental results showed that the technique could lower the groundwater level and accelerate settlement without requiring external energy input. Wu et al. (2018) [121] later conducted comparative experiments and confirmed that siphon drainage significantly enhanced consolidation performance, even exceeding predictions from the ideal sand drain theory. Zheng et al. (2021) [122] proposed a new velocity calculation formula that incorporated bubble release effects, thereby improving the accuracy of siphon pipe design and operation.

For parameter optimization, Shen et al. (2024) [123] examined the influence of siphon hole spacing on drainage performance. They found that reducing hole spacing substantially lowered groundwater levels and accelerated pore water pressure dissipation in soft soils. More recently, researchers have introduced the siphon–vacuum drainage method (SVD, see Figure 7) [115]. This approach improves drainage efficiency by automatically generating vacuum pressure under siphon action. Shu et al. (2024) [124] demonstrated that the SVD system could generate vacuum pressures above 80 kPa. As a result, it achieved higher drainage rates while reducing reliance on sealing membranes and vacuum pumps, thereby offering a greener, low-carbon, and more efficient solution.

3.1.3. Engineering Application Progress

The feasibility of siphon drainage for soft soil reinforcement has been demonstrated in engineering practice. Wang et al. (2023) [125] conducted field tests in Zhoushan, Zhejiang, and reported that the groundwater level dropped by 3.44 m within 25 days, accompanied by 242 mm of surface settlement. Shu et al. (2024) [115] performed siphon–vacuum drainage (SVD) tests at the same site and found that the SVD system could establish a stable vacuum zone within the soft soil. Shen et al. (2024) [123] extended this work by optimizing siphon hole spacing through field trials. When the spacing was reduced to 0.9 m, the average groundwater drawdown increased by 75.9% compared with 1.8 m spacing, and the time required to achieve the same groundwater decline was shortened by 63.6%.

3.2. Aerosol Injection Technique (AIT)

3.2.1. Technical Overview

The AIT is a novel active drainage consolidation method. It can be seen from Figure 8 that its system mainly consists of high-pressure pumps, rotary injection pipes, vertical drainage channels, and monitoring devices [126]. In soft soils already equipped with vertical drains, high-pressure gas or high-pressure aerosols are injected horizontally through rotary spraying to induce groundwater discharge and soil consolidation. The mechanism of AIT can be summarized in three aspects:

• Deep-layer three-dimensional drainage: High-pressure gas cuts through the soil to form multiple horizontal drainage channels at different depths, accelerating pore water dissipation.
• Deep pressurization effect: The gas pressure generated within drainage channels enhances vertical drainage efficiency.
• Air-lift effect: The injected high-pressure gas strongly interacts with the water inside the drains, producing numerous bubbles that carry water to the surface. This significantly reduces pore water pressure and promotes consolidation.

Compared with conventional surcharge preloading and vacuum preloading, the AIT offers several advantages. It overcomes the limitations of treatment depth and long consolidation duration. In addition, it improves soil structure and reduces post-construction settlement effectively.

3.2.2. Research Status

Wu et al. (2019) [127] conducted model tests on AIT. Results showed that rotary spraying of high-pressure gas could form multiple horizontal cutting layers in soft soil, significantly improving permeability, accelerating pore water discharge, and thereby increasing consolidation. However, the study also found that cutting layers created solely by gas disturbance gradually closed over time, reducing long-term drainage effectiveness. Thus, the introduction of functional aerosol materials was proposed to enhance and extend consolidation performance. Building on this, Wu et al. (2021) [128] systematically investigated the air-lift effect of AIT. Using a self-developed large-scale model apparatus, they studied the influence of injection pressure, injection depth, vertical drain diameter, and surrounding soil medium on air-lift efficiency. Results indicated that the maximum dewatering depth depended on the matching relationship between injection pressure and drain diameter. A regression formula for dewatering depth was established, providing quantitative guidance for engineering design.

3.2.3. Engineering Application Progress

The AIT has been field-validated for soft ground improvement in coastal regions such as Ningbo. Wu et al. (2020) [129] reported that high-pressure aerosol injection significantly enhanced drainage consolidation effects within a depth range of 3–15 m. Settlement rates increased markedly during construction, with cumulative settlement exceeding that of conventional surcharge preloading by more than 130 mm. At the same time, deep lateral soil displacement was effectively suppressed, and the foundation bearing capacity met design requirements.

In subsequent engineering practice, Wu et al. (2021) [126] combined AIT with PVDs and surcharge preloading to form a composite drainage system. Field monitoring showed that the cumulative settlement in the AIT test section was about 20% greater than that with PVDs alone, pore water pressure dissipated more rapidly, and the consolidation influence depth exceeded 15 m. Meanwhile, lateral displacement was significantly reduced, and cone penetration test results confirmed a marked improvement in undrained shear strength.

3.3. Other Exploratory Techniques

In recent years, to address the problems of high energy consumption and limited applicability in traditional drainage consolidation methods, researchers have explored several new techniques. Many of these approaches are inspired by natural processes or bionic principles. They emphasize reduced energy demand, improved environmental sustainability, and structural innovation, and thus demonstrate considerable potential for future development.

3.3.1. Osmosis-Based Drainage Technique

An osmosis-based drainage device developed by Tianjin University [130] incorporates a core structure of dialysis membranes and a liquid storage tank (see Figure 9). The dialysis membranes are wrapped around the main structure of the liquid storage tank. Operating on the principle of forward osmosis, it creates a stable osmotic pressure difference across the membrane. This pressure gradient spontaneously drives pore water discharge from soft soils. In contrast to vacuum preloading, which relies on pumps to maintain negative pressure, this method harnesses natural osmotic pressure. The driving forces can even exceed atmospheric pressure. As a result, the device not only reduces energy consumption but also shortens consolidation time.

The system is further equipped with an intelligent monitoring and control unit. It can sense liquid level changes in real time, adjust solution concentration automatically, and extract liquid when necessary. These features enable remote operation and optimize system performance. With its advantages of low energy demand, environmental compatibility, and construction simplicity, the device represents a novel and sustainable approach to green soft soil improvement.

3.3.2. Cellular Fluidics Drainage

Dudukovic et al. (2021) [131] proposed a cellular fluidics drainage device based on the principle of capillarity, and Figure 10 provides its schematic representation. This device mimics natural systems, such as water transport in plants, by utilizing architected three-dimensional unit-cell structures (e.g., body-centered cubic or isotruss cells) fabricated via additive manufacturing. It establishes efficient water migration pathways between soil and air through deterministic porous media: small pores within the cells contain and transport liquid via capillary action, while larger pores remain open to the gas phase, facilitating evaporation. Soil moisture continuously rises along these capillary channels—driven by surface tension and adhesive forces between the liquid and solid struts—and evaporates rapidly into the atmosphere at the gas–liquid interfaces, achieving energy-free, long-distance water migration and discharge.

The structural design substantially enlarges the water–air interfacial area, accelerating evaporation efficiency by optimizing cell type, size, and relative density to ‘program’ flow paths. For instance, during operation, liquid infiltrates the tetragonal pyramid formed by diagonal struts, accelerating along the central axis towards the cell node, where inertial effects may cause interface oscillations before stabilization. Furthermore, through modular assembly and dendritic (tree-branch-like) structural arrangements, drainage capacity can be enhanced for scalable applications in soft soil consolidation.

4. Innovations and Advances in Drainage Elements

4.1. Novel Drainage Element Structures

The structural design of drainage elements plays a critical role in soft soil improvement. It determines drainage channel smoothness and vacuum transfer stability. It also directly influences consolidation efficiency and reinforcement performance. In recent years, researchers have proposed various structural innovations to address the shortcomings of traditional PVDs, such as bending deformation, clogging, and insufficient consolidation rates.

4.1.1. Geotextile Encased Columns (GEC)

The GEC are granular piles encased in high-strength geotextile sleeves, providing structural support and drainage in soft soils with undrained shear strengths below 15 kPa [132]. The geotextile confines the infill under horizontal stresses, forming a stiff structure that transfers loads to deeper strata and limits stress on soft ground. Installation involves vibrating a steel casing into the soil, inserting the sleeve, filling with granular material, and retracting the casing to densify the column. Advantages include immediate loadability, up to 90% consolidation during construction, and 50–75% settlement reduction, suiting marine soft soil applications in roads, railways, ports, and reclamation.

Early studies focused on pressure relief and stability. Schnaid et al. (2017) [133] used GEC in a bridge abutment on soft soil, with monitoring showing 50% reduction in horizontal pressures on piles during embankment construction, enhancing consolidation in marine clays. Subsequent research integrated sustainable materials for performance enhancement. Pandey et al. (2021) [134] evaluated stone columns encased with conductive jute geotextile under k0 stress conditions, using electrokinetic processes with a 0.1 V/mm voltage gradient. Results showed increased undrained shear strength, higher modulus of subgrade reaction, and reduced settlement and compression index compared to ordinary or non-conductive encased columns. The chemical and mineralogical analyses confirming alterations in pH, composition, and microfabric for greener, low-carbon applications. Advancing eco-friendly designs, Liu et al. (2025) [135] analyzed geotextile-encased cinder gravel columns via DEM-FDM under triaxial compression. Cinder gravel as a volcanic byproduct enhanced bearing capacity with higher density and coarser gradations, balancing efficiency in marine soils. Recently, Erten et al. (2025) [136] compared CO₂ emissions of stone columns versus GEC, showing GEC’s lower footprint due to reduced material and transport needs.

4.1.2. Integrated Drainage Boards

Traditional separated PVDs are prone to vacuum loss during installation because the filter membrane detaches from the core board. To address this issue, Cai et al. (2017) [137] developed an integrated PVD, in which the filter membrane and core are tightly bonded using thermal fusion technology. This improves bending and tensile resistance and prevents Z-shaped folding or breakage in soft sludge. Zhang et al. (2021) [138] further validated the superiority of integrated PVDs in a “freeze–thaw and vacuum preloading” sludge treatment project. Results showed that the consolidation rate of integrated PVDs was significantly faster than that of conventional separated drains, with more uniform dewatering performance. Such structural innovations have greatly improved the mechanical stability of PVDs and the continuity of vacuum transfer. On this basis, Chen et al. (2023) [63] proposed a horizontal drainage enhanced geotextile (HDeG). This design combines prefabricated drains with geotextiles to form a plate-like integrated structure, further extending the integrated approach.

4.1.3. Multi-Channel Expanded Drainage Boards

To increase the drainage influence zone, researchers have designed multi-channel drainage boards. Two typical examples are radial and winged drainage boards (see Figure 11). Feng et al. (2021) [139] designed a radial board that combines vertical and horizontal channels into a radial drainage network. This accelerated pore water pressure dissipation and expanded the consolidation area. Fu and Chai (2020) [140] introduced the winged PVD. In this design, auxiliary wings were added on both sides of the board to provide extra horizontal drainage paths. These wings enlarged the effective drainage diameter and improved consolidation efficiency. Although different in structure, both designs share the goal of enhancing horizontal drainage to increase coverage and improve vacuum transfer. Such innovations move beyond the limits of single-channel PVDs and represent an important direction for improving consolidation efficiency.

4.1.4. Tubular EKG

The EKGs have introduced another type of structural innovation. As demonstrated in Figure 12, compared with traditional plate EKGs, tubular EKGs offer higher stiffness and larger drainage cross-sections. These features reduce the risk of bending and clogging [141]. Experiments showed that tubular EKGs had 1.33 times higher drainage efficiency than plate EKGs. They also produced greater soft soil improvement, with shear strength gains of about 1.76 times. Because of their high structural stability and clogging resistance, tubular EKGs are particularly suitable for thick hydraulic fills and high-water-content soft soils.

4.2. Novel Materials for Drainage Elements

Although plastic PVDs are widely used in soft soil improvement, they are mainly made of polyethylene (PE) and polypropylene (PP). These materials are non-degradable and can remain in the soil for long periods. As a result, they create environmental burdens and may interfere with later foundation construction. To support green, low-carbon, and sustainable development, recent studies have focused on biodegradable natural materials and the reuse of waste-derived drainage elements. Both approaches have shown promising results in laboratory and field applications.

4.2.1. Biodegradable Drainage Elements

Natural fibers are renewable, biodegradable, and have a low-carbon footprint. They are therefore regarded as important alternatives to plastic drainage boards. Nguyen et al. (2018, 2020) [142,143] developed biodegradable BPVDs using jute and coir. Laboratory and field tests showed that their consolidation effects were comparable to conventional plastic drains. Asha and Mandal (2015) [144] tested natural PVDs (NPVDs) made of jute and coir in marine clays. They found that NPVDs achieved similar consolidation rates to synthetic materials and significantly accelerated settlement. Although performance declined under high stress, it was still sufficient under medium and low stress. These findings suggest that natural-fiber drainage elements provide effective drainage during service and then degrade naturally, reducing environmental pollution.

Agricultural by-products such as rice straw and wheat straw have also been used as biodegradable drainage materials. Xu et al. (2017, 2020, 2022) [145,146,147] confirmed that straw-based drains performed well in vacuum preloading and sludge consolidation. Their drainage capacity was similar to plastic drains, and they adapted well to soil deformation. Yuan et al. (2024, 2025) [148,149] further examined their degradation behavior in soft soils. They found that straw drains met consolidation requirements during construction and gradually degraded after service. This ensured both effective ground improvement and environmental sustainability.

Recent reviews on sustainable vertical drains highlight biobased alternatives for soft soil consolidation. Dijkstra and Bodamer (2024) [150] note that biodrains, made from poly-lactic acid with additives, offer 300–400% higher discharge capacities, up to 220 mL/s under 120 kPa, compared to conventional PVDs at 75 mL/s. They reduce CO₂ footprints by over 35% using optimized extrusion processes compatible with standard equipment. Fiber drains, using jute and coconut fibers, have lower discharge at 5–15 mL/s but show comparable consolidation rates in field trials. Their negative CO₂ emissions come from plant-based carbon storage, supporting a low-carbon approach in marine environments.

4.2.2. Waste-Recycling Materials

In addition to natural fibers, researchers have explored recycling solid waste into drainage materials. Lou et al. (2025) [151] proposed combining face mask fibers (FMFs) with drainage boards to create a composite system. Tests showed that FMFs accelerated pore water pressure dissipation and settlement rates. At the same time, they enabled the reuse of plastic waste. This research opened new opportunities for drainage materials, demonstrating that strong drainage and consolidation performance can be achieved together with environmental protection.

4.3. Multifunctional Composite Drainage Elements

With the growing demands of soft soil improvement, traditional drainage elements that provide only a single function can no longer meet engineering needs. They fall short in terms of consolidation efficiency, environmental safety, and low-carbon development. In recent years, researchers have explored coupling external energy fields such as air, heat, and electricity to develop various multifunctional composite drainage elements. These innovations provide new pathways for soft soil reinforcement.

4.3.1. Air–Drainage Composite Elements

Cai et al. (2018) [152] proposed booster PVDs. These drains not only provide conventional drainage but also act as channels for compressed air, replacing traditional booster tubes. Field trials in 20 m deep marine clay in Wenzhou showed that this method alleviated deep PVD clogging, improved vacuum transfer, and significantly increased consolidation. The results confirmed its strong applicability in deep marine soft soils.

4.3.2. Thermal–Drainage Composite Elements

Tang et al. (2025) [153] introduced prefabricated vertical thermo-drains. In this design, U-shaped heating pipes are attached externally to conventional PVDs, and circulating hot water transfers thermal energy into the soil (see Figure 13). The heating process elevates pore water pressure, reduces water viscosity, and generates thermo-osmosis. Together, these mechanisms accelerate consolidation.

4.3.3. Electroosmotic–Vacuum Composite Elements

Zhang et al. (2025) [154] proposed combining vacuum preloading with electroosmosis by embedding EKG within vertical drains. This configuration allows both drainage and current-driven flow. Results showed that it accelerated pore water pressure dissipation, reduced water content, and improved shear strength, which enhanced the reinforcement of deep soft soils. Similarly, Feng et al. (2025) [155] developed EKG radial drainage boards. These combine vertical and horizontal drains with conductive carbon fiber geotextiles, giving them both drainage and conductive functions. Tests showed that this method not only increased the consolidation rate of soft soils but also removed heavy metals such as copper (Cu).

5. Discussion and Perspectives

5.1. Summary of Current Technological Advances

5.1.1. Evolution of Traditional Techniques

Traditional drainage consolidation methods for soft ground (including surcharge preloading, vacuum preloading, electroosmotic drainage, dynamic consolidation, dewatering preloading, and thermal consolidation) have long served as fundamental techniques in engineering practice. With relatively mature construction experience and reliable reinforcement outcomes, these techniques constitute the mainstream system of soft ground improvement. As engineering demands increase and green, low-carbon concepts gain prominence, these methods have undergone continuous optimization and innovation.

For example, surcharge preloading now uses water bags and soil bags to reduce reliance on sand and gravel, supporting local material use and recycling. Vacuum preloading has advanced with membrane-less systems, cyclic vacuum applications, and air-boosting measures, achieving energy savings and higher efficiency. Optimization of electroosmotic drainage has centered on electrode materials and power supply modes. The use of EKG has reduced electrode corrosion, while intermittent power supply and polarity reversal have lowered energy use. Thermal consolidation has tested solar and industrial waste heat as energy sources. Although still experimental, it shows strong potential for sustainable development. Overall, traditional methods are moving toward greener, low-carbon, and more efficient approaches while maintaining reliability.

5.1.2. Exploration and Potential of Emerging Techniques

As traditional methods near their limits, researchers have developed new techniques to address challenges such as deep consolidation, high energy consumption, and environmental pollution. The siphon drainage method requires no pumps or energy-intensive equipment. Instead, it uses natural water-level differences to drive pore water discharge. Field trials showed a groundwater drop of 3.44 m in 25 days, proving its feasibility. Its evolution into SVD generates a vacuum automatically within the soil, greatly increasing efficiency. Tests showed that SVD can reach vacuum pressures above 50 kPa.

The AIT creates multilayer drainage channels with high-pressure gas and an air-lift effect. It efficiently discharges deep pore water and overcomes the shallow treatment limits of surcharge and vacuum preloading. AIT accelerates consolidation in medium-to-deep soils and reduces post-construction settlement. Bionic intelligent drainage and cellular fluidics drainage represent new low-energy directions. The former uses osmotic pressure differences instead of pumps and integrates intelligent sensing for control. The latter relies on bio-inspired porous structures to enlarge the water–air exchange surface, enabling energy-free evaporative drainage. Together, these methods reflect an interdisciplinary trend, pointing toward integration with materials science, environmental science, and bioengineering.

5.1.3. Synergistic Effects of Combined Techniques

Multi-technology combinations have become an important strategy to increase efficiency and adapt to complex conditions. The vacuum–surcharge method combines external load and vacuum suction, producing more uniform consolidation. The vacuum–electroosmosis method is particularly effective in low-permeability soils, as electroosmosis offsets the depth limits of vacuum preloading. Vacuum–dynamic and vacuum–thermal methods have also proven effective in speeding pore pressure dissipation. More complex approaches, such as vacuum–surcharge–electroosmosis, have been tested in reclamation projects, producing faster consolidation while controlling energy use and risks. These results highlight the benefits of composite methods and point to future research in multi-field coupling.

5.1.4. Innovations and Breakthroughs in Drainage Elements

Drainage elements are core components of consolidation systems, and innovations in their structure and materials are now research frontiers. Integrated, radial, and multi-channel drainage boards improve vacuum transfer and consolidation uniformity. Tubular EKGs combine electroosmotic and drainage functions, increasing adaptability. In materials, natural fibers (such as jute, coir, and straw) and waste-derived products (such as face mask fibers) address plastic drains’ durability and pollution problems. Multifunctional composite elements further improve effectiveness. These advances suggest that future drainage elements will emphasize diverse structures, multifunctionality, and sustainable materials.

5.2. Challenges Facing Current Techniques

5.2.1. Ranges and Limitations of Technical Applications

Traditional methods suit shallow to moderate-depth marine soft soils, typically within 20 m. Surcharge preloading works for large-area soft ground. It reduces post-construction settlement with loads over 200 kPa. However, it requires massive materials, leading to high resource use and carbon emissions. New vacuum preloading advances in green, low-carbon, and efficient aspects. Yet, issues persist in maintaining vacuum pressure, PVD clogging, and vacuum loss at depth. Electroosmotic drainage fits ultra-soft, low-permeability soft soils. It enables fast dewatering and strength gains. But high energy use remains a problem. Electrode corrosion in saline environments and uneven effects limit large-scale application. VDCM does not suit ultra-low-permeability soft soils. It needs heavy machinery, generates much noise, and risks uneven densification. Engineering costs are also high. Thermal consolidation stays at the laboratory stage. It lacks extensive field applications.

Emerging technologies break some limits but face challenges in promotion and use. For example, siphon drainage achieves ultra-low-energy consolidation in low-permeability soils. But supporting drainage elements and construction processes are immature. And it still lacks large-scale application experience. AIT helps create multiple horizontal drainage layers at various depths. This speeds up pore water dissipation and soil consolidation. However, induced channels often close over time, reducing effectiveness. The method requires advanced high-pressure gas injection equipment. This raises costs and complexity. Osmosis-based drainage and cellular fluidics drainage are only at the conceptual stage. They are far from testing and practical application.

5.2.2. Environmental and Sustainability Concerns

The environmental persistence of conventional plastic drains is increasingly prominent, and their non-degradable nature may cause long-term impacts on subsurface environments. In addition, they can complicate subsequent construction and site rehabilitation. Natural fibers and recycled materials show promise, but their durability, degradation rate, and long-term mechanical performance still need further field verification. Chemical flocculants and additives can improve drainage consolidation, but they may introduce secondary pollution, creating an urgent need to balance environmental safety with engineering efficiency.

5.2.3. Obstacles to Green and Low-Carbon Transition

The main barrier to green transition is energy dependence. Vacuum pumps and electroosmosis systems still rely on conventional energy sources, producing high energy consumption and carbon emissions. This conflicts with sustainability goals. Solar, wind, and industrial waste heat show potential in experiments, but their large-scale use is limited by energy stability and local conditions. Furthermore, reducing energy use while maintaining effective consolidation remains unresolved. Progress will require advances in both equipment optimization and energy transition.

5.3. Future Directions and Prospects

5.3.1. Development and Optimization of Green and Low-Carbon Technologies

Future work should focus on developing drainage consolidation technologies powered by low-energy and renewable sources. The progress in siphon drainage offers a new perspective for green and low-carbon soil treatment. By using the gravitational potential of liquid columns to drive pore water discharge, this method reduces energy use and costs compared with vacuum or dewatering preloading. Its further development may involve optimizing drainage elements, improving adaptability under different soil conditions, and combining it with other methods.

For energy use, clean energy should replace fossil fuels. For example, solar-powered vacuum preloading or electroosmosis systems may be applied. Thermal consolidation driven by solar or industrial waste heat shows great potential but still requires more research and system optimization. In addition, high-efficiency pumps, optimized power supply modes, and new low-carbon additives (such as chitosan-modified materials) can further reduce emissions. To maximize these benefits, future efforts should prioritize the development of scalable solar-energy infrastructures and the standardization of low-carbon additives to ensure widespread adoption across diverse marine environments. Moreover, pilot projects integrating siphon drainage with existing vacuum systems could provide critical data on energy savings and soil response, offering actionable insights for immediate implementation in coastal reclamation sites. With the integration of renewable energy and energy-saving technologies, soft soil improvement is expected to meet the goals of “low-carbon, high-efficiency, and environmental sustainability.”

5.3.2. Multi-Field Coupling and Synergistic Mechanisms

A major direction is to clarify the microscopic mechanisms of soil consolidation under multiple fields—physical, chemical, and biological. For example, electro–chemo–bio methods may combine MICP with electroosmosis to strengthen soils at depth. Early studies have explored this, but future research must examine the interactions among pores, water, ions, and microbes more systematically. Beyond this, the coupling of heat, high-pressure gas, vacuum, and dynamic loads also deserves attention. The goal of multi-field synergy is to achieve “1 + 1 > 2,” where efficiency and depth exceed those of single methods. At the same time, research should ensure that combined effects reinforce each other rather than conflict. To advance this, comprehensive numerical simulations and field experiments are recommended to quantify synergistic effects, particularly under varying salinity and temperature conditions prevalent in marine soils. Current applications could benefit from hybrid pilot tests combining thermal and electroosmotic methods in low-permeability clays, providing a basis for refining design parameters and addressing conflicting interactions, such as electrode corrosion or uneven heat distribution. With deeper studies, future improvement methods will integrate multiple energy fields more effectively, reaching higher targets at lower costs.

5.3.3. Sustainable Materials and Waste Utilization

Sustainable materials will play a key role. One direction is to increase the use of recyclable, natural, and industrial by-product materials in drainage elements and reinforcement agents. Existing studies show that natural fiber drains (jute, coir, straw) can replace conventional PVDs in practice. More field trials are needed to verify long-term performance under different soil conditions, monitor degradation, and confirm that they meet design life requirements.

Other auxiliary materials can also adopt sustainable alternatives. For instance, vacuum preloading membranes and high-carbon binders such as cement and lime could be replaced. When using waste materials, it is essential to avoid harmful substances and secondary pollution. This requires testing for leaching toxicity, long-term stability, and developing technical standards. To enhance practical adoption, immediate recommendations include establishing standardized testing protocols for biodegradable drains to assess their durability under marine conditions, alongside pilot projects utilizing industrial by-products like fly ash in drainage systems to validate cost-effectiveness and environmental safety. Future research should focus on developing composite materials that combine natural fibers with synthetic reinforcements, potentially extending service life while maintaining sustainability. Overall, sustainable materials reduce costs, ease environmental burdens, and promote waste reuse, making them a key focus for future research.

5.3.4. Deep Integration of Intelligence and Digitalization

Future soft soil improvement will increasingly rely on intelligent and digital technologies. The introduction of artificial intelligence (AI), big data, and the Internet of Things (IoT) will enable real-time monitoring, prediction, and optimization of consolidation processes. For example, AI-driven dynamic control systems can automatically adjust the operation of vacuum pumps or power supply modes, minimizing energy consumption while ensuring consolidation effectiveness. The probabilistic observational method can effectively quantify uncertainties during the design stage, avoiding excessive surcharge loading and resource waste.

At the same time, virtual modeling and digital twin technologies can provide further support. By building digital twin models of site-specific ground–drainage systems and integrating real-time sensor data, these tools can predict the remaining construction time. They can also simulate the effects of different control strategies, which helps guide on-site optimization. To accelerate this transition, current efforts should focus on deploying IoT sensor networks in ongoing projects to collect high-resolution data, enabling the calibration of digital twin models for immediate operational improvements. For the future, developing AI algorithms tailored to predict soil behavior under multi-field conditions—such as thermal-electroosmotic coupling—could revolutionize design precision, with recommended investment in interdisciplinary collaborations to integrate geotechnical and data science expertise. This shift from “experience-driven” to “data-driven” practices will substantially enhance the scientific rigor and precision of soft soil improvement.

5.3.5. Concluding Remarks

In summary, drainage consolidation technologies for soft ground are currently undergoing parallel development across the stages of “traditional, improved, and innovative.” Traditional methods are continuously optimized, emerging technologies are rapidly developing, and composite techniques and material innovations are proliferating. Future development will place greater emphasis on intelligence, low-carbon practices, and sustainability, while showing a clear trend toward interdisciplinary integration. The implications of this review extend to providing a roadmap for engineers to adopt sustainable practices now, such as integrating renewable energy and biodegradable materials, while future prospects hinge on overcoming technical barriers through targeted research and policy support for intelligent systems. This dual focus ensures that soft soil improvement not only meets current infrastructure needs but also aligns with global sustainability goals, promising qualitative leaps in efficiency, cost-effectiveness, and environmental harmony. Through a combination of theoretical breakthroughs and engineering practice, soft soil improvement is expected to achieve qualitative advances in efficiency, cost-effectiveness, and environmental sustainability.

6. Conclusions

As an effective means of treating soft soils, drainage consolidation technologies have been widely applied worldwide. This review summarized progress in four areas: the optimization of traditional methods, the rise in emerging techniques, the combined application of multiple technologies, and innovations in drainage elements. While these advances highlight a shift toward greener, low-carbon, and efficient solutions, several gaps and challenges remain. The main conclusions are as follows:

(1)

Established techniques such as surcharge preloading, vacuum preloading, electroosmosis, dynamic loading, dewatering, and thermal consolidation are now optimized in several ways. These include refined loading schemes and staged application, pressure-variable or intermittent operation, improved sealing and vacuum-transmission control, energy-efficient pump management, probabilistic or AI-assisted design and monitoring, and the use of renewable heat or power. Emerging methods, including siphon drainage and aerosol injection, provide alternatives for deeper or more energy-efficient treatment. Hybrid approaches that combine multiple methods show promise for complex soils. However, the applicability of these techniques under highly variable field conditions and their long-term reliability remains insufficiently validated.

(2)

Innovations in drainage elements. As the core of drainage consolidation systems, innovations in the structure and materials of drainage elements have become research hotspots. Integrated, multi-channel, radial drainage boards and tubular electrokinetic geosynthetics (EKG) have demonstrated remarkable improvements in drainage efficiency and stability. Meanwhile, studies on natural fibers (e.g., jute, coir, straw), waste-recycled materials (e.g., face mask fibers), and biodegradable drains provide new pathways to address the environmental burdens of conventional plastic drains. These innovations have laid a foundation for the sustainable development of soft soil improvement. Yet, their durability, degradation rates, and mechanical performance in large-scale projects still require systematic testing and long-term monitoring.

(3)

Challenges and future focuses. At present, the applicability of drainage consolidation technologies remains limited under special conditions such as thick soft clay layers and ultra-soft sludge. Large-scale applications of emerging methods still face cost and technical barriers. Issues related to environmental safety, long-term material performance, and energy dependence also require urgent solutions. Future development should focus on low-energy and renewable-energy-driven green technologies; multi-field coupling mechanisms integrating electro, chemical, bio, thermal, air, and dynamic effects; the development of sustainable materials and waste reutilization; and the deep integration of intelligent and digital technologies to enable real-time monitoring and dynamic control of consolidation processes.

In conclusion, drainage consolidation methods stand at a turning point. They are evolving from traditional approaches toward greener, more efficient, and intelligent systems. Future progress depends not only on technical innovation but also on addressing cost, environmental safety, and scalability. With interdisciplinary integration, the field can deliver more reliable and sustainable solutions for complex soft ground worldwide.