Search Results (13)

Accurate determination of constitutive model parameters is crucial for reliable numerical simulation in deep foundation pit engineering. This study presents an inverse analysis method using Multioutput Least-Squares Support Vector Regression (MLSSVR) optimized by the Gray Wolf Optimization (GWO) algorithm to invert key parameters of the Hardening Soil (HS) model. A case study on a foundation pit in the dredger fill stratum of Xiamen Railway integrates finite element simulation with machine learning. The proposed GWO-MLSSVR model demonstrates high predictive accuracy, with lateral displacement predictions closely matching field monitoring data and relative errors within 5% at various depths of measurement point. Compared to traditional inversion methods and MLSSVR models optimized by other algorithms, this approach significantly reduces prediction errors. Additionally, the influence of construction stages, input layer nodes, and training sample size on inversion performance is investigated. This method provides a practical and efficient solution for accurately obtaining soil parameters under complex soil conditions, thereby enhancing the reliability of geotechnical numerical simulations and offering valuable guidance for foundation pit design and safety assessment. Full article

Anisotropic permeability is of great significance for assessing the consolidation and drainage mode of soil layers in reclamation areas, as well as for preventing and controlling ground settlement after project construction. This paper analyzes the anisotropic permeability of the inland and nearshore soil layers in Chongming East Shoal, Shanghai, and the formation mechanism of anisotropic permeability through permeability and scanning electron microscope (SEM) tests. The results highlight that compared with dredger fill and sandy silt, the horizontal permeability coefficient of underlying soft clay (USC) is significantly higher than its vertical permeability coefficient, which is more significant in nearshore USC. Interestingly, the upper clay (21.5 m) in the thickest clay layer shows greater anisotropic permeability than the lower clay (41.5 m). Due to the instability of seepage channels, the USC anisotropic permeability increases in a fluctuating manner as the hydraulic gradient increases. Microstructural parameters are used to reveal the mechanism of anisotropic permeability, which shows that a simple soil skeleton and structure, strong particle orientation, decreased particle abundance, increased particle roundness, decreased particle contact area, and increased pore area all contribute to the enhancement of permeability. Moreover, micro-parameters have been proposed to evaluate anisotropic permeability in terms of the effective seepage-pore area. This approach addresses the constraint of water films on the permeability efficiency of USC particles. Full article

An effective method widely used in geotechnical engineering to solve the shrinkage and cracking issues in cement-stabilized soil (CS) is evenly mixing randomly distributed fibers into it. Dredger fills stabilized with cement and polypropylene fibers (PFCSs) are exposed to rainwater immersion and seawater erosion in coastal areas, influencing their mechanical performance and durability. In this study, direct shear and consolidation compression tests were conducted to investigate the influence of different curing environments on the mechanical properties and compressive behavior of PFCSs. Dominance and regression analyses were used to study the impact of each factor under different curing regimes. The reinforcement mechanism of different curing environments was also explored using scanning electron microscopy (SEM) imaging. The results show that the cohesion and elastic modulus of the specimens cured in seawater were reduced compared with those cured in freshwater and standard curing environments. The best fiber content for the strength and compressive modulus of PFCSs was determined to be 0.9% of the mass of dredged fill. The results of value-added contributions and the relative importance of each factor in different curing environments show that the overall average contribution of cement content in the seawater curing environment is reduced by 6.79% compared to the freshwater environment. Multiple linear regression models were developed, effectively describing the quantitative relationships of different properties under different curing conditions. Further, the shear strength was improved by the coupling effect of soil particles, a C-S-H gel, and polypropylene fibers in the PFCSs. However, the shear strength of the PFCSs was reduced due to the structural damage of the specimens in the freshwater and seawater curing environments. Full article

In contrast to the marine environment, coastal regions encompass substantial saline soils characterized by complex corrosive chemical compositions. This poses notable challenges to the durability of concrete structures erected in coastal dredger fill silty soil environments. This research undertook concrete chloride corrosion assessments in both a dredger fill silty soil environment and a simulated solution environment. The findings demonstrated a progressive escalation in the free chloride concentration within concrete specimens, as the exposure duration was extended from 60 to 120 d, and discernible convection zones were observed with depths ranging from 6 to 8 mm. The investigation revealed a diminishing trend in the apparent chloride diffusion coefficient, corresponding to the elongation of exposure time and the augmentation of burial depth. Paradoxically, the burial depth and exposure duration exhibited converse effects on the apparent surface chloride concentration. Empirical formulations were derived to express the apparent surface chloride concentrations and apparent chloride diffusion coefficients as dependent on the exposure time and burial depth variables. These models exhibited an excellent goodness of fit, reaching up to 0.96. Notably, concrete specimens interred at a depth of 0.0 m displayed a favorable likeness to the simulated solution environment throughout the 60 d exposure period. Full article

In this study, the step vacuum preloading method was used to reinforce high clay content dredger fill in the laboratory. The pore structures and permeability characteristics of dredger fill under different vacuum pressures were tested. The correlation between the pore structure parameters and permeability coefficient was analyzed using the grey T’s correlation analysis method. The research results indicate that the pore ratio, large pore (the diameter with a range of 4–40 μm) content, and permeability coefficient of dredger fill decreased with the increase in vacuum pressures, but the decrease rates of the pore ratio were different at various sampling locations. The contents of micropores (the diameter with a range of <0.04 μm) and small pores (the diameter with a range of 0.04–0.4 μm) increased with the increase in vacuum pressure. The results of the correlation analysis showed that a large pore content had a strong correlation with the permeability coefficient and could be used to describe the permeability characteristics of soil. The research results can provide reference for the improvement of the reinforcement method and for the evaluation of the reinforcement effect of dredger fill in engineering practice. Full article

This paper puts forward a new soft soil reinforcement technology—microbial-induced calcite precipitation (MICP) technology—which considers the problem of dredger fill soft-soil reinforcement in Dalian Taiping Bay. In this paper, the calcium carbonate content (CCC) and unconfined compressive strength (UCS) of microbial solidified dredger fill (MSDF) samples were determined using laboratory experiments. The microstructure and chemical composition of MSDF samples were studied by SEM–EDS and XRD. The failure and reinforcement mechanism of MSDF under different experimental conditions (ambient temperature, cementation solution concentration, and clay content) were investigated. The results showed that there was a certain residual strength after the peak strength of MSDF. With the increase of ambient temperature, the number of microorganisms increased, but the activities of urease, CCC, and UCS decreased. The UCS and CCC increased with the increase of cementation solution concentration, while they first increased and then decreased with the increase of clay content. The clay content enhanced the compactness of MSDF samples but reduced the soil permeability and weakened the mineralization. There were significant differences in the morphology of microbial-induced precipitation caused by different concentrations of cementation solution. Full article

Most areas of Caofeidian (Tangshan Caofeidian New District in Hebei Province, China) were formed by land reclamation, where the dredger fill has a high water content and a high salt content. The solidification of Caofeidian’s dredger fill is difficult because of salinisation, as well as environmental and economic factors. This article proposes a composite slag solidifying agent. The optimal proportion of various additives was determined by an unconfined compression test and orthogonal design. Next, a microanalysis was carried out by SEM and XRD tests to characterise the solidification mechanism of the composite slag solidifying agent. The results reveal that the composite slag solidifying agent can substantially improve the unconfined compressive strength of Caofeidian’s saline dredger fill by imparting a good microstructure: a compact overall structure was obtained and few voids were observed in the solidified soil. The optimal proportion was determined as 10% slag + 1.0% quicklime + 0.8% sodium silicate + 1.5% gypsum powder. In addition, the composite slag solidifying agent can effectively reduce the content of soluble salts in saline dredger fill and substantially improve the engineering characteristics of solidified soil. Full article

The land reclaimed from the seaside may have a long-term subsidence trend, which poses a potential geohazard in the future land use. Xiamen Xiang’an New Airport (XXNA) is built on reclaimed land since 2016. Based on the spaceborne Sentinel-1 data between January 2018 to April 2019 and the time series interferometric synthetic aperture radar (InSAR) technique, this paper analyzed the reclaimed land subsidence evolution at XXNA in this period. InSAR measurements show that XXNA is suffering from severe subsidence, mainly in three regions because of the earth and sand compacting. By analyzing the spatial subsidence characterizations of the main subsiding areas combined with historical land reclamation and future land use planning, we find the potential threat of subsidence to future land use. Correlation between subsidence and the period of reclamation was found, indicating that the consolidation and compression in dredger fill is the main cause of subsidence. By combining subsidence monitoring results with different land use types and adopting the Expectation (Ex) and Entropy (En) methods, we analyzed the key area with potential subsidence geo-hazard. This work shows that with SAR interferometry, it is possible to find the large area ground subsidence in the airport reclaimed area. The areas with potential subsidence geo-hazards are consistent with the deep reclaimed earth, which means high subsidence risk in the future. Full article

With the development of the economy, land reclamation, as a result of dredged soil, has become an effective measure to alleviate land scarcity in many coastal cities around the world. Chongming East Shoal (CES), a typical reclamation area in Shanghai that is formed by multi-phase reclamation projects, was selected as the study area. The small baseline subset–interferometry synthetic aperture radar (SBAS-InSAR) method was applied to derive the map of velocity distribution and accumulated deformation with 70 Sentinel-1 synthetic aperture radar (SAR) images collected from 22 March 2015 to 2 December 2019. In addition, 25 undisturbed soil samples, including dredger fill and underlying soil layers, were collected from five boreholes (maximum depth 55 m) through a field investigation. Laboratory tests were then performed on all soil samples in order to facilitate an understanding of geological features, including the measurement of basic physical properties, cation exchange capacity, compressibility, microscale structure, and pores. The present results show that the whole CES was undergoing differential ground deformation, with a velocity ranging from −47.5 to 34.6 mm/y. Fast (−3.4 mm/y) to slow (−0.3 mm/y) mean subsidence velocities were detected in multi-phase reclamation areas from inland areas to the coastline, and were controlled by building load and geological features of soil layers. Urbanization is the main factor that triggers accelerated subsidence and should receive special attention for reclamation areas that have been finished for a long time (over 20 years in this study). The geological features indicated that poor drainage conditions in offshore soil layers resulted in slow subsidence. The field investigation and laboratory test can be powerful explanatory tools to monitor the results from a mechanical perspective. Full article

As a solution to avoid the blockage of the drainage pipe by traditional vacuum preloading, step vacuum preloading (SVP) has been progressively studied. However, the effectiveness of this technique has yet to be systematically analyzed. In this study, an indoor model test was conducted in which vacuum pressure was applied in five stages (10, 20, 40, 60, and 80 kPa) to dredger soil with high clay content at a reclamation site in Binhai New Area, Tianjin, China. The extent of the consolidation effect of the soil was determined, and the effectiveness of the step vacuum preloading method to address drainage pipe blockage was evaluated. The results indicate that soil settlement increases at each stage of vacuum pressure treatment and the degree of vertical consolidation at each stage exceeds 90%. At the end of the treatment stage with vacuum pressure of 80 kPa, the weakly bound water was discharged. Dissipation of pore water pressure occurred in all stages. On the basis of these results, it is shown that SVP can efficiently reinforce dredger fill. Moreover, after SVP, the grain size of the soil and void ratio are still uniformly distributed. Regardless of their location from the drainage pipe, soil exhibits permeability coefficients within the same order of magnitude. The consolidation effect of soil in each stage and the increased drainage rate in the initial stage of vacuum preloading with 80 kPa indicate that the test in the current study can decrease the horizontal displacement of fine particles and can avoid drainage pipe blockage. Full article

Salt release from dredger filling sediment is a significant threat to freshwater resources in coastal regions around the world. This study focuses on the estimation of the field-scale salt-release process from the low-permeability sediment–water interface under different hydrological and hydrodynamic conditions. In situ sampling tests and physical experiments were implemented to calculate hydrogeological parameters and monitor sediment and water salinity. Numerical modeling was used to calibrate the molecular diffusion coefficient, of which the correlation coefficient was over 0.9, and explore the salt-release process across the sediment–water interface in Yuehu Lake, China. Furthermore, we discuss the influence of hydrologic conditions in terms of the lake stage and hydrodynamic conditions with water-exchange on the process of salt exchange between the sediment and water based on numerical simulations. Our findings showed that water-exchange accelerated the process of salt release from the sediment and maintained a relatively low salinity in the surface water. The salt-release rate decreased gradually as the concentration gradient between the water and sediment decreased. A frequency of water-exchange of 90 d maintained a rapid salt-release rate with fewer water-exchange steps. The influence of the lake stage was weak on the salt-release process at low-permeability area and salt release was not impeded before the salt capacity of water reached the maximum value. When the water–sediment salinity reached equilibrium, the salt-release process between the water and sediment equilibrated as the supply from the lower layers equaled the release to the water at the interface. These results are important in regard to controlling surface water salinization in coastal reclamation areas. Full article

Adding a curing agent can enhance the mechanical properties of soil including its compressive strength. However, few studies have quantitatively analyzed the compressive strength and microstructure of soils to explore the impact of changes in the microstructure on compressive strength. In addition, the cost of curing agents is too high to be widely used. In this study, curing agents with different proportions of fly ash were added to dredger fill to reduce the amount of curing agents needed. The quantitative analysis of the relationships between the modulus of compression Es and microstructures of stabilized soil samples is presented. The modulus of compression Es was gauged from compression tests. Microscopic images acquired using a scanning electron microscope were processed using the Image-Pro Plus (IPP) image processing software. The microscopic parameters, obtained using IPP, included the average equivalent particle size Dp, the average equivalent aperture size Db, and the plane pore ratio e. This research demonstrated that the fly ash added to the curing agent achieved the same effect as the curing agent, and the amount of curing agent required was reduced. Therefore, the modulus of compression for stabilized soil can be improved. This is due to the hydration products (i.e., calcium silicate hydrate, calcium hydroxide, and ettringite), produced by the hydration reaction, and which adhere to the surface of the particles and fill the spaces among them. Thus, the change in the pore structure and the compactness of the particles helps to increase the modulus of compression. In addition, there was a good linear relationship between the modulus of compression and the microscopic parameters. Using the mathematical relationships between the macroscopic and microscopic parameters, correlations can be built for macro–microscopic research. Full article

The deterioration of soil-cement in a saline environment leads to a reduction in strength and an increase in permeability. Effective methods of determining the deteriorated layer permeability coefficient of soil-cement are currently lacking. A laboratory test method for measuring the permeability coefficient of the deteriorated layer was proposed using the modified permeability coefficient testing apparatus. According to the proposed method, the permeability coefficient of the deteriorated layer could be obtained after testing the permeability coefficient of the soil-cement specimen in acuring room and testing the equivalent permeability coefficient and deterioration depth of the soil-cement specimen in a deteriorated environment. Using the marine dredger fill from Jiaozhou Bay as a case study, the deteriorated layer permeability coefficients of soil-cements with different cement contents were tested. It turned out that the permeability of the deteriorated layer increases with age. At the beginning of the curing age, higher cement content led to a smaller permeability coefficient of the deteriorated layer of soil-cement. As the curing age increased, the deteriorated layer permeability coefficient of the soil-cement with higher cement content increased. The evolution of the permeability coefficient of a deteriorated layer with age can be formulated as the Logistic function. This study provides support for anti-permeability designs of soil-cement structures in saline environments. Full article