Research on Polyurethane-Stabilized Soils and Development of Quantitative Indicators for Integration into BIM-Based Project Planning

Abstract

This research presents the results of studies on the physical and mechanical properties of the soil–polymer composites developed by the Scientific and Production Company “Special Polymer Technologies” SPT^® by injecting polyurethane material into clay soils to strengthen the foundations of erected structures. A novel method is proposed to determine the strain characteristics of these composites, embracing the preparation of model specimens in cylindrical containers with subsequent static and dynamic load testing. The results of static tests showed a significant increase in the strain modulus in comparison to that of the soil, resulting in soil stabilization due to a decrease in the initial content of moisture squeezed out of the modified soil. A coefficient of increase in the deformation modulus (K_E) is introduced to quantitatively assess the soil stabilization efficiency. An original technique is also proposed for assessing composite durability, and it is based on analyzing the mass loss after cyclic wetting and drying. The proposed soil stabilization approach promotes and improves digital construction technologies such as Building Information Modeling (BIM) by enabling the accurate simulation and prediction of the behavior of loaded soil in foundation systems. The introduced quantifiable metrics can be integrated into Digital Twin- or BIM-based project planning tools, contributing to sustainability, safety, and reliability in modern construction practices.

1. Introduction

The load-bearing capacity of structural foundations in construction technologies is determined by the physical and mechanical properties of the underlying soils. And, very often, these properties require certain improvements due to problematic soils, in particular, loose clays that exhibit significant volume changes (swelling and shrinkage) under the action of some climatic factors, particularly during wetting and drying cycles [1,2]. Natural soils are in fact capillary-porous materials, and one of the factors of their stability is the presence of water or so-called soil moisture in the pore space. As a matter of fact, drying and swelling can change the pore pressure and capillary tension, resulting in crack propagation and ultimately reducing the bearing capacity of the foundation, potentially causing structural damage [3,4]. At low moisture levels, soils are more stable but are also prone to shrinkage and cracking.

Moisture content significantly affects soil stability under dynamic loading conditions [5]. With an increase in moisture content, the adhesive forces between fine soil particles are weakened. This results in increased strain and a reduced bearing capacity of the foundation. In water-saturated soils, a liquefaction effect may occur, whereby the soil loses its structural integrity and behaves like a liquid. Wet, sandy, and clayish soils are especially sensitive to this phenomenon [6,7]. Furthermore, high moisture content contributes to pore pressure buildup, enhancing deformation under vibrational loading [8]. Dynamic and vibrational loads are rather detrimental to transport infrastructure, often causing the rapid subsidence of clay- and loam-based underlying soils [9,10]. Soil moisture also contributes to the propagation of stray currents and the acceleration of electrocorrosion processes in transport infrastructure [11,12]. Therefore, it is essential not only to strengthen building foundations, but also to control and reduce their moisture content.

The scientific community has shown a marked increase in the number of publications on soil improvement methods, especially over the last decade (2015–2025), reflecting a growing interest in innovative solutions [13,14,15]. Traditional stabilization methods, such as mixing soil with cement and lime, are widely used for various engineering projects, including the strengthening of dams, embankment slopes, and road surfaces, as well as foundation stabilization [16]. These methods are known as those that increase compressive and tensile strength, reduce the plasticity index (PI), and improve soil compaction characteristics [13]. For example, the Deep Mixing Method (DMM), which makes use of lime and cement, has proven to be effective in increasing soil strength and reducing soil subsidence [17].

One of the ways to increase the bearing capacity of soils is to stabilize them by injecting hardening solutions [15,18]. The polymers used for soil stabilization are typically categorized as geopolymers (inorganic polymers), biopolymers, and synthetic organic polymers. Each class exhibits different properties and mechanisms of interaction with soil particles. A polymer’s efficiency depends on various factors such as molecular weight, particle size, charge, conformation, solubility, viscosity, pH, and moisture availability [19]. Geopolymers stabilize soils by a series of reactions resulting in the formation of cementing materials, in particular sodium and/or calcium aluminosilicate gel, which bind soil particles and form a denser matrix [20]. The mechanisms of stabilization by organic polymers of any nature include the formation of thin films covering soil particles, the formation of polymer bonds connecting non-contacting neighboring particles, and the origination of adhesion between the particles [19]. Each of these methods of soil stabilization is applicable under certain conditions and has its own advantages and disadvantages. For example, injecting liquid glass (silicification) or water-soluble urea resins (resinification) stabilizes the soil by impregnating the material. As a result, the application of silicification and resinification is limited in loamy soils due to their low permeability [18].

The diversity of polymer stabilization mechanisms is indicative of the efficiency of a particular polymer, which depends on its chemical composition and the way it interacts with a particular type of soil.

Numerous studies prove that polymers significantly improve the mechanical properties of soils, including strength, stiffness, and elastic modulus. Commercial polymers such as ROCAMIX, TECOFIX, and EPS PM50–PM70 have shown improvements in water retention and strength, with performance increasing proportionally to polymer concentration [21]. The authors of [22] showed that the addition of polymers consistently improved the uniaxial compressive strength (UCS) and elastic modulus in different types of soils, sometimes outperforming traditional additives such as lime and cement. In addition to the direct addition of polymers, there are techniques for incorporating various fibers (e.g., papyrus, coir, multi-walled carbon nanotubes) into soil–polymer composites to further improve properties such as shear strength and strain reduction, and even to impart self-sensing properties [23].

Fine-grained clay soils with a large specific surface area and specific mineralogy interact with polymers differently from coarse-grained soils [24]. It is indicative of the fact that a one-size-fits-all approach to polymer stabilization is ineffective, and that targeted research is critical to understand the specific interaction mechanisms and optimize the application of a specific soil–polymer combination. It is advisable to use for such soils the method of injecting polyurethane materials that break up soil layers, compact them, and displace water, forming reinforcing components [25,26]. Polyurethane injection technology is new and increasingly used to strengthen shallow foundations, reduce differential settlements, lift structures, and improve the mechanical properties of soil. This technology has demonstrated its efficiency even for critical applications, such as the restoration of historic buildings [27]. The unique advantage of polyurethane is its chemical nature: it requires no external pressure to expand. Expandable polyurethane resins are formed through the exothermic reaction between the polyol and isocyanate, producing a large amount of carbon dioxide (in the presence of water or a foaming agent), which results in volumetric expansion and the formation of a porous hardened structure in a very short time [28]. Previous studies [25,26,27,28,29] have shown that the injected polyurethane material penetrates the soil through shrinkage cracks and their germs, and it breaks the soil into blocks and forms a cohesive soil–polymer composite. This technology is based on the use of expandable polyurethane resins, and it has significant practical advantages due to the minimum disturbance of existing structures and the topsoil, its rapid polymerization, and its ability to stabilize soil at various depths [26,27]. The ease of injection and short curing times make this technology particularly attractive for time-sensitive maintenance applications. A notable field of application is for the stabilization of railway track ballasts, where polyurethane enhances rail capacity and subgrade maintenance efficiency [30]. These properties collectively define a unique and extremely valuable strategic niche for polyurethane injection technology. Unlike conventional methods involving extensive excavation, long curing periods, or major disruption, polyurethane offers a fast, less invasive, and highly effective solution for existing structures that are prone to subsidence or require immediate stabilization. This makes it particularly suitable for urban environments, historic preservation projects, or critical infrastructure where downtime must be minimized.

According to the data reported in [31,32], polyurethane elastomers are susceptible to photo, thermal, ozonolytic, hydrolytic, chemical, oxidative, biological, and mechanical degradation. Therefore, it is necessary to investigate the extent of aggressive environmental impacts under specific service conditions. Durability is a critical factor for the long-term performance and service life of stabilized soil layers, particularly for their ability to maintain strength and rigidity during wetting–drying cycles and the moisture exposure cycle [13]. Moisture penetration is a major catalyst for causing damage to transportation structures [33]. Common laboratory durability tests include wet–dry cycles, capillary rise tests, and humidity test methods [34]. Although polymer treatments can reduce soil hydraulic conductivity and improve strength properties, susceptibility to moisture fluctuations can vary significantly for different polymer types [35]. For example, some studies have shown that cement-stabilized soil is less sensitive to moisture variation, whereas VAE copolymer-stabilized soil showed higher sensitivity to moisture changes [22,33]. This underscores the need for specific durability testing for each soil–polymer combination. Polymer stabilization of soil does not imply a universal moisture resistance; it rather implies a durability that is highly dependent on the specific polymer type and its unique interactions with water and the soil matrix. Detailed durability testing of a specific soil–polymer composite is necessary, as such specific testing is indispensable to accurately predict the long-term performance and reliability of a selected material under realistic conditions of variable moisture. Despite promising developments, challenges to the wider application of polymer stabilization include limited evaluation standards and unresolved durability issues, emphasizing the need to adopt standardized testing protocols and carry out further investigations of stabilization mechanisms [19,35].

Recently, the integration of Building Information Modeling (BIM) and Digital Twin technologies into geotechnical engineering has become a key strategy for ensuring infrastructure resilience and sustainability. These digital tools allow for the simulation of soil behavior and foundation performance over time, including interactions with polymer-based reinforcement [29,35]. Incorporating such technologies enables parametric design, performance monitoring, and lifecycle-based decision-making in soil stabilization projects [19].

The main purpose of this research is to develop and experimentally validate an original methodology for evaluating the deformation properties of a soil–polymer composite developed by the scientific production company “Special Polymer Technologies” SPT^® (Kyiv, UA 01033, Ukraine) by injecting polyurethane material into clay soil, as well as to establish the dependence of the deformation modulus of the composite on injection parameters, including initial state and soil moisture. Additionally, the durability of the resulting composite is assessed by analyzing mass loss during cyclic wetting and drying. The obtained quantitative parameters of the physical and mechanical characteristics of the soil–polymer composite can be integrated into BIM systems and Digital Twin systems to improve the efficiency of parametric simulation, predictive maintenance, and stability assessment in foundation design and geotechnical planning.

2. Materials and Methods

2.1. Soil–Polymer Composite Models

Polyurethane materials SPT^® Resins, in particular, fast-reacting, expanding multicomponent polyurethane materials, were used to produce and investigate soil–polymer composites for the foundations of artificial structures. SPT^® materials comply with the Ukrainian technical specification TU U 20.1-40781863-001:2016 and are manufactured by SPT Ukraine LLC (Kyiv, UA 01033, Ukraine). Polyurethane resin is a two-component system consisting of liquid polyol and isocyanate, which react to form a solid polymer. In this reaction, the polyol acts as the expansive agent, whereas the isocyanate provides the strength and density of the resulting material. An optimal 1:1 ratio ensures a balance between volumetric expansion and compressive strength. The curing process is accompanied by an exothermic reaction. In the presence of water, the reaction with isocyanate generates carbon dioxide, leading to substantial volumetric expansion and the formation of a cellular structure. In the absence of water, an inert blowing agent is used to initiate the expansion. As a result, the mixture rapidly transforms from a liquid into a high-strength polymer, making it highly effective for soil injection applications [26]. The principle of stabilization and strengthening of the underlying soils using these materials is as follows. Upon injection, the expanding polyurethane fills soil cavities and cracks. In the presence of water lenses, it displaces water, which is replaced by compacted soil and polymer, converting the former void into a high-strength zone. Additionally, soil compaction occurs under the action of the expanding force of polymaterials. As a result, the weak and loose soil zone is transformed into a reinforced mixture of compacted soil and durable polymer. Reinforcing bodies are formed and the bearing capacity of the base soils is increased several times, as shown in Figure 1. A certain waterproof barrier is also formed from compacted soil and polymaterials, which is a zone with much lower filtration characteristics than those of the surrounding soil.

To select soil for the composite models, we analyzed soil characteristics of the foundations of railway structures and buildings damaged due to insufficient bearing capacity. Clay soil with characteristics generalized for buildings and structures damaged in different years was selected for the models.

Hence, to manufacture and study the models, a soil was selected that is close to sandy loam in terms of properties, with the following consistency indicators: plasticity limit PL = 0.12–0.14; liquid limit LL = 0.17–0.19; plasticity index PI = 0.05–0.07.

Various models were developed for static and dynamic testing, considering the composite structure and the constraints of the testing equipment. Cylindrical clay soil models with a volume of 100 litres and a diameter of 600 mm were prepared for static testing (5 units), while the models with the same volume and a diameter of 400 mm were used for dynamic tests (5 units).

The soil was conditioned to a medium plastic state with a moisture content of w = 0.15 (15%) prior to model fabrication. Throughout the study, the determination of soil moisture was carried out using the standard gravimetric (weight-based) method with a moisture balance. Containers were filled with prepared soil in 50–70 mm thick layers, each compacted with 20 strokes using a 10 kg metal tamper with a 150 mm round base.

In three 100 litre models with a diameter of 600 mm for static tests and three 100 litre models with a diameter of 400 mm for dynamic tests—P1; P2; and P3—SPT^® material was injected into the soil according to the manufacturer’s technology. Two containers, i.e., C1 and C2, were used as control samples without polymer injection.

2.2. Methods of Static Testing of Soil–Polymer Composites

For static tests of soil models, a gantry device was arranged to allow static loading from the top, as shown in Figure 2. A 100 litre container with a 600 mm diameter soil model was placed in the gantry, and a square metal stamp with 200 mm side dimensions was placed on the soil along the model axis. A 4 tonne hydraulic jack and a 3 tonne dynamometer were placed between the stamp and the upper beam of the gantry. Four tripods were fixed to the container, each holding four clock-type linear displacement indicators in such a way that the indicator probes touch the upper edge of the die at the center of its sides, as shown in Figure 2.

The model was loaded incrementally using the jack, by applying 67 kg steps (10 dynamometer divisions) up to a maximum of 600 kg, which is equivalent to the subgrade load typical for culverts, with an overload factor of 1.5. At each loading stage, the force (stress) was measured by the dynamometer, and displacement was measured using the four indicators. The arithmetic mean of their readings was used for further tests.

Three composite samples and two control samples were tested sequentially, with moisture levels monitored during testing. The loading procedure comprised two phases, in particular, primary loading to compress surface irregularities, and secondary loading to construct force–displacement, stress–displacement, and stress–strain diagrams.

2.3. Methods of Dynamic Testing of a Soil–Polymer Composite

Dynamic testing was conducted using the MUP-50 testing machine (Tochmashpribor, Armavir, Russia), shown in Figure 3. The MUP-50 is a versatile hydraulic testing machine employed for evaluating materials under tension, compression, and bending. The load is applied through hydraulic oil pressure, ensuring smooth and precisely controlled force adjustment. A steady piston movement enables a uniform load transfer without shocks, while the load is delivered via a steel plate shaped to fit the specimen geometry.

A 200 mm square metal stamp was positioned centrally on the surface of each model. The stamp was subjected to dynamic loading at 3 Hz using the MUP-50, with a maximum force of 400 kg (combined permanent and temporary load) and a minimum force of 200 kg (permanent load). The dynamic load was applied for 109,000 cycles. Every 9900 cycles (approximately 30 min), testing was paused, and the settlement of the soil (stamp displacement) was measured.

Three composite models and two control models were tested sequentially, with moisture content monitored during testing. Three soil–polymer composite models were prepared with varying polymer concentrations per cubic meter: P.1—6.6 kg/m³; P.2—50.1 kg/m³; P.3—62.6 kg/m³. The polymer concentration was selected according to the manufacturer’s recommendations for the respective soil types and moisture contents: 6.6 kg/m³ was obtained by injection to refusal (complete saturation of pores and voids) in the control soil sample P1 with a moisture content of 6.9% (stiff consistency); concentrations of 50.1 kg/m³ and 62.6 kg/m³ were applied to soil samples P2 and P3, with a moisture content of 15% (plastic consistency).

2.4. Methods for Assessing the Durability of a Soil–Polymer Composite

The durability assessment method for the soil–polymer composite is based on simulating real service conditions that drive material degradation. According to the manufacturer’s catalogue and the technical specifications TU U 20.1-40781863-001:2016 (SPT, Kyiv, UA 01033, Ukraine), the investigated SPT^® polyurethane material demonstrates stability under environments of moderate aggressiveness. The durability of a soil–polymer composite in the absence of insolation depends to the greatest extent on alternate water saturation and dehydration. Under the operating conditions of railway transport facilities and buildings, two cycles typically occur per year. These are water saturation in spring and autumn, and dehydration in summer and winter.

To evaluate the durability of a soil–polymer composite, we propose to determine the mass loss of composite samples based on the number of cycles of alternate water saturation and drying. To do this research, eight (8) samples were cut from each model of soil–polymer composite on a stone-cutting machine, shown in Figure 4. The methodology involves weighing the specimens before the experiment and after the completion of each cycle to precisely capture mass changes. At each stage, the specimens are first saturated with water and then dried in an oven to a constant weight. Before and after each cycle, all samples are weighed, and their mass loss (Δm) is calculated. It is proposed to assess the durability of the soil–polymer composite by comparing the mass loss of its samples with the mass loss of similar samples of stabilized soil composites of known durability. This comparative approach enables extrapolation of the long-term behavior of the new composite and provides an estimate of its expected lifespan.

3. Results

3.1. Static Testing of Soil–Polymer Composites

Initially, it was noticed that polymer injection reduced soil moisture from 15% to 10.9%. It is likely that this reduction resulted from water displacement and soil compaction during injection. Based on static test results, stress–strain (σ-ε) graphs were plotted, and the deformation modulus (E) was determined for various moisture contents (w), as shown in Figure 5.

Figure 5 and

Table 1. Static test results for unreinforced samples (C.1, C.2) and reinforced samples (P.1, P.2, P.3).

Soil Condition	Sample Model	Sequence Number of the Load	Soil Moisture (w), %	Deformation Module (E), MPa
Natural	C.1	1	15	0.7
	C.2	1	15	0.5
		1	11.5	3.9
		1	9.5	33.5
		2	9.5	26.8
Reinforced	P.1	1	10.9	11.2
		2	10.9	19.4
	P.2	1	10.9	19.2
		2	10.9	49.0
		1	8.7	37.5
		2	8.7	74.9
	P.3	1	9.5	33.4
		2	9.5	56.2
		1	6.9	58.3
		2	6.9	89.0

show that the stabilization significantly decreased the soil deformability, resulting in higher deformation modulus values. For unreinforced soil with w = 15%, the deformation modulus ranges from 0.5 to 0.7 MPa. Following stabilization, the deformation modulus increases to at least 11.2 MPa at w = 10.9% and 33.4 MPa at w = 6.9%.

Figure 5d presents the relationship between the deformation modulus (E) and the soil moisture (w). Table 1 and Figure 5d show that the stabilization has not significantly increased the deformation modulus for dry sandy loam soils with moisture content below 8.6%. For wetter soils, the stabilization increases the deformation modulus by 2.5 times at 10% moisture (from 17.3 to 42.8 MPa), by 20 times at 14% (from 0.98 to 19.4 MPa), and by 30 times at 15% (from 0.54 to 16.4 MPa).

According to [36], based on the obtained experimental relationship of the deformation modulus (E) as a function of the moisture content (w), it is proposed to introduce a coefficient of increase in the deformation modulus due to the soil stabilization, K_E. This coefficient reflects the ratio of the deformation modulus of stabilized soil to the deformation modulus of soil in its natural state at the same moisture content, and it is intended for quantitative assessment of the effect of stabilization on soil deformability. By analogy, the values of the coefficient of increase in the deformation modulus were compared with the soil consistency index based on the obtained experimental data, as shown in Figure 6.

The coefficient of increase in the soil deformation modulus (K_E) due to the stabilization is given for soils of the following consistencies (I_C): very stiff to hard—1 (no effect) to 7; stiff—7 to 27; firm—27 to 37; soft—37 to 49; very soft—49 to 65; liquid—65 to 130.

Experimental findings confirmed that polyurethane injection significantly increases the deformation modulus (E) in soil–polymer composites. To quantify the increase in E, it is proposed to use the coefficient of increase in the modulus of deformation due to the stabilization. K_E is the ratio of the modulus of deformation (E) of the soil–polymer composite to that of natural soil at the same moisture content.

3.2. Dynamic Testing of a Soil–Polymer Composite

The dynamic testing data are processed and presented as diagrams, shown in Figure 7. As illustrated, the deformability of the soil–polymer composite is much lower than that of the soil in its natural state. The diagrams enable a determination of the vibrational shear modulus of the composite for any given number of dynamic load cycles.

Thus, an original method for studying the deformation properties under the action of static and dynamic loads of a soil–polymer composite obtained by injecting SPT^® polyurethane into clay soil has been developed and tested. The methodology includes the fabrication and testing of composite models.

3.3. Assessing the Durability of a Soil–Polymer Composite

As control specimens for durability assessment, soil samples from an embankment stabilized by silicatization were selected, with a confirmed service life of 15 years. Comparable conditions were then created for soils stabilized with a silicate solution and with polyurethane.

Based on the results of the research, a graph of mass loss (∆m) versus the number of cycles of alternate wetting and drying (N) was plotted, as shown in Figure 8. The durability of the soil–polymer composite was assessed by comparing the mass loss of its samples with the mass loss of similar soil samples stabilized by silicification.

The analysis of the structure of the models and samples revealed two distinct zones in the soil–polymer composite: an inner narrow zone of incompletely foamed polyurethane and an outer zone of a polyurethane–soil mixture adjacent to it, in which the polyurethane-to-soil proportion is decreased with an increase in distance from the inner zone. The mass loss of the soil–polymer composite samples was explained by the leaching (“falling out”) of soil particles from its outer zone. After 10 cycles, soil–polymer composite samples exhibited a mass loss of 5%. Subsequently, the leaching of soil particles practically stops, and the weight loss does not exceed 6%. In contrast to the soil stabilization by silicification, the mass loss exceeds 10% after 2 cycles and continues to increase, surpassing 23% after 10 cycles.

In addition to the deformation properties of the composite, we also assessed its durability, and it is proposed to determine the mass loss of composite samples based on the number of cycles of alternate drying and wetting, and to compare it with the mass loss of similar materials with known durability. Provided that silicification offers a service life of approximately 15 years, the soil–polymer composite is expected to provide a service life for at least 60 years.

4. Conclusions

This study presents a validated methodology for evaluating the strain and durability characteristics of soil–polymer composites formed via the injection of SPT^® polyurethane into clay soils. Static and dynamic testing data indicate a significant increase in the deformation modulus and durability due to the polymer stabilization. The conducted static tests demonstrated that the injection of SPT^® polyurethane into clay soils provides a significant improvement in stiffness. For untreated soil at a moisture content of 15%, the deformation modulus was only 0.5–0.7 MPa, whereas after stabilization it increased to 11.2 MPa at w = 10.9% and up to 33.4 MPa at w = 6.9%, corresponding to an increase by a factor of 16–60.

A new coefficient of increase in the deformation modulus (K_E) is proposed for quantitative analysis. The introduced coefficient K_E quantitatively confirmed the effectiveness of polyurethane stabilization, reaching values of 7–27 for stiff soils and up to 65–130 for very soft and liquid soils.

Dynamic tests under cyclic loading up to 109,000 cycles showed that the settlement of the reinforced composites was 2–3 times lower compared with control samples. At a polyurethane concentration of 50–62 kg/m³, the relative deformation was reduced by more than 50%, indicating an enhanced resistance to vibrational loading.

Durability studies confirmed the high stability of the polyurethane–soil composite under wetting–drying cycles. Mass loss of the composite specimens did not exceed 5–6% after 10 cycles, whereas silicatized soil lost more than 23%. This comparative approach allows us to extrapolate the durability characteristics of the composite under study and estimate its expected service life. Considering that silicification ensures a service life of at least 15 years, the expected service life of the polyurethane composite is at least 60 years.

The laboratory tests were conducted to verify the applicability of the proposed methodology and to establish the relationship between the deformation characteristics of the composite, the material dosage, and the soil consistency. The required dosage of the injected composition should be defined by the intended outcome: maintaining foundation stability, increasing the deformation modulus, or, conversely, expanding the soil volume to transfer displacements to the structure for its realignment. The optimization of the formulation should be considered in the context of specific engineering conditions and objectives, which is consistent with the practical applications of expanding polyurethane systems in foundation engineering.

In addition to traditional testing, this research introduces the concept of integrating experimental parameters into BIM-based digital platforms. This integration enables the use of parametric models for foundation reinforcement planning, enhancing predictive capabilities and long-term performance tracking through Digital Twin environments. The obtained results contribute to smart construction practices by enabling data-driven soil stabilization, offering a practical approach to infrastructure sustainability and resilience.