Geotechnical evaluation of soil composition and mechanical properties for foundation stability

2.1. Field sampling and site characterization

Field investigations were conducted at multiple representative sites to capture spatial variability under local climatic and geological conditions. At each site, disturbed and (where possible) undisturbed samples were collected from the surface and subsoil layers at depths relevant to foundation construction. Special attention was paid to zones with pronounced variability in natural moisture and particle-size distribution, as these factors strongly affect compaction quality, stiffness, and settlement behavior [5-7]. Stratigraphic descriptions, lithological features, and groundwater-related observations were documented during fieldwork.

2.2. Laboratory testing program

Collected samples were transported to the laboratory for detailed characterization. The laboratory program included the following groups of tests:

(i) Particle-size distribution (granulometry). Coarse fractions were evaluated by sieving, while fine fractions were determined by sedimentation (pipette method). The granulometric composition was expressed as the mass percentage of each size fraction (gravel, sand, silt, clay), which forms the basis for soil classification and provides an initial indication of permeability and compressibility tendencies [7], [8].

(ii) Physical and hydro-mechanical indices. Bulk density, natural water content, plasticity indices, and shrink-swell potential were measured using standard geotechnical procedures to characterize moisture sensitivity and volume-change behavior of fine-grained soils.

(iii) Permeability and water-retention/capillarity. Permeability and capillary rise/water-retention tests were performed to assess drainage capability and moisture migration mechanisms, which are critical for foundation stability in cohesive and fine-grained soils.

(iv) Compaction and deformation behavior. Laboratory compaction tests were carried out to determine optimum moisture content and maximum dry density for each soil type. Compressibility and deformation characteristics were evaluated to interpret settlement susceptibility under structural loading. Where applicable, stiffness and deformation response were additionally assessed using oedometer testing and field plate load testing to support engineering interpretation of settlement behavior.

2.3. Methodological considerations

The study design accounts for regional factors relevant to Uzbekistan, including seasonal moisture fluctuations, semi-arid conditions, and alluvial deposition processes, which may alter soil fabric, moisture regime, and mechanical response. By integrating field observations with laboratory measurements, the work develops a site-relevant geotechnical profile that supports engineering assessment of soil suitability for foundation construction.

2.4. Main research methods (summary)

The research is based on an integrated approach combining (i) field sampling and site characterization, (ii) granulometric assessment, (iii) hydro-mechanical testing (permeability, capillarity, swelling/shrinkage), and (iv) compaction and deformation testing (including oedometer/plate load where applicable). This combination provides the parameter set needed to evaluate both bearing capacity and settlement-related aspects of foundation stability.

2.5. Theoretical and calculation basis

The interpretation follows classical soil mechanics concepts linking composition, hydraulic behavior, and strength/deformation response under effective stress conditions.

Particle-size distribution. The fraction content is defined as:

where $m_i$ is the mass of fraction $i$, and $m$ is total sample mass.

Soil structure (void ratio). The structural state of the soil skeleton is described by the void ratio:

where $V_v$ is the volume of voids and $V_s$ is the volume of solids. This index governs compressibility and stiffness trends.

Hydraulic behavior (Darcy’s law). Permeability is quantified using Darcy’s relationship:

where $k$ is permeability, $Q$ is discharged volume, $L$ is sample length, $A$ is cross-sectional area, $h$ is hydraulic head difference, and $t$ is elapsed time.

Effective stress principle. Mechanical response is interpreted using effective stress:

where ${\sigma }^{\text{'}}$ is effective stress controlling strength and deformation, $\sigma$ is total stress, and $u$ is pore-water pressure.

Deformation modulus. Settlement-related behavior is characterized using the deformation modulus:

where $\mathrm{\Delta }{\sigma }^{\text{'}}$ is effective stress increment and $\mathrm{\Delta }\epsilon$ is strain increment.

Shear strength (Mohr-Coulomb). Shear resistance is evaluated as:

where $c^{\text{'}}$ is effective cohesion and ${\phi }^{\text{'}}$ is effective friction angle.

These relationships provide a consistent framework linking laboratory measurements to engineering design parameters and supporting assessment of foundation stability in terms of both bearing capacity and settlement susceptibility.

3.1. Swelling, shrinkage, and water sensitivity

Fine-grained soils may exhibit swelling upon wetting and shrinkage upon drying. Swelling potential is largely controlled by mineral composition and the content of active clay minerals; montmorillonite-rich soils typically show the highest expansion. In this study, swelling was assessed using the A. M. Vasilev apparatus. As an engineering criterion, soils with $\delta >$ 0.04 are treated as highly expansive.

Shrinkage occurs due to moisture loss through evaporation and/or vegetation uptake. This process reduces soil volume and can induce surface cracking, which alters both mechanical and hydraulic behavior and may contribute to non-uniform settlement.

3.2. Capillary rise and moisture migration

Capillary action is critical for fine-grained soils such as clays and loess. Small pore sizes generate capillary pressures that can raise water above the groundwater level. In loess soils, capillary rise may reach up to approximately 3 m, which can lead to progressive wetting of foundation soils and time-dependent reduction in stiffness and shear strength [6].

Table 1 summarizes representative particle settling velocities and corresponding particle diameters interpreted using Stokes, Sabanin, and Atterberg approaches.

3.3. Capillary height measurement procedure

Capillary height was measured using a laboratory setup consisting of a vertical glass tube connected to a funnel and reservoir. The soil was compacted incrementally inside the tube to minimize layering effects. The tube was fixed on a tripod and immersed into a water container. The wetting front was tracked visually by changes in soil color. To ensure consistency, the water level was maintained constant, and capillary height was recorded after 1, 2, 3, 5, 10, 20, and 30 minutes, as well as after 1 hour and 24 hours, until stabilization.

3.4. Coagulation (water resistance) and permeability

Soil behavior during immersion varies with texture and composition. Some soils disperse rapidly in water, while others retain structural integrity for extended periods. This behavior (water resistance/coagulation) is critical for structures where foundation soils are frequently exposed to water, including embankments, bridge approaches, and hydraulic structures [6-8].

Permeability was evaluated using standard laboratory devices (e.g., permeameter-type setups for sands and cohesive soils). For cohesive soils, specimens were prepared according to standard procedures to obtain representative filtration coefficients. The permeability coefficient is a key parameter for engineering calculations, influencing drainage conditions, consolidation rate, and long-term settlement development [4]. In field conditions, permeability may be assessed by pumping tests or water injection tests, providing hydraulic characteristics under in-situ conditions [9], [10].

3.5. Porosity–strength relationship

Under external loading, soil particles rearrange and interparticle forces mobilize resistance. A strength-related coefficient $M$ was used as an indicator associated with porosity and particle interaction. Table 2 presents the relationship between porosity level and $M$ considered in this study.

The data show a monotonic increase in $M$ with porosity from 26 % to 42 % (from 0.01187 to 0.05789), highlighting the role of soil structure in controlling mechanical response.

4.1. Bearing capacity implications

From a practical design perspective, the bearing resistance of soils is controlled by shear strength parameters mobilized under effective stress conditions. Moisture increase may reduce effective stress and weaken aqueous-colloidal bonding in fine-grained soils, which can decrease the mobilized shear resistance and reduce the margin of safety against bearing failure. Therefore, bearing-capacity verification should consider the most unfavorable moisture regime for soils exhibiting high capillarity or swelling sensitivity, especially where capillary rise can progressively wet the foundation zone [6].

4.2. Settlement and serviceability

Settlement susceptibility is directly related to compressibility and soil structure. Wetting may lead to microstructural rearrangement and loss of stiffness, which increases settlement under working loads. In addition, permeability controls drainage and the time scale of deformation development; therefore, filtration characteristics must be considered to evaluate the likelihood of time-dependent settlement and changes in pore-water conditions [4], [9], [10]. For moisture-sensitive deposits, compaction quality and moisture control (drainage) are essential to reduce long-term deformation.

4.3. Moisture control and durability-oriented measures

Because long-term performance is controlled by coupled “soil-foundation-structure” interactions, stability assessment should consider the combined effect of moisture on soil parameters and on structural materials in contact with the ground. Environmental interaction of structural elements with internal/external conditions may influence long-term performance, supporting the need for moisture control and durability-oriented design measures [11]. Furthermore, the influence of water on cement stone and concrete properties highlights the importance of accounting for moisture-related mechanisms when interpreting long-term foundation behavior and serviceability [12].