Numerical Investigation on the Bearing Capacity Improvement of Shallow Foundation on Salinity Soft Clay Layers

Foundations play a pivotal role in structures by effectively transferring loads from the surface to the underlying soil, thereby preventing failure of either the soil or the foundation itself. Thus, it's essential to evaluate the soil's bearing capacity. This aspect has garnered significant attention from geotechnical researchers both historically and presently [1-5]. Soft soil foundations often encounter issues like excessive settling and inadequate load-bearing capacity, particularly in soils with Modern engineering practices frequently entail the replacement of weak topsoil with stabilized soil to mitigate the effects of small shear strength, for instance consolidated or considerably over-consolidated clays. However, the load-bearing capacity of layered soil is dependent on the capacities of both its top and bottom layers. Along the banks of the Euphrates River, fine sediment deposits like silt or silty clay, along with mud accumulation, create steep barriers that obstruct river flow. Utilizing river water for various engineering projects like car parks, dams, and bridges is facilitated by stable river banks; unstable banks can cause delays in exploitation. In the southern region of Iraq, soils along the Euphrates River are characterized as saline soft soils, posing challenges for civil engineering applications, particularly when dealing with fine subgrade soils. Using various chemicals to stabilize soil has been the subject of several investigations. Lime and cement stabilization is the standard procedure for stabilizing clayey soil. However, there is reason to investigate low-cost chemicals that may be used to alter soil properties. Ground improvement or ground modification refers to the application of mechanical, hydrological, physicochemical, biological, or any combination of these techniques to alter particular characteristics of naturally occurring soil deposits. Enhancing the permeability or reducing the amount of settlement existing soils is the primary objective of ground improvement. In order to improve the consistency limits and shear strength of saline soft soils, Al-Kinani and Fattah [6] investigated the effects of adding various salt compounds, such as NaCl, MgCl₂, Na₂SiO₃, and CaCl₂, in varying percentages (2%, 4%, 8%, and 10%), together with Portland cement. The unconfined compressive strength of the soil was found to increase from 290 to 814, 506, 404, 574, and 422 kPa, respectively, and to decrease the consistency limits when cement materials and a group of chlorides, NaCl, MgCl₂, Na₂SiO₃, and CaCl₂, were added.

Moayedi et al. [7] studied the effect of sodium silicate system binders on the physicochemical characteristics of soft soil. A number of batch tests were conducted. According to the results, adding 3 mol/L of Na₂SiO₃ may raise the unconfined compressive strength (UCS) of soil in group testing by up to 220% of its reference strength, while adding activators CaCl₂ and/or Al₂(SO₄)₃ can boost UCS values by up to 270%. Additionally, adding CaCl₂ at greater concentrations (such as 1 mol/L) had no appreciable impact on the UCS findings.

Al Asadi and Al-Kinani [8] studied the soft soil was treated with salt concentrations of 2%, 4%, 8%, and 10% to observe the effects on consistency limits and compaction properties. The study's conclusions showed that while the optimal moisture content decreased, the highest dry density increased as the amount of each chloride component increased. The plastic limit, plasticity index, and liquid limit all decrease with increasing salt concentration.

For instance, Bhardwaj and Sharma [9] used FEM to expect the bearing farthest reaches of shallow foundations on settled layered soil with various point degrees (L/B) and soil layers. They saw that raising the degree of unsterilized clayey soil impelled a reduction in balance bearing breaking point.

Szypcio and Dołżyk [10] used PLAXIS to analyze four specific earth conditions with two-layered structures of varying thickness ratios, aiming to determine the ultimate bearing capacity of strip and square footings. Their findings identified an optimal thickness ratio (H/B = 2) for evaluating the bearing capacity of such subsoils. Ground improvement efforts focus on enhancing soil strength, minimizing settlement, and modifying permeability. Saline soils, often containing hydrated gypsum (CaSO₄·2H₂O), SiO₂, calcite (CaCO₃), or NaCl, are typically found at the surface. High moisture content, caused by capillary action and diffusion, along with increased water salinity due to salt sedimentation from evaporation, are key factors contributing to saline soil formation [3, 4]. Sharp soils with chloride salt levels beating 3% show high conductivity [5]. Saline particles soak penetrable electric media through water migration and hence team up with soil particles. Chloride particles other than have a wide hydration range and can ingest water. Without fitting treatment, such soil would forget to satisfy improvement rules on account of issues like salt turn of events, breaking down, and immersion ingestion. Chloride particles impact both the microstructure and strength properties of treated soil, applying a fundamental impact on its short and huge length strength [7, 8].

The substantial presence of saline content suggests potential alterations in the geotechnical attributes of clayey soils when subjected to water infiltration. Experiments involving sodium chloride mixed with clayey soils revealed a decrease in both plasticity index and unconfined compressive strength with increasing salt concentrations [9]. Introducing chloride salt notably enhanced the structure of lime-soil blends, augmenting the proportion of coarse soil particles while reducing overall surface area. However, elevating salt levels diminished the homogeneity degree, though a linear relationship was observed between salt quantity and microstructural features such as bone area, appearance ratio, and roundness [10]. Incorporating up to 8% calcium chloride proved beneficial for lateritic soil improvement, although it's not ideal as a standalone stabilizer but rather serves well as a modifier or admixture in cement stabilization for lateritic soil. Research conducted in Shatt Al-Arab Southern Iraq explored the impact the impact of salinity on the engineering properties of fine-grained soil was explored, involving laboratory tests such as Atterberg parameters, standard compaction, consolidation, and soil shear strength analysis. It is worth noting that the presence of discernible levels of suspended salts in water can result in changes to the soil's engineering attributes [11].

The effects of salt solutions with varying concentrations of seawater on the geotechnical characteristics of soft soil scattered in various regions from Mong Cai to Thanh Hoa province have been thoroughly examined by Nguyen et al. [12]. Prior to testing, the undisturbed soil samples were steeped for 10 days in various salt solutions. The findings of the study demonstrate that a rise in seawater proportions raises the soil's salt content. In terms of some geotechnical characteristics, as soil salt content rises, the compression index and vertical coefficient of consolidation rise while the Atterberg limits and the soil's undrained shear strength decrease.

Different studies evaluated the impact of Magnesium Chloride (MgCl₂) emulsion on the geotechnical characteristics of clay soils. Turkoz et al. [13] found that increasing the MgCl₂ content led to a decrease in soil consistency (Atterberg limits). The addition of NaCl, CaCl₂, and MgCl₂ was observed to fill the voids between soil particles because the particle size of these compounds is smaller than that of soil particles, facilitating their replacement of voids. As a result, the compression guide and swelling index reduced with increasing chloride compound percentage [14, 15].

In order to determine the subsurface stratification, the field works for the site's geotechnical soil investigations include drilling three boreholes to a depth of 10 meters below the current ground level, performing the required field tests, and gathering undisturbed and disturbed soil samples for laboratory analysis. Soil samples are tested in a lab to ascertain its engineering and physical characteristics at certain predetermined depths.

Grayish silty clay soils, typical of saline areas in southern Iraq, were sourced from the south of the country. Soil specimens were gathered from a depth ranging between 0.5 to 1 meter within Al-Nasiriyah city, located in southern Iraq (Coordinates: Latitude = 628069, Longitude = 3427088). The study area was selected between the Euphrates riverbank and the public estuary project, a significant development endeavor in Iraq. This project plays a crucial role in conveying saline water for irrigation purposes, as depicted by the blue mark in Figure 1. This location exhibits a strong likelihood of heightened salinization due to the area being exposed to drought conditions. Per the standard specification ASTM D 2487-11, the soil falls under the classification ML (Fine-grained soil). After transferring the samples to the Civil Engineering Departments' laboratories at the University of Thi-Qar, comprehensive geotechnical and chemical analyses were conducted. Table 1 outlines the essential geotechnical and chemical characteristics of the soil.

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Figure 1. Satellite image of the location of the test points for samples extraction

Table 1. The geotechnical properties of the tested soil

5.1 The natural soil

The shear strength of soils treated with various additives was evaluated through UCS testing. To achieve the maximum dry unit weight and optimum water content, the samples were compacted in three layers using a Harvard miniature compaction device. Each compacted specimen had a diameter of 33 mm and a length of 70 mm.

The undrained shear strength of the treated soils was assessed by incorporating different additives, including Portland cement, NaCl, MgCl₂·6H₂O, CaCl₂·2H₂O, and sodium silicate, at varying percentages 0%, 2%, 4%, 8%, and 10%. The results of these tests are presented in Table 2.

Table 2. The undrained shear strength results for different chemical agents

5.2 Portland cement

According to the unconfined compressive strength test, the unconfined compressive strength increased as the percent of common cement increased. However, it was discovered that when ordinary cement was mixed with 8% and 10% of cement mixing, the results were unsatisfactory due to the absence of bonding between the cement and the soil.

5.3 Sodium chloride

The results indicate that sodium chloride—a chemical that forms clusters of tiny particles and binds them together—can dissolve rapidly in water, providing a sufficient supply of salt ions for ionic exchange interactions with clayey soil [18, 19].

Sodium chloride was added at varying dosages of 2%, 4%, 8%, and 10% by weight of the soil. The UCS initially increased from 290 kN/m² to 574 kN/m² with the addition of 2% sodium chloride. However, when the sodium chloride content was further increased to 4%, 8%, and 10%, a decline in undrained shear strength was observed.

5.4 Calcium chloride

Suresh and Murugaiyan [20] provided evidence of solid calcium chloride's remarkable water-absorbing capability. At a relative humidity of 95%, solid CaCl₂ can absorb 16.6 times its weight in water. Even at a humidity level of 30%, it can still absorb nearly its entire weight in water.

Calcium chloride has been widely used as a stabilizer due to its ability to modify a material’s permeability, compressibility, and strength. Its primary function is to bind and agglomerate fine particles, enhancing soil stability. The addition of 2% calcium chloride increases the UCS of the soil to 422 kN/m². However, when the calcium chloride content was further increased to 4%, 8%, and 10%, a decline in UCS was observed.

5.5 Magnesium chloride

Magnesium chloride, a sea salt, is utilized as a green stabilizer because it doesn't harm humans or animals and doesn't erode concrete, asphalt, or automobiles. Conversely, magnesium has an electrical configuration of 2, 8, 2 and an atomic number of 12. As a result, it possesses two more electrons above the nearest stable noble gas electrical configuration, neon. Magnesium cation (Mg²⁺) is formed when magnesium prefers to shed two electrons from its outermost shell and acquire a stable electronic state. A MgCl₂ molecule is created when two Mg atoms combine with two Cl atoms, transferring two electrons from the Mg to the Cl. Because of this, each atom has a stable octet electronic configuration.

When 2% MgCl₂ is added to soil, the UCS increases to 404 kN/m². However, it was discovered that when MgCl₂ was mixed with 4%, 8%, and 10% of the salt mix, the results showed a decrease in UCS.

5.6 Sodium silicates

Sodium silicates are polymeric, alkaline, inorganic compounds based on silica. Three fundamental components make up all silicates: water, alkali Na₂O or K₂O (raw material = soda ash or potash), and silica SiO₂ (raw material = sand). The UCS of soil rises to 574 kN/m² when 2% sodium silicates are used. Nevertheless, it was shown that the UCS decreased when MgCl₂ was combined with 4%, 8%, and 10% of the salt mixture.

The commercial Plaxis finite element program was utilized as the numerical technique in this investigation. A three-dimensional finite element analysis is called Plaxis 3D. Unless otherwise noted, all analyses in this publication are based on 15 node analysis. The linear soil model, or Mohr Coulomb (MC) model, was used.

7.1 Constitutive models and parametric study

This section does not aim to provide a comprehensive overview of all constitutive models applicable to soft clay soils, nor does it seek to determine the most reliable soil models for clay. Instead, its objective is to outline various constitutive models that effectively represent the mechanical behavior of soft clays and to highlight the importance of specific numerical modeling approaches in accurately describing soft clay behavior.

The MC model (Method B) is employed to simulate geological layers, where stiffness and strength are specified in terms of effective properties. The MC model assumes elastic behavior before failure. The key parameters utilized include E', Cᵤ, or C' for undrained and drained conditions, respectively. It is assumed that clay is normally consolidated (OCR = 1). The MC model is chosen for its simplicity and its ability to predict yielding and strain softening.

Additionally, a linear-elastic model is used for footing analysis. The mesh for all models in Plaxis-3D is generated using a medium element distribution, with a maximum of 256 cores. This study investigates the behavior of two-layered soils beneath a rigid rectangular footing. The footing is modeled as a linear elastic material, while the MC model is used to simulate the saline soil, and a soft soil model represents the lower layer.

The geotechnical properties of soft clay, stabilized clay, and footing materials used in the FEM analysis are detailed in Table 4. Additionally, the study examines a reference case, where the subsoil consists solely of soft soil. Table 3 outlines the specific soil types present in the upper and lower layers beneath both types of footings in two-layered soil systems.

The ultimate bearing capacity of the footing is determined from the load-settlement curve using the 10% B method. Furthermore, the impact of the water table on bearing capacity calculations is not considered in this study.

Table 4. Properties of the materials used in the analysis