Mechanical Performance of Expansive Clay Subgrades Under Heavy Compaction and Soaked Loading Conditions

Abstract

Expansive clay subgrades are highly moisture-sensitive, and their mechanical response under saturated conditions often governs the performance of flexible pavements and thickness design. This study experimentally evaluates the short-term mechanical performance of an untreated, remoulded expansive clay compacted with high compaction energy and tested under soaked loading conditions. Laboratory testing was carried out according to relevant Indian Standards and comprised of Modified Proctor compaction (IS 2720 Part 8), soaked California Bearing Ratio CBR testing after soaking for 96 h at 2.5 kg surcharge (IS 2720 Part 16), and unconsolidated-undrained UU direct shear testing as three runs to add normal stress. The results of the Modified Proctor tests showed a well-defined compaction peak: Maximum Dry Density (MDD) = 1.780 g/cm3 at Optimum Moisture Content (OMC) = 17.80%. The soil under saturated conditions showed low penetration resistance, with CBR = 4.96% at 2.5 mm and CBR = 3.86% at 5.0 mm, indicating that the saturated bearing capacity is limited under stable, ductile load penetration. UU direct shear response showed progressive mobilisation of shear resistance with increasing confinement, with peak shear stress increasing from approx 0.54 kg/cm2 (Run 1) to 0.80 kg/cm2 (Run 2) and 0.94 kg/cm2 (Run 3). The Mohr-Coulomb failure envelope was linear in the stress range tested, yielding shear. strength c = 0.36 kg/cm2, ph = 21.9deg (shear strength parameters given in kg/cm2 consistent with laboratory output). Overall, the sum of the results suggests that the short-term shear behaviour of the soil is predictable, while the soaked bearing resistance is mechanically limiting the soil and thus strength-based parameters of the soil (soaked CBR and shear parameters) may be used to select between pavement thickening and subgrade improvement/stabilisation in moisture-prone service conditions.

Introduction

This study investigates the engineering performance of expansive soil as a pavement subgrade, with emphasis on its behavior under moisture-sensitive conditions. Expansive clays undergo significant strength loss, swelling, and deformation during seasonal wetting, leading to pavement failures such as rutting and cracking. Since subgrade strength directly influences pavement thickness and service life, evaluating bearing capacity (CBR) and shear strength is more meaningful than relying solely on soil classification properties.

The literature highlights that moisture infiltration reduces soil suction and effective stress, weakening expansive soils through hydromechanical coupling. Previous research demonstrates that stabilizing expansive soils improves California Bearing Ratio (CBR), compressive strength, shear strength, and resilient modulus while reducing swelling. However, determining the untreated soil's baseline mechanical performance remains essential before selecting stabilization techniques. Consequently, this study aims to establish practical benchmarks for pavement subgrade evaluation based on heavy compaction characteristics, soaked CBR, and direct shear strength.

Materials and Methods

The experimental program used a remoulded expansive soil sample identified as "Soil, Lime and Coal" and consisted of three laboratory tests conducted according to Indian Standards:

• Modified Proctor (Heavy Compaction) Test (IS 2720 Part 8) to determine the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD).
• Soaked California Bearing Ratio (CBR) Test (IS 2720 Part 16) performed on specimens compacted at OMC and soaked for 96 hours.
• Direct Shear Test under Unconsolidated Undrained (UU) conditions using three different normal stress levels to determine shear strength parameters.

Results

1. Heavy Compaction Characteristics

The Modified Proctor test showed the typical compaction behavior of cohesive soils:

• Dry density increased with moisture content until reaching the optimum condition.
• Beyond the optimum moisture level, additional water reduced compaction efficiency.

The optimum compaction properties were:

• Optimum Moisture Content (OMC): 17.80%
• Maximum Dry Density (MDD): 1.780 g/cm³

The well-defined compaction curve confirmed reliable laboratory measurements and provided suitable conditions for subsequent strength testing.

2. Soaked California Bearing Ratio (CBR)

The soaked CBR test evaluated the soil's bearing capacity after 96 hours of saturation.

Results:

• CBR at 2.5 mm penetration: 4.96%
• CBR at 5.0 mm penetration: 3.86%

Since the higher value governs pavement design, the soaked CBR is 4.96%.

The load-penetration curve exhibited:

• Rapid initial increase in resistance due to particle interlocking.
• Gradual increase in load with penetration after 2.5 mm.
• No sudden failure, indicating ductile deformation typical of compacted fine-grained soils.

Although stable under loading, the relatively low soaked CBR indicates moderate bearing capacity, suggesting that stabilization may be required for heavy pavement applications.

3. Direct Shear Behavior

Direct shear tests under UU conditions demonstrated that:

• Shear resistance increased progressively with displacement.
• Higher normal stress produced greater peak shear stress.
• Specimen thickness decreased only slightly, indicating minor compression during shearing.
• Failure was ductile rather than brittle, with strain-hardening behavior observed.

Peak shear stresses increased systematically across the three tests:

• Run 1: approximately 0.54 kg/cm²
• Run 2: approximately 0.81 kg/cm²
• Run 3: approximately 0.94 kg/cm²

The shear stress-displacement curves confirmed a typical Mohr-Coulomb response, with confinement significantly improving shear strength.

Discussion

The results demonstrate that the tested expansive soil can be compacted effectively at its optimum moisture content but still exhibits only moderate bearing capacity under soaked conditions. The soaked CBR value of 4.96% indicates that moisture substantially limits pavement support, reinforcing previous findings that expansive soils lose strength upon saturation.

Direct shear testing showed stable short-term behavior with increasing confinement, indicating that while the soil possesses reasonable shear resistance, its performance remains highly dependent on moisture conditions.

Overall, the study confirms that:

• Modified Proctor compaction provides reliable reference conditions for subgrade evaluation.
• Soaked CBR is an effective indicator of pavement design suitability.
• Direct shear parameters provide essential information about deformation resistance and subgrade stability.
• Performance-based evaluation using mechanical properties offers a stronger basis for pavement design than relying solely on soil classification.

Conclusion

This study was conducted to experimentally evaluate the mechanical performance of a remoulded expansive clay subgrade under heavy compaction and soaked loading, using Modified Proctor compaction, soaked CBR test, and UU direct shear test, based on relevant IS procedures. The Modified Proctor results yielded a well-related compaction curve with a maximum dry density (MDD) of 1.780 g/cm3 and an OMC of 17.80%, indicating a Good compaction response for preparing repeatable laboratory specimens under heavy compaction conditions for subgrade construction. Under soaked conditions, the soil showed low CBR, controlled by the 2.5 mm penetration value of 4.96% (higher values at 3.86% for 5.0 mm). The load-penetration response revealed a stable, ductile increase in resistance without sudden failure. However, the magnitude of the CBR implies that bearing capacity is limited under saturated conditions, which has direct implications for increasing pavement thickness requirements or the need for subgrade enhancement when a soaked condition is anticipated. The UU direct shear results showed progressive mobilisation of shear resistance with displacement and definite dependence on applied normal stress. The Mohr-Coulomb envelope was observed to be linear over the tested stress range, and the derived parameters were c = 0.36 kg/cm2 and ph = 21.9 degrees, consistent with a soil exhibiting a certain amount of apparent cohesion and a moderate frictional contribution. Collectively, the results of the soaked CBR and shear strength tests confirm that this soil is moisture-sensitive and that its untreated soaked behaviour is mechanically limiting for subgrade service conditions in pavements. Therefore, for practical pavement engineering, the study supports considering strength-based performance as decision variables (soaked CBR and shear parameters in this case) to rationalise whether to proceed with subgrade improvement/stabilisation or design with increased pavement layer thickness to manage wet-season performance risks.

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Copyright

Copyright © 2026 Mr. Chetan Karekar, Dr. Swati Agrawal. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.