Factor Affecting the Shear Strength of Cohesionless Soil & Cohesive Soil

Introduction

• Geotechnical engineering relies on soil shear strength.
• It assesses soil stability and behaviour under load.
• Two primary soil types: cohesive and cohesionless.
• This presentation examines cohesive and cohesionless soil shear strength parameters.

Cohesive Soil

• Definition: Particles in cohesive soil have electrochemical forces that cause them to stay together.
• Characteristics: Cohesive behavior is shown by fine-grained soils like clay and silt (ACS, 2023)
• Shear strength: The capacity of cohesive soil to withstand deformation and shear stress.
• Moisture, effective stress, porosity, void ratio, and clay mineralogy all have a role in the shear strength of cohesive soil.

Shear Strength of Cohesive Soil

• Laboratory tests, for instance direct shear test, vane shear test, as well as triaxial test, are used to determine the shear strength of cohesive soil.
• Parameter of Shear strength : Cohesion (C) as well as internal friction angle (φ) (Adhadh et al., 2021)
• The cohesion represents the electrochemical forces between particles, while the internal friction angle indicates the ability of the particles to transfer shear stress.

Factors Affecting Shear Strength of Cohesive Soil - Moisture Content

• Moisture content affects the shear strength of the cohesive soil.
• Dry soil has a higher shear strength compared to fully saturated soil (Jiang et al., 2021).
• As enhance in moisture content, the soil will be softer and weaker.
• Optimum moisture: the moisture content at which the soil achieves maximum shear strength.

Factors Affecting Shear Strength of Cohesive Soil - Effective Stress

• Effective stress is the contrast among the complete pressure also the pore water pressure.
• Increasing effective stress leads to an enhance in shear strength of cohesive soil.
• Effective stress influences the bonding forces between soil particles (Kang et al., 2022).
• Higher effective stress helps in developing larger inter-particle contact forces.

Factors Affecting Shear Strength of Cohesive Soil - Porosity

• Porosity is the measure of void space within the soil.
• Low porosity (high density) leads to higher shear strength.
• Porosity affects the soil's ability to transmit and dissipate shear stress.
• Compaction of soil reduces porosity and increases shear strength.

Factors Affecting Shear Strength of Cohesive Soil - Void Ratio

• Void proportion is the proportion of the volume of voids to the volume of solids in a dirt example (Zhang et al., 2020).
• Low void ratios indicate higher density and greater particle interlocking.
• Higher void ratios lead to lower shear strength due to increased particle movement and reduced contact forces.

Factors Affecting Shear Strength of Cohesive Soil - Clay Mineralogy

• Clay mineralogy influences the shear strength of cohesive soil.
• Different types of clay minerals have varying cohesive properties.
• The presence of certain minerals, such as montmorillonite or kaolinite, can significantly affect shear strength.

Cohesionless Soil

• Definition: Soil with no cohesive properties and particles that do not stick together.
• Characteristics: Coarse-grained soil, such as sand and gravel, exhibits cohesionless behavior.
• Shear strength: The ability of cohesionless soil to resist shear stress and deformation.
• Factors affecting shear strength of cohesion less soil: particle size and shape, packing arrangement, and relative density.

Shear Strength of Cohesionless Soil

• The cohesionless soil’s shear strength of is measured using laboratory tests, for instance the triaxial test also direct shear test.
• Shear strength parameters: friction angle (φ) and angle of internal friction.
• The angle of friction represents the inter-locking of particles and the ability to transfer shear stress.

Factors Affecting Shear Strength of Cohesionless Soil - Particle Size and Shape

• Particle size and shape affect cohesionless soil shear strength.
• Angular and non-uniformly shaped particles provide greater interlocking and hence higher shear strength.
• Fine particles or rounded particles have lower shear strength due to reduced interlocking.

Factors Affecting Shear Strength of Cohesionless Soil - Packing Arrangement

• The packing arrangement of particles affects shear strength.
• Dense packing leads to higher interparticle forces and increased shear strength.
• Loose packing results in lower particle contacts and reduced shear strength.

Factors Affecting Shear Strength of Cohesionless Soil - Relative Density

• Relative density is a measure of how densely packed the soil particles are.
• Higher relative density corresponds to higher shear strength.
• Loose or dense states of cohesionless soil affect the interlocking and particle contact forces.

Case Study: Factors Affecting Shear Strength of Sand

Assuming a cohesionless soil is exposed to a pore liquid stream condition that outcomes in nothing (or close to nothing) successful pressure, the strength of the dirt goes to nothing. This is frequently alluded to as a "fast" condition.10 Under such circumstances, the leakage powers defeat the gravitational powers and the pore pressure rises to the complete pressure. The overabundance pore pressure then powers the overlying soil mass to rise and hurl. In cohesionless soils, the dirt air pockets in a "bubble"; since the dirt has no strength, it frequently cleans out.

There are two Normal Circumstances in which the Pore Pressure Approaches the Absolute Pressure, Apecifically as follows:

Assuming a cohesionless soil is exposed to a pore liquid stream condition that outcomes in nothing (or close to nothing) successful pressure, the strength of the dirt goes to nothing. This is frequently alluded to as a "fast" condition.10 Under such circumstances, the leakage powers defeat the gravitational powers and the pore pressure rises to the complete pressure. The overabundance pore pressure then powers the overlying soil mass to rise and hurl. In cohesionless soils, the dirt air pockets in a "bubble"; since the dirt has no strength, it frequently cleans out.

There are two Normal Circumstances in which the Pore Pressure Approaches the Absolute Pressure, Specifically as follows:

1. On account of up leakage (Figure 6.12), where the drainage force approaches the lowered load of the dirt.
2. At the point when an unexpected stacking is applied to a free soaked soil; this causes a volume decline in the dirt and results in the successful pressure being moved to the pore pressure.

Comparison of Shear Strength in Cohesive and Cohesionless Soils

• Cohesive soil typically has lower shear strength than cohesionless soil.
• Cohesionless soil relies on frictional forces between particles for its shear strength.
• Cohesive soil depends on interparticle bonding in addition to frictional forces.

Conclusions/Summary

• Moisture content, effective stress, porosity, void ratio, and clay mineralogy are factors affecting the cohesive soil shear strength.
• Particle size and shape, packing arrangement, and relative density influence the shear strength of cohesionless soil.
• Cohesive soil generally has lower shear strength compared to cohesionless soil.
• Understanding and considering these factors is crucial for designing and analyzing geotechnical structures and foundations.

References

• ACS. (2023). Factor influencing the shear strength of cohesionless soil. Factor Influencing The Shear Strength of Cohesionless Soil. https://civil-engg-world.blogspot.com/2008/12/factor-influencing-shear-strength-of.html 
• Adhadh, A. R. A., Abbas, B. J., & Ali, A. M. (2021). Factors influencing the shear strength of clays: A Review. IOP Conference Series: Materials Science and Engineering, 1090(1), 012009. https://doi.org/10.1088/1757-899x/1090/1/012009 
• Jiang, Q., Cao, M., Wang, Y., Wang, J., & He, Z. (2021). Estimation of soil shear strength indicators using soil physical properties of paddy soils in the plastic state. Applied Sciences, 11(12), 5609. https://doi.org/10.3390/app11125609 
• Kang, Q., Xia, Y., Li, X., Zhang, W., & Feng, C. (2022). Study on the effect of moisture content and dry density on shear strength of silty clay based on direct shear test. Advances in Civil Engineering, 2022, 1–9. https://doi.org/10.1155/2022/2213363 
• Zhang, M., Sang, S., Wang, Y., & Bai, X. (2020). Factors influencing the mechanical characteristics of a pile–soil interface in clay soil. Frontiers in Earth Science, 7. 

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