Intro to Geotechnical Science

🏔️Intro to Geotechnical Science Unit 7 – Soil Shear Strength

Soil shear strength is crucial in geotechnical engineering, determining how soil behaves under stress. It's influenced by factors like soil composition, stress conditions, and water content. Understanding shear strength helps engineers design safe structures and predict soil behavior. Key concepts include cohesion, friction angle, and effective stress. Testing methods like direct shear and triaxial compression provide vital data. Real-world applications range from slope stability analysis to foundation design, making shear strength essential for civil engineering projects.

Key Concepts and Definitions

  • Shear strength represents the maximum shear stress a soil can withstand before failure occurs
  • Cohesion (cc) measures the soil's intrinsic shear strength due to electrostatic forces between particles
  • Friction angle (ϕ\phi) quantifies the soil's frictional resistance to shearing, dependent on the normal stress
    • Typically ranges from 0° (very loose sand) to 45° (dense sand or gravel)
  • Effective stress (σ\sigma') accounts for the stress carried by the soil skeleton, equal to total stress minus pore water pressure
  • Mohr-Coulomb failure criterion relates shear strength to normal stress, cohesion, and friction angle: τ=c+σtanϕ\tau = c + \sigma'\tan\phi
  • Drained conditions allow pore water to dissipate during loading, while undrained conditions prevent pore water dissipation
  • Critical state soil mechanics describes the behavior of soil at large strains, where it continues to deform without changes in stress or volume

Soil Composition and Properties

  • Soil consists of solid particles (sand, silt, clay), water, and air voids
  • Particle size distribution (gradation) influences soil behavior and shear strength
    • Well-graded soils have a wide range of particle sizes, leading to higher density and shear strength
    • Poorly-graded soils have a narrow range of particle sizes, resulting in lower density and shear strength
  • Plasticity index (PI) measures the range of water contents where clay exhibits plastic behavior
  • Void ratio (ee) represents the ratio of void volume to solid volume in a soil sample
  • Relative density (DrD_r) expresses the degree of compaction for granular soils, affecting shear strength and stiffness
  • Soil structure refers to the arrangement of particles and pores, impacting strength and permeability
    • Flocculated structure (clay particles in edge-to-face contact) leads to higher void ratios and lower shear strength
    • Dispersed structure (clay particles in face-to-face contact) results in lower void ratios and higher shear strength

Stress in Soils

  • Total stress (σ\sigma) is the sum of effective stress (σ\sigma') and pore water pressure (uu): σ=σ+u\sigma = \sigma' + u
  • Effective stress governs soil behavior and shear strength, as it represents the stress carried by the soil skeleton
  • Pore water pressure develops in saturated soils under undrained loading, reducing effective stress and shear strength
  • Overburden stress increases with depth due to the weight of overlying soil layers
  • Lateral earth pressure is the horizontal stress acting on soil elements, influenced by soil properties and loading conditions
    • At-rest earth pressure (K0K_0) occurs when no lateral strain is allowed (retaining walls)
    • Active earth pressure (KaK_a) develops when soil is allowed to expand laterally (excavations)
    • Passive earth pressure (KpK_p) mobilizes when soil is forced to compress laterally (pile foundations)
  • Shear stress (τ\tau) acts parallel to the soil element faces, causing angular distortion and shear deformation

Shear Strength Theory

  • Mohr-Coulomb failure criterion defines the shear strength envelope in terms of cohesion and friction angle
  • Peak shear strength is the maximum shear stress a soil can sustain before failure, corresponding to the peak of the stress-strain curve
  • Residual shear strength is the minimum shear strength reached after large displacements, important for slope stability analysis
  • Dilation occurs in dense granular soils, where particles must climb over each other during shearing, leading to volume increase
  • Contraction happens in loose granular soils, where particles collapse into void spaces during shearing, resulting in volume decrease
  • Critical state is reached when soil continues to deform at constant stress and volume, representing the ultimate shear strength
  • Strain softening describes the reduction in shear strength after the peak, common in overconsolidated clays
  • Strain hardening refers to the increase in shear strength with increasing strain, typical of normally consolidated clays

Testing Methods

  • Direct shear test applies a normal stress and measures the shear force required to cause failure along a predetermined plane
    • Suitable for granular soils and quick estimates of shear strength parameters
    • Limited control over drainage conditions and stress path
  • Triaxial compression test confines a cylindrical soil sample in a pressurized cell and applies axial load until failure
    • Allows control over drainage conditions (drained, undrained) and stress path (compression, extension)
    • Provides more reliable and representative shear strength parameters
  • Unconfined compression test applies axial load to an unconfined cylindrical soil sample until failure, measuring the unconfined compressive strength
    • Used for quick estimates of shear strength in cohesive soils (clays)
    • Not suitable for granular soils or soils with significant confining stresses
  • Vane shear test measures the in-situ undrained shear strength of soft clays by rotating a vane and measuring the torque at failure
    • Provides a quick and direct measurement of undrained shear strength
    • Limited to soft, saturated, cohesive soils
  • Cone penetration test (CPT) pushes a instrumented cone into the soil, measuring tip resistance and sleeve friction
    • Provides continuous profiles of soil strength and behavior with depth
    • Correlations available to estimate shear strength parameters from CPT data

Factors Affecting Soil Shear Strength

  • Soil type and mineralogy influence intrinsic shear strength properties (cohesion, friction angle)
    • Granular soils (sands, gravels) derive shear strength primarily from friction and interlocking
    • Cohesive soils (clays) exhibit shear strength due to electrostatic forces and cohesion
  • Density and void ratio affect the shear strength of granular soils, with denser soils having higher strength
  • Water content impacts the shear strength of cohesive soils, with increasing water content leading to reduced strength
  • Confining stress increases the frictional component of shear strength in granular soils
  • Drainage conditions during loading determine the development of pore water pressures and effective stresses
    • Drained loading allows dissipation of excess pore water pressures, maintaining effective stresses
    • Undrained loading generates excess pore water pressures, reducing effective stresses and shear strength
  • Stress history and overconsolidation ratio (OCR) influence the shear strength and stiffness of cohesive soils
    • Overconsolidated soils (OCR > 1) exhibit higher shear strength and brittleness
    • Normally consolidated soils (OCR = 1) display lower shear strength and ductility
  • Anisotropy results in different shear strengths depending on the direction of loading relative to the soil fabric

Real-World Applications

  • Slope stability analysis assesses the risk of landslides and designs safe slopes using shear strength parameters
    • Factor of safety (FoS) compares the available shear strength to the shear stresses driving failure
    • Limit equilibrium methods (Bishop, Janbu, Spencer) calculate FoS for potential failure surfaces
  • Foundation design relies on shear strength to ensure adequate bearing capacity and limit settlements
    • Shallow foundations (spread footings, mats) transfer loads to the soil near the surface
    • Deep foundations (piles, drilled shafts) transfer loads to stronger soil or rock layers at depth
  • Retaining wall design uses shear strength to calculate lateral earth pressures and ensure stability against overturning and sliding
    • Gravity walls resist lateral pressures through their own weight and friction at the base
    • Cantilever walls consist of a vertical stem and base slab, using the soil's passive resistance for stability
  • Excavation support systems (bracing, tieback anchors) rely on shear strength to maintain stability and prevent collapse
  • Soil liquefaction assessment predicts the loss of shear strength in saturated, loose sands during earthquakes
    • Cyclic shear stresses can cause a rapid increase in pore water pressure, leading to a temporary loss of strength
    • Liquefaction potential is evaluated using in-situ tests (SPT, CPT) and laboratory cyclic shear tests

Common Challenges and Solutions

  • Sample disturbance during collection and preparation can alter soil structure and lead to underestimated shear strengths
    • Minimize disturbance by using thin-walled samplers and careful handling techniques
    • Use in-situ tests (CPT, vane shear) to measure shear strength directly in the field
  • Spatial variability of soil properties can result in uncertainty and potential failures
    • Conduct thorough site investigations with sufficient sampling and testing locations
    • Use geostatistical methods (kriging) to interpolate soil properties between data points
  • Strain incompatibility between laboratory tests and field conditions can lead to overestimated or underestimated shear strengths
    • Select appropriate strain rates and stress paths in laboratory tests to mimic field conditions
    • Use back-analysis of case histories to calibrate shear strength parameters
  • Time-dependent behavior (creep, consolidation) can cause long-term deformations and strength changes
    • Conduct long-term oedometer tests to assess consolidation and secondary compression
    • Use time-dependent constitutive models (modified Cam Clay, viscoelastic) in numerical simulations
  • Multiphase interactions between solid particles, water, and air can complicate shear strength behavior
    • Use coupled hydro-mechanical models (Biot theory) to capture the interaction between pore fluids and soil skeleton
    • Consider unsaturated soil mechanics principles for soils above the water table
  • Environmental factors (temperature, chemistry) can degrade soil shear strength over time
    • Assess the impact of temperature fluctuations on soil properties, especially in cold regions
    • Evaluate the potential for chemical reactions (dissolution, precipitation) that may alter soil structure and strength


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.