What is the shear strength of soil? I Geotechnical Engineering I TGC Ask Andrew EP 5

Why Shear Strength is Fundamental to Soil Mechanics
📌 Geotechnical engineers focus on shear strength because soil is a granular material where failure occurs by particles sliding relative to each other, not by particle crushing (compressive strength) or separation (tensile strength).
💪 Soil strength fundamentally comes from friction and interlock between grains, making its characterization complex as friction varies with applied stress.
🚧 Examples of shear failure in ground engineering include the collapse mechanism beneath a foundation load or sliding planes developing behind a retaining wall.

Characterizing Frictional Soil Strength (Granular Soils)
🔺 For granular materials, shear stress ($\tau$) at the point of failure is directly related to the effective normal stress ($\sigma'$) applied across the interface.
📈 The relationship is often plotted as a straight line (Mohr-Coulomb criterion), where strength is expressed as the friction angle ($\phi'$), as zero effective stress results in zero shear strength.
🔍 Cohesion ($c$) may appear as an intercept, but a very high cohesion value in granular soils should be treated with suspicion, potentially indicating apparent cohesion or reliance on moisture content.

Undrained Strength in Clays (Short-Term Loading)
💧 In saturated clays, an immediate load addition transfers stress to the incompressible pore water, meaning the effective stress ($\sigma'$) between particles does not change initially.
➡️ This results in a constant shear strength ($\tau_u$ or $s_u$), known as the undrained shear strength, which is applicable for short-term design scenarios under loading.
⏳ Clay strength is not a fundamental soil property; it depends heavily on the stress and moisture state and changes over time as consolidation occurs.

Long-Term Strength Changes in Clay
📈 When loading clay (e.g., foundation), effective stress increases over time, leading to a gain in strength; thus, the short term is often more critical.
📉 When unloading clay (e.g., excavation or cutting slope), effective stress decreases as the clay swells, causing the strength to decrease over time; long-term conditions are more critical in these unloading scenarios.

Key Points & Insights
➡️ The shear strength of soil is the critical parameter because soils fail through particle movement/sliding, governed by friction.
➡️ For granular soils, strength is a function of the effective normal stress ($\sigma'$) expressed via the friction angle ($\phi'$).
➡️ For clays in the short term, the strength ($\tau_u$) is constant because the applied load is initially carried by the pore water, not the soil skeleton.
➡️ Always be cautious of high cohesion ($c$) intercepts in granular soil testing, defaulting to zero if unsure, as moisture dependency can make this strength unreliable.

📸 Video summarized with SummaryTube.com on Mar 12, 2026, 13:14 UTC