7.4 Factors affecting shear strength (drainage conditions, soil type, stress history)

Drainage Conditions and Shear Strength

Shear strength governs how soil resists sliding, deformation, and failure under load. Three factors shape it more than almost anything else: drainage conditions, soil type, and stress history. Getting these right is the foundation of slope stability analysis, foundation design, and retaining wall calculations.

Drainage conditions control pore water pressure, which directly affects effective stress. Soil type determines whether strength comes from friction, cohesion, or both. And stress history (especially overconsolidation) dictates how stiff, strong, and brittle a soil will be when you load it again.

Drainage and Pore Water Pressure

Whether water can escape from soil pores during loading has a huge effect on shear strength. The key question is: does the water have time to drain, or is it trapped?

• Undrained conditions occur when loading is fast relative to the soil's permeability. Water can't escape, so excess pore water pressure builds up. This reduces effective stress and, in turn, shear strength. You'll see this in rapid construction on clay, for example.
• Drained conditions occur when loading is slow enough (or the soil is permeable enough) that excess pore water pressure dissipates as it develops. Effective stress changes correspond directly to applied loads.
• Partially drained conditions fall in between and require more complex analysis. These happen at intermediate loading rates.

The relationship between loading rate and soil permeability is what determines which condition applies. Sandy soils drain quickly, so they're almost always in a drained state. Clays drain slowly, so short-term loading on clay is typically undrained.

For analysis:

• Undrained conditions use total stress analysis
• Drained conditions use effective stress analysis, built on the effective stress principle: $\sigma' = \sigma - u$

Analysis Methods for Drainage Conditions

Engineers use lab and analytical tools to evaluate shear strength under different drainage scenarios:

• Triaxial tests simulate specific drainage conditions. The three standard types are consolidated-drained (CD), consolidated-undrained (CU), and unconsolidated-undrained (UU). Each gives you different strength parameters depending on the field situation you're modeling.
• Skempton's pore pressure parameters (A and B) quantify how much pore pressure develops in response to stress changes during undrained loading.
• Terzaghi's consolidation theory estimates how long it takes for excess pore pressure to dissipate, which helps you judge whether a real loading scenario will be drained or undrained.
• Stress path method lets you visualize how stresses change during loading under different drainage conditions, making it easier to track where the soil is heading relative to failure.
• Numerical modeling (e.g., finite element analysis) handles complex drainage scenarios where simple hand calculations aren't enough.

Cohesive vs. Cohesionless Soil Shear Strength

Shear Strength Mechanisms

Soil type fundamentally determines where shear strength comes from.

Cohesionless soils (sands and gravels) derive their strength almost entirely from friction between particles. They have high permeability, so they drain quickly and almost always behave in a drained manner. Dense sands tend to dilate (expand in volume) during shearing, which temporarily increases their strength. Loose sands compress instead, and at large strains, both dense and loose sands converge toward the same critical state, where shearing continues at constant volume and effective stress.

Cohesive soils (clays and silts) get strength from both friction and cohesion (the attraction between fine particles). Their low permeability means they often behave in an undrained manner under short-term loading. Cohesive soils typically show more ductile stress-strain behavior, deforming gradually rather than failing suddenly. The critical state concept is especially important here because it defines the ultimate condition the clay is heading toward regardless of its starting point.

The Mohr-Coulomb failure criterion describes shear strength for both soil types:

$\tau_f = c + \sigma' \tan\phi$

where $c$ is cohesion, $\sigma'$ is effective normal stress, and $\phi$ is the friction angle. For clean sands, $c$ is essentially zero, so strength depends entirely on $\sigma' \tan\phi$.

Soil Classification and Testing

Different soil types call for different tests:

• Unified Soil Classification System (USCS) categorizes soils by grain size distribution and plasticity, giving you a first indication of expected behavior.
• Atterberg limits (liquid limit, plastic limit, plasticity index) characterize how cohesive soils transition between solid, plastic, and liquid states. A high plasticity index generally means more pronounced cohesive behavior.
• Direct shear test is commonly used for cohesionless soils to measure the friction angle.
• Triaxial tests work for both soil types and provide the most comprehensive strength parameters ($c$, $\phi$, pore pressure response).
• Unconfined compression test is a quick way to estimate undrained shear strength of cohesive soils. The undrained shear strength equals half the unconfined compressive strength.
• Vane shear test measures in-situ undrained strength of soft clays directly in the ground.
• Cone penetration test (CPT) works for both soil types in the field and correlates tip resistance and sleeve friction with shear strength parameters.

Overconsolidation Ratio Impact on Shear Strength

OCR and Soil Behavior

Stress history refers to the maximum effective stress a soil has ever experienced compared to what it's under now. The overconsolidation ratio (OCR) captures this:

$OCR = \frac{\sigma'_p}{\sigma'_0}$

where $\sigma'_p$ is the maximum past effective stress (preconsolidation pressure) and $\sigma'_0$ is the current effective stress.

• Normally consolidated (NC) soils have $OCR = 1$. They've never been under more stress than they are right now.
• Overconsolidated (OC) soils have $OCR > 1$. At some point in the past, they carried a higher load (from a glacier, eroded overburden, or lowered water table, for example) and have since been unloaded.

Why does this matter for shear strength? OC soils are denser and stiffer than NC soils at the same current stress. This leads to several important differences:

• OC clays have higher undrained shear strength than NC clays at the same effective stress.
• OC soils tend to dilate during shearing, while NC soils tend to compress. This dilation can generate negative excess pore pressures during undrained loading, which actually increases effective stress and strength temporarily.
• OC soils show stiffer, more brittle stress-strain behavior. They reach peak strength quickly, then may soften toward a residual or critical state value.
• The cohesion intercept in the Mohr-Coulomb model is typically higher for OC soils, and the peak friction angle can also differ from NC soils.

OCR Determination and Applications

• The oedometer (consolidation) test is the standard lab method for finding preconsolidation pressure. You load a soil sample incrementally and plot void ratio vs. log effective stress ($e$-$\log \sigma'$). The break in the curve marks the preconsolidation pressure.
• In-situ tests like CPT and pressuremeter can be correlated with OCR, though these are empirical relationships that vary by soil type.
• The SHANSEP method (Stress History and Normalized Soil Engineering Properties) estimates undrained shear strength as a function of OCR. It assumes the ratio of undrained strength to effective stress is consistent for a given soil when normalized by OCR.
• Settlement calculations depend on OCR because the recompression index ($C_r$), used for reloading within the preconsolidation range, is much smaller than the compression index ($C_c$) used beyond it. OC soils settle less under the same load increase.
• Slope stability analysis must account for OCR when selecting strength parameters. Using NC parameters for an OC soil would underestimate strength (conservative but potentially wasteful), while ignoring strain-softening in brittle OC clays could be unconservative.
• Lateral earth pressure coefficients ($K_0$, $K_a$, $K_p$) vary with OCR. The at-rest coefficient $K_0$ is higher for OC soils, which directly affects retaining wall design pressures.

Additional Factors Affecting Soil Shear Strength

Soil Structure and Composition

Beyond drainage, soil type, and stress history, several compositional factors influence shear strength:

• Soil fabric and bonding matter a great deal in natural clay deposits. Undisturbed clays can have structured bonds between particles that give them extra strength. Remolding (disturbing) the clay breaks these bonds, sometimes dramatically reducing strength. The ratio of undisturbed to remolded strength is called sensitivity.
• Cementation from natural agents (like calcium carbonate) or artificial ones (cement, lime stabilization) increases cohesion and stiffness. This is why lime-treated subgrades are stronger than untreated ones.
• Anisotropy means strength varies with direction. Soils deposited in layers or subjected to directional stress histories will be stronger in some orientations than others. This matters when failure surfaces aren't horizontal.
• Clay mineralogy affects interparticle forces. Montmorillonite (swelling clay) has much weaker interparticle bonds and higher water-holding capacity than kaolinite, leading to lower shear strength and more problematic behavior.
• Organic matter (peat, organic clays) reduces shear strength and increases compressibility. Highly organic soils are among the weakest and most compressible you'll encounter.
• Void ratio and relative density control strength in cohesionless soils. Denser soils (lower void ratio, higher relative density) have higher friction angles and greater dilatancy.
• Particle shape and size distribution influence how well grains interlock. Angular, well-graded particles produce higher friction angles than rounded, uniform ones.

Environmental and Time-Dependent Factors

Shear strength isn't fixed forever. Several environmental processes change it over time:

• Cyclic loading from earthquakes or machine vibrations can degrade strength in loose, saturated sands, potentially triggering liquefaction, where the soil temporarily loses all shear strength and behaves like a liquid.
• Aging and thixotropy cause time-dependent strength gains in some clays. A disturbed clay may slowly regain some of its lost strength over weeks or months as particles rearrange.
• Chemical changes in pore fluid (pH shifts, salt concentration changes) alter interparticle forces in clays. Leaching salt from marine clays, for instance, can dramatically reduce their strength (this is a factor in quick clay landslides).
• Freeze-thaw cycles disrupt soil structure in cold regions, generally reducing strength after repeated cycles.
• Temperature changes affect pore water pressure and can alter clay mineral properties, though this is usually a secondary concern except in specialized applications (energy foundations, nuclear waste storage).
• Weathering gradually transforms rock and soil minerals over long timescales, changing composition and strength. Fresh rock fill, for example, may weaken over decades as minerals break down.
• Biological activity such as plant roots can reinforce soil (root cohesion in slope stability), while burrowing animals can create preferential drainage paths or weaken the soil fabric.