soil strength

Soil strength

The maximum stress a soil can sustain before it fails.

Importance of soil strength

Determines whether soil can safely support loads from foundations, retaining walls and other structures without failure.

Shear strength

The maximum internal resistance of soil to applied shearing forces.

Tensile strength

The maximum stress soil can withstand when being stretched or pulled before breaking or fracturing.

Soil failure

Occurs when the applied stress exceeds the soil's available strength, resulting in deformation or collapse.

Examples of soil failure

Slope failure and foundation failure, as illustrated in the lecture.

Direct shear test

A laboratory test used to measure soil shear strength by applying a normal load and shearing the soil along a predetermined plane.

Direct shear test apparatus

Contains a soil specimen in a shear box, a normal loading system and a horizontal shear loading mechanism.

Normal stress in a direct shear test

The stress acting perpendicular to the shear plane.

Shear stress in a direct shear test

The stress acting parallel to the shear plane.

Triaxial test

A laboratory test used to determine soil shear strength by applying controlled confining pressure and axial loading to a cylindrical specimen.

Triaxial test specimen

A cylindrical soil sample surrounded by a protective membrane and subjected to confining pressure and axial load.

Purpose of the triaxial test

To investigate soil strength and behaviour under controlled stress conditions.

Peak shear strength

The maximum shear stress reached during shearing.

Critical-state shear strength

The shear strength reached when soil continues to deform at approximately constant volume and stress.

Residual shear strength

The lower shear strength that may develop after substantial displacement along an established shear surface; the lecture notes that only clays exhibit residual strength in its illustrated comparison.

Peak versus critical strength

Peak strength is the maximum resistance reached during shearing; critical strength is the resistance maintained during continued deformation.

Peak strength of dense soil

Dense sand and heavily consolidated clay commonly show a pronounced peak in shear strength before approaching critical-state strength.

Peak strength of loose soil

Loose sand and normally or lightly consolidated clay generally show less pronounced peak behaviour and tend towards critical-state strength.

Loose sand

Sand with relatively open particle packing that tends to contract during shearing.

Dense sand

Sand with tightly packed particles that tends to dilate during shearing.

Normally consolidated clay

Clay whose current effective stress equals its maximum past effective stress; it tends to contract during shearing according to the lecture.

Lightly consolidated clay

Clay that has undergone limited consolidation and tends to contract during shearing, as described in the lecture.

Heavily consolidated clay

Clay that has undergone substantial consolidation and tends to dilate during shearing, according to the lecture.

Dilation

An increase in soil volume during shearing, associated in the lecture with dense sand and heavily consolidated clay.

Contraction

A decrease in soil volume during shearing, associated in the lecture with loose sand and normally or lightly consolidated clay.

Effect of dilation on shear behaviour

Dense soil may develop a peak shear strength before reaching a lower, approximately constant critical-state strength.

Effect of contraction on shear behaviour

Loose soil tends to compact during shearing and generally approaches critical-state strength without a pronounced peak.

Shear strain (γ)

A measure of angular distortion caused by shearing.

Shear stress (τ)

The stress acting parallel to a plane within the soil.

Normal effective stress (σ′)

The normal stress carried by the soil skeleton after accounting for pore pressure.

Coulomb's frictional law

Relates frictional resistance to the normal load: H = μW, where H is horizontal frictional resistance, μ is the coefficient of friction and W is the normal load.

Coefficient of friction (μ)

The ratio of frictional resistance to normal load; the lecture gives μ = tan φ′.

Effective angle of shearing resistance (φ′)

A soil strength parameter describing frictional resistance between particles under effective stress.

Frictional shear strength equation

τ = σ′ tan φ′, representing frictional shear strength where cohesion is taken as zero.

Cohesion (c)

A parameter representing the cohesive component of shear strength in the drained strength equation presented in the lecture.

Drained shear strength equation

τ = σ′ tan φ′ + c, where σ′ is effective normal stress, φ′ is the effective friction angle and c is cohesion.

Undrained shear strength (Su)

The shear strength of fine-grained soil under undrained conditions, represented in the lecture by τ = Su.

Dry sugar analogy

The lecture uses dry sugar to represent coarse-grained soil, with frictional strength described by τ = σ′ tan φ′.

Damp sugar analogy

The lecture uses damp sugar to represent drained fine-grained soil, with strength described by τ = σ′ tan φ′ + c.

Butter analogy

The lecture uses butter to represent undrained fine-grained soil, with strength described by τ = Su.

Coarse-grained soil strength

The lecture illustrates frictional shear strength using τ = σ′ tan φ′, with cohesion taken as zero.

Drained fine-grained soil strength

The lecture gives τ = σ′ tan φ′ + c, incorporating friction and cohesion.

Undrained fine-grained soil strength

The lecture gives τ = Su; the undrained shear strength depends on the initial confining stress or initial void ratio.

Drained condition

A condition in which pore water can drain during loading, allowing changes in water pressure through drainage.

Undrained condition

A condition in which water cannot drain during the loading process.

Drained versus undrained strength

Drained strength is expressed using effective stress, friction angle and cohesion; the lecture represents undrained strength using Su.

Peak, critical and residual states

Three stages of soil shear behaviour: maximum strength, approximately constant strength during continued deformation, and lower strength after substantial displacement, respectively.

Soil strength parameters

The lecture focuses on effective friction angle (φ′), cohesion (c) and undrained shear strength (Su).

Main shear strength laboratory tests

The lecture identifies the direct shear test and the triaxial test.

Main factors affecting soil shear behaviour

The lecture highlights soil density or consolidation state, contraction or dilation, effective stress and whether drainage occurs.

Main topics in the soil strength lecture

Importance of soil strength, laboratory testing, peak versus critical strength, dilation and contraction, Coulomb's frictional law, and drained versus undrained shear strength.