Effective Stress

Total Stress

The stress acting on a soil element below the ground surface due to the weight of everything lying above it, including soil, water, and surface loading.

Effective Stress

The portion of the total stress which is actually carried by the soil grains themselves through particle-to-particle contact.

Effective Stress

The strength and compressibility of the soil depend on the stresses within the solid granular fabric.

Pore Water Pressure

The pressure of the water filling the interconnected void spaces within the soil mass.

Buoyant Unit Weight

The unit weight of soil calculated by subtracting the unit weight of water from the saturated unit weight

Permeability

The specific property of soil that determines the rate of water drainage (flow out of the soil) when pressure increases within the pore water.

Normal Stress

The component of stress determined by dividing the normal force (N_v or N_h) by the normal area

Shear Stress

The component of stress determined by dividing the tangential force (T_h or T_v) by the parallel area

Water Table (Phreatic Surface)

The natural static level of water in the ground where the magnitude of the pore pressure is exactly zero.

Saturated Soil

The condition of a soil layer where all voids are filled with water, which requires the use of the saturated unit weight for stress calculations.

Coefficient of Lateral Stress

The factor expressing the ratio of horizontal stress to vertical stress in a soil mass

Coefficient of Lateral Stress at Rest

No lateral strain, k_o (0.4-0.5)

Passive Coefficient of Lateral Stress

With strain, soil compressing, kp, its about (3)

Active Coefficient of Lateral Stress

With strain, soil stretching, ka, and its equal to (1/kp)

Negative Pore Pressure

The state of pore water pressure that exists above the water table when the soil is saturated by capillary action.

Capillary Rise

The height above the water table to which the soil is saturated

Coarse Sand

Capillary rise: 0.1-0.2

Fine sand

Capillary rise: 0.3-1.2

Silt

Capillary rise: 0.75-7.5

Clay

Capillary rise: 7.5-23

Degree of Saturation

The variable used as a percentage to determine the pore water pressure in a layer of soil that is only partially saturated by capillary rise.

Hydrostatic Conditions

The state in the ground where there is no seepage flow and the water table remains horizontal like the surface of a lake.

Upward Seepage

A directional condition of water flow that mathematically decreases the effective stress in the soil

Downward Seepage

A directional condition of water flow that mathematically increases the effective stress in the soil

Multi-layer Soil System

A ground profile consisting of distinct layers of different unit weights, where total stress is computed by summing the stresses contributed by each individual layer

Zero Effective Stress

A dangerous geotechnical condition where large upward seepage forces counteract the downward buoyant weight of the soil, potentially causing heavy erosion.

Water Table Fluctuation

A change in the subsurface water level that directly results in corresponding changes to the effective stresses below ground.

Surface Water Fluctuation

A change in water level above ground (such as in lakes or rivers) that does not cause changes in effective stresses in the ground below it.

Boiling

Gradient is so high at the ground surface that "sand boils" (i.e. sand volcanoes) appear.

Piping

Typically occurs beneath the ground due to high gradients. Erosion "channels" begin to form which undermine your hydraulic structure.

Heaving

Usually at the base of a slope or excavation. The gradient is so high that the ground "heaves" up. Can be very dangerous to construction workers.

Liquefaction

Applied shear forces ( ... usually from a quake) cause rapid pore pressure increase and drop in effective stress. This phenomenon is not due

to steady state seepagell

Critical Hydraulic Gradient

The specific seepage gradient at which effective stress becomes exactly zero, mathematically approximated as 1.0.

2.0

The standard numerical value generally required for the Factor of Safety against heaving and piping to ensure stability.

Factor of Safety

A structural safety ratio evaluated against heaving, piping, and boiling by dividing the critical hydraulic gradient by the maximum exit gradient

Shear Strength Determination

One of the critical geotechnical design calculations that relies purely on effective stress to determine a soil's maximum resistance to failure.

Settlement Calculations

Geotechnical computations regarding the amount and rate of ground compression that fundamentally depend on effective stress rather than total stress.

Bundles of Capillary Tubes

The physical model used to conceptualize the continuous void spaces of variable cross-sections in a soil mass, allowing water to rise against gravity

Drainage

The increase in pressure within the pore water causes _______