Essay on Soil Mechanics

Submitted By paras077
Words: 1731
Pages: 7

Drained and Undrained Conditions
• Drained condition occurs when there is no change in pore water pressure due to external loading.
• In a drained condition, the pore water can drain out of the soil easily, causing volumetric strains in the soil. • Undrained condition occurs when the pore water is unable to drain out of the soil.
• In an undrained condition, the rate of loading is much quicker than the rate at which the pore water is able to drain out of the soil.
• As a result, most of the external loading is taken by the pore water, resulting in an increase in the pore water pressure.
• The tendency of soil to change volume is suppressed during undrained loading.

Undrained and Drained
Shear Strength
Lecture No. 11
October 22, 2002


Drained and Undrained Conditions (Continued..)

Drained and Undrained Conditions (Continued..)

• The existence of either a drained or an undrained condition in a soil depends on:
– The soil type (e.g. fine-grained or coarse-grained)
– Geological formation (fissures, sand layers in clays, etc.)
– Rate of loading

• For a rate of loading associated with a normal construction activity, saturated coarse-grained soils (e.g. sands and gravels) experience drained conditions and saturated fine-grained soils (e.g. silts and clays) experience undrained conditions.
• If the rate of loading is fast enough (e.g. during an earthquake), even coarse-grained soils can experience undrained loading, often resulting in liquefaction. 3

Drained Condition

Undrained Condition

• A soil with a tendency to compress during drained loading will exhibit an increase in pore water pressure during undrained loading, resulting in a decrease in effective stress.
• A soil with a tendency to expand or dilate during drained loading will exhibit a decrease in pore water pressure during undrained loading, resulting in an increase in effective stress.

Undrained Shear Strength

Undrained Shear Strength (Continued..)

• The shear strength of a fine-grained soil under undrained condition is called the undrained shear strength and is denoted by su.
• su is the radius of the Mohr’s Circle of Total Stress:

• Unlike the critical confining stresses state angle of φ’cs τ su2 friction, the undrained shear su1 strength is not a fundamental soil σ parameter.
• Its value depends on the values of the φ’cs Lower effective effective confining confining stresses stresses. • An increase in effective confining stresses causes a decrease in void ratio and an increase in undrained shear strength as shown in the figure

su =

(σ1 )f − (σ 3 )f (σ1 )f − (σ ′3 )f τ




Effective Stress Circle


(σ 1 )f
(σ 3 )f
• The undrained shear strength depends only σ, σ’ on the initial void
(σ ′3 )f

(σ 1 )f ratio or the initial u water content of the
Total Stress Circle soil. [Note that the horizontal tangent to the two circles is NOT a failure envelope.]


Higher effective


Undrained Shear Strength (Continued..)
• The Atterberg limits
(Liquid Limit and Plastic
Limit) define the range of 1.0 undrained shear strengths for a fine-grained plastic soil. • At its Liquid Limit (i.e.
Liquidity Index IL = 1), a
150 log(su) clay has su approximately kPa kPa equal to 1.5 kPa.
• At its Plastic Limit (i.e. IL = 0), a clay has su approximately equal to 150 kPa.
• Therefore, approximate estimate of su can be obtained by knowing the water content of the soil.

• TSA stands for Total Stress Analysis.
• A TSA uses undrained shear strength (su) for the analysis of soil strength and soil stability problems. • TSA derives its name from the fact that su value for a fine-grained soil can be obtained using total stresses (see description and figure on page 5).
• ESA stands for Effective Stress Analysis.
• An ESA uses critical state angle of friction (φ’cs) for the