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

2

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.
4

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
6
above.

su =

′
(σ1 )f − (σ 3 )f (σ1 )f − (σ ′3 )f τ
2

=

2

su

Effective Stress Circle

φ’cs

(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.]

5

Higher effective

TSA and ESA

Undrained Shear Strength (Continued..)
• The Atterberg limits
IL
(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
0
1.5
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.
7

• 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