The Importance of Effective Stress

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At any point within the soil mass, the magitudes of both total stress and pore water pressure are dependent on the
ground water position. With a shift in the water table due to seasonal fluctuations, there is a resulting change in the
distribution in pore water pressure with depth.
Changes in water level below ground result in changes in effective stresses below the water table. A rise increases the
pore water pressure at all elevations thus causing a decrease in effective stress. In contrast, a fall in the water table
produces an increase in the effective stress.
Changes in water level above ground do not cause changes in effective stresses in the ground below. A rise above
ground surface increases both the total stress and the pore water pressure by the same amount, and consequently
effective stress is not altered.
In some analyses it is better to work with the changes of quantity, rather than in absolute quantities. The effective
stress expression then becomes:
Ds´ = Ds - D u
If both total stress and pore water pressure change by the same amount, the effective stress remains constant.
Total and effective stresses must be distinguishable in all calculations.Ground movements and instabilities can be
caused by changes in total stress, such as caused by loading by foundations and unloading due to excavations. They
can also be caused by changes in pore water pressures, such as failure of slopes after rainfall.
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