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Geostatic Stress :
is the stress acting at a point in a soil mass witha horizontal top surface. The total stress is
computed as the total weight of a column of unit
area above the point.
v z
Z
Unit area
Ground Surface
unit weight of soil () is
constant throughout the
depth (z)
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Pore Water Pressure
Hydrostatic or geostatic pore water pressure is
the pore water pressure in soil due to geologic
conditions.
Excess pore water pressure is the pore waterpressure that results when soil is loaded.
Back pressure is the pore water pressure
applied directly to soil specimens duringlaboratory testing.
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Effective Stress
Effective soil stress is not the actual stress
acting at the areas of contact between soil
particles.
Effective soil stress corresponds to the stress
transmitted through the soil mineral skeleton.
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Karl von Terzaghi (1883 -1963):
An Austrian civil engineer andgeologist, called the father of soil
mechanics.
He started modern soil mechanicswith his theories of consolidation,
lateral earth pressures, bearing
capacity, and stability
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Effective soil stress is a stress state variable that is
useful to characterize behavior occurring in saturated
soils including volume change, permeability and shear
strength
The difference between total stress () and the porewater pressure (u) is equal to the effective stress ().
'u
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Consolidation of Saturated Soil
When saturated soils are loaded, they
develop excess pore water pressures that
dissipate over time.As water flows from the soil the excess pore
water pressures dissipate resulting in
settlement.This process is referred to as primary
consolidation.
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Compaction - Consolidation
Compaction means
the removal of
air-filled porosity.
Consolidation meansthe removal of
water-filled porosity.
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V z
z
GL
A
Vertical stress distribution:
If the water table is below the depth z;
Where, v= max vertical stress (total vertical stress)
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'
v subz
v sat sub wz z z wu z
z
GL
A
WT
E l 1
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Ground Water Table
Dry Sand, dry = 16.5 KN/m3
Clay, sat = 19.25 KN/m3
Impermeable layer
3m
3m
13m
Example 1:
A soil profile is shown below. Calculate Total Stress, Pore water Pressure and
Effective stress at A, B, C and D.
A
. B
. c
. D
E ample 2
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A layer of saturated clay 4 m thick is overlain by sand 5 m deep, the water table being
3 m below the surface. The saturated unit weights of the clay and sand are 19 and 20
kN/m3 respectively; above the water table the unit weight of the sand is 17 kN/m3. Plot
the values of total vertical stress and effective vertical stress against depth. If sand to aheight of 1 m above the water table is saturated with capillary water, how are the
above stresses affected?
Example 2:
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