pore pressure-engineering geology

7/24/2019 Pore Pressure-Engineering Geology

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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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