Consolidation Settlement - Part 01

Excess pore water pressure diagram in the above diagram : Pore water pressure distribution immediately after placing the surcharge, but before the beginning of the dissipation of the excess pore water pressure. (under undrained condition)
   

In sand layers, pore water pressure dissipate quickly due to high permeability. Therefore at boundary of clay layer pore water pressure = 0, then pwp will dissipate in clay layer to sand layers.



Consolidation :
The increase in the pore water pressure causes a head gradient in the pore water, resulting in a transient flow of pore water pore water towards a free draining boundary of the soil layer.

This flow or drainage will continue until the pore water pressure a gain becomes equal to a valve governed by a steady position of the water table.

The reduction of the excess pore water pressure to the steady state value is described as dissipation and when this has been completed the soil is said to be in a drained condition.

The time taken for drainage to be completed depends on the permeability of the soil. in soils of low permeability, such as saturated clays, drainage will be slow and the whole process is referred to as consolidation.

Oedeometer test 

Consolidation properties of soil is obtained by carrying out one dimensional consolidation test commonly referred as oedeometer test. ( 1-D consolidation test : it can compressed in one direction can't expand laterally )

first watch this video
https://www.youtube.com/watch?v=5kuw6-axQIw

50 mm diameter and 20 mm high cylindrical similar size sample is used, because otherwise 24 hrs won't be sufficient for this test.

Sample should be always under the water.

Generally we take an undisturbed sample
   - drill a bore hole
   - push a tube to the soft clay layer using a drilling machine
   - take it out and seal with parapine wax. (to stop evaporation of water)

Load is applied on the sample in increments and each loading increment is kept for 24 hrs. the settlement of the sample is measured at 0, 15 sec, 30 sec, 1 min, 2 min, 4 min, 15 min, 30 min, 1 hr, 2 hr, 3 hr, 4 hr, 8 hr, 16 hr and 24 hr after placing the load.

After 24 hrs the next load increment is applied on the sample ( double the earlier load ). water content Wf of the sample at the end of the consolidation test should be measured. the dry weight of the specimen, ms is measured at the end of the test.

No air since the sample is saturated.

Thickness of solids (Hs),

where,
A = gross sectional area of the soil samples
Gs = specific gravity of the solids of the soil (2.65)
Ms = dry weight of the sample
ro w = density of water

Sample volume will reduce when sample is compressed. Hs (thickness of the solid phase) will be constant through the test. sample height reduction is due to reduction in height of water. (water drain out from the sample through the porous stone)

If the thickness of the soil sample at the end of each loading increment H1, is known the void ration e1,

e1 = (H1-H2)/Hs

Since the thickness of the soil sample is known at the end of each loading increment, the void ration at the end of each loading increment may be estimated.

If we know the Ms,wet and moisture content (mc) at the beginning of the test.

Ms = (Ms,wet)/ (1 + mc%/100)

gamma dry = gamma wet / (1 + w/100)

when we increase the load, the void ratio will decrease. Typical plot of void ratio (e) after consolidation against effective stress for a saturated clay are shown in the following figure. and also, after 24 hrs we assume that pore water pressure is dissipated and pwp in the sample is zero. therefore effective stress is same as total stress applied.