What Is Consolidation Of Soil?
When soil is loaded because of the construction of a structure, the volume of the soil will decrease due to the rearrangement of soil particles. If the soil is in a completely saturated state, volume reduction takes place due to the expulsion of pore water from voids. This process is called the consolidation of soil. It is a time-dependent phenomenon.
The terms “consolidation” and “compaction” are frequently interchanged. Compaction reduces the volume of air in the voids, increasing the density of unsaturated soil. Consolidation, on the other hand, is a time-consuming procedure for raising the density of saturated soil by draining some of the water from the spaces.
To assure the serviceability of structures built on a compressible soil layer, consolidation theory is essential for the prediction of both the magnitude and rate of consolidation settlements.
Type Of Consolidation Of Soil
Total consolidation of soil can be divided into two types.
- Primary consolidation
- Secondary consolidation
1. Primary Consolidation
This process is started when the soil is fully saturated. When effective stress over-saturated soil mass increases, then the pore water pressure increases. If a drainage facility is provided, the expulsion of pore water will occur.
Primary consolidation is completed when pore water expulsion stops. During this process of consolidation, the soil remains in saturated condition and the flow of water is under laminar condition i.e<1. The volume of expelled water is equal to the change in volume of soil.
The soil mass is considered semi-infinite. So the change in volume is equal to the change in depth. Due to primary consolidation, settlements occur which is time-dependent. The time required for settlement depends upon the following factors:
- Rate of application of load.
- Coefficient of permeability of the soil.
- Availability of drainage facility (one way or two way).
- Length of the drainage path.
2. Secondary Consolidation
After completion of primary consolidation, when the expulsion of pore water is stopped and load continues to act, then at a very slow rate volume changes may be recorded.
This is due to plastic readjustment of soil. This process is called secondary consolidation. It is a time-dependent process and much slower than the primary consolidation process.
Secondary consolidation is more seen in plastic soil and in highly plastic clays. It is nearly 10-20% of total volume change. In coarse-grained soil like gravels and sands, secondary consolidation is negligible.
Classification Of Soil Based On Consolidation
a) Normally Consolidated Soil
These are the soils that are loaded for the first time to the present applied effective stress. It also means that the past applied effective stress was lower than the present applied effective stress. These are more compressible soils.
b) Over Consolidated Soil
Over consolidated soils are those which have been subjected to effective stress in the past greater than the present applied effective stress. It is also called pre-consolidated or pre-compressed soil.
Over consolidated soils are less compressible and have greater shear strength and more stability. Causes of Over-consolidation or Pre-consolidation are:
- Overburden pressure or surcharge was placed in the past which is removed later.
- Continuous erosion of overburdened soil.
- Melting of the glacier which was covered by the soil mass in the past.
- Effect of capillary pressure which is later destroyed by the rising of the water table.
- Dessication of soil. i.e., due to drying of soil, effective stress reduces and the soil becomes overconsolidated.
- Soil is subjected to downward seepage pressure previously but when it stops later, effective stress reduces.
- Due to the effects of tectonic forces.
Parameters involved in the calculation of Consolidation of soil:
Coefficient of Compressibility (av): It is the ratio of change in void ratio to the change in effective stress. It is also the slope of effective stress v/s void ratio curve. i.e
Coefficient of Compressibility decreases with increase in the effective stress.
Coefficient of Consolidation (cv):
cv is not a constant parameter. It is determined empirically generally on the basis of comparison between the experimental time-compression curve with the theoretical curve. The methods are also known as time fitting method.
Where, d = Length of drainage path
T= Time factor
t= Time taken for the consolidation process
How To Test The Consolidation Of Soil?
To determine the consolidation properties, laboratory consolidation tests are carried out on a suitable undisturbed soil layer. Besides these, from this test permeability and swelling behaviour of soil can also be determined.
The test simulates one-dimensional consolidation with double drainage conditions. The test is carried out in a device called Oedometer or Consolidometer. The device consists of a loading mechanism and a specimen container (or consolidation cell).
Two types of consolidation cells are used: floating ring cell and fixed ring cell. In the floating ring test, compression occurs from the top and bottom portions.
But in the fixed ring test, the soil moves only downward, relative to the ring. The drainage from the bottom porous stone can only be measured in the case of a fixed ring test.
The procedures used in this test are according to IS: 2720(Part XV)-1965. The procedures of fixed ring tests are explained below:
1. An undisturbed soil specimen representing an in-situ soil layer is first carefully trimmed and placed in a metallic confining ring.
2. Since the ring doesn’t allow any lateral deformation of the soil, so porous stone discs are provided at the top and bottom of the sample to allow drainage in the vertical direction, both ways.
3. The diameter of the specimen used should be from 65 to110 mm and thickness of the specimen should be 15 to 30 mm is used. The ratio of diameter to height of the specimen is between 2.5 to 5.
4. The soil sample is loaded in increments of vertical loads. Under each stress increment, the soil sample is allowed to consolidate till the excess pore water pressure is being completely dissipated. The load is kept for 24 hours. The commonly used stresses are 25, 50,100,200,400 and 800 kN/.
5. The vertical deformation of the specimen is measured by a dial gauge. The dial gauge readings are taken at the different elapsed times after a load is placed. The last reading is taken at 24 hours. The 24-hour reading gives the final compression under each stress increment.
6. After the consolidation under the last stress increment is over, the specimen is unloaded in two or three stages and the soil is allowed to swell.
7. To obtain the void ratio vs. pressure relationship, the equilibrium void ratio is computed with the help of 24-hour reading.
8. Only the final swell readings are taken at each unloading stage and after completion of swelling the consolidation ring with the soil specimen is taken out, dried in the oven, and the weight of the solids and final water content is determined.