The consolidation test for soil samples is conducted in laboratory scale to determine the compressibility behavior of the soil specimen. The test is performed in the consolidation test apparatus, known as the consolidometer or an oedometer. Consolidometer consists of a loading device and a cylindrical container which is called as a consolidation cell. The soil specimen is placed in the consolidation cell between the top and bottom porous stones. The consolidation cells are of two types as follows:
1. Floating or free ring cell in which both the top and bottom porous stones are free to move. The top porous stone can move downward and the bottom stone can move upward as the sample consolidates.
Source: Soil Management India
2. Fixed ring cell in which the bottom porous stone cannot move. Only the top porous stone can move downwards as the specimen consolidates. The fixed ring cell can also be used as a variable- head permeability test apparatus. For this purpose, a piezometer is attached to the base of the cell. The inside surface of the fixed ring should be smooth and polished, mainly to reduce friction when the sample is placed. During the test, the ring creates a condition of zero lateral strain on the soil sample placed in the cell. The internal diameter of the cell is usually 60 mm, but the cells with a diameter up to 100 mm are also available. The thickness of the sample is fixed from the following considerations:
- The thickness of the sample should be as small as possible to reduce side friction, but a minimum thickness of 20mm is usually required to get uniform distribution of pressure on the sample.
- The diameter to the thickness ratio should be a minimum of 3
- The thickness of the specimen should not be less than 10 times the maximum size of the particle.
Therefore, the thickness of the sample for a 60 mm diameter cell is taken as 20 mm. The specimen of diameter 50, 70, and 100 mm may also be used in special cases.
The consolidometer has arrangements for the application of the desired load increment, saturation of sample and measurement of change in thickness of the sample at every stage of consolidation process. The consolidation cell is placed in a water jacket or water trough so that water has free access into and out of the sample. The consolidation cell is provided with a perforated pressure pad at its top surface for the application of axial load. The load is applied either by suspending weights from a hanger resting at the centre of the pressure pad or by a lever arrangement. The arrangement for saturation of the sample consists of a small water reservoir connected to the consolidation takes place. The sample is kept submerged under water to prevent evaporation from the surface.
Before conducting the test, the porous stones are saturated either by boiling them in distilled water for about 15 minutes or by keeping them submerged under water for 4 to 8 hours. The bottom porous stone is first placed in the consolidation cell and a filter paper is fixed on the porous stone. The fixed ring containing the soil sample is then placed on the bottom porous stone. Then a filter paper is kept on the top of the sample and then the top porous stone is placed over it. The loading pad is placed on the top porous stone. The bolts are tightened so as to hold the entire assembly, and then the consolidation cell is kept under loading unit. It should be centered carefully so that the load is applied axially. The dial gauge is mounted and adjusted. The mould assembly is joined to the water reservoir to saturate the sample. For complete saturation, the level of water in the reservoir should be approximately same or equal as that of the sample assembly.
An initial setting pressure of about 5.0 kN/m2 (for very soft soils, 2.5 kN/m2) is applied to the sample. The initial setting pressure is chosen such thatthere is no swelling. The load is allowed to stand till there is no change in the dial gauge reading or 24 hours whichever is less. The final dial gauge reading under the initial setting pressure is noted.
Then the second increment of the load is applied. It is usual practice to double the previous load in each increment. The successive pressured usually applied are 20.40,80,160,320 and 640 kN/m2 etc., till the desired maximum required load intensity is reached. The maximum load intensity is governed by the actual loading on the soil in the field after the construction of the structure.
After the consolidation under the final load increment is complete, the load is reduced to one-fourth of the final load (160 kN/m2 in above case) and allowed to stand for 24 hours. The sample takes water and swells. The reading of the dial gauge is noted when the swelling of the sample is complete. The load is further reduced to one-fourth intensity (40 kN/m2) and the swelling recorded after 24 hours. The load is then reduced to 10 kN/m2 and the swelling is noted. The load is finally reduced to the initial setting load for 24 hours and the final dial gauge reading is taken. Throughout the consolidation test, the container should be kept filled with water.
Immediately after complete unloading the ring with the sample is taken out. The excess water in the soil surface can be dried using a blotting paper. The weight of the ring and the sample is taken. The sample is then dried in an oven (maintained at 110 degree Celsius) for 24 hours and its dry mass,Ms and the water content are determined.
The results of a consolidation test are plotted in the form of a plot between the void ratio and the effective stress. For this purpose, it is essential to determine the void ratio at various load increments. There are mainly two methods which are used for this purpose, they are enumerated as follows:
1. Height of solids method
2. Change in void ratio method
The first method is a general method and is applicable to both saturated and unsaturated soil samples. The second method is applicable only to saturated soil samples.
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