Question

1. (2 pts) What things do we need to be measured during the consolidation test?

2. (2 pts) What is the difference between primary consolidation and secondary consolidation?

3. (4 pts) Using a semi‐log plot of time versus dial readings describe the procedure that are used to determine the coefficient of consolidation cv?

4. (2 pts) Does the laboratory sample in the consolidation test deform laterally?

5. (2 pts) How many drainage paths are there in a consolidation test?

6. (4 pts) Describe how an oedometer can be used to determine the volumetric changes in a soil sample due to wetting over a range of vertical pressures?

7. (2 pts) What assumption do we make to permit us to compute settlement (vertical changes in ground surface) using the volumetric changes in a soil tested in the consolidation test?

Answer 1

Solution 1.

The four most important soil properties determined by a consolidation test are:

a. Pre-consolidation stress- This is the maximum stress that the soil has felt in the past.

b. Compression index- It indicates the compressibility of a normally consolidated soil.

c. Recompression index- It indicates the compressibility of an over consolidated soil.

d. Coefficient of consolidation- It indicates the rate of compression under a load increment.

-By this test the amount of change in height of sample and the rate at which it happens when a load is applied on a sample is measured.

-The amount of change in heights is the measure of reduction in voids.

-This test is performed on saturated soil specimen and thus the reduction in voids will give properties such as void ratio, porosity etc.

-This test helps to study the amount of settlement and rate of settlement in the soil.

Reference:

Consolidation of Soils: Testing and Evaluation : a Symposium by Raymond Nen Yong, Frank C. Townsend.

Solution 2.

Primary consolidation is caused as a result of volume change in saturated cohesive soils due to exclusion of water occupied in the void spaces. It is a very slow process and continues over a very long period of time.

Secondary consolidation is an additional form of compression that occurs at a constant effective stress at a very slow rate. It occurs in saturated cohesive soils as a result of the plastic adjustment and rearrangement of the soil fabrics. It occurs after the primary consolidation settlement has ended.

Solution 3.

Procedure to determine Coefficient of Consolidation:

Reference: IS 2720(Part 15): 1986 Methods of test for soils: Determination of consolidation properties.

a. Plot the dial gauge readings versus square root of "t" for each load increment and draw a smooth curve joining the points.

b. The dial reading corresponding to zero primary consolidation do is found by extrapolating the straight line portion of the curve DC back to t=0.

c. Straight line is then drawn from do such that the abscissae of this line are 1.15 times the abscissae of the straight line CD.

d. The point at which the drawn line intersects the experimently obtained curve is d90, that is corresponding to 90 percentage consolidation.

e. Time required for 90 percentage consolidation is read off the curve as t90.

f. The coefficient of consolidation for the load increment under consideration may be calculated from the formula below:

CV = {0.848 * (Hav / 2)2}/t90

where Hav = average specimen thickness for the load increment.

Solution 4.

-The consolidation parameters obtained by this test are used to determine the consolidation settlement and time of consolidation for a given loading rate.

-Consolidation test is used to determine the rate and magnitude of soil consolidation when the soil is restrained laterally and loaded axially.

-Thus the laboratory sample in the consolidation test does not deform laterally.