How to determine pre-compression stress in a soil

When a soil mass is loaded, it undergoes compression/consolidation which is due to the stress from the load it is subjected to. The higher the load, the bigger the stress. If in the course of time, the load causing the compression is reduced by action of man or through natural actions such as erosion, the soil now experiences stress that is less than the initial stress it experienced in the past and the curve of void ratio against the pressure adjusts its shape as shown below (Figure 1). Such soil is said to be overconsolidated. Before the removal of the load causing compression, the soil is said to be normally consolidated.

To show whether a soil is overconsoliaded or normally consolidated, overconsolidation ratio (OCR) is determined.

Mathematically, OCR = P_c/ P_o

Where P_c = precompression stress

and P_o = present effective stress of the soil

When OCR ≈ 1, the soil is said to be normally consolidated

When OCR ˃˃ 1 (far greater than 1 or large), the soil is said to be overconsolidated.

The study of overconsolidation of soils is important when considering the possible settlement of a stratum under load to know how much pressure an overconsolidated layer has been subjected to in the past. In that case, it becomes necessary to determine the pre-compression stress of the soil.

How to determine the pre-compression stress of soil

The method commonly used to determine this parameter is the one devised by Casagrande in 1936. It involves some construction of the e versus log P curve of the soil using some data such as presented in Table 1 below.

Table 1: pressure versus void ratio data for the soil

Procedure

1. Plot e versus P curve on a semi-logarithmic graph as shown below (Figure 2).
2. Locate the point of greatest curvature on the recompression branch by eye (point X).
3. Draw a tangent through this point to form line XZ.
4. Also draw a horizontal line through the point to form line XY.
5. Bisect the angle formed by the two lines (angle YXZ) and form line XA.
6. Produce the straight line portion of the curve backwards to meet line XA at O.
7. Extend the point down vertically to the log P scale to get the corresponding value.
8. Read off the value to get the precompression stress. In our case, the value = 210 kPa.

Causes of precompression in soils

1. Overburden soil which had been removed by erosion.
2. Loads of buildings or other structures which had been demolished.
3. Melting of glaciers which covered the soil deposits in the past.
4. Capillary pressure which acted on the soil in the past but was later destroyed due to rise in water table.
5. Desiccation of the clay deposit.
6. Sustained downward seepage forces which later stopped.
7. Tectonic forces caused by the movement of earth’s crust which later becomes less severe.

Determination of compression index from the curve

Compression index, C_c is the ratio of change in void ratio to the logarithm of corresponding change in pressure or the slope of the void ratio versus pressure (log scale).’

Mathematically, C_c = (e₀ – e₁)/ log (P/P₀). The value can be determined from Figure 3.

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