Effect of Stress History on Compression of Clays | Soil Engineering

As the consolidation settlement depends on both the initial and final stress levels and because there is significant difference in consolidation settlement due to virgin compression and re-compression, stress history has a significant effect on compression of clays.

Influence of Stress Cycle:

The relation between soil compression and effective stress is not unique even for the same soil but it is a function of stress history. The compression of a soil subjected to an effective stress for the first time in its stress history is known as virgin compression. The compression of soil subjected to an effective stress for the second or subsequent time, after removal of the previously applied load, is known as re-compression. Due to plasticity of soils, re-compression is always much less than virgin compression even for the same stress increment.

Figure 11.25 shows the void ratio-effective stress relationship for a soil subjected to successive cycles of loading and unloading in a laboratory consolidation test. In the first cycle, the soil subjected to the increase of effective stress from σ₀‘ to σ₁’ for the first time (A to B) undergoes virgin compression (curve AB). When this stress σ₁’ is reduced to σ₀‘, the soil expands but reaches to a void ratio e_C, which is much less than the initial void ratio, e_A, though the effective stress, is the same, indicating that most of the deformation is plastic deformation.

This expansion of soil, with removal of stress, represented by the curve BC, is known as rebound. A stress cycle is said to be completed when the soil is subjected to compression followed by stress release and expansion. The second stress cycle begins when the soil is subjected to the same stress second time.

In the second cycle, the soil subjected to the increase of effective stress from σ₀‘ to σ₁’ for the second time (C to D) undergoes re-compression (curve CD). The decrease in the void ratio (e_C to e_D) during re-compression is much less compared to that (e_A to e_B) during virgin compression, though the effective stress increment is the same in both cases, that is, from σ₀’ to σ₁’. However, the void ratio after re-compression, e_D, is observed to be somewhat lower than the void ratio after virgin compression, e_B, for the same stress level of σ₁.

Similarly, the curves BE, EF, and FG represent the virgin compression, rebound, and re-compression during the stress range of σ₁‘ to σ₂‘.

Influence of Pre-Consolidation Pressure:

Clay deposits may be classified based on the pre-consolidation pressure into the following types:

1. Normally Consolidated Clays:

If the effective stress acting on a clay deposit is more than the pre-consolidation pressure, such a clay is called normally consolidated clay. Normally consolidated clays undergo virgin compres­sion and their compressibility behavior is represented by the curves AB, DE, or GH shown in Fig. 11.25.

2. Over-Consolidated Clays:

If the effective stress, acting on a clay deposit, is less than the pre-consolidation pres­sure, such a clay is called over-consolidated clay. Over-consolidated clays undergo re-compression and their compressibility behavior is represented by the curves CD or FG shown in Fig. 11.25. For over-consolidated clays, the degree of over-consolidation is expressed by the over-consolidation ratio (OCR), defined as follows –

OCR = σ’_p/σ’ …(11.50)

It should be noted that whether a clay deposit would undergo virgin compression or re-compression depends on the value of stresses acting on the deposit relative to the pre-consolidation pressure in its past history.

Suppose a normally consolidated clay is subjected to an effective stress σ’, which is more than its pre-consolidation pressure, σ’_p, the clay would undergo re-compression as the stress increases from 0 to σ’_p. It would be then subjected to virgin compression when the stress increases from σ’_p to σ’. Hence, the terms normally consolidated clay and over-consolidated clay are only relative terms depending on the magnitude of the effective stresses acting on the clays relative to that of its pre-consolidation pressure.

For the same effective stress increment, Δσ’, the decrease in the void ratio (Δe) is found to be much larger for a normally consolidated soil than that for an over-consolidated soil. Thus, the normally consolidated clays possess much higher compression index than that of over-consolidated clays. Similarly, soils are able to withstand different effective stresses σ’₁ and σ’₂, even at the same void ratio, because of the difference in their past stress history.

Influence of Stress Level:

Referring to Figs. 11.26 and 11.27, when the effective stress increases by Δσ’ from σ’₁ to σ’₂ the soil initially in loose state with a void ratio e₁ would undergo reduction in the void ratio by Δe₁ from e₁ to e₂.

Now, in the same stress cycle, when the soil is applied an additional but equal stress increment of Δσ’, from σ’₂ to σ’₃, the soil being denser with a void ratio e₂ would undergo reduction in the void ratio by Δe₂ from e₂ to e₃, which is much less compared to Δe₁ even though the stress increment is the same.

Hence, the coefficient of compressibility goes on decreasing with increase in effective stress and thus is not con­stant but a function of the effective stress (stress level). The actual value of coefficient of compressibility to be used for estimation of void ratio should correspond to the stress range actually encountered in the field. The relation between the coefficient of compressibility and the effective stress should be established by conducting a consolida­tion test over a wide range of effective stress actually expected in the field.