Compressibility and Consolidation of Soils

Compressibility and Consolidation of Soils

Copyright: © 2015 |Pages: 52
DOI: 10.4018/978-1-4666-6505-7.ch008
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The total compression of soil under load is composed of three components (i.e. elastic settlement, primary consolidation settlement, and secondary compression). The consolidation component is time-dependent and its analysis is usually based on Terzaghi's theory. The chapter considers the consolidation characteristics of a soil and their experimental determination. The coefficient of consolidation can be determined by the Casagrande Logarithm-of-Time Fitting Method or the Taylor Square-Root of Time Method. The concepts of preconsolidation and overconsolidation are discussed while ways of determining the preconsolidation pressure, compression index, precompression index, and the coefficient of volume compressibility are explained. Ways to compute the settlement using coefficient of volume compressibility and e-logs methods for both normally consolidated and overconsolidated soils are provided. The chapter also explains Schmertmann (1955) graphical procedure for approximating the field compression index from the laboratory curve. It includes the derivation of Terzaghi's 1-D theory of consolidation and its solution using both analytical and graphical methods. Finally, the phenomenon and way of computing the secondary compression index are treated.
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8.1 Compressibility Of Soils

Soil is a multi-phase medium made up of mineral grains which enclose voids that may be filled with gas, liquid or a combination of both. When stress is applied to a soil sample its volume decreases. Such a change in volume may be due to: (a) a compression of the solid soil particles, (b) a compression of water and air within the voids, or (c) an escape of water and air from the voids.

The solid particles and the pore water are relatively incompressible and therefore under the loads usually encountered in geotechnical engineering, they will not undergo appreciable volume changes. Therefore the decrease in volume of a saturated soil mass when subjected to stress increase is due almost entirely to an escape of water from the voids. Some compression also occurs as a result of shifting of position by the soil particles by rolling and sliding under the influence of the applied load. This aspect of the compressibility of a soil mass depends on the rigidity of the soil skeleton. The rigidity, in turn, is dependent on the structural arrangement of the soil particles, and in fine-grained soils, on the degree to which adjacent particles are bonded together.

As compression occurs in soils the pore water escapes. The escape of water, according to Terzaghi (1943), takes place in accordance with Darcy's law. This process which involves a slow escape of water from the pores, gradual compression and a gradual pressure adjustment is called consolidation. When the compression takes place in unsaturated soils by mechanical means such as rolling or tamping, it is termed compaction and mostly air rather than water is driven out from the pores of the soil.

In sand, most of the consolidation takes place during construction and the after-effects are therefore much smaller than in fine-grained soils. However, the impact on foundation engineering depends on the sensitivity of the structure to small differential settlements. Therefore, even for sands, it may still be necessary to estimate the settlement of some structures.

Clay as found in nature has normally undergone a natural process of consolidation, having originally been deposited in water and then gradually compressed by the weight of the material deposited above it. The soil is said to be fully or partially consolidated depending on whether or not a state of equilibrium has been reached under the existing overburden pressure. Some clay deposits are over-consolidated, that is, they have been compressed at some time in their geologic history by super-imposed loads, such as the ice sheets of the Pleistocene period or have consolidated because of free draining conditions as in some lodgement tills, by capillary suction or by lowering of ground water table. It is also possible for some of the original overburden pressure, which caused the deposit to consolidate, to be removed in the cause of geological history e.g. by erosion. This will also give rise to overconsolidation.

The total compression of soil under load is therefore composed of three components i.e.Total Settlement = Se + Sp + Ss(8.1) where

Se = Elastic Deformation (Immediate settlement)Sp = Primary ConsolidationSs = Secondary Compression

The elastic deformation is due to the deformation of soil and rock grains and the compression of air and water in voids. It is fully recoverable; the primary consolidation is drainage of water and air from the voids allowing compression of soil skeleton. It is inelastic, time dependent and only partially recoverable; while the secondary compression is due to creep movements – plastic adjustment of soil fabric under a constant effective stress. This is also inelastic, time dependent and unrecoverable. The elastic deformation of soils is discussed in Chapter 4.

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