SOIL MECHANICS - CHAPTER 4 ppt

Chapter 4

STRESSES IN SOILS

4.1 Stresses

As in other materials, stresses may act in soils as a result of an external load and the volumetric weight of the material itself. Soils, however, have a

number of properties that distinguish it from other materials. Firstly, a special property is that soils can only transfer compressive normal stresses,

and no tensile stresses. Secondly, shear stresses can only be transmitted if they are relatively small, compared to the normal stresses. Furthermore

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σxx

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σxy

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Figure 4.1: Stresses.

it is characteristic of soils that part of the stresses is transferred by the water in the pores. This will

be considered in detail in this chapter.

Because the normal stresses in soils usually are compressive stresses only, it is standard practice to

use a sign convention for the stresses that is just opposite to the sign convention of classical continuum

mechanics, namely such that compressive stresses are considered positive, and tensile stresses are

negative. The stress tensor will be denoted by σ. The sign convention for the stress components is

illustrated in Figure 4.1. Its definition is that a stress component when it acts in a positive coordinate

direction on a plane with its outward normal in a negative coordinate direction, or when it acts in

negative direction on a plane in positive direction. This means that the sign of all stress components

is just opposite to the sign that they would have in most books on continuum mechanics or in applied

mechanics.

It is assumed that in indicating a stress component σij the first index denotes the plane on which the

stress is acting, and the second index denotes the direction of the stress itself. This means, for instance,

that the stress component σxy indicates that the force y-direction, acting upon a plane having its normal in the x-direction is Fy = −σxyAx,

where Ax denotes the area of the plane surface. The minus sign is needed because of the sign convention of soil mechanics, assuming that the

sign convention for forces is the same as in mechanics in general.

4.2 Pore pressures

Soil is a porous material, consisting of particles that together constitute the grain skeleton. In the pores of the grain skeleton a fluid may be

present: usually water. The pore structure of all normal soils is such that the pores are mutually connected. The water fills a space of very

complex form, but it constitutes a single continuous body. In this water body a pressure may be transmitted, and the water may also flow

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