6.4 Pore Pressures
Subsurface water is divided into zones of positive and negative pore pressures. The dividing line is the groundwater table (also known as phreatic surface) where the pressure is equal to atmospheric pressure. Below the groundwater table, the soil is fully saturated, and the pore pressure is above atmospheric pressure and positive in value. Above the groundwater table where the soil is unsaturated, the pore pressure is below atmospheric pressure and hence is negative in value. In this zone, the pore water is continuous or semi continuous and the pore water pressure is below atmospheric pressure. The magnitude of the negative pore pressure (sometimes called soil suction) is controlled by surface tension at the air-water boundaries within the pores and is governed by grain size. In general, the finer the soil particles, the larger the saturation capillary head, and hence the higher the negative pore pressure. Rainfall infiltration from the ground surface may rapidly reduce the magnitude of negative pore pressure. Any change in these pore pressures alters alter the shear strength of soil and therefore has a tremendous effect on the slope stability.
The water level measured in a piezometer within the saturation zone coincides with the water table. However, the pore pressures are no longer hydrostatic if there is a flow. In this instance, the pore pressure from any point within the soil mass is computed by means of a flow net, from the difference in head between the point and the free water surface.
By lowering effective stress, positive pore pressure reduces the available shear strength within the soil mass thereby decreasing the slope stability. Increase in positive pore pressure can be rapid after a period of heavy rainfall. That is a major reason why many slope failures occur after heavy rainfall. The rate of increase, however, depends on many factors such as the rate of rainfall, the nature of the ground surface, the catchment area, and the soil permeability.
Pore pressure below the groundwater table can be assessed using analytical, numerical, and graphical methods. Various analytical methods are available for determining flow nets and pore pressure distributions in slopes. Numerical techniques using finite difference or finite element method provide powerful tool for obtaining pore water distributions in slopes. They are the only means by which transient flow situations can be fully modeled.
Negative pore pressures increase the effective stresses within a soil mass and improve the stability of a slope. Ho and Fredlund (1982a) suggested increase in shear strength due to negative pore pressure as
Where c= total cohesion of the soil
= the slope of the plot of matrix suction when - is held constant
Here, a matrix suction increases the shear strength by . The increase in soil strength can be represented by a three-dimensional failure surface using stress variable , as shown in Figure. These negative pore pressures reduce in magnitude when the degree of saturation increases and become zero when the soils are fully saturated, The major problem in evaluation of stability in unsaturated soils is associated with the assessment of reduction in negative pore pressure and possible increase in positive pore pressure as a function of rainfall history.
The total stress is either due to self-weight of the soil or the externally applied forces or both. the neutral stress at any point inside a soil mass is defined as the stress carried by the pore water and is same in all directions when there is static equilibrium, since water cannot take static shear stress. This is also called ‘pore water pressure’ and is designated as u. This is equal to γw.z at a depth z below the water table i.e.
u = γw z
Effective stress is defined as the difference between the total stress and the neutral stress. This is also referred to as the inter granular pressure and is denoted by:
σ' = σ – u
let us consider full saturation of the rock, including the joint, where no drainage of water is allowed. If we assume that water is incompressible and that no flow of water into or out of the joint is allowed, the volume of the test specimen including the joint must remain constant. under this condition, the water must sustain stresses sufficient to prevent volume change of the specimen. The total applied stress across the joint will be transmitted by the rock asperities and by the water. If the water carries some of the normal stress, then the rock asperities carry less normal load and therefore has less shear strength than it would be if drained. The normal stress transmitted by the water is equal to the joint water pressure. The stress transmitted through the rock asperities is, therefore, equal to the applied stress minus the joint water pressure. The joint shear strength will now be reduced proportionally. The reduced normal stress acting through the rock contacts is termed as the effective normal stress and is given by
=effective normal stress
The total stress imposed on such a soil will be sustained by the soil (the effective stress,) and the pore water (the pore pressure, u). A reservoir can be used to create an upward seepage through the soil sample. For this purpose, we assume that the valve leading to the upper reservoir is closed. Thus, there is no water flowing through the soil sample (figure 8). This is the case of no seepage the Effective stress is
Figure 8: Effective stress when there is no water flow
Upward seepage conditions can be induced in the laboratory using constant-head permeability test apparatus, in figure 9. The upper reservoir causes the water to flow upward through the soil sample. If the hydraulic gradient is large, the upward-seepage force will cause the effective stress within the soil to become zero, thus causing a sudden loss of soil strength in accordance with the effective-stress principle. However, if downward seepage is allowed, effective stress sigma' is
Figure 9: Effective stress when there is no water flow
In the analysis of stability of slopes in terms of effective stresses, the pore water pressure distribution is of fundamental importance and its evaluation is one of the prime objectives in the early stages of any stability study.