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Figure 3.3 Schematic illustration of the stress state at different points along a potential planar and a potential curved failure surface in a rock slope.

3.1.2 Groundwater and Effective Stress

The stress state in a slope is also dependent upon the groundwater conditions. Groundwater is defined as water below the water table, which is also known as the zone of saturation. Factors influencing groundwater pressure will be discussed later; however, for now it will be assumed that groundwater is present in a certain rock slope, thus resulting in varying groundwater pressure distribution throughout the slope.

The primary effect of groundwater pressure is through the principle of effective stress. The stress state at a point in the rock mass is governed by the principal stresses and the acting water pressure. Since the water pressure is equal in all directions, it acts to reduce the effective stress at a given point in the rock mass. This principle applies to all permeable

materials and both soils and rocks. A reduction in the effective stress state has a bearing on the shear strength of the rock mass. The shear strength of a discontinuity is directly

proportional to the applied normal stress. A reduction of normal stress consequently leads to a reduction of shear strength of the failure surface Furthermore, existing groundwater pressures can act as additional driving forces on failure surfaces for certain failure modes.

Secondary effects of having water present is that some minerals react unfavorably with water thus reducing the material strength of a filled discontinuity for certain rock types. Erosion brought about by flowing water could also result in reduced strength (Morgenstern, 1971;

Sage, 1976; Sharp et al., 1977; Hoek and Bray, 1981).

The sources of groundwater and the groundwater pressure distribution in slopes also need discussion. The undisturbed water table (before excavation) in a rock mass depends on (1) the infiltration of rainfall and melting snow, (2) the surrounding topography, (3) nearby rivers and lakes, and (4) the geohydrological characteristics of the rock mass. It follows from this that the water table changes with time, for example, during spring runoff or heavy rainfalls. As an open pit is being mined, the initial water table will change (be drawn down) due to inflow of water into the excavation. The water table, or the phreatic surface, will thus change

constantly depending upon the development of the excavation (Morgenstern, 1971; Sharp et al., 1977). This drawdown will result in a water table as illustrated schematically in Figure 3.4 for a homogeneous rock mass. The actual shape and location of the water table will depend upon the slope geometry, the permeability characteristics, and recharge from the surrounding rock mass. Furthermore, freezing of water during wintertime can prevent flow into the pit and thus increase groundwater pressures in the slope. There may also be zones with contained, or perched water, in a rock mass which further complicate the groundwater pressure distribution.

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Figure 3.4 Drawdown due to mining of an open pit and the resulting phreatic surface (partly after Sharp et al., 1977).

Points in the rock mass below the water table are subjected to groundwater pressure.

Consequently, these points are also subjected to a lower effective stress than points which are located above the water table, as shown in Figure 3.5.

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Figure 3.5 Comparison of the effective stress state in a partly saturated slope (top figure) and an almost drained slope (bottom). Stress states are shown for the same point in the slopes and for two different phreatic surfaces.

The groundwater pressures along a surface in the saturated zone represents the piezometric surface, which is a profile of water pressure on a given surface. For static conditions, i.e., no flow or uniform horizontal water flow, the piezometric surfaces all coincide with the water table, but in all other cases there will be a different piezometric surface for each physical surface through the zone of flow (Sharp et al., 1977). It follows that in order to correctly assess the groundwater pressure on an actual or potential failure surface, the water pressures cannot be calculated from the location of the water table but must instead be measured. It is

often difficult to precisely determine water pressure distributions along failure surfaces and in practice, an assumption of static conditions is often made and the groundwater pressures are thus calculated from the vertical distance to the water table.

Measurement of groundwater pressure can be conducted using various types of piezometers, ranging from simple standpipe tubes to electrical piezometers. Open holes and standpipes can be used for rocks with high permeability, whereas a closed system, and preferably an electrical piezometer is required for less permeable rocks (Hoek and Bray, 1981). Geophysical

techniques, preferably resistivity measurements, have also been used to quantify flow paths and groundwater levels in the rock mass (Kusumi, Taniguchi and Nakamura, 1993). The application of these techniques is new and promising but not yet fully tested and verified.

The permeability, or hydraulic conductivity, of the rock determines its ability to transmit a fluid. Compared to a granular material such as soil, intact rock has a relatively low

permeability. This applies in particular to hard, igneous rocks. However, since the rock mass also contains numerous discontinuities, the mass, or bulk, permeability can be significantly higher, as the flow of water concentrates along pre-existing discontinuities (Louis, 1969;

Londe, 1973b). The combined rock mass permeability can, depending upon the degree of fracturing, be several magnitudes of orders higher than the permeability of intact rock. By only considering an equivalent permeability for the rock mass, the actual flow pattern in the rock mass is grossly simplified. If larger and more dominate structural features exist, this simplification cannot be justified.

The permeability of individual discontinuities is very sensitive to changes in the joint aperture (opening) which in turn depends upon the normal stress acting on the joint surfaces. Thus, a coupled behavior between stresses and groundwater flow arises. For instance, a slope which is being destressed close to the face will permit a higher flow of water in this portion, thus changing the location of the phreatic surface which in turn changes the effective stress. The same could occur in a rock mass which is strongly fractured during failure. On the other hand, a decrease in the overall permeability can be expected in regions of high compressive stress, for example, at the toe of the slope (Sharp et al., 1977).

The coupled effects discussed above are difficult to quantify, and illustrate the difficulty in accurately determining the groundwater pressure in a rock slope. In most cases, it is not economically or practically feasible to measure the actual groundwater pressure distribution, other than at a few selected points. To be able to assess the groundwater pressures at other locations in the rock mass surrounding the pit, an analysis tool must be utilized. Previously, graphical flow nets have been used extensively but this technique is very time-consuming for

more complex problems, involving anisotropic and heterogeneous materials. Today, numerical analyses are commonly used for assessing the resulting groundwater pressures, thereby also accounting for coupled effects and flow in discontinuities.

In the presentation so far, only the two-dimensional case of groundwater flow has been discussed. In reality, the groundwater pressure will vary in all three dimensions as well as around the slope outline, just as was discussed with respect to the stress state. The three-dimensional flow of water is difficult to analyze theoretically (Louis, 1969), in particular for jointed rock masses and accounting for coupled effects between stress and water pressure.

To summarize, the stress state in the rock mass surrounding an open pit depends on (1) the virgin stress state before excavation, (2) the slope geometry in three dimensions, (3) the groundwater conditions, and (4) to some extent the local geology of the rock mass. An assessment of the true three-dimensional stress state around an open pit requires good measurements of both the virgin stress state and the groundwater pressures, and a good analysis tool for calculating stress redistributions due to mining. The effective stress state will obviously influence both the failure modes and the resulting strength parameters for a rock slope. Preferably, the analysis tool should be able to handle groundwater pressures and coupled effects but this is seldom practically feasible. In many cases, it might be necessary to assess the effects of existing groundwater pressures separately or in a simplified manner. The accuracy required obviously depends on the accuracy with which other parameters which govern the stability of a slope can be determined.