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Slope stability analyses may be performed using either total stresses or effective stresses. The use of total stress as opposed to effective stress analyses and the various ways in which design shear strengths can be selected can produce a wide range of safety factors. In general, these questions are more important than the choice of the method used for analyzing stability (Johnson, 1974). Bishop and Bjerrum (1960) set forth the following basic guidelines on the specification of shear strength for use in limit equilibrium slope stability analyses:

1. "Effective stress analysis is a generally valid method for analyzing any stability problem and is particularly valuable in revealing trends in stability which would not be apparent from total stress methods. Its application in practice is limited to cases where the pore pressures are measured or can be estimated with reasonable accuracy, such as long-term stability where the pore pressure is controlled either by the static water table or by a steady-state flow pattern."

2. "Where a saturated clay is loaded or unloaded at such a rate that there is no significant dissipation of the excess pore pressures set up, the stability can be determined by the _u =  0 analysis, using the undrained strength obtained in the laboratory or from in-situ tests. This is essentially an end of construction method, and in the majority of foundation problems, where the factor of safety increases with time, it provides a sufficient check on stability. For cuts, on the other hand, where the factor of safety generally decreases with time, the long term stability must be calculated by the effective stress method."

3. "For saturated soils the values of c' and ' are obtained from drained [triaxial] tests or consolidated undrained tests with pore pressure measurements, carried out on undisturbed samples. The range in stresses at failure should be chosen to correspond to those in the field. Values measured in the laboratory appear to be in satisfactory agreement with field records with two exceptions. In stiff fissured clays the field value of c' is lower than the value given by standard laboratory tests; in some very sensitive clays the field value of ' is lower than the laboratory value."

These 1960 guidelines are still generally valid but increases in our understanding, particularly of undrained strengths, since that time now allow us to do more accurate analyses albeit at the expense of some complications!

Problems in slope stability can be broadly grouped in two classes: short-term problems and long-term problems. When a saturated or partially saturated soil with a low permeability undergoes a change in stress there will generally be a corresponding change in pore pressure. The stage at which the excess pore pressures (positive or negative) resulting from the change in stress are fully developed is referred to as the short-term condition. With the passage of time these out-of-balance pore pressures are redistributed until eventually they are everywhere in equilibrium with the steady state pore pressures appropriate for the new stress conditions. This final stage is referred to as the long-term condition and the continuing stability of the slope under gravity or applied loads is a problem with drained loading conditions.

Long-term or drained stability problems are usually simpler than short-term or undrained stability problems since they always involve drained or effective stress strength parameters and for a given soil these do not vary very much with the type of test that is used to determine them. However, it should be noted that even the effective stress Mohr-Coulomb envelope is curved, rather than straight, for most soils and that the values of ' are thus lower at higher confining pressures. The curvature of the Mohr-Coulomb envelope should normally be taken into account for slopes higher than about 100 feet since use of values of ' determined at the usual confining pressures of about 1 t.s.f. will then lead to errors on the unsafe side.

In general, short-term stability problems involve undrained loading and they can be addressed using total stresses and undrained strengths or effective stresses, drained strengths and pore water pressures. It is commonly believed that both approaches should give the same answer but this is not necessarily so. As noted by Bishop and Bjerrum (1960):

"for factors of safety other than 1 the two methods will not in general give numerically equal values of F. In the effective stress method the pore pressure is predicted for the stresses in the soil, under the actual loading conditions, and the value of F expresses the proportion of c' and tan ' then necessary for equilibrium. The total stress method on the other hand implicitly uses a value of pore pressure related to the pore pressure at failure in the undrained test."

Since the limit equilibrium method is most applicable at failure, in effective stress analyses one should in fact use the pore pressures "at failure," rather than the "actual" pore pressures for the short-term loading condition, and then both approaches will give similar answers. In this connection one should note that the pore pressures specified in effective stress analyses affect only the resisting forces that are computed and not the driving forces. This occurs because the total normal force at the base of each slice is an unknown and an increase in the specified pore pressure decreases the effective normal force but has no effect on the total normal force. Similarly, changes in the pore pressures created by shearing under undrained loading conditions are not included as driving forces in total stress analyses.

The traditional argument for using effective stress analyses for short-term, undrained problems is that it is the effective stresses which really count in determining deformations and therefore effective stress analyses provide greater insight into the problem at hand. However, effective stress analyses require determination of either the "actual" pore pressures or the pore pressures "at failure" and this is no easy task. Indeed, it is about as easy as it is to determine the undrained strength that should be used in a total stress analysis since the reason that undrained strengths vary with sample orientation, the type of test and the details of the loading conditions is largely that the excess pore pressures are sensitive to these factors. In other words, it is about equally as difficult to predict excess pore pressures for use in effective stress analyses as it is to determine the appropriate undrained strengths for use in total stress analyses.

In practice various methods may be used to either predict excess pore pressures or to determine undrained shear strengths for use in short-term stability analyses. Excess pore pressures in fully saturated soils are most commonly predicted using the pore pressure coefficients A and B (Skempton, 1954). Prediction of excess pore pressures in partly saturated soils is extremely difficult. Fredlund and Morgenstern (1977) and Fredlund (1979) discuss various approaches. Special methods have been developed for particular problems such as the end-of-construction condition for embankments, as discussed subsequently.

The preferred method for determining undrained strengths has changed over the years. In 1960 Bishop and Bjerrum recommended the use of UU triaxial tests and cautioned against the use of CU triaxial tests. For a time, use of the vane shear test was popular but it is now recognized that correction factors should normally be applied to the measured strengths (Bjerrum, 1972; Duncan and Buchignani, 1973; Larsson, 1980). Deficiencies in the UU test were also subsequently recognized (e.g. Bjerrum, 1973) and it is now generally agreed that use of UU triaxial tests should normally be restricted to those cases where local experience has shown that use of UU strengths leads to safe and economical construction. More generally, S_u should be obtained from tests on reconsolidated samples using the test equipment and rate of loading which best reproduces the in-situ stress and deformation conditions, using anisotropic consolidation if necessary to represent the initial shear stresses on potential failure planes in the field and using procedures such as the SHANSEP method (Ladd and Foott, 1974) to minimize the effects of sample disturbance.

Some of the issues involved in selecting appropriate pore pressures and/or strengths for slope stability analyses are discussed further in the following sections in terms of the common classes of slope stability problems.

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