ABSTRACT

A nomogram has been constructed for analyzing slope stability in fine-grained sedimentary environments. It was designed to aid in interpreting the muses of mass movement in modern and ancient settings, to provide a basis for evaluating and predicting slope stability under given conditions, and to further the understanding of the relationships among the several key factors that control slope stability.

Design of the nomogram is based on effective stress and combines consolidation theory as applicable to depositional environments with the infinite-slope model of slope-stability analysis. The link between the two combined theories is a term representing the effective overburden stress, which may be predicted from consolidation theory and a knowledge of sedimentation rate, time, and the coefficient of consolidation. In turn, if infinite-slope conditions are assumed to exist, the effective overburden stress can be used to derive a factor of safety against static slope failure by using the angle of internal friction and the slope angle. Values of the variables may be determined directly from measurement, or, depending on the objectives or limitations of the application, they may be speci ied or estimated. Information supplied with the nomogram is intended to assist in estimating values where necessary.

The nomogram applies to depositional settings in which fine-grained sediment has accumulated at a relatively constant rate upon a base that is essentially impermeable. The model further assumes that the lateral extent of sediment affected by any mass movement will be great compared to its thickness and that no outside agents (e.g., cements, gas) are influencing the section. The nomogram is applicable to static conditions (inherent stability of the slope) and certain dynamic conditions (such as earthquakes). It may be used to investigate mass movements in the geologic past as well as those in modern environments.

Although the nomogram was not designed to be more than an interpretive aid and was not intended for solving slope-stability problems of any complexity or where precision is required, it does provide a basis for interpretation and an adequate first approximation in most cases.