Rational design treatment of slides in overconsolidated clays and clay shales

Abstract

During the early part of the past century, engineers were repeatedly surprised by unforeseen movements of valley slopes on overconsolidated, flat-bedded clays and clay shales. Movements have occurred, and still persist, throughout the Cretaceous Sea area of North America. During the past 20 years the profession has been developed to the point where the element of surprise should not be an issue. However, historically, during construction of bridges and dams, intensive effort was required to moderate and control the movements. Research in the geotechnical community over the past 50 years has led to an understanding of the causes and treatment of the movements. Highly overconsolidated deposits are associated with coefficients of earth pressure at rest of about 2, induced by geologic activities such as erosion and glaciation. The horizontal forces that prevail are a major contributing factor to movement along depositional weak layers beside or beneath valleys. Overconsolidated clays display distinct peak and residual strength values. Progressive reduction of strength from the toe of a slope along a weak layer results in a global reduction of shearing resistance, finally leading to a factor of safety of unity. It has been the experience of the authors that the geologic details and hydrologic environment which prevail in most slides are complex and usually cannot be defined well enough to justify great effort in computational analyses. Nevertheless, evaluation of case records over the vast area of Cretaceous deposition with the aid of geomorphology, air photographs, and records of movements allows useful conclusions to be drawn, leading to a rational design philosophy. This review focuses attention on such a philosophy. Movements in slide areas are progressive or episodic. Often an historic slide area is dormant and may be reactivated by relatively minor construction activity. Other cases involve ground that is gradually reaching a global factor of safety of unity. Inherent in the design philosophy proposed is a decision whether to accept the movements and design the structures to tolerate them (passive design), or to restrain or stop the movements of the slide mass (aggressive design).This approach is described. Detailed and sophisticated computational analyses have been useful in supplementing an understanding of failure mechanisms. In practice, however, there has been an increasing tendency to turn to computer programs, sometimes quite elaborate, without adequate knowledge of geologic, hydrologic, and observational data. Often, if such data are collected and digested, very simple analyses may suffice for the design of preventive or remedial measures. Observations of active slide masses show that they most often move at a rate of about 100 mm/year. In these cases only simple analyses are justified in evaluating possible corrective measures. Key words : slides, clay, shale, overconsolidated, movements.