ABSTRACT:

Typical practice within Southern California requires that while estimating the slope stability of dip slopes in weak, bedded rock, one of two toe-failure mechanisms be considered: (1) shearing at the toe along a prominent joint or (2) shearing at the toe through intact rock. Both toe failure mechanisms may be valid depending on the geologic conditions encountered at the toe of the slope. It is generally agreed that failure/shearing along the upper portion of the slope will occur along bedding. Slope stability evaluations are carried out by applying anisotropic strength relationships and the Mohr-Coulomb failure criterion. This paper presents a case history where the two mechanisms typically used to model failure through the toe did not apply because of the combined influence of non-persistent geologic structures and intact rock strength (i.e. rock mass strength). Instead, it was decided to use the Hoek-Brown Failure Criterion to estimate the shear strength of the rock mass at the toe of the dip slopes. This paper illustrates the application of the Hoek-Brown Failure Criterion to one geologic environment that was not conducive to the typical failure mechanisms experienced in Southern California. The results of this study suggest that given the proper geologic conditions, the Hoek-Brown Failure Criterion is appropriate to estimate the strength of a bedded rock mass where dip slope failures may be expected.

INTRODUCTION

High-density residential development within Southern California has becoming increasingly difficult for a number of reasons. The geology of the area is tectonically disturbed (folded and faulted), and seismically, is very active. Reviewing agencies are becoming more and more stringent in their requirements for grading permits because of the public?s willingness to bring legal action. Yet the publics? demand for residential development motivates developers to maximize the amount of developed land above steep slopes.

A residential development consisting of 375 single family residential lots has been proposed on the 230-acre site above Devil Canyon, north of the City of Chatsworth, California. The general location of the proposed development is shown in Fig. 1. Developments such as these are not unique. Because the geology of the area consists of some of the oldest and most highly indurated rock encountered within Southern California, the typical procedures employed to characterize rock masses for the purposes of evaluating slope stability may not be appropriate Given the value of the land, the most challenging aspect of the geotechnical design was providing recommendations for the minimum setback distances along the slope crests above Devil Canyon; i.e. how close to the slope crests may development proceed, balancing issues of public safety and maximizing land use.

The south side of Devil Canyon consists of shallowdipping slopes that are coincident with bedding and reach heights of 60 m. The north side of the canyon is steeper, with bedding dipping into the slope and heights approaching 85 m. At the base of the canyon is an intermittent stream. For the purposes of discussion the southern slopes will be referred to as dip slopes. The north slopes are not coincident with bedding and are referred to as anti-dip slopes.

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