During construction of a bridge project, it became important to check the stability of one of the bridge pier foundations. The spread-footing foundation was located at the crest of a 60-ft-deep bedrock channel formed by a series of steeply dipping joints. The overall slope of the channel face was about 50 degrees, and rock mass conditions exposed during construction prompted concerns about potential instability of the channel wall beneath the footing. As a consequence, the foundation was modified: a keyway was excavated beneath the footing and a series of long rock bolts designed to guard against possible sliding of the block of rock was installed beneath the footing.At the request of the owner, independent investigations and analyses were undertaken to evaluate the stability of the modified foundation. The approach used for the stability evaluation was first to assess which mechanisms of failure could be considered kinematically feasible on the basis of field investigations of geological conditions.Then, for each potential failure mechanism, appropriate strength parameters were assigned to the sliding surfaces involved. Finally, the stability of the system against each postulated failure mechanism was evaluated. In this process, the nonlinear barton criterion for shear strength of a rough discontinuity was applied to a potential block sliding mechanism, and the nonlinear hoek-brown criterion for the strength of a pervasively jointed rock mass was applied to a potential circular failure surface passing beneath the footing. In the latter case, stability analysis methods developed by bishop and sarma were used, because the latter method allows specific geological structural features to be incorporated in the body of the slide mass with shear strength parameters that differ from those along the basal plane of sliding. This approach is considered to more closely reflectthe actual conditions often present in a jointed rock mass. The methods used and the results obtained from the stability evaluation aresummarized, with particular reference to the selection of barton and hoek-brown shear strength parameters. This paper appears in transportation research record no. 1330, Behavior of jointed rock masses and reinforced soil structures 1991
Abstract