In areas of weathered and decomposed rock profiles, the definition of soil parameters needed for the analysis and design of laterally loaded drilled shafts poses a great challenge. The lack of an acceptable analysis procedure is compounded by the unavailability of a means for evaluating the weathered profile properties, including the lateral subgrade modulus, which often leads to the conservative design. Results from this research revealed that currently proposed P-y approaches to design drilled shafts embedded in weathered Piedmont profiles do not provide reasonable estimates of load-deflection response. Results in this report are used to develop and validate a procedure for the analysis of laterally loaded drilled shafts embedded in a weathered rock mass. The developed procedure is based on the P-y method of analysis in which the shape and magnitude of the P-y function are defined. The research proceeded along four complementary tracks: i) Finite Element modeling , ii) Laboratory work, iii) Field testing using full scale shafts; field work also included estimation of in situ modulus of subgrade reaction using rock dilatometer, and finally iv) Performance predictions. The proposed P-y curves are developed as hyperbolic functions. A method to evaluate in situ stiffness properties of the weathered rock by utilization of the rock dilatometer, as well as by using geologic information of joint conditions, RQD, and the strength properties of cored samples, is proposed. A computational scheme for lateral behavior is advanced by which different lateral subgrade responses are assigned in the model based on the location of the point of rotation. Above the point of rotation, a coefficient of lateral subgrade reaction is assigned on the basis of evaluated modulus as computed from rock dilatometer data or from index geologic properties. A stiffer lateral subgrade reaction is assigned below the point of rotation in order to model the relatively small shear strains in this region. Predictions based on the proposed P- y model for weathered rock show good agreement with field test results, which were performed in various rock profiles. The proposed method is also verified by comparisons with published results of an additional field test. Concepts of the proposed weathered rock model have been encoded into the computer program LTBASE.
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