Importance of Residual Strengths in Factors of Safety and Reliability

Reliability analyses are becoming more common place. They play an important role in geosynthetic design because they provide for a rational and defensible means of managing uncertainty in field performance. Residual shear strengths that are less than the peak strength are also common place with many geosynthetic materials. Conventional reliability analyses do not explicitly incorporate residual strengths; they are based on the mean and standard deviation for the strength. The residual strength is only considered implicitly as a data point in establishing the mean or standard deviation. However, a residual strength is more than just a data point in the distribution of possible strengths. It represents the lowest possible strength that the material will have. From a reliability standpoint, this lowest possible strength plays a significant role. A slope cannot fail if the applied stress is less than the residual strength; that is, the probability of failure is zero in this case. This paper describes ongoing work at The University of Texas at Austin to establish residual strengths for different geosynthetic materials, to develop tools to include residual strengths in reliability analyses, and to assemble practical methodologies so that residual strengths can rationally be included in design.