Cyclic Shear Strength of Soft Railroad Subgrade

A laboratory study was conducted to investigate the behavior of a soft compacted clayey soil subjected to repeated loading. The work was part of a comprehensive investigation of the Low Track Modulus section at the Facility for Accelerated Service Testing, operated by the Association of American Railroads Transportation Technology Center in Pueblo, Colorado. The goal of the study was to quantify the stress-strain behavior of the LTM subgrade soil subjected to physical and stress conditions existing under train traffic. To achieve this goal, cyclic triaxial tests were conducted on tube samples at their natural water content and on samples subjected to back-pressure saturation. Results indicate that the normalized cyclic shear strength is sensitive to drainage conditions and degree of saturation. Data trends indicate that cyclic strength may decrease by approximately 80% as the initial degree of saturation is increased from 90 to 100%. Field observations of persistently accumulating shear deformations in the the LTM subgrade are consistent with the strength criteria developed in this study.