Loading-unloading shear behavior of rammed earth upon varying clay content and relative humidity conditions

This paper investigates the impact of clay and moisture contents on the shear behavior of compacted earth taking into account loading-unloading cycles. Fine sand was added to a natural soil, thereby obtaining three different soils with clay contents of 35%, 26%, and 17%, respectively. A series of triaxial tests was conducted on samples previously equilibrated at three different values of relative humidity (RH). The evolution of failure strength f_c, Young's modulus E, and residual strain ε_res was investigated according to the clay content and the RH, the last two parameters being measured during the loading-unloading cycles. Firstly, the relative humidity at which the samples were fabricated and conditioned was seen to have a strong impact on the mechanical characteristics of the earthen material. An increase in RH led to a decrease in both failure strength f_c and Young’s modulus E, and an increase in plastic strain. The tendencies were found to depend on the clay content of the samples. Secondly, with an increasing stress level, a progressive decrease in Young’s modulus and an increase in residual strain ε_res (after a loading-unloading cycle) appeared. Thirdly, within the range of the investigated clay contents, both failure strength f_c and residual strain ε_res increased with an increasing clay content at constant values of RH and confining pressure, the rate of this increase being a function of the RH. Young’s modulus E was relatively insensitive to changes in the clay content, its variation being less than 20% for all cases. Finally, based on a particular definition of Bishop's effective stress, involving a specific functional form χ(s), the failure states of all the samples were observed to lie approximately on a unique failure line crossing the origin in the (p′-q) plane regardless of the matric suction and confining pressure.