Considerations on the shear strength behavior of weathered rockfill

A comprehensive investigation on the mechanical behavior of rockfill has been carried out in large-size shear devices in the laboratory. Triaxial and direct shear tests were performed on basalt riprap specimens from Marimbondo Dam in Brazil, which was constructed in 1975. The main results and conclusions on the shear strength envelopes are presented in this paper. Weathered riprap particles were collected at the region of water level fluctuation, where natural alteration by disintegration and decomposition of the basalt rockfill was noted to be more intense. An artificially weathered rockfill material was produced in the laboratory by continuous lixiviation in large Soxhlet equipment. Lixiviated basalt rockfill exhibits marked changes of surface chemical concentration, number of joints and deformability of rock particles. Processes of basalt weathering in the field and in the laboratory are compared in the paper. The methodology for accelerated weathering in the laboratory proved to adequately simulate the effects of field weathering, making possible a prediction of the long-term mechanical behavior of basalt rockfill. A significant reduction of the shear strength of weathered rockfill may be noted when compared to the intact material. The upstream rip-rap at Marimbondo Dam is predicted to remain competent as a slope protection material for at least more 50 years.