When the forensic analysis of the earth movement and partial collapse of the retaining wall system in the Hills of Rivermist in Northwest San Antonio is complete, a clear picture of the cause and origin of what most residents of San Antonio rarely see: An apparent slope failure and retaining wall collapse. But did the retaining wall collapse cause the massive movement of soil downhill, or did the fill material on the slope overwhelm the retaining wall?
Slope failures are quite common throughout the United States, but they are a rare occurrence in the San Antonio area. Robert Day in his book “Forensic Geotechnical and Foundation Engineering”, notes that slope failures may be related to periods of prolonged moisture. During the hot and dry summer period, the slope face (earth or fill) can dry and shrink. During the rainy season or when excess water is introduced into the slope surface, the shear strength of the soil or fill diminishes downhill creep begins. Slope failure occurs when the pull of gravity is greater than the soil’s ability to resist it, and the earthen mass moves laterally or parallel to the slope of the hill. Structural barriers such as retaining walls function to prevent the uncontrolled movement of earth or fill on slopes. Gravity type retaining wall failure can occur when the weight of the earthen slope mass exceeds the resistance capacity of the retaining wall.