SPATIAL VARIABILITY OF SATURATED HYDRAULIC CONDUCTIVITY AND THRESHOLDS FOR SLOPE FAILURE

Shallow landslides are commonly triggered by positive pore-fluid pressures linked to heavy rainfall. Positive pore-fluid pressures are generated at low-conductivity layers that occur within the soil profile or at the soil-bedrock interface. In this study, in situ measurements of saturated hydraulic conductivity (Ksat) were performed at three historic debris flow sites to determine its variability and implications for triggering debris flows at Sedgwick Reserve, Ca. Falling head tests were performed in the soil column within each horizon and at the soil-bedrock interface to estimate Ksat. It was found that the failure plane exhibited Ksat values ranging from 4 to 33 mm/hr, whereas overlying soils of the A and B/Bt horizons exhibited higher infiltration rates, ranging from 37-138 mm/hr and 24-127 mm/hr, respectively. Below the failure plane, Ksat ranged from 4 to 120 mm/hr, signifying a pattern of fast-slow-fast infiltration. Measurements of grain size distributions and bulk density do not provide insight into the cause of low-conductivity at the soil-bedrock interface. Rather, field observations suggest that the infilling of fractures with fine-grained particles at the soil-bedrock interface may be the cause of the low-conductivity layer. Statistical analyses also show that Ksat has a strong dependence on soil horizon. Hillslope parameters and measurements of soil mechanical properties were used in a finite slope-stability model to predict thresholds for failure. Results of the stability analysis predict failure at 35-50% saturation of the soil column above the failure plane, and suggests that the vertical patterns and spatial variability in hydraulic conductivities of landslide-prone soils are important factors in predicting slope stability.