Spatial variability of saturated hydraulic conductivity and measurement-based intensity-duration thresholds for slope stability, Santa Ynez Valley, CA

Abstract

Shallow landslides are commonly triggered by positive pore-fluid pressures generated at the soil-bedrock interface and are linked to heavy rainfall. In this study, in situ measurements of saturated hydraulic conductivity (Ksat) were performed at three historic landslide sites to determine its variability and implications on predicting rainfall-rate thresholds for shallow landslides in the Santa Ynez Valley, CA. Falling head tests were performed in the soil column and at the soil-bedrock interface to estimate Ksat. It was found that overlying soils from the A and B horizons exhibited higher infiltration rates ranging from 37 to 138 mm/h and 24 to 77 mm/h, respectively, whereas the failure plane (coincident with the soil-bedrock interface) exhibited Ksat values ranging from 4 to 33 mm/h. Below the soil-bedrock interface, Ksat ranged from 5 to 123 mm/h, 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 and pore spaces with fine-grained particles and caliche are a possible cause of the low-conductivity layer. Measurements of soil mechanical and hydrologic properties were used in a finite slope-stability model as well as a simple 1-D flow model to predict intensity-duration (I-D) thresholds for shallow landsliding in this area. Results of this analysis indicate rainfall intensities >20–30 mm/h that are sustained for >2–4 h are capable of triggering widespread shallow landsliding. This study also suggests that the variability in Ksat at the failure plane can produce significant changes in I-D thresholds for failure and is an important factor in predicting slope stability over any spatial scale.