GIS-enabled Spatial Analysis and Modeling of Geotechnical Soil Properties for Seismic Risk Assessment of Levee Systems.

Abstract

Flood protection systems are complex, interconnected engineered systems, where failure at one location means the failure of the entire system. Earthen levees, the systems’ major component, are at risk from many causes of failure including seepage, erosion and instability due to seismic loading, yet there are currently no guidelines available for the seismic design of levees.

Levees stretch for long distances and are formed through various geologic processes and human activities over time, however information regarding soil properties is collected only at limited point locations and varies significantly both laterally and with depth. Levee vulnerability analyses are currently performed only at locations with known soil properties. Prediction of levee performance in locations where no soil data is available becomes a limitation for system risk assessment studies.

A simplified methodology is proposed to predict soil variability in riverine geologic environments for the seismic risk assessment of earthen levee systems. A key step in this methodology is to provide a continuous characterization of soil conditions throughout the system. The proposed model correlates soil properties to preselected regional variables and is implemented, using geostatistical kriging, in a Geographic Information Systems (GIS) environment. GIS was crucial in this research and proved to be the appropriate platform for input, manipulation, analysis, and output presentation of spatial and non-spatial data.

Correlation relationships between soil strength parameters and geological and river geometry factors are presented for a pilot study area in California. Global observations that apply across the study area included the increasing trend of shear strength, Su, with increasing distance from the river, and decreasing trend of Su with increasing river Sinuosity Index levels. Only local trends were observed in the relation of friction angle, ϕ, with Sinuosity Index, as well as in the relation of Su and ϕ with geological formations. The proposed methodology also includes steps for seismic response analysis of levee segments, and flood scenarios in protected areas. Since seismic response of earthen structures is controlled primarily by input ground motions, a methodology for selecting ground motions based on their mean period, Tm, for liquefaction triggering assessment of levees is also developed.