Investigative procedures for assessing subsidence and earth fissure risk for dams and levees

Abstract

Investigative procedures for assessing land subsidence and earth fissure risk for dams and levees have been developed for projects in arid regions of the USA. The overall assessment includes evaluation of the overall subsidence experienced in the vicinity of a subsidenceimpacted structure, detailed investigation to search for earth fissures, prediction/modeling of future subsidence and related earth fissure development, delineation of risk, and recommendations for moving forward, such as engineered solutions and monitoring. Utilizing the findings of subsidence investigation, additional investigative methods for earth fissure search include photogeologic (lineament) analysis, assessment of the capability of near-surface soils to develop an earth fissure, assessment of the degree of ground disturbance, detailed site inspection, seismic refraction profiling for concealed earth fissures, and excavation of trenches. Such an investigation must include investigative techniques capable of detecting earth fissures that lack significant surficial expression. Satellite-based interferometry by repeat pass synthetic aperture radar (InSAR) provides unique information about active land subsidence over large areas. The subsidence or deformation image known as an interferogram can also, with proper interpretation, reveal some preliminary subsurface information about alluvial basin geometry, lithology and hydrology where active land subsidence is interpreted. Effective subsidence risk assessment and mitigation requires understanding and quantifying historic subsidence, and estimating potential future subsidence that could impact the dams and levee infrastructure. Basin alluvium and bedrock interface geometry, and basin alluvium lithology changes and interfaces, profoundly influence patterns and degree of subsidence. Characterization includes collection and synthesis of historic survey and well data, surface geophysical methods for basin and bedrock characterization, and when available, InSAR to document recent or current subsidence patterns. Utilizing a synthesis of this information, subsidence modeling matchingdocumented historic subsidence and estimating potential future subsidence can be developed to assess potential impacts on dam and levee infrastructure. Utilizing the results of site characterization and subsidence modeling, a finite-element stress-strain model can be developed to estimate past and future ground strain. Estimated tensional strain values can be utilized to predict where earth fissures are likely to initiate with future subsidence and reduce the risk of failure.