Evaluating karst geotechnical risk in the urbanized area of Sete Lagoas, Minas Gerais, Brazil

Abstract

An increase in groundwater consumption in the municipality of Sete Lagoas (Minas Gerais, Brazil) has induced subsidence and collapse in the last three decades. The area is associated with natural karst conditions. The primary objective of this research was to evaluate and identify the potential subsidence or collapse risk zones. Aerial photographs, lithologic well profiles, optical well logs, and geologic mapping were utilized to categorize risk factors influencing karst subsidence and collapse, which were then applied to an index system. The study showed that the majority of the urbanized area overlies mantled limestone from the Sete Lagoas Formation covered with unconsolidated sediments, contained within a graben, resulting in barrier boundaries for groundwater flow. This structure, together with natural karst processes, explains the location of solutionally enlarged bedding-plane conduits and high hydraulic conductivity in the limestone. Five risks zones in the municipality were identified (negligible, low, moderate, considerable, and high risks) related to geologic and hydrologic risk factors. The urbanized area is located largely in the high risk zone where the majority of the collapse features are located. Additional intensive groundwater extraction in that area will likely generate additional events.