Assessment of groundwater lowering for urban infrastructure works, a case study in Sumbe, Angola/Africa

Abstract

This paper describes the hydrogeological study developed for Sumbe city, in Angola, Africa, to evaluate the feasibility of employing existing wells to drawdown the water table in the central area of the city both to facilitate the urban infrastructure implementation and to increase its lifespan. The hydraulic conductivity of the unsaturated zone was estimated based on the open hole and double ring infiltrometer tests conducted in several points of the site to capture the variability of the hydrogeologic condition of the zone of aeration. The transmissivity and the hydraulic conductivity of the saturation zone were estimated based on an pumping test performed in a pumping well and two monitoring wells. Hand auger, SPT and rotary boring were carried out and combined with all field data information to produce a simplified conceptual model of the hydrogeologic system for the study site. Based on the survey data and by applying the Dupuit approximation of the Boussinesq equation for unconfined aquifers, the piezometric surface and the radius of influence of the well were computed for an operational steady condition. The study demonstrated that the entire site location is on a thick layer of clay and employing wells for groundwater lowering is unfeasible for either permanent or temporary purposes. For temporary groundwater lowering, collecting the water drained at the excavation by direct pumping was suggested. Finally, additional considerations regarding the sewerage/stormwater systems and the drainage system for the pavement base were also addressed. Keywords: groundwater lowering; infrastructure; hydraulic conductivity; influence radius of wells.