Geotechnical treatment analysis for headrace tunnel in gneissic rock mass

Abstract

This article presents the methodology adopted for the geotechnical design of the support applied to the headrace tunnels of the Laúca Hydropower Plant (AHE Laúca), located in Angola. The plant's headrace system consists of six parallel tunnels, each with an average length of 1900 meters, excavated in a gneissic rock mass. To determine the supports required to maintain excavation stability during the construction phase and the plant's operation, geological and geotechnical tests and investigations were conducted. The results of these investigations supported the studies presented here, providing parameters such as compressive strength and deformability of the rock mass, as well as shear strength of the rock mass discontinuities. The definition of appropriate support to ensure excavation stability was based on the Q-system rock mass classification. After the preliminary design of the support for excavations in competent rock mass, classified as Classes I to III, the proposed support was subjected to verification and validation through local stability analyses. These analyses were performed using the Unwedge software for the End of Construction load case, and a combined approach of local stability and coupled flow analysis using the Unwedge and Phase software for the operational phase load condition. The results highlighted the necessity of a drainage system consisting of perforated drains in the rock mass, in addition to the standard treatment comprising anchors and shotcrete. This system was crucial for reducing pore pressures acting on potentially unstable wedges around the excavations, particularly in scenarios where one tunnel remained empty while the others were in operation. Currently, all tunnels in the project are in operation and have undergone emptying processes due to operational demands. On none of these occasions were issues observed with the tunnel floor, nor were there any reports of block falls from the walls or crown.