Geological challenges at the Nam Ngum 3 and 4 schemes in Lao PDR

The 480 MW Nam Ngum 3 (NNG3) and the 240 MW Nam Ngum 4 (NNG4) hydro schemes are part of a cascade development in Lao PDR. At both projects, the main underground structures are the headrace tunnels, with lengths of 11 km and almost 17 km, respectively. Close to the powerhouse, both pressure tunnels are connected to a surge shaft to balance the dynamic pressure effects of waterhammer. The NNG3 project includes a drainage tunnel 60 m above the penstock, to release the water pressure above the steel-lined section. The downstream end of the NNG4 project is fully lined from the surge shaft to the powerhouse, while at NNG3, the steel liner is only installed several hundred metres upstream of the powerhouse, at a point where in-situ stress surrounding the tunnel allows for an acceptable safety level regarding hydrojacking risk. The authors present some key challenges encountered during excavation and construction of the tunnels.