Application of High-pressure Jet Grouting in Construction of Cofferdam at Diversion Tunnel Outlet,Nuozhadu Hydropower Project

The design elevation of the floor of Diversion Tunnel 2,Nuozhadu Hydropower Project,is 576 m.The riverbed elevation at downstream of the dam is 594 m.So,the floor is lower than the riverbed by 18 m.After Diversion Tunnel 2 is put into operation,sediment at the tunnel outlet seriously accumulates with thickness of 5-8 m.Before the cofferdam at the tunnel outlet is built,the sediment is cleaned by application of sand-pump dredger and negative suction pipe.But the sediment at the riverbed bottom cannot be removed completely because of restriction of the river water return and the dredger itself.Therefore,a couple of seepage passages in the cofferdam at the outlet are available and serious seepage occurs.The cut-off wall area of the cofferdam is treated by high-pressure jet grouting for impermeability and outstanding effect is resulted in to assure safety and stability of the cofferdam at the outlet of Diversion Tunnel 2.