Abstract: The peripheral concrete structure of the spiral case of turbine unit No.27 in the Three Gorges underground power station  connects with bedrock in the upstream, downstream and left side. Concrete deformation is constrained by the bedrock, and the force situation is unusual. In order to provide scientific basis for the selection of embedding manner of the spiral case and the reinforcement of the concrete structure, we carried out 3-D nonlinear finite element analysis on the flow channel, peripheral concrete, and surrounding bedrock of the turbine unit. Results show that the scheme of spiral case embedded directly is not economical because of  high tensile stress and high reinforcement ratio; the scheme of spiral case embedded with cushion layers has the advantage of low tensile stress but could not well perform the function of joint-bearing; whereas for the  scheme combining the above two, the tensile stress fall between those of the above two schemes, but the relative rising displacement of the lower rack base approaches to the control standard. In view of this, measures should be taken to reduce the relative rising displacement of the lower rack base to prevent from penetrating cracks.

Key words: Three Gorges underground power station, unit No.27, embedding manner of spira1 case, static force analysis