The supporting and back structures are important temporary structures in jacking construction of bridges and culverts, and are of great importance in ensuring smooth construction and guaranteeing the time limit for a project.

Ground penetrating radar is a kind of economical and high efficient detection tools in geotechnical engineering and can detect the thickness of sprayed concrete for initial lining,back cavity and compacting status of backfilling for overbreak in tunnel.

In this paper,In case of the Three Gorges Project,the finite element model of the penstock downstream of dam with cushion course is built by using the software of ANSYS;the stress state of penstock is calculated and analyzed.