Abstract: Baihetan hydropower station is another large hydropower station with a total installed capacity over 10 000 MW, following the Three Gorges and Xiluodu hydropower station. The water retaining structure of the station is a double-curvature arch dam, with the rock mass of dam foundation belongs mainly to Permian system thick bedded basalt and some microlite and crypto-crystal columnar jointed basalt. Deformation characteristics of the rock mass are important basis in dam design. We introduced the principle and method of borehole jack, and compared its measured results with in-situ bearing plate test results, and analyzed the deformation characteristics and variation regularity of columnar jointed basalt. Results showed that the deformation results measured by borehole jack well consists with that by in-situ bearing plate test, which indicates that the measured results are reliable. Moreover, the horizontal deformation modulus is evidently larger than vertical deformation modulus, and the deformation moduli in both banks are subject to influences of geological factors, geostress, and bank slope unloading. Deformation modulus of test holes near the bank slope is smaller than that far from the bank slope. The deformation parameters measured by borehole jack can be applied directly to engineering.

Key words: Baihetan hydropower station, test, deformation modulus, borehole jack