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Abstract After the excavation and formation of high arch dam foundation base of Gupitan hydropower station in Wujiang River，a large amount of ultrasonic wave tests have been carried out on the foundation base. Through these tests with different elevations at different places，the unloading depth is determined according to the comprehensive relationship curve between wave speed and slope distance；and the wave speed distribution pattern and characteristics are also analyzed. In-situ deformation tests are made at 35 points in the unloading zone with conditions of loading path and the maximum pressure similar to that of the arch dam acting on the rock mass. According to dynamic and static comparative tests of the 35 deformation points，the relation curve between unloading rock mass deformation parameters and wave speed is established；and the rock mass deformation parameters in the unloading zone are evaluated based on results of large-scale acoustic tests. The weakening of rock mass deformation parameters in the unloading zone on the high arch dam foundation base is analyzed and studied in combination with the regularity for change of wave speed and deformation parameters before and after excavations. It is shown by experimental study that the foundation base unloading depth is 6 m；the wave speed of the unloading rock mass declines 7.0% on average；and the average deformation modulus reduces from 29.1 GPa to 16.2 GPa，down by 43.6% on average. Finally，the unloading rock mass deformation parameters and the calculating method are put forward.
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Received: 25 February 2010
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