Under the condition of excavation and unloading

the induced relaxation and cracks of columnar jointed basalts (CJB) were the main challenges for the stability of the dam foundation of Baihetan hydropower station. To analyse the loosening characteristics of CJB

the temporal-spatial evolution of the loosening depths and degrees of class Ⅲ1 and class Ⅲ2 of CJB were studied by the single-hole acoustic wave test and borehole camera inspection which were conducted in the CJB test section of the left bank dam foundation of Baihetan hydropower station. The reason of loosening characteristics was then analysed in detail. The results showed that the loosening depth was mainly caused by excavation. The loosening degree of CJB adjacent excavation surface grew progressively with time by the negative exponent type

with a semi-annual growth period. The loosening characteristics were closely associated with the rock mass quality. The loosening depth

the time-dependent characteristics and distribution with borehole depths of loosening degree were largely different from class Ⅲ1 to class Ⅲ2. The time-dependent loosening was caused by the ageing degradation of strength parameters of rock mass due to the opening of internal joints. This study is useful to understand the unloading mechanical behaviour of CJB and to optimise the excavation and support of CJB in the dam foundation.