ABSTRACT
In the Baihetan hydropower station of China, periodic volcanic eruptions in history have resulted in current geological coexistence of columnar jointed rock mass with high mechanical strength and volcanic tuff soil with weak mechanical property in the same stratum. The unstable geotechnical problems of columnar jointed rock mass and volcanic tuff soil, such as unloading loose, crack and collapse of columnar jointed basalt, as well as plastic deformation and collapse of volcanic tuff soil induced by underground excavation, have challenged the design and construction of large underground engineering structures. Detailed field investigation was conducted to expose their geological characteristics and corresponding mechanical properties of columnar jointed basalt. Field exploration and experiments were also implemented to estimate the properties of tuff soil with ‘hard-soft-fracture’ structure, which consists of basalt, tuff soil and cinerite. Our practice indicated that the accurate geological exploration of the columnar jointed rock mass and tuff soil is the key to minimize their adverse effects on the underground engineering. Moreover, a careful construction design that considers blasting control, small opening bench and timely support, is necessary.
