Seismic safety analysis of Kukuan underground power cavern

Abstract

The Kukuan power plant is one of the six hydropower schemes constructed along the Tachia River in central Taiwan. The 13.8 m wide, 34.5 m high, 87.27 m long underground cavern with reinforced concrete crown and sidewalls was slightly damaged by the ML7.3 Chi-Chi earthquake of September 21, 1999 and was subsequently flooded due to blocking of river channel by debris flow in a heavy rainfall. To rehabilitate the power plant, the floor structures and the secondary concrete around the power generation equipments will be removed and reconstructed. To evaluate the safety of the cavern during rehabilitation work and in the event of future strong earthquake, static and seismic analyses were conducted. The rock masses encountered at the cavern site consist mainly of thick-bedded quartzitic sandstone interbedded with thin layers of slaty quartzite and slate. The rock mass characteristics were obtained by surveying the unlined drainage gallery above the cavern roof. Laboratory tests were also conducted on specimens to obtain the basic properties of the rock materials. Good to very good quality rock masses were encountered with two major weak planes of bedding and a dipping joint set. Computer code 3DEC was employed to carry out discrete element analysis considering the relatively rigid blocky structure formed by the weak planes. Empirical relationships suggested by Hoek et al. (2002), Barton et al. (1977) and Chern et al. (1997) using the basic rock test results, rock mass rating and weak plane properties measured were used to estimate the strength and deformation characteristics of rock blocks and discontinuities. Two separate analyses corresponding to the longitudinal and transversal sections to the cavern axis were performed to simulate static and dynamic performances of the underground cavern. Modeling steps included the re-establishment of stress condition prior to earthquake, Chi-Chi earthquake shaking, cavern rehabilitation and future design earthquake event. Acceleration time history with PGA value of 0.43 g assessed from Chi-Chi earthquake was adopted for the analysis. The numerical results obtained were compared with the field observations due to Chi-Chi earthquake and monitored data during cavern rehabilitation. Comparable results obtained indicate the numerical simulation can adequately capture the behavior of the cavern under static and seismic loading conditions. Based on the results obtained, the following conclusions may be drawn: (1) Significant shearing and opening of ground observed in the field after Chi-Chi earthquake only occur on the surface layer of slope, and it would not affect the cavern safety. Surface treatment by using shotcrete is suggested to prevent the infiltration of surface water from occuring. (2) Rehabilitation of cavern would result in very small rock movement and hence would not endanger cavern stability. (3) During design earthquake event, the cavern would be subject to significant shearing and result in sidewall closure and moderate increase in concrete stress and cable force. Although cavern stability can be maintained, installation of additional cables in the sidewalls is still recommended to enhance the stability of the cavern. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.