Performance assessment and optimization of seepage control system: A numerical case study for Kala underground powerhouse
Introduction
Located in Muli County, Sichuan Province, China, the Kala Hydropower Project was planned to be built in the middle reach of Yalong River, mainly for power generation purpose. During construction, a large-scale underground cavern system consisting of a machine hall (232 m in length, 30 m in width and 54 m in height), a transformer room, a surge chamber, auxiliary caverns, tunnels, and tailraces will be excavated in the rock masses at the right bank to host four generator units, with an installed capacity of 980 MW. The surrounding rocks contain weak carbonaceous slate prone to softening, and geological structures such as bedding planes, faults and fractures are well developed in the cavern area. Furthermore, the excavation of the cavern complex induces stress redistribution and permeability variation in the surrounding rocks, leading to formation of the so-called excavation disturbed zone or excavation damaged zone (EDZ) [1]. In the EDZ, existing fractures may open or undergo shear dilatancy, and new cracks or fractures may be induced, which contributes to a significant increase in hydraulic conductivity. Existing studies [2], [3], [4], [5] have shown that the excavation-induced variation of hydraulic conductivity in the disturbed zone of deeply-buried tunnels may reach up to or even over 3 orders of magnitude, which evidently leads to change in local distribution of the seepage field and varying performance of the seepage control system.
With the poor geological conditions in the cavern area, seepage control is critical for maintaining the stability and safety of the cavern complex. For this purpose, a seepage control system containing a grouting curtain, drainage tunnels and drainage hole arrays was suggested for the underground powerhouse, but its performance remains an important issue to be addressed. This case study presents a systematic performance assessment of the seepage control system designed for Kala underground powerhouse. Particular concerns are paid to geological characterization of the site and quantitative determination of the hydraulic conductivities of the rock masses. The excavation-induced variations in the hydraulic conductivities of the surrounding rock masses are assessed with a strain-dependent hydraulic conductivity tensor model [2] by considering the effects of stress release and fracture orientation. Sensitivity analyses are conducted to yield optimized parameters for the layout design of the seepage control system.
Given the fact that the deployment of the seepage-proof and drainage systems in the rock masses may reduce the unfavorable effect of individual discontinuities on the concentrated seepage flow behavior, as evidenced in [6], the equivalent continuum method is adopted in this case study. Furthermore, considering that the long-term effect of seepage flow is the major concern in the design of the seepage control system, the steady-state flow model is used for assessment. Typically, the variational inequality (VI) formulation [7], together with a substructure technique and an adaptive penalized Heaviside function, is employed for its effectiveness in numerical modeling of the seepage flow behaviors of drainage hole arrays. The remainder of this paper is arranged as follows: In Section 2, the VI method for unconfined seepage problems is briefly introduced. Section 3 presents geological characterization of the site and determination of the initial hydraulic conductivities of the rock masses. In Section 4, the performance of the seepage control system is assessed with consideration of the excavation-induced variation in conductivity, which is followed by conclusions presented in Section 5.
Section snippets
A brief review of the variational inequality method
The variational inequality (VI) formulation adopted in this study was proposed by Zheng et al. [8] for steady-state seepage problems by extending the traditional Darcy’s law defined in the wet domain to the entire domain of interest and specifying the boundary condition on the potential seepage surfaces as the complementary condition of Signorini’s type. This condition is represented as follows:where Γs is the potential seepage boundary, qn the flux out of the
General description
As shown in Fig. 2, the strata at the construction site of the Kala Hydropower Project consist of a series of epizonal metamorphic rocks of the Zagunao group of upper Triassic system, corresponding to the second member of the regional strata (). The strata are tilted in a monoclinal manner, striking roughly parallel to the flow direction of the river and dipping at 45–70° toward the left bank. The rocks in the second member mainly consist of sandy slate, marble, metamorphosed
Performance assessment of the seepage control system at Kala underground powerhouse
Given the geological and hydrogeological conditions of the site and the layout of the project, a seepage control system containing a grouting curtain, drainage tunnels and drainage hole arrays was designed for the underground powerhouse. This section assesses the performance of the seepage control system with the finite element method, in which the excavation-induced variations in hydraulic conductivity of the surrounding rocks are particularly characterized.
Conclusions
Planned to be built in the middle reach of Yalong River, China, the Kala Hydropower Station consists of a large-scale cavern complex due to the outstanding geomorphic feature of deeply-cut valleys at the construction site. The cavern area is characteristic of poor geological conditions and is rich in groundwater. This study presents a systematic case study of the performance of the seepage control system designed for the cavern system using the variational inequality formulation and the finite
Acknowledgements
The authors thank the anonymous reviewers for the valuable comments in improving this study. The financial supports from the National Basic Research Program of China (No. 2011CB013503) and the National Natural Science Foundation of China (Nos. 51222903, 51079107 and 51179136) are gratefully acknowledged.
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