Numerical modeling is a standard tool employed on civil engineering projects to simulate complex foundation structure interaction problems, particularly those involving strong seismicity. A key to a representative model is incorporating detailed knowledge of the geologic conditions and an understanding of potential failure modes. During excavation of the stilling basin at the Neelum Jhelum Hydroelectric Project in Pakistan a rock slide undermined the sedimentation basin foundation halting work in the area. A redesigned slope allowed construction to resume. Most important, the post-slide evaluation identified a potential failure mode that had not previously been considered that had the potential to undermine the global stability of the foundation and structures. The ensuing finite element analyses employed jointed material properties to simulate weak discontinuities in the foundation. Sensitivity analyses along with concurrent detailed mapping and monitoring during construction of the new redesigned slope provide greater confidence that global stability is satisfied for this potential failure mode.
The Neelum Jhelum Hydroelectric Project (NJHP), located near Muzaffarabad in the state of Azad Jammu and Kashmir in northeastern Pakistan, is being developed by the Water & Power Development Authority (WAPDA) of Pakistan. The project, shown on Fig. 1, utilizes a gross hydraulic head of about 430 m by diverting water from the Neelum River with a dam and intake works at Nauseri to the lower branch of the Jhelum River through a 32-kmlong tunnel system and underground powerhouse complex. The installed generating capacity of the project is 968 MW.
The project is divided into three construction lots: Lot C2 includes the headrace tunnel; Lot C3 covers the surge shaft, penstocks, powerhouse complex, and tailrace tunnel; and Lot C1, which is the area of focus in this paper, includes the headworks and the initial segment of the headrace tunnel. Lot C1, shown on Fig. 2, includes a central 56-m high concrete gravity dam with a concrete debris channel and rockfill dam section forming the right abutment. A gated spillway leads to a 100-m-long by 32- m-high stilling basin between El. 986 m and its foundation near El. 954 m. The dam diverts water through the intake structure and a 300-m-long by 76-m-wide sedimentation basin. The sedimentation basin is located immediately west of the stilling basin and is founded at El. 986 m with walls that extend to El. 1019 m. After flowing through the sedimentation basin, the water is routed to the headrace tunnel.