Design and construction of cast in-situ steel fibre reinforced concrete headrace tunnels for the Neelum Jhelum Hydroelectric project

Abstract

Twin 10 km long parallel headrace tunnels were excavated as part of the headrace tunnel system for the Neelum Jhelum Hydroelectric Project, using two Tunnel Boring Machines (TBMs). The permanent support normally comprised a shotcrete lining applied over initial support elements. However, concrete lining was required in certain areas of extremely poor ground conditions. Once placement of conventionally reinforced concrete commenced, it was found to be taking longer than anticipated, prompting a search for alternative solutions. Steel Fibre Reinforced Concrete (SFRC) proved to be the most viable option for accelerating the concrete lining programme. This paper briefly outlines the requirement for a concrete lining in the aforementioned areas. It includes an assessment of the suitability of a conventionally reinforced concrete lining versus an SFRC lining, the design basis, and the actual design itself. A comparison of the costs and durations to install the two types of lining is also presented. 1 PROJECT DESCRIPTION AND OVERVIEW The Neelum Jhelum hydroelectric project is located in the Muzaffarabad district of Azad Jammu & Kashmir (AJK), in northeastern Pakistan. Geographically, the area consists of rugged terrain between 500 and 3 200 m in elevation within the Himalayan foothill zone known as the Sub-Himalayan Range. The project is a run-of-river one, employing 28.6 km long headrace and 3.6 km long tailrace tunnels to cut off a major loop in the river system, transferring the waters of the Neelum River into the Jhelum River, for a total head gain of 420 m (Figure 1). The headrace tunnels comprise both twin (69 %) and single (31 %) tunnels, while the tailrace tunnel consists of a single tunnel. Design capacity of the waterway system is 283 cumecs. The project, which was completed in 2018, has an installed capacity of 969 MW, generated by four Francis-type turbines located in an underground powerhouse. At commencement of construction in 2008, all tunnels were to be excavated using conventional drill & blast techniques. However, it soon became apparent that with the equipment being employed, a 13.5 km long section of the headrace twin tunnels underlain by high terrain that precludes construction of additional access adits, would take too long to excavate. Consequently, the construction contract was amended to allow the operation of two 8.5 m diameter open gripper hard rock TBMs to each excavate some 10 km of the twin headrace tunnels (Figure 1), with an initial centre-to-centre lateral spacing of 33 m, later increased to 55.5 m. The tunnel excavation diameter was 8.5 m diameter giving a total face area of 56.75 m. Excavation direction was upstream to promote drainage, with a typical gradient of 0.8 %.