The Rajamandala Hydro Electric Power Plant (the Project) includes excavation work for tunnel of 1.1 km in length, which connects the intake with the powerhouse area. The excavation work from the outlet side started in January 2016 and that from the inlet side started in April 2016. Some areas, especially downstream side, were excavated with a bench cut method, consequently, the upper part of the tunnel had a breakthrough in March 2017. The geology along the tunnel consists of limestone of the Rajamandala layer in the Paleogene period and tuff-breccia of the Citarum layer in Miocene. In addition, mudstone also exists, and some of the mudstone are fractured at the boundary between the limestone and tuff-breccia areas due to the compressive stress from both the sides. Actually, stress release of surrounding rock due to excavation work caused deformation of tunnel excavated surface continuously. The supporting system for the excavated tunnel surface was enhanced several times by such as longer steel pipe reinforced grouting (S.P.R.G.) or double steel ribs, in order to secure the safety of tunnel works and the required sectional area of the water passing with no pressure. Finally, the tunnel excavation for full section completed in December 2018 successfully.
The Project is located at around 100 km southeast of Jakarta in Indonesia as shown in Fig. 1. and consists of weir, intake, headrace tunnel, headrace open channel, head tank, spillway, penstock, powerhouse, tailrace, outlet, switchyard, transmission line and access road. The Rajamandala Hydro Power Plant is a run of river type with the output of 47 MW which is mainly generated by the discharged water from the Saguling Hydro Electric Power Plant located at just upstream of the Project as shown in Fig. 1.1)
Site layout of the Project is shown in Fig. 2, and the tunnel is located between intake and powerhouse area. The tunnel dimension of the Rajamandala HEPP is 8.5 m in height and 8.6 m in width and 1.1 km in length, which leads the water volume of 168 m3/s with non-pressure flow.