John Hart Generating Station Replacement Project: System Solutions for difficult Ground Conditions in an Access Tunnel
The John Hart Generating Station in Campbell River on Vancouver Island in Western Canada has been operating since 1947. In 2014, a large-scale modernization project was initiated that includes the construction of an underground powerhouse and associated facilities. Upon completion, expected in 2018/2019, the facility will have an increased installed capacity of 132MW featured by 3 new turbines. Besides an increase of structural safety against potential earthquakes, the visual impact of the hydro station will be reduced by removing the 3 current surge towers and wood stave penstock lines. One key element of the project is the L20 main access tunnel, which provides truck access to the 40m high and 94m long by 20m wide underground power house. In May 2015, weak ground was encountered about half way through the tunnel. As later confirmed, this zone was part of an ancient riverbed traversing the previously excavated basaltic formation likely deposited by an ancient glacier. These features are sometimes referred to as buried valleys. As opposed to the original ground support concept employed using poly reinforced shotcrete, lattice girders, and CT-Bolt™ combination rock bolts, excavation and support of this weak ground zone required additional measures. Bolstered by technical support from DSI and various tunnel expert teams, a contingency excavation and ground control concept was developed within a short period of time. Among other things, this concept included a double layer of the AT – 139 Pipe Umbrella Support System, DYWI® Drill spiles and face bolts as well as a heavy-duty set of 3-bar Lattice Girders. The pipe umbrella support system used features a so-called squeezed connection type, which allows a safer and faster completion of installation work besides an increased load-bearing capacity. By using an AT – Automation Unit, operational effectiveness and occupational health and safety were improved further on. Within a few weeks’ time, all required ground control systems were designed and transported to the remote project site. Afterwards, excavation works were successfully accomplished.