Tujuh Bukit Decline Tunnel Project in Banyuwangi is a part from advanced study on copper and gold mining activity, operated by PT Bumi Suksesindo. The tunnel is constructed in order to collect comprehensive data consist of geology, geochemical, geotechnical, and metallurgy, therefore, it is conducted in the biggest deposit in Tujuh Bukit District. Decline tunnel was predicted to intersect with E-W normal fault which contain big water zone for up to 1300 L/min of flow rate from historical drilling. Thus, this amount of water surely becomes a challenge that has to be encountered as it also leads to a significant decrease on Q-value.
Pre-excavation grouting (PEG) method was conducted since CH. 1018 m and it was done as a preventive effort to encounter the expected water bearing structure ahead of the tunnel by creating grouting curtain surround tunnel perimeter. With scattered information upon pre-grouting technology around the world, preliminary assessment before the grouting works were done, such as rock mass classification, hydrogeology monitoring and modeling, and trial on grout material behavior to determine GIN value as a stop criteria, grout hole pattern, and specified grout material mix design.
During PEG works, qualitative data were obtained as a proof for a successful methodology. However, the quantification of the data still limited only to the decrement flow rate of water, but lack of definitive increment on rock mass quality and also the effectiveness of grout infiltration. Hence, some ideas were developed to get better valuation of pre-grouting effectiveness, synthetizing from both engineering and practical approach.
Tujuh Bukit porphyry and high sulfidation epithermal project is the largest deposit in the Tujuh Bukit district. The porphyry resource contains 1.9 billion tonnes @0.45% Cu and 0.45 g/t Au, for 28.1 Moz Au and 19 billion lbs of Cu. There are additional 2.1 Moz Au and 72.9 Moz Ag on high-sulfidation epithermal mineralization. Tujuh Bukit porphyry is located along NW-SE mineralized structural corridor and EW normal fault zone. During feasibility stage, PT. Bumi Suksesindo as the owner, decided to build decline tunnel to collect comprehensive data including geology, geochemical, geotechnical, and metallurgy.