The Application of Self Drilling Anchor Bolt at the Tala Hydro Project for Tunneling in Poor Rock Mass Conditions

The Tala hydroelectric project is located in Chukha Dzongkhag in western Bhutan. The dam site is in the Thimphu gneiss, and the section of HRT up to 14km downstream from C-1 is in gneiss with quartzite bands and biotite schists. There are joint sets, and the rock is highly faulted, with foliation shears of varying thickness up to 30cm. From 14km to 23km, the rock type is biotite gneiss with bands of quartzite, mica schist, and sericite schist with amphibolite. Rock conditions merit 3 - 4 on the NGI Q system, with pretty poor conditions affecting parts of C2, C-3 and all of C-4. The rock formation in the power house and penstocks, envisaged to be in classification 2 - 3, showed a large variance during actual excavation.
Self drilling anchor bolts were in widespread use at Tala for stabilization of the desilting chamber walls and for tunneling in poor rock mass conditions. They are reinforcement tools which, unlike other types of rockbolts, are equipped with a drillbit of their own which allows them to be installed in one operation in rock, highly fractured weak formations, and even soil where the drill hole collapses and conventional anchor systems cannot be installed.

Self drilling anchor bolts have been successfully used for stabilizing the RCC wall of desilting Chamber No.3, behind which enormous cracks had been observed by borehole camera investigation. The strata encountered was so fragmented that the drilled holes would not stay open long enough to install longer rock bolts. The wall of the chamber has been anchored to the deeper competent rock using one row of 114 anchors with 20m length and 38mm diameter at 3m centers, and another row of 36 anchors with 24m length and 51mm diameter at 3m centers.Mining is the extraction

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