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  Design and Construction related issues of Adit by DBM for Launching 9.86m diam eter Double Shield Tunnel Boring Machine

  With the success of long distance Tunnel boring carried out for Kishanganga Hydroelectric Project with complex Himalayan geology, more and more tunnels in Hydropower Project are being bored by Tunnel Boring Machine. One such Hydro Power Project is coming up in Uttarakhand, India, wherein the Head Race Tunnel (HRT) is to be bored for a distance of about 12km. The bore diameter of the HRT is 9.86m. The HRT is to be approached by an Adit having similar cross-section. In the initial reaches of the TBM Adit, the Geology was found to be extremely poor consisting of Clay with Quartzite Boulders. Thus, it was proposed to construct the Adit with NATM till the proper geological strata is encountered for launching the Double Shield Unit (DSU) Tunnel Boring Machine (TBM) from that point onwards. This NATM stretch of the Adit was worked out to be about 50m inside the hill based on Geological and Geophysical tests carried out in this area. After weighing various options of supporting Adit, it was decided to design the Adit as a shell structure consisting of Self Drilling Anchors and Lattice Girders embedded in 350 thick SFRS layer. The Design was carried out ensuring availability of SFRS shell at all intermediate stages of excavation of the Adit. The Adit was envisaged to be excavated in three stages; heading of 7m followed by benching of the remaining portion of the tunnel cross-section in two benches. The excavation of the heading was achieved by supporting the crown by 114mm diameter pipe roofing. The face was kept in stable condition by FRP bolts/SDA anchors along with facial SFRS. The deflections at crown and invert of the Adit were kept under permissible values with the type of arrangement provided above.The Construction of Adit is presently under progress. The 114mm dia pipe roofing were intuitively installed with the help of one arm Boomer by adding an attachment to hold the Pipe roof and its drilling bit. This resulted in saving of Costly Equipment normally required for putting in- place large diameter pipe roofing. The heading was achieved at an average rate of about 20m per month. The Benching work was taken up followed by extending the side lattice girders and spraying SFRS as per Designs. After completion of the excavation work, in-situ 20m long RCC lining will be done near the face of the TBM Adit for launching the Tunnel boring machine. The paper discusses the analysis and design of large size Adit in extremely poor rock condition and innovative measures undertaken for the construction of the said Adit. TBM, FRP bolts, SDA, SFRS, Lattice girders.

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