The Application of Self Drilling Anchor Bolt in the Soft Ground Tunneling at Middle Marsyangdi

Date: 2015-04-18view:

 Middle Marsyangdi Hydroelectric Project, the second largest hydroelectric project in Nepal, lies in the middle reaches of the river Marsyangdi in the district of Lamjung in western Nepal.  The major components consist of: a 95m long and 62m high concrete gravity with part rockfill dam; three intake tunnels to three different underground desilting chambers and so on.

The project area is located at an elevation of 500-700m in a zone where the valley is wide and characterized by large accumulations of Quaternary and recent terrace deposit which is mainly comprised of alluvial materials, mudflow deposits and moraines. The Quaternary soils cover the most important part of the project area. Fluvial, mudflow (glacio-fluvial), colluvium, talus deposits and residual soil are representative for these soils.

Self drilling anchor bolt is a very common flexible rock support to anchor the fractured rock mass in the tunnel and gives the strength and stability to the ground. Since the ground condition was completely loose and the drill holes collapsed immediately during the drilling, the Self Drilling Anchor and post grouting method was applied. 

Installation of pipe roof umbrella and grouting before excavation, 50mm thick protection shotcrete immediately after the excavation of each 1m advance, steel rib support at 1m c/c, 30cm thick steel fiber reinforced shotcrete at wall and arch, 20cm thick shotcrete with two layer wiremesh at invert and Self Drilling Anchor (IBO Bolt) at 1m c/c below the pipe canopy are the applied support combination of the penstock tunnel. This combination provides immediate stabilization of an opening as well as permanent stabilization.


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