A hydroelectric headrace tunnel of 7m dia in the Himalayas encounters schist and quartzite rock with nearly 1000 m overburden. The tunnel is now advancing through tightly jointed quartzitic rock (upper-class II and Lower class III of RMR) and minor seepage is also observed. Now the tunnel is experiencing moderate to severe burst at the right side just above SPL. Is there any technique that can reduce the severity of the bursting? (though pressure relief holes are being done regularly)
Drilling and blasting for tunnel progress has to be stopped and lining upto tunnel face to be completed. Tunnel Boring machine should be procured of the tunnel dimension.Fibre reinforced Shotcrete should be applied at week joints in lining of the tunnel. Lining method of the tunnel should be reviewed to withstand the occasional bursts.
I would cut short (~ 5m deep) drifts on both sides every 20 m or so to "absorb" the rock bursting away from the main haulage.
Thank you Michael Issigonis for your suggestion. We are having incidences of rock bursting near the face of the tunnel, mostly within a range of 8 m behind the face (within 0-48 hrs of excavation). What would be an economical suggestion for the same? Is there any specifications to the length, dia, position and orientation of the pressure relief holes to effectively reduce the incidences?
There are many experts in this field at RG than I am, but I have practical experience from deep mines.
You are probably digging into a mountain under 1000 m of rock (not overburden which is usually soil and boulders). Hard rocks like quartzite would react to an opening under them by rockbursts. The orientation of relief holes is a difficult subject. You have to experiment with several directions and learn from that.
I would think that rockbursts would slow down with time as the mountain adjusts to the equilibrium changes from your tunnel. You will be lucky if no one gets hurt in this operation.
Himalayan geology is fragile, as created by tectonic shift and thrust unlike mountains of volcanic origin. Safety against rockburst in tunneling should be assured by continuous tunnelling with strong lining support .
Hi Kundu Sir!
Along with all the information given above, the answer will also depend on factors such as confining pressure, stiffness of surrounding rocks (the area surrounding the burst volume), excavation direction wrt to bedding, and where in the section of the tunnel does the rockburst occur, etc. Considering RMR lower bound class III, we can go for end anchored rockbolts of 4m long, spaced 1.5m apart, arranged in crown and side walls, steel mesh reinforcement (Wire mesh + lattice girder arrangement), forepoling in the advance direction and 10+10+35cm lining all around. Another option would be to use a type of yielding rockbolt (but that's made with special order and takes months to produce) spaced more than 1.5m apart along with 10cm shotcrete and wire mesh, if you want to be economical. You would also need to consider the excavation method, you can go for NATM method, top heading (I) and benching (II) method. In case of any seepage, you can opt for waterproofing system, such as panel (take 4 hours to act) or membrane water proofing system. In case of heavy water ingress, you may need to pump out water.
For best help, Sir I would suggest you approach IIT Delhi, Prof. K. S. Rao. He has executed projects like this before, and I would also get the chance to see it.
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