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Waste and its disposal
Published in F.G. Bell, Geological Hazards, 1999
Granite is less easy to excavate than rock salt or shale but is less likely to offer problems of cavern support. It provides a more than adequate shield against radiation and will disperse any heat produced by radioactive waste. The quantity of groundwater in granite masses is small, and its composition is generally non-corrosive. However, fissure and shear zones, along which copious quantities of groundwater can flow, do occur within granites. Discontinuities tend to close with depth and faults can be sealed. For location and design of a repository for spent nuclear fuel, the objective is to find ‘solid blocks’ that are large enough to host the tunnels and caverns of the repository. The Pre-Cambrian shields represent stable granite—gneiss regions.
Tunneling challenges in Himalayas: A case study of Head Race Tunnel of 720 MW Mangdechhu Hydro-Electric Project, Bhutan
Published in Daniele Peila, Giulia Viggiani, Tarcisio Celestino, Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art, 2019
Shear zones are characterized by highly deformed, sheared/pulverized, water charged poor rock mass conditions is the biggest nightmare of tunnel engineer. Serious tunneling problems have been experienced when the rock mass is affected by shear zone. The most common problems associated with shear zones are, loose fall, muck flow, chimney formation, squeezing and heaving of the ground, face and crown collapse (Panthi, 2007, Maurya et. al., 2010, Sharma and Tiwari, 2015). These problems are attributed to less standup time of poor rock mass of shear zone where RMR value is less than 20 (Bieniawski 1989). Along the entire length of the HRT only two major shear zones were encountered. A foliation parallel shear zone of 7 to 8 m thickness was encountered at RD 287 of Face-7 and a 4 m thick at RD 1424 of Face-9. Both the shear zones comprised of rock fragments along with clay and rock flour. The encountered shear zones resulted in formation of 7 to 8 m high cavity between RD 287 to 293 of face-7 and RD 1424 to 1426m along Face-9, with continued loose fall. The shear zone along face-9 was accompanied with heavy ingress of water resulting in muck flow and face collapse. The continued loose fall was restricted by spraying many layers of shotcrete at a regular interval till falling of loose/sheared material stopped, followed by cautiously erecting steel ribs and back filling the cavity. After treatment of the cavity the extent of shear zones was probed by aid of drilling 15m long probe holes. Based on the probe hole data thickness of shear zone along Face-7 was estimated to be of the order of 7 to 8m and its extent along the tunnel was about 63m i.e. upto RD 350 (figure 11), while the thickness of shear zone along Face-9 was deciphered to be 4m and extending for a length of 43m along the face i.e. upto RD 1467 (figure 12).
Groundwater Problems for Excavations in Rock
Published in Pat M. Cashman, Martin Preene, Groundwater Lowering in Construction, 2020
It has been discussed previously that the permeability of rock is controlled largely by fractures on various scales (Figure 8.2). On a small scale, the fracture networks that might be observed in core recovered from investigation boreholes have an important effect on groundwater flow; this is discussed later. On a larger scale, geological structures such as faults and shear zones can also control groundwater flow; geological structures are discussed in Section 8.3.2.
Evaluation of in-situ stress state along the shotcrete lined high-pressure headrace tunnel at a complex Himalayan geological condition
Published in Geosystem Engineering, 2021
Chhatra Bahadur Basnet, Krishna Kanta Panthi
In general, there are two categories of weakness zones in the rock mass (Nilsen & Thidemann, 1993; Palmstrom, 1995; Panthi, 2006). The first category includes the layer of weak rocks within the series of strong rocks. The second category includes the zones of crushed and sheared rocks formed by faulting or tectonic activities. The weakness zones along the river valleys are essentially the crushed zones formed along the tensional crack developed in hard rock possibly during the large earthquakes. If the valleys are striking along foliation of the rock, the weakness zones are characterized as shear zones. The shear zones can also be formed in the slope topography along the foliation due to shearing within the rock mass.