Rajinder Bhasin

Static and dynamic simulation of a 700-m high rock slope in western Norway

Abstract Static and dynamic rock slope stability analyses were performed using a numerical discontinuum modelling technique for a 700-m high rock slope in western Norway. The rock slope has been investigated by the Geological Survey of Norway (NGU), which has been carrying out rock slide studies for the county More and Romsdal in western Norway. The purpose of numerical modelling was to estimate the volume of the rock mass that could potentially slide under static and dynamic forces. This estimation was required to assess the run-up heights (tsunami) in a fjord that could potentially be caused by the rockslide. Three cases have been simulated for predicting the behaviour of the rock slope. First, an initial static loading is applied in the numerical model to simulate the prevailing rock mass conditions at the site. Second, saturated and weathered joint conditions are modelled by reducing the residual friction angle along the discontinuities of the rock mass. In doing so, the model simulates the effect of degradation of discontinuities in the rock slope. Third, a dynamic loading, based on peak ground accelerations expected in the area, is applied to simulate dynamic earthquake conditions. These numerical studies have provided some useful insights into the deformation mechanisms in the rock slope. Both sliding and rotation of blocks start to occur once the residual friction angle along the discontinuities is reduced and when the region is shaken by a strong earthquake. The results indicate that, due to variations in the inclination of discontinuities, the entire slope does not become unstable and that down-slope sliding and rotation of blocks occur mainly on the top layers of the slope. Within the range of parameter values considered for this study, it is unlikely that the whole rock slope can be destabilised. The study provides an illustration of how the geo-mechanical properties of a rock mass can be integrated in a discontinuum rock slope model, which is used for predicting the behaviour of the slope under existing environmental and earthquake conditions. This model has helped not only to better understand the dynamics of the rockslide but also to estimate the potential rock volume that can become unstable when subjected to static and dynamic loads.

The use of stress-strength relationships in the assessment of tunnel stability

Abstract A method of predicting probable ground behaviour considering stress-strength relationships is presented. The influence of overburden pressure and rock mass strength which are implicitly used in calculating the Q value (Q-system of Barton et al. 1974) are explicitly considered in forecasting stability problems in tunnels. In particular, squeezing, spalling and loosening of the rock mass in tunnels constructed through the complex geological set-up of the Himalayas have been analysed. A new Q-system correlation which takes into account the dimension of an opening is suggested for evaluating support pressures in squeezing ground.

Parametric study for a large cavern in jointed rock using a distinct element model (UDEC—BB)

Parametric studies are performed using the distinct element computer program UDEC—BB for a large cavern in the Himalayas. The studies provide insights into some important deformation mechanisms in a numerical system of blocks. The sensitivity studies involving changes in joint spacings (block size) revealed that the deformations around an opening are dependent on the size or the number of blocks adjacent to the excavation. Large size blocks deform mainly through translational shear upon which rotational shear may occur. In a model in which the block size is small compared to the tunnel dimensions, the tendency to dilation across an increased number of non-planar joints may contribute to interlocking of blocks. The BB joint behavioural model, which allows the modelling of the dilation accompanying shear and hence the build-up of higher normal stresses, predicts smaller deformations than the Mohr—Coulomb model in which the dilation angle is constant. The sensitivity studies involving changes in key BB joint shear strength parameters have shown that, within the range of values considered, the model output is relatively insensitive to assumed joint strength parameters joint roughness coefficient and r. However, an increase in the joint wall compressive strength contributes to an increase in the joint shear strength resulting in a marked reduction of displacements around the opening.

Predicted and measured performance of the 62 m span Norwegian olympic ice Hockey Cavern at Gjøvik

Abstract The feasibility of excavating caverns of very large span for underground siting of nuclear power stations in Norway was investigated in the early 1970s. In the end, the 1994 Winter Olympic Games provided the necessary impetus for utilizing a very large engineered rock cavern and proving its general feasibility. The 62m span Olympic Ice Hockey Cavern was constructed in Gjovik by Veidekke-Selmer JV in 1991. It is located in a jointed gneiss of average RQD = 67%. The Q-values range from 1 to 30, with a weighted mean of about 9, i.e. fair quality rock. The cavern has a rock cover of only 25–50m, thus posing challenging design problems. The investigations prior to construction included two types of rock stress measurements, cross-hole seismic tomography, geotechnical core logging, Q-system classification and numerical modelling with UDEC-BB. Predicted maximum deformations were 4–8 mm; these were surprisingly small due to the high horizontal stresses recorded. Extensometer (MPBX) installations from the surface prior to construction, precision surface levelling and MPBX installed from inside the cavern gave a combined measure of maximum deformations in the range 7–8 mm with the 62m span fully excavated, and three adjacent caverns for the Postal Services also completed. Permanent rock reinforcement based on the Norwegian method of tunnelling (NMT), consisted of 10cm wet process steel fibre reinforced shotcrete, and systematic bolting and cable bolting in alternating 2.5 and 5.0 m c/c patterns. Both the cables and bolting were untensioned and fully grouted.

Landslide hazards and mitigation measures at Gangtok, Sikkim Himalaya

Landslides and other mass movements are serious geo-environmental hazards in the Himalayas. Massive landslides killing tens of thousands of people with catastrophic damages have occurred in the Eastern Himalayan State of Sikkim, which shares common borders with Tibet, Nepal, and Bhutan. This paper describes the investigations carried out on recent landslides in Gangtok, Sikkim, India, with emphasis on the triggering mechanisms that have contributed to the release and creep of natural slopes in the region. It is believed that the intense rainfall in the region not only contributes to rapid erosion and weathering of the rock mass, but also increases the groundwater level that leads to reduction in the stability of natural slopes. A landslide instrumentation programme that includes placement of settlement pillars and piezometers is underway to predict the behaviour of landslides in the area.

Engineering geological characterization of low strength anisotropic rocks in the Himalayan region for assessment of tunnel support

Abstract A detailed engineering geological assessment of low strength (6–35 MPa) anisotropic rocks at an ongoing Hydroelectric Project in the Himalayan Region has been carried out. The project (the Nathpa Jhakri Hydroelectric Project) will have one of the worlds largest and longest water conducting systems, consisting of a 10.15 m diameter and 27.3 km long head race tunnel, a 942 000 m 3 underground desilting complex and a 20 × 49 × 216 m powerhouse cavern in the area. Because these constructions are made in low strength metamorphosed anisotropic rocks like quartz mica schists, biotite schists and muscovite schists, it seemed necessary and worthwhile to undertake a comprehensive study of such rocks. The studies include petrographic and petrofabric analyses of the rocks, geo-mechanical properties, rock stress measurements, rock mass classification using the Q-System and data concerning joint geometry, joint roughness and joint strength. The use of mapped geological and geotechnical data along with the experimental and modelling studies have helped to better understand the behaviour of these rocks in underground openings.

Comparison of predicted and measured performance of a large cavern in the Himalayas

Abstract Detailed investigative and performance monitoring studies have been carried out at the site of an underground powerhouse cavern in the Himalayan Region of India. The updated empirical (Q-system) and numerical (UDEC-BB) approaches, applied for predicting the behaviour of the rock mass prior to the construction of the underground cavern (20 × 49 × 216 m), have been compared with the instrumentation data from multi point borehole extensometers (MPBX). Upon completion of the first numerical excavation step (20 m span arch), a relatively high stress-strength ratio and a maximum deformation of approx. 18 mm was predicted in the roof of the cavern. MPBX readings in the arch have indicated maximum deformations in the range 19–24 mm with the 20m span fully excavated. The results of numerically excavating the cavern to its full height (49 m), have indicated maximum deformations in the range 43–45 mm in the walls of the cavern. Upon completion of the ongoing benching operations, the measured performance from the walls of the cavern will be available for comparison with the existing numerical results. Permanent rock support in the cavern consists of systematic bolting of alternating lenghts and mesh reinforced shotcrete S(mr). However, rock support design recommendations based on the Norwegian Method of Tunnelling (NMT), which employs wet process fibre reinforced shotcrete S(fr) instead of S(mr), have been numerically tested and verified.

Numerical modelling of block size effects and influence of joint properties in multiply jointed rock

Abstract Two-dimensional numerical experiments are conducted for investigating the effects of block size and joint properties on the behaviour of multiply jointed rock. The paper explains how the size of the individual blocks controls both the shear strength of the assembly (rock mass) and its deformational characteristics. A closely jointed rock mass, in which block rotations occur, exhibits a lower stiffness but a higher strength than a roch mass with widely spacedjoints. These numerical results are similar to those from reported physical model tests on jointed slabs of a rock model material. The paper shows how parametric studies on a single joint using the Barton-Bandis (BB) formulation are useful for providing information about the response of a jointed rock mass.

Finite element analysis of failed slope by shear strength reduction technique: a case study for Surabhi Resort Landslide, Mussoorie township, Garhwal Himalaya

ABSTRACT Finite element analysis of failed slope of the Surabhi Resort landslide located in the Mussoorie township, Garhwal Himalaya has been carried out using shear strength reduction technique. Two slope models viz. debris and rock mass were taken into consideration in this study and have been analysed for possible failure of slope in future. Critical strength reduction factor (SRF) for the failed slope is observed to be 0.28 and 0.83 for the debris and rock mass model, respectively. A low SRF value of the slope revealed significant progressive displacement in the zone of detachment. This has also been evidenced in the form of cracks in the building of Surabhi Resort and presence of subsidence zones in the Mussoorie International School. These results are consistent with the study carried out by other workers using different approach.

Surface displacement estimation using multi-temporal SAR Interferometry in a seismically active region of the Himalaya

The Indian subcontinent is one of the most earthquake-prone regions of the world. The Himalayas are well known for high seismic activity, and the ongoing northwards drift of the Indian plate makes the Himalaya geodynamically active. During the last three decades, several major earthquakes occurred at the plate interiors and boundaries in this subcontinent causing massive losses. Therefore, one of the major challenges in seismology has been to estimate long recurrence period of large earthquakes where most of the classical Probabilistic Seismic Hazard Approaches fail due to short catalogues used in the prediction models. Therefore, during the past few decades, the Himalayan region has been studied extensively in terms of the present ongoing displacements. In this context the present study has been carried out to estimate the surface displacement in a seismically active region of the Himalaya, in between Ganga and Yamuna Tear, using multi-temporal Synthetic Aperture Radar (SAR) Interferometry. A displacement rate of 6.2–8.2 mm/yr in N14°E direction of the Indian plate towards the Tibetan plate has been obtained. It has been noted that the estimated convergence rate using Differential SAR Interferometry technique is relatively low in comparison with those obtained from previous classical studies. The reported low convergence rate may be due to the occurrence of silent/quite earthquakes, aseismic slip, differential movement of Delhi Hardwar ridge, etc. Therefore, in view of the contemporary seismicity and conspicuous displacements, a study of long-term observations of this surface movement has been recommended in future through a time-series SAR Interferometry analysis.