Bhawani Singh

Correlation between observed support pressure and rock mass quality

Abstract The correlation between rock mass quality and support pressure proposed by Barton et al. (1974) has proven useful, except in cases of squeezing ground conditions. Field data collected systematically from 20 tunnel sections indicate a clear need for correction factors to account for height of overburden and tunnel closure, which do not seem to be adequately accounted for by the stress reduction factor. As expected, the support pressure decreases rapidly with tunnel closure and then increases beyond a limiting closure. The fact that the observed wall support pressures were always close to zero except in squeezing ground conditions has been taken care of by slightly modifying wall factors for Q-wall. A criterion derived from the field data shows that squeezing ground conditions would be encountered where the height of the overburden is greater than 350 Q 1 3 . The data reported herein confirm the earlier findings of Barton et al. (1974) that the support pressure is independent of the tunnel size.

Schmidt hammer rebound data for estimation of large scale in situ coal strength

The paper reports an investigation to determine whether a correlation exists between the Schmidt hammer rebound and the in situ large-scale strength. The results showed a reasonable correlation between the large-scale in situ crushing strength of 0.3 m cubes of coal and the lower mean of rebound values obtained. The regression is seen to be linear within the range 2.75 - 13.14 MPa of in situ strength. In situ large-scale testing procedures are cumbersome and expensive, and the Schmidt hammer rebound method can offer a quicker and cheaper means of estimating this strength. Laboratory 25 mm cube coal strength shows a greater scatter with rebound values than the large-scale in situ strength, and gives a non-linear regression.

Landslide hazard and risk assessment mapping of mountainous terrains — a case study from Kumaun Himalaya, India

Landslides are studied systematically in order to evaluate the nature of hazard and the damages to the human life, land, roads, buildings and other properties. This can be expressed in terms of risk, which is a function of hazard probability and damage potential. A risk map will indicate the priorities for landslide hazard management. A new approach to risk assessment mapping using a risk assessment matrix (RAM) is presented.

ROCK MASS STRENGTH PARAMETERS MOBILISED IN TUNNELS

Abstract Experience suggests that there is an enhancement in the strength of rock mass around the tunnels due to constraints in fracture propagation. In this paper, mobilised strength parameters and modulus of deformation have been deduced from back analysis of the field experience for the purpose of realistic non-linear stress analysis of arched underground openings in nearly dry rock masses, and modified correlations have been suggested. It is also inferred from field observations that sympathetic failure of rock mass may take place at all points almost simultaneously within the failure zone if deviatoric strain exceeds a critical value at any point. Block shear tests support Hoek and Browns (1980) criterion, which is recommended for analysis of rock slopes and open cut mines.

Effect of tunnel depth on modulus of deformation of rock mass

SummaryDeformability of rock mass significantly influences its behaviour and is, therefore, an important consideration for the design of underground openings. The modulus of deformation of rock mass is, however, normally obtained from expensive and time-consuming uniaxial jacking tests, whose results often have a large scatter. An empirical correlation has, therefore, been proposed for a quick and inexpensive preliminary estimation of the modulus of deformation of rock mass on the basis of field instrumentation carried out in tunnels in India.

Coal pillar strength estimation from failed and stable cases

A new pillar strength equation is proposed for all practical values of width-height ratio based on 23 unstable and 20 stable pillar cases. Strength data of a number of coal seams are used to show that the in situ large-scale strength is more likely to be affected by depth of cover than the laboratory small-specimen strength. The latter is therefore incorporated in the new equation. Performance of some of the better known pillar strength formulae is compared with the new equation against the case studies. It is proposed that the safety factor should be changed with depth of cover and width-height ratio of pillars. Safety factor values are accordingly recommended for stowed and unstowed pillar arrays and chain pillars. An alternative simpler equation is also proposed for slender pillars.

Landslide Hazard Zonation

Landslide hazard zonation (LHZ) map is an important tool for designers, field engineers and geologists, to classify the land surface into zones of varying degree of hazards based on the estimated significance of causative factors which influence the stability (Anbalagan, 1992) . The landslide hazard zonation map, in short called LHZ map, is a rapid technique of hazard assessment of the land surface (Gupta and Anbalagan, 1995) . It is useful for the following purposes: (i) The LHZ maps help the planners and field engineers to identify the hazard prone areas and therefore enable one to choose favourable locations for site development schemes. In case the site cannot be changed and it is hazardous, the zonation before construction helps to adopt proper precautionary measures to tackle the hazard problems.

A nonlinear criterion for triaxial strength of inherently anisotropic rocks

Rocks encountered at many underground construction sites are laminated and exhibit direction-dependent strength behavior. It is also a well-established fact that the strength varies in a nonlinear manner with confining pressure. There is a need of strength criterion which could capture the nonlinearity as well as the anisotropy in the triaxial strength behavior of the rocks. It is essential that the criterion should be simple and must involve minimum testing to the extent possible. Further, the parameters of the criterion should have wide acceptability among the geotechnical fraternity. In the present study, a nonlinear strength criterion for transversely isotropic rocks is presented. Critical state concept Barton (Int J Rock Mech Mining Sci Geomech Abstr 13(9):255–279, 1976) has been used to define the curvature of the criterion. With a correctly defined curvature and starting from a reference point (UCS), it is possible to accurately assess the triaxial strength for given confining pressure. An experimental study conducted on triaxial strength behavior of three types of anisotropic rocks namely phyllite, slate and orthoquartzite has been discussed. A data base comprising more than 1140 triaxial tests conducted worldwide on anisotropic rocks has been compiled. Statistical evaluation of goodness of fit of the proposed criterion to the data base has been carried out. Further, the predictive capabilities of the proposed criterion have been evaluated by determining the error in estimation of triaxial strength if only few triaxial test data are available for determining the criterion parameters. The data base has also been back analyzed to assess the critical confining pressure for anisotropic rocks. Statistically, the critical confining pressure for anisotropic rocks can be taken nearly equal to 1.25 times the maximum UCS (obtained by applying load either parallel or perpendicular to planes of anisotropy). It is concluded that reasonable estimates of the triaxial strength of anisotropic rock can be made through the proposed criterion using minimum amount of triaxial test data available.

Support pressure assessment in arched underground openings through poor rock masses

Abstract Himalayan tunnels provide challenging opportunities for working on the problem of assessment of support pressure in tunnels. A resume of the efforts by Indian researchers on this topic is presented in this paper, with a brief review and comparison with the approaches of Bieniawski (1979) and Barton et al. (1974; Analysis of Rock Mass Quality and Support Practice in Tunnelling, and a Guide for Estimating Support Requirements, Report by NGI, June). It is noted that Goels classification of tunnelling conditions (Goel, 1994; Correlations for Predicting Support Pressures and Closures in tunnels. Ph.D. Thesis, Nagpur University, India, p. 308) is complementary to the Q-system. Contrary to Terzaghis theory, support pressures are observed to be independent of opening size.

Thrust-wedge mechanics and coeval development of normal and reverse faults in the Himalayas

Abstract: The coeval development of the South Tibetan Detachment (a regional-scale normal fault in Tibet) and the Main Central Thrust together with the observed dominance of thrusting in the Himalayas, is modelled using stress simulation analysis. 2D non-linear elastic and homogeneous wedge models, representing cross-sections of the Himalayas and Tibet are used. Simulated stresses for a set of boundary conditions in which the stress magnitudes are sufficient to cause failure along the wedge base (lower boundary) and reverse faulting at its toe (updip end of the base), invariably lead to the simultaneous development of intra-wedge normal faults. Further, a decrease in shear strength of the wedge base relative to its interior favours the development of normal faults and/or reduction in the magnitude of thrusting stresses within the wedge. These results suggest that the presence of a relatively strong Main Himalayan Thrust, the plate boundary fault below the Himalayas, would have favoured the occurrence of thrusting in the wedge. Moreover, a weak Main Himalayan Thrust below Tibet along with initiation of the Main Central Thrust can explain coeval development of the South Tibetan Detachment.