ESTIMATION OF ENGINEERING PARAMETERS OF ROCK MASS

Abstract

With remarkable increase in CIVILENGINEERING activities, availability of sites with favourable geological conditions is becoming increasingly restricted posing tremendous challenges to the geotechnical engineers. This is particularly so in India where the developmental works for harnessing the water resources need gigantic dams, long tunnels, large underground cavities, especially in the young Himalaya. The rock formations in the Himalaya are intricately folded, faulted or thrusted, comprising different types of rocks with intrusions and water-charged regions. Besides these geological complexities, the region is also highly seismic. Because of the complicated geology, geotechnical problems have been encountered in a number of river valley projects built in this region. For safe and sound design of structures in this complex geological area, understanding of the strength"and deformation behaviour of the rocks is of paramount importance. At present only scanty information is available regarding the engineering behaviour of the Himalayan rocks. Some empirical relationships exist in literature but these are based mostly on the geological data of comparatively better rocks. As a number of hydroelectric projects are under construction in the Lesser Himalaya (lower Himalayan region adjacent to river plain), there is an urgent need that experience in this area is documented as early as possible. Rock mass behaviour can be assessed through adequate number of field and laboratory tests. However, such tests can (.1) be carried out only at project sites because of being time consuming and expensive due to their massive scale of operation. Moreover, expertise is almost always needed for performance of these tests which is available for these projects. The most significant properties of rocks involved in the design process are as follows: 1. Deformability characteristics - required for the design of tunnels, underground openings and dam foundations; 2. Shear strength characteristics - required for the design of rock slopes, foundations and dam abutments; 3. Support pressure - important in the design of tunnels and underground cavities ; 4. Allowable bearing pressure - necessary for the design of foundations. Construction of a number of hydroelectric projects in the Lesser Himalaya provided an excellent opportunity to author to conduct the tests to study the properties and rock mass behaviour of different rock types encountered in the region. An attempt has been made through this research study, to correlate the following design parameters with the RMR and Q systems. The new relationships which have been developed, on the basis of the data from the Lesser Himalaya, are as follows: 1. Correlation between modulus of deformation and RMR; 2. Correlation between support pressure on tunnels and rock mass quality Q; 3. Correlation between RMR and shear strength parameters of rock mass; 4. Failure envelopes for the rock mass; and 5. Correlation between RMR and the allowable bearing (ii) pressure on rocks. The effect of saturation on the properties of rock mass has been studied and relationships developed for both, the naturally moist and the saturated rock mass. On the basis of field and laboratory test data of six major hydroelectric project sites of Lesser Himalaya, it has been possible to examine the applicability of existing relationships for these Himalayan rocks. The results of field and laboratory investigations show that the rocks in the Lesser Himalayan region are generally of 'poor' quality. On the basis of extensive uniaxial jacking tests, a relationship has been developed between the rock mass rating (RMR) and the modulus of deformation. The trend of the relationship shows that it is not in agreement with the existing relationships of Bieniawski (1978), and Serafim and Pereira (1983). Bieniawski's relationship is based on the experience of 117 tunnel sites in hard rock areas. Hence, it is reasonable to expect that it may not be applicable to Lesser Himalayan rocks which are generally poor and weak. Moreover, Bieniawski's curve is valid for RMR values more than 50 showing that the relationship is not applicable to poor rock masses. It is further found that the relationship proposed by the author has a similar trend as that given by Serafim and Pereira (1983) but the two are not in good agreement. The correlation given by Serafim and Pereira (1983) would highly overestimate the modulus of deformation of these rocks. It is thus inferred that the existing relationships of Bieniawski (1978) and Serafim and Pereira (1983) are not suitable for application to (iii) the Lesser Himalayan rocks. The effect of saturation on the modulus of deformation has been found to be significant. On saturation, the reduction in the modulus values may be as high as 90 per cent for the poor rock masses and 70 per cent for the fair quality rock masses. Modulus of deformation has been found to have a definite correlation with the support pressure on tunnels. So, if one knows the modulus of deformation accurately by tests, support pressure could be predicted objectively. A correlation has, therefore,been suggested by the author to evaluate the support pressure for tunnels in the Lesser Himalayan region. It is found that the estimated support pressure decreases with increase of modulus of deformation. Initially, the support pressure decreases rapidly until a modulus value of 3.5 GPa is reached. Beyond the modulus of 3.5 GPa, the decrease in the support pressure is relatively at a slower rate. It is noted that for the modulus values of 10 GPa or more, the support 2 pressures are stabilised more or less at 0.20 kg/cm for the 2 rock masses at natural moisture content and 0.30 kg/cm for the rock masses at saturation. Results have led to show that the moisture condition significantly affects the self supporting capacity of the rock mass, thereby affecting the support pressures. For the poor rock masses, as in the Himalayan region, saturated condition may be considered to be critical for maximum support pressure because of the possible increase in the pore water pressure and decrease in the angle of internal friction, cohesion and the modulus of deformation of the rock mass. In case of saturated rock mass wide scatter of test data indicates that for modulus values below 3.5 GPa, the pressure distribution in the rock mass may not remain uniform. This may be responsible for non-uniform pressure distribution around the tunnel lining due to non-uniform flow of water through a few open joints only. Thus proper drainage is essential for execution of work especially when excavating tunnels in the low modulus ground (< 3.5 GPa). The prediction of support pressure through the modulus of deformation as brought out in this study would prove helpful in understanding the tunnel-support interaction in the rock mass. The relationship developed is simple and easy to apply. Results of extensive in-situ block shear tests performed under drained condition show that both the shear parameters, cohesion and angle of internal friction of the rock mass increase simultaneously. It is observed that initially the angle of iriternal friction increases rapidly but attains the maximum value of 57° at a cohesion value of 450 kPa. It is further noted that shear strength parameters of the rock mass increase with increase in RMR. However, no appreciable increase is found in the values beyond RMR value of 60. It is noted that the angle of internal friction decreases with decrease in RMR and becomes asymptotic at 32 . The effect of saturation has been found to be more striking on cohesion than the angle of internal friction. A maximum reduction of 70 per cent in cohesion and 38 per cent in (v) the angle of internal friction has been observed in poor rock masses. In case of fair category rock masses a maximum reduction of 52 per cent in cohesion and 22 per cent in the angle of internal friction has been observed. Mohr failure envelopes have been deduced from the data of large scale block shear tests on different rock types. The failure envelopes,which have been derived for both dry and saturated rock masses , show stress dependent behaviour. The failure envelopes show in many cases similar trend as given by Hoek and Brown (1980). The envelopes may be used for strength determination at the desired magnitude of normal stress and may prove helpful in carrying out stability analyses of rock masses. The rock mass rating (RMR) lias also been used to provide relationship for net allowable bearing pressure on 'poor' and 'fair' category rock masses (RMR 21-60). A non-linear relationship has been obtained on the basis of the criterion of permissble settlement. This may be used for estimating the net allowable bearing pressures for shallow foundations on the rocks. The various relationships developed on the basis of rock mass classification and the results of extensive field and laboratory tests would provide a rational basis to examine the geotechnical feasibility of future projects expeditiously in the Lesser Himalayas. (