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ABSTRACT ABSTRACT A seismic fissuration index (K) is defined as the ratio of the difference in the P and S wave velocities obtained when a dry rock specimen is tested both under a load equal to half the uniaxial compressive strength of the rock (Vo) and without an applied load (Vd) to the velocity obtained without an applied load (Vd), as K = (Vo - Vd)/Vd. A similar seismic fissuration index is also proposed for classifying the dry rock mass in terms of field seismic velocities. The field equation is extended by including the intact rock seismic velocities. K is found to be related to the porosity of rocks. The proposed equation can equally be used for characterizing saturated rocks, using S-wave velocities. However, the time average equation should be used for characterizing saturated rocks, using P-wave velocities, even though this equation gives upperbound estimates of the rock material and fracture porosities. INTRODUCTION Geophysical seismic techniques are generally employed to characterize and determine the dynamic properties of rocks. As these tech- niques are non-destructive and easy to carry out, they are increasingly being used in geotechnical engineering. The seismic methods, in general, give an overall estimate of the rock mass velocity and they can be carried out from the ground surface, underground, or in boreholes. Seismic surveys are generally carried out in the early stages of site investigation in order to delineate the zones of interest and areas where further investigation is required. Excavatability of the rock mass is suggested by the equipment manufacturers in terms of their seismic velocities (Anon, 1980). Attempts have also been made to assess grouting, rockbolt reinforcement and blasting efficiencies in the rock mass by the seismic velocity determinations (Knill, 1970; Price et al., 1970 and Young et al., 1985). Some other interesting applications of seismic techniques have included the prediction of rock mass deformation, stress and the extent of fracture zones developed around underground openings (Onodera, 1963; Gladwin, 1982; Hudson et al., 1980). The seismic properties of the rock mass are influenced by the intrinsic rock material properties and external factors such as temper- ature, pressure, pore fluids and fractures. The influence of these factors is best studied in the laboratory under controlled conditions. Such a study would enable the influence of jointing or weathering to be determined by comparing the seismic velocity of the rock material at the pressure, temperature and moisture content corresponding to the rock mass conditions. Variation in seismic velocity may not only be due to quality variation in the rock mass, but also a change of rock type, depth, and the presence of groundwater. In this paper, initially the factors influencing seismic velocity and the suggested methods for characterizing the rock mass are briefly presented. Then a new method for characterizing rocks in terms of seismic velocity is given. According to this method a seismic fissuration index has been defined as the ratio of the difference in velocities obtained when a dry rock specimen is tested under a load equal to half the uniaxial compressive strength and without an applied load to the velocity obtained without an applied load.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type (0.89)
ABSTRACT Abstract The theoretical relations between the strain and stress Mohr circles make it possible to establish a relation between the internal friction angle (Ø) and the Poisson's ratio (¿) of rocks. The internal friction angle is related to the Poisson's ratio as Sin Ø = (1 - ¿)/(1 + ¿). This relation gives lower friction angle values than the triaxial Mohr envelopes and higher values than those given by surface sliding friction tests. INTRODUCTION The shear strength properties of rocks are generally determined either by carrying out direct shear box test or triaxial strength tests on specimens in the laboratory. Usually Mohr circles are drawn to represent these test results. The friction angles of the rocks are obtained from the slopes of the Mohr envelopes drawn as a tangent to the Mohr circles. The theory for the graphical representation of stresses as Mohr circles is given in most standard textbooks on elasticity, strength of materials and rock and soil mechanics (Ford and Alexander, 1977; Jaeger and Cook, 1979; Jumkis, 1969). A number of authors have also carried out studies to find the best fitting line representing the Mohr envelopes (Balmer, 1952; Hoek, 1968; Franklin, 1971; Bieniawski, 1974; Bland, 1983). Rocks undergo deformation when they are subjected to pressure or loading. The deformation measurements are made by means of strain guages or linear voltage digital transducers (LVDT) and the results are generally expressed as horizontal and vertical strain ratios of the original test specimen dimensions. The elastic modulus and Poisson's ratio of rocks are obtained by plotting the vertical strain value against the applied pressure and the horizontal strain against the vertical strain respectively. The International Society of Rock Mechanics (ISRM 1981) gives guidelines for making the deformation measurements and method for evaluating the test results. The deformation properties of rocks can also be represented by Mohr circles. The stress and strain Mohr circles of a material are similar except that their origins and radii are different (Ford and Alexander, 1977; Jaeger and Cook, 1979).Thus, the friction angles of rocks should not only be estimated from the stress Mohr envelopes but also from the strain Mohr envelopes. However, such use of the strain Mohr envelopes is not made in rock mechanics for one reason or another. Here an attempt is made to investigate shear strength properties of rocks in terms of strain properties under uniaxial and triaxial loading. It is shown that the internal friction angles of rocks can be estimated from the Mohr strain circles. This means that the frictional properties of rocks can be found from strain measurements on rocks during uniaxial compressive testing, instead of carrying out a number of triaxial strength tests. This saves time and money in rock testing and the method becomes particularly important where triaxial test equipment is not available or when it is not possible to obtain a sufficient number of specimens in order to carry out the required number of triaxial tests. In this paper, first the theoretical relation between the strain and stress Mohr circles is given.