Homogeneous Rock Mass Research Articles

Strength theory of homogeneous jointed rock mass

Strength theory of homogeneous jointed rock mass

Strength theory of homogeneous jointed rock mass

Strength theory of homogeneous jointed rock mass

Classification of geological conditions by types and estimate of the stability of pit edges

Homogeneous, heterogeneous, and composite-structural rock masses and the characteristic failure mechanisms are identified. Methods of calculating the stability of pit edges under various geological conditions are presented. Stage-by-stage estimation of the stability degree of the pit edges as the mining operations deepen is recommended to prevent their large-scale deformations.

Finite element modelling of temperature gradient driven rock alteration and mineralization in porous rock masses

We present finite element simulations of temperature gradient driven rock alteration and mineralization in fluid saturated porous rock masses. In particular, we explore the significance of production/annihilation terms in the mass balance equations and the dependence of the spatial patterns of rock alteration upon the ratio of the roll over time of large scale convection cells to the relaxation time of the chemical reactions. Special concepts such as the gradient reaction criterion or rock alteration index (RAI) are discussed in light of the present, more general theory. In order to validate the finite element simulation, we derive an analytical solution for the rock alteration index of a benchmark problem on a two-dimensional rectangular domain. Since the geometry and boundary conditions of the benchmark problem can be easily and exactly modelled, the analytical solution is also useful for validating other numerical methods, such as the finite difference method and the boundary element method, when they are used to deal with this kind of problem. Finally, the potential of the theory is illustrated by means of finite element studies related to coupled flow problems in materially homogeneous and inhomogeneous porous rock masses.

原位置せん断試験で得られる均質な岩盤の変形・強度特性

To investigate the fracture process and to interprete the strength parameters of the homogeneous soft rock mass obtained by the in-situ block shear test, two dimensional finite element simulation study has been conducted under the assumption that the rock mass behaves as the strain softening material. At the same time, three dimensional physical model experiment has been performed using plaster as the modeling material. Main results are as follows:(1) Results of the in-situ shear test obtained both by the simulation and model study were expressed as the linear curve in the σ-τdiagram, and the value of the shear strength indicated from these data lay between the peak shear strength and the residual shear strength esitimated from the triaxial tests.(2) In the model study, three types of shape on the shear load versus vertical displacement curve, depending on the vertical stress level, were recognized.(3) In the simulation study, progressive failure has started from the early stage of loading and in accordance with this, load-displacement curve bent from the straight line observed in the inital loading stage. Similar tendency for the corresponding curve was recognized both in the field tests and in the model study.(4) The corrugated shaped fracture surface located deeper than the assumed shear plane was observed in the field experiment as well as in the model study. It was also found in the model and in the simulation study that the thickness of the yield zone developing under the fracture surface was not uniform.

Simulation of microseismic emission during rock failure

In order to interpret the microseismic data proceeding rockbursts, it is important to understand the emission process of microseismic events. A numerical, phenomenological model has been developed to simulate seismic events and energy release during rock failure. This model consists of a 1-D multiple-mass-spring system and it is based on the stick-slip process using a slip-velocity-dependent friction law. Although no constitutive law of seismic emission was used, this model generates results that are of practical value. The simulated seismic events and energy release are very similar to those recorded during laboratory tests on rock specimens. Some important features obtained from these results can also be observed from the microseismic data monitored during some rockbursts. The effects of material properties and loading conditions on microseismic emission are examined using this model, which is otherwise not possible in the field. The effect of inhomogeneities is also assessed. For the case of a hard intercalation in the rock mass along a joint, the modelling results have showed excessive microseismic events and the event rate shows more than one peak value when compared with the case of a homogeneous rock mass. This may explain why periods of high seismic activity are sometimes not followed by rockbursts. With a soft intercalation, failure seems to follow the peak of microseismic emission very closely. There is little warning time for this case. This may be the reason of some rockbursts not showing seismic precursor.

深部地下空洞を対象とした岩盤分類法 石油地下備蓄基地における岩盤分類と総合評価

On national energy storage projects, the underground oil storage projects are under construction at three sites in Japan. As to large-scale caverns excavated by NATM in deep, fresh and homogeneous rock masses, an attempt of rock classification and evaluation systems with an abbreviation “CHIKABI Classification System” was made. This classification system is composed mainly of “Rock Hardness” and “Joint Spacing”, with supplementary geological factors of “Weathering and/or Alteration degree”, “Joint plane conditions” and “Water seepage conditions”.The design criteria of the caverns and their supporting measures were made based on the said classification and evaluation systems practically. In order to correlate the actual rock conditons with the rock evaluation results, in situ measurements of P-wave velocity have been carried out in the excavated caverns.In this paper, a principal thought and practical applying conditons of the classification and evaluation systems and a part of the results are described.

A method for evaluating the representative elementary volume based on joint survey of rock masses

The representative elementary volume of a statistically homogeneous rock mass is defined as the minimum volume beyond which any submass behaves essentially like the whole rock mass. A simple, but still general, method for the determination of the minimum volume is given on the basis of the crack tensor concept; it is concluded that the size must be at least three times larger than a typical length of joint traces. Key words: computer simulation, rock hydraulics, site investigation.

Similarity rule of crack geometry in statistically homogeneous rock masses

An nth rank tensor called the generalized fabric tensor is introduced to express crack geometry due to discontinuities like joints and faults in rock masses. Statistically homogeneous rock masses can be regarded as geometrically similar bodies if they are characterized by a common fabric tensor. In order to say that they are also similar in mechanical properties, the crack geometry must be similar in the sense of the generalized fabric tensor. The generalized fabric tensor can be expressed by (1) the number of cracks crossed by a unit length of a scanning line, (2) the number of cracks associated with a unit area of a scanning plane, and (3) the density function E( n) to describe the orientation of crack normal unit vectors n. Since these are all determined by conventional field surveys, one can say that the similarity rule for crack geometry is ready to be used in practical rock mechanics.

Theoretical study of pile-rock socket behaviour : Rowe, P K; Pella, P J N Proc International Conference on Structural Foundations on Rock, Sydney, 7–9 May 1980, P253–264. Publ Rotterdam: A A Balkema, 1980

The behaviour of piles socketed into a homogeneous rock mass is studied using a finite element technique. A pointwise strain-softening model of the interface behaviour is adopted. This model corresponds to the situation where there is a break in the cement bond between the concrete and rock once the peak shear strength is reached at that point. Consideration is given to the influence of socket geometry, the relative stiffness of the pile and rock, the angle of friction at the interface, the initial horizontal stresses at the interface prior to loading, and the roughness of the socket. It is shown that under certain combinations of these fundamental parameters, a brittle response of the pile may be anticipated. Other combinations of these parameters are shown to give rise to a simple plastic or even plastic workhardening response of the pile even though the interface behaviour is pointwise strain-softening. The results of a simplified analysis are presented in a chart form which may be used in the design of socketed piles. These charts provide an estimate of the pile head settlement, the load transfer to the pile base and the shear mobilisation along the socket for a given pile load. (Author/TRRL)

Incipient Joint Motion Due to Spherical Explosion

The problem of incipient joint/fault motion due to a spherical explosion in an infinite homogeneous isotropic rock mass is considered analytically. The mechanical response of intact rock is assumed to be elastic. The shear failure criterion for the joint or fault is taken to be of the usual Mohr-Coulomb form, which results in a single scalar quantity for the definition of the stress states at which failure occurs. In some cases it is found that there exist certain unexpected orientations for which shear failure is inhibited, even though the fault is very close to the source. Also, a quantitative description of catastrophic shear failure on a fault or joint depends on detailed time-resolved stress histories; simple quantitative models based solely on total explosive energy, for example, are incomplete.

Use of photoelastic analysis of models to estimate the fracture of the rock near an underground working

We propose a new approach to the estimation of fracture of rocks near underground workings on the basis of a photoelastic analysis of models. We verify our views and concepts with several series of experiments which show that our proposals have promise. In order to carry out our recommendations, it is necessary to know the diagrams of mechanical state of the prototype and model materials, to prepare and test a photoelastic model, and to make an experimental estimate of its behavior in static or dynamic conditions. Thus, we can analyze the processes of fracture of the rock near various openings with various cross-sectional shapes. In the elastic formulation the problems for unsupported workings have been studied comparatively thoroughly and their results in each particular case must be used, without even appealing to new experiments. For problems which do not yet posses photoelastic solutions, it is necessary to carry out additional experiments, and from their results to construct stress diistributions, mainly on the periphery of a working. As a result of an analysis of models and from our knowledge of the limiting strength characteristics of actual rock, we can predict the regions of possible loss of stability and fracture of themore » rocks, and can indicate the beginning and sequence of development of inelastic deformation of a working. According to the views advanced in this article, the failure of a homogeneous isotropic rock mass, which occurs by propagation of cracks in it, is always confined to the points of maximum stress concentration.« less

Stresses and displacements around openings in homogeneous rock

An extensive program of finite element analyses was conducted to investigate the significance of material properties, initial stresses, excavation sequences and opening shapes on the resulting stresses and displacements around underground openings in homogeneous rock masses. These analyses simulated excavation operations and employed both linear as well as nonlinear, stress-dependent material properties. The results of these analyses showed that the most significant factors affecting the final stresses and/or displacements are the opening shape, initial in-situ stress magnitude, orientation and variation with depth, and the rock modulus and its stress-dependency; all other factors caused minor, if any, variations.

QUANTITATIVE SLOPE ANALYSIS

Statistics describing ground surface slope have geomorphic, hydrologic, engineering, and military applications, because slope steepness influences rates of runoff, soil creep, and soil flowage and the ease of cross-country movement of men and vehicles. Slope maps can be drawn to show the areal distribution of (1) degree of slope and (2) magnitude of the downslope component of gravitational acceleration. The first uses lines of equal slope tangent (isotangents) and is therefore a form of first-derivative map. Isotangents are drawn from a large number of slope readings determined from a large-scale contour map or by field measurement with surveying instruments. Planimeter measurement of areas between successive isotangents yields a percentage-frequency distribution from which slope-tangent mean, variance, and standard deviation are computed as population parameters. The second type of map uses lines of equal sine of slope (isosines) and shows the distribution of the downslope component of gravitational acceleration, which determines the intensity of shear stresses acting upon soil, surface water, and any surface objects. Measurement of areas between successive isosines yields a frequency distribution from which sine mean, variance, and standard deviation are computed as population parameters. Isotangent and isosine maps were drawn and associated frequency distributions computed for drainage basins in three areas unlike in size and stage of erosional development: (1) a small, youthful basin in weak clays of badlands at Perth Amboy, New Jersey; (2) two mature basins strongly controlled by massive sandstones of the Catskill plateau; (3) two basins with smooth slopes in the vicinity of Emporium, Pennsylvania, where Carboniferous strata are maturely dissected. Frequency distribution of sine of slope is strongly skewed and has a high mean in the Perth Amboy locality because of immaturity of stage of basin development. It is bimodal in the Catskill locality, where structural benching is pronounced, but is smoothly symmetrical with almost normal form in the Emporium locality, which may be representative of typical mature erosional topography developed on a homogeneous rock mass. Because of time and labor needed to prepare slope maps and measure frequency distributions by planimeter, two methods of rapid point sampling of slope were tested for reliability: (1) location of sampling points by drawing random pairs of rectangular coordinates; (2) use of sampling points uniformly spaced on a rectangular grid. A test area of about 1 square mile was selected near the center of the Emporium, Pennsylvania, topographic quadrangle. Population parameters of tangent of slope were computed by planimeter measurements of a detailed isotangent map of the test square. Samples of 100 points each were then taken directly from the topographic map of the test square by both random co-ordinate and grid methods and compared with the slope population. In tests using the statistic χ 2 /d.f. and the statistic z the hypotheses that random co-ordinate sample and population variances are equal and that sample and population means are equal were accepted at the 5 per cent level. Confidence limits based on population variance were determined for distribution of means for samples of various sizes. A chi-square test of goodness of fit required acceptance of the hypothesis that random-coordinate sample distribution is similar to the population distribution. Operator variance was tested by means of a t-test of paired differences in tangent values read by two operators at the same points. A low mean of differences, not significantly different from zero, established that variations introduced by operator judgment are small compared with natural slope differences. It is concluded that the random-coordinate method of point sampling can be applied to determine slope mean, variance, standard deviation, and general form of slope distribution within predictable small limits.