Mechanical Behavior Of Rock Mass Research Articles

On modelling of incident boundary for wave propagation in jointed rock masses using discrete element method

Numerical modelling is an effective alternative for studying the mechanical behaviour of rock masses. Very often, in solving the same problem with the same code, different models yield remarkably different results. There are many reasons. Amongst them, the boundary conditions are often overlooked but it has significant influence on the stress wave propagation in jointed rock masses. Usually, in solving dynamic problems, the incident boundaries are described either by stress-history input (SHI) or velocity-history input (VHI). This paper presents the study on the effect of these two approaches using discrete element method (DEM) on wave propagation and wave attenuation in jointed rock masses. Two cases were designed to illustrate the points. The first one shows one-dimensional P-wave propagation along a rock bar having an internal cross joint. The second one shows the blasting wave propagating through jointed rock masses. The blasting wave is induced by an underground explosion. The rock bar/masses are discretized into elements and the analyses adopt the DEM, which is available in the code ‘UDEC’. The results reveal that the VHI approach can yield results in accord with field-test data, while the SHI approach may lead to unreasonable results. The two approaches may yield the same results for homogeneous rock, but definitely not the same for jointed rock masses. In addition to the numerical illustrations, this paper also presents some insights of the reason behind it.

岩盤モデルおよび解析手法の歴史的な変遷と特徴

A lot of methods have been developed in order to evaluate mechanical behavior of rock mass containing discontinuities. The methods can be classified roughly into three categories. First one is to model the rock mass which contains many and unspecific discontinuities by replacing it with an equivalent continuum. Second one is to analyze failure behavior of rock mass, modeling all of the discontinuities as they are, for the rock mass in which specific joints exist distinctly. Third one is to analyze formation of shear band as the result of successive development of micro fractures and accumulation of those, which depend on heterogeneity of rock mass.These numerical methods have been rapidly developed since the decade of 1960 when FEM analyses were introduced into rock engineering and computer came into wide use, and further development are still being carried out. Historical changes in development of the numerical methods and the significant features of those are described in this report.

Mechanical Tests on Pervasively Jointed Rock Material: Insight into Rock Mass Behaviour

In the exploration phase for the design of an underground cavern in a limestone formation a large number of triaxial compression tests were carried out on laboratory specimens which were characterized by a variable degree of fracturing. The data were analyzed to investigate the influence of fracture intensity and of confining stress on the mechanical parameters. In particular, investigations focused on the relationships between the parameters of the strength criteria, respectively in residual and peak conditions, on the decay of the Young modulus with stress level in the prepeak phase of the test, and on the brittleness of the rock in the postpeak phase. The tested rock can be considered as a small-scale model of a jointed rock mass, and the laboratory data therefore provide useful insight into the mechanical behaviour of rock masses, especially the relationships between residual and peak strength parameters, which are required in many analytical models and in numerical codes for the analysis of underground excavations.

Statistical Properties of the Bukit Timah Granite in Singapore

AbstractThe mechanical behavior of rock mass is affected strongly by its material properties. This paper carries out statistical analyses of the material properties of a rock mass. Statistical analyses of the material constants and the initial damage parameter of the rock mass are performed by using both field and laboratory test data. Based on the measured properties of rock specimens, the elastic modulus, uniaxial compressive strength, and the Poisson's ratio of rock are each found to have normal distributions and the tensile strength to have the gamma distribution. By using Monte Carlo simulation, the equivalent critical tensile strain is found to have the normal distribution. The initial damage of the rock mass, which describes the effects of naturally existing geological discontinuities in a rock mass, is estimated using the field-measured longitudinal and transverse elastic wave velocities. It is found to have the beta distribution. These distributions of rock properties can be used in statistical analyses of the rock mass responses to both static and dynamic loads.

Photoelastic study of the contact mechanics of fractal joints

The Earth's crust exhibits numerous fractures and weaknesses which significantly affect the mechanical behaviour of rock masses, such as the deformation, strength and conductivity, in which the surface roughness of rock fractures plays a dominant role. To take the influence of roughness into account, efforts have been made to characterize the surface roughness of rock joints. The brittle elastic properties of rock joints can be approximately simulated by using photoelastic material and an investigation has been performed on epoxy specimens cast against natural rock joints. In order to cntrol the roughness and to display directly the fractal effects on the mechanical behavior, a series of synthetic fractal joints with different fractal dimensions is tested in the present study by using the photoelastic method under uniaxial compression and direct shear.

Elastic moduli for fractured rock mass

The presence of joint discontinuities has long been recognized as an important factor influencing the mechanical behavior of rock masses. This paper proposes a stress-strain model for an assemblage of intact rock blocks separated by joint planes. The stress-strain relationship accounts for spacings and orientations of the joint sets. Closed-form expressions of elastic moduli for a rock mass with three intersecting sets of joints are derived explicitly in terms of properties of joints and intact rock. Applicability of the derived expressions are evaluated by comparing the predicted results with experimental results from physical model tests.

Rock slope stability analysis : Giani, G P Rotterdam: A A Balkema, 1992, 361P

This English edition represents an updated and revised version of the first Italian edition published by the Associazione Mineraria Subalpina of Turin (1988). The book deals with the methods of assessing the stability of rock slopes and the techniques of improving the stability conditions of natural and artificial slopes which are at risk. The book also deals with the description of survey and measurement methods used to model the mechanical behavior of rock masses and the outlining of field observations to calibrate numerical or analytical methods of slope analysis. The main topics of the book are as follows: Slope instability movement classification and description in order to establish a connection between engineering geology and rock and soil slope engineering fields; Different modes of slope instability and the correspondent types of slope analysis; The geometrical and physical features of the rock mass and the rock discontinuity; Shear strength; Rock mass modelling for flow and mechanical analysis; Rock slope stability analysis in static and dynamic fields; Rock fall modelling; and Methods of improving rock slope stability and the protection methods for rockfalls.

EFFECT OF CLAYEY SEAM ON MECHANICAL BEHAVIOR OF ROCK MASS

The mechanical behavior of saturated model rock mass with a clayey seam was investigated by conducting a series of undrained triaxial compression tests. In the tests, a rock mass was modeled by a specimen made by sandwiching an alluvial clay seam between two pieces of Ohya-stone (tuff). The effects of the thickness of clayey seam, the inclination of seam plane to the maximum principal stress direction, the shear strain rate, the confining pressure and the over-consolidation ratio on the mechanical behavior of rock mass were studied based on the effective strees concept. From the experimental results, even if the thickness of clayey seam was so thin, the shear strength of rock mass is found to be governed by that of the clayey seam.

AN APPLICATION OF DAMAGE TENSOR FOR ESTIMATING MECHANICAL PROPERTIES OF ROCK MASS

Distributed discontinuities essentially affect to the mechanical behaviour of rock mass. Such discontinuities can be modelled by introducing the damage tensor of Murakami et al. which was originally proposed for creep failure of metal-like materials. In this paper, the concept is extended to the rate-insensitive rock mass behaviour involving joint sets. An in-situ observation method of the damage tensor is proposed.

Potential applications of the corejacking test to characterize the mechanical behavior of rock masses : Blankenship, D A; Amadei, B; Stickney, R G Proc 24th US Symposium on Rock Mechanics, College Station, Texas, 20–23 June 1983P335–342. Publ: AEG, 1983

Potential applications of the corejacking test to characterize the mechanical behavior of rock masses : Blankenship, D A; Amadei, B; Stickney, R G Proc 24th US Symposium on Rock Mechanics, College Station, Texas, 20–23 June 1983P335–342. Publ: AEG, 1983

A method for evaluating the effect of crack geometry on the mechanical behavior of cracked rock masses

Discontinuities like faults and joints (called cracks) are of widespread occurrence in rock masses in situ, with very complicated geological setting. The complexity, especially in their geometry, is no doubt a major obstruction to develop a useful theory for evaluating the mechanical behavior of cracked rock masses. An index measure (called fabric tensor) which has been introduced to show crack geometry is further discussed in this paper to see if it is useful for evaluating the mechanical behavior of rock masses in situ. Based on some acceptable simplification, an overall elastic compliance for cracked materials is successfully formulated in terms of the fabric tensor. Furthermore, with the help of geometrical probability, the fabric tensor is expressed in terms of in situ measurable quantities. These results strongly suggests that the concept of fabric tensor is useful in the analysis of cracked rock masses.

Fracture Patterns and Anisotropy of San Manuel Quartz Monzonite

This report presents an interrelationship of the structure and anisotropy of the porphyry copper deposit of the San Manuel mine, Arizona. Analysis of orientations of the fractures mapped in the quartz monzonite of the ore body established the existence of an orthogonal fracture pattern. Oriented compression and Brazilian tension tests on the quartz monzonite showed that the mappable fracture pattern is reflected in planar weaknesses in the rock. Analysis of thin sections indicated that the existing microfractures controlled failure of the test specimens. These microfractures likely originated from fluid inclusion planes that were formed by stress relief. The fracture patterns mapped in the underground workings can be generalized as four sets: E–W and N–S, Both vertical; N. 80°E. to E–W, vertical; and horizontal. These sets are rotated up to 30°, with respect to both the depth and horizontal location in the mine. This suggests a location control within the pluton. Rock fractures are important controls on the mechanical behavior of rock masses and, if properly analyzed, are helpful in the design and safe operation of mining excavations.

Fracture in rock

Fracture is perhaps the most important topic in rock mechanics today, primarily because it is the dominant mechanism of rock failure at the relatively low pressures and temperatures at shallow depths in the earth's crust and because existing fractures strongly influence the physical and mechanical behavior of the rock mass. Fracture considerations therefore pervade all of man's interests dealing with exploration and production of natural resources, engineering projects under or on the earth's surface, and the prediction, modification, and control of earthquakes. For example, (1) estimates indicate that about 5% of the energy generated in the United States is consumed in fracturing rocks [Lewis, 1966]; (2) in the petroleum industry an understanding of fracture is essential in relation to ‘fracture porosity’ reservoirs, hydraulic fracturing, secondary recovery programs, structural inferences from existing fractures, underground storage of gas, and drilling technology; (3) in certain geothermal projects, fractures serve both as subsurface permeability channels and as surfaces to heat circulating waters; (4) the in situ ‘mining’ of oil shale or of the gasification of coal is primarily a matter of man's ability to fracture the rock mass under controlled conditions; (5) in mining and tunneling operations, fractures influence the stability of the openings as well as the extraction of rock or ore; and (6) recent developments in earthquake research have focused attention on fracture (dilatancy) as the source of a host of premonitory events from which it may someday be possible to predict earthquakes [Nur, 1972; Whitcomb et al., 1973; Scholz et al., 1973]. Accordingly, there is ample mission orientation for even the most basic research designed to gain a better understanding of fracture processes in rock and of the mechanical behavior of rock masses containing fractures.