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Proceedings Papers

Paper presented at the ISRM International Symposium, November 19–24, 2000

Paper Number: ISRM-IS-2000-332

..., obtained equations express the deformation and the

**stress****distribution**of the geonet. INTRODUCTION Sand replacement on dredged and reclaimed marine clay has been used for some time. It is easy to construct the land on the soft clay, the replacement method dumping the sand into the reclaimed marine clay...
Abstract

ABSTRACT: The ground replacement method reinforced by geonet is based on field experiences so that a theoretical consideration has been required to adapt for use with the geonet. In this study, we have considered the mechanical behavior of the geonet based on the cable theory. As a result, obtained equations express the deformation and the stress distribution of the geonet. INTRODUCTION Sand replacement on dredged and reclaimed marine clay has been used for some time. It is easy to construct the land on the soft clay, the replacement method dumping the sand into the reclaimed marine clay easily. However, this conventional replacement method has been gradually decreasing because of the unreliable sand shape. In recent years, budgetary cutback of public undertaking has occurred so that this method is new employed by using the geonet, which can restrict the replaced sand and achieved a sand shape (Yasuhara et al., 1982). Hereafter, we call it the Geonet Replacement Method (GRM). GRM is shown in Figure 1. Theoretical consideration of this method was based on the application of the expression of the sheet method on the soft clay ground (Yamanouchi et al., 1979). However, it seems that the mechanism of GRM is different from the sheet method on the soft clay ground. Therefore, a theoretical consideration has been required to adapt to the geonet. In this study, we have considered the mechanical behavior of the geonet on a basis of the cable theory and derived the differential equation from the equilibrium condition of the geonet on the soft clay ground. COMPARISON OF THE CALCULATION AND OBSERVATION Verifying the validity of both the geonet deformation and the tensile stress distribution that were obtained by the above theoretical considerations, we compared the calculation result and the observation data.

Proceedings Papers

Paper presented at the ISRM International Symposium, November 19–24, 2000

Paper Number: ISRM-IS-2000-548

... movement and

**stress****distribution**must be reassessed for seam with deep dip angle. In view of this, based on field monitoring, model test and discrete element method are used in the study of strata behavior in inclined coal seam, relevant strata control method is proposed in the paper. MODEL TEST OF...
Abstract

ABSTRACT: Strata behavior and control in inclined coal seam face are studied with the following methods: field monitoring of strata behavior is carried out at a working face; Model test on equivalent material is made along dip direction of face and both strata movement and stress redistribution are analyzed and described; Overlying strata movement and fracture are modeled by discrete element method (DEM). The following conclusions may be drawn: Inclined dip angle of coal seam leads to asymmetry of the movement and fracture of overlying strata as well as the abutment pressure distribution; The gob is filled with caved debris differently; the lower end is fully packed, whereas the upper end is however in vacant condition. In the dip direction, the roof is supported by the caved debris quite differently, which results in differentiate roof deflection. The roof at the lower part will not break, but will certainly break at the upper face end, which leads to intensive strata movement in the upper end; Roof movement is much more remarkable in inclined coal seam than that in gently inclined seam. It is therefore strongly recommended that the supports in longwall working face be set with higher stability to keep them sound. INTRODUCTION Meanwhile the rolling blocks may have impact action to the face support; (2) Face floor may fail and shear displacement may occur along bedding plane in sip direction, which as a results has negative effects on support stability; (3) Due to uneven filling of gob by the caved debris, roof behaves quite differently in the dip direction characterized by intensive roof movement at the middle and upper section; (4) More remarkable anisotropy rockmass, which makes gateway deform and fail with distinguished direction.

Proceedings Papers

Paper presented at the ISRM International Symposium, November 19–24, 2000

Paper Number: ISRM-IS-2000-047

.... Consequently, the displacement of each crack and

**stress****distribution**around each crack are the same. An evaluation of the macroscopic strain field is possible by estimating the displacement distribution of a representative crack in the elemental region (Figure 2). COMPLIANCE OF THE MODEL In the model, the...
Abstract

ABSTRACT: A Homogenized Multi-Crack Model is proposed in order to estimate the deformability of a rock mass which has a high crack density. In this model, all cracks are parallel and infinitely arranged at equal intervals in all directions, and the crack pattern around a crack is always the same at any part of the model. In this study, the effective compliance of HMCM is estimated by applying the 2-Dimensional Displacement Discontinuity Method and linear fracture mechanics. It was found that the deformability of the model in the direction normal to the cracks is strongly affected by crack length, crack intervals in the normal direction of cracks and crack patterns. INTRODUCTION The deformation and failure of a rock mass is largely dependent on the presence of geological discontinuities such as cracks or faults. In the case of high crack density the mechanical properties of the rock mass, such as compliance or Young's modulus, are strongly affected by the mechanical interaction between the cracks, and therefore a quantitative evaluation of this interaction is necessary (Anderson, 1974; Walsh 1974; Kaneko, 1990). However, there are few studies about these basic problems, especially in the compressive stress field or mixed mode loading problem. Here, a Homogenized Multi-Crack Model (HMCM) is proposed in order to estimate the deformability of a rock mass which has a high crack density. In HMCM, all cracks are parallel and infinitely arranged at equal intervals in all directions, and the crack pattern around a given crack is always the same at any part of the model. In this study, the effective compliance of HMCM is estimated by applying the 2-Dimensional Displacement Discontinuity Method (2D-DDM) (Crouch, 1983) and linear fracture mechanics. The influence of the mechanical interaction of each crack towards the effective compliance is also discussed.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 30–September 2, 1989

Paper Number: ISRM-IS-1989-175

.... Fluid-skeleton interaction is described using Biot' s coefficients which are dependant upon deviatoric stress. Examples are then given for

**stress****distribution**around a borehole. I- INTRODUCTION The study of the**stress**-**distribution**around a drilled hole is of great importance in several situations...
Abstract

ABSTRACT: In this paper, experimental results for Fontainebleau sandstone are presented: skeleton behaviour through triaxial tests and interaction between pore fluid pressure and skeleton deformation. To modelize the skeleton behaviour an hypoelastic constitutive law is assumed for sandstone. Fluid-skeleton interaction is described using Biot' s coefficients which are dependant upon deviatoric stress. Examples are then given for stress distribution around a borehole. I- INTRODUCTION The study of the stress-distribution around a drilled hole is of great importance in several situations, such as, while drilling, during production or injection and when calculating fracturing pressures. To that end, a good knowledge of the rheological behaviour of rocks is required. Conventional triaxial tests on Fontainebleau sandstone have been carried out with various confining pressures, under monotonous and loading-unloading sollicitations. The crack behaviour is studied through crack closure tests, and their interaction with fluids is described by IKOGOU (1897), SIBAI (1987). The results obtained proved that the non-linear behaviour is essentially due to microcracks. The crack extension is characterized by a very early start and a very pronounced orientation in the direction of loading inducing a strong anisotropic dilatancy. Loading-unloading cycles show a strong decrease in material stiffness in the direction perpendicular to loading, and zero axial permanent strains. Usual elasto-plastic constitutive laws are inefficient to describe this type of behaviour, NGUYEN (1988), SANTARELLI (1986), SHAO (1987). Within the framework of proportional loading sollicitations to structures, an orthotropic incremental model is proposed. The modulus are determined from conventional triaxial tests and their derivatives. This model is then used to determine the stresses around cavities. II MODELISATION OF SKELETON 2.1 Principle of the model The model is used to describe the behaviour of rocks whose deformation is essentially due to microcracking. We use an hypoelastic orthotropic incremental model chosen according to the crack extension mechanism (figure 1). 2.3 Experimental results 2.3.1 Tests Characterization tests are conventional triaxial axisymetrical tests under the following confinement pressures: 5, 10, 20, 30 and 40 MPa. The tests have been conducted on specimen of 37,5 mm diameter and 75 mm height. The specimens are saturated with methanol under vacuum and are instrumented with two axial and two transversal strain gages. The test rate is έ = 1,2. 10–6 s-l. 2.3.2 Results Paths A Figure 2 shows the results obtained for a confining pressure of 10 MPa. The following comments can be made: - Dilantancy induced by microcracks decreases when confining pressure increases. - The threshold of microcrack initiation increases with the confining pressure. - The axial modulus is almost constant up to 75 % of peak strength. This shows that crack extends axially up to stresses very close to the peak strenght value. Paths B For confining pressures less than or equal to 10 MPa, permanent transverse strains appear. Beyond 10 MPa, there are no permanent strains but hysteretic loops start growing with the confining pressure, until infinite on negative unloading modulus are reached. It is therefore proposed to use secant modulus to characterize these paths.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 30–September 2, 1989

Paper Number: ISRM-IS-1989-167

... broken coal with resin is analyzed. A circular area around a gallery could be injected. A material model is described for injected coal (Mohr-Coulomb envelope with a small cohe- Sion) and again an exact solution is found for the

**stress****distribution**and deforma- tions. Calculations show that only a small...
Abstract

Abstract: At the coal-mine of "Les Houilleres de Blanzy", France, extraction galleries are constructed at a depth of approximately 700 m, entirely in coal of very poor mechanical properties. To understand the problem of the severe convergence that occurs in these galleries a mathematical simulation is made, using a simplified shape (cylindrical tunnel) of the underground opening. The material model is described as follows: Linear elasticity up to a linear Mohr-Coulomb failure criterion. After reaching the Mohr-Coulomb criterion the material fails with a sudden volume increase. The broken material then behaves following soil-mechanics with no further volume change. The given mathematical simulation is an exact solution of the above problem. The thus calculated deformations are in good agreement with the convergence observed underground. The presence of a large zone of broken material around the gallery is verified by in situ observations and measurements. Therefore, it can be concluded that the simplifications made are acceptable. It is obvious that man-made measures are necessary to avoid or limit the problems related to the strong convergence. Some experiments carried out are described shortly. As a preparation to an underground experiment, the possibility of injecting the broken coal with resin is analyzed. A circular area around a gallery could be injected. A material model is described for injected coal (Mohr-Coulomb envelope with a small cohesion) and again an exact solution is found for the stress distribution and deformations. Calculations show that only a small improvement of the material property, by means of injection, already gives a strong stabilizing effect on the galleries. It remains to be seen if this is applicable in practice. 1. Introduction The underground coalmine of "Les Houilleres de Blanzy" is located in the coal basin of Blanzy, between Rozelay and Montceau les Mines. This formation belongs to the north-east section of the Massif Central, France. The Blanzy formation is generally folded in the north-east/south- west direction. The basin is a depression of 100 Km long, 10 Km wide with a minimum depth of 1500 m, see fig. 1. The coal found in this region belongs to the upper carbon period. The coal was subjected to tectonic movements and metamorphism at low pressures and high temperature. The result of this geological history is that coal seams occur of thicknesses up to 40 m which are of very poor mechanical properties. The production method used in this mine (a special retreating longwall caving method) requires that extraction galleries are constructed entirely in the coalseam and are maintained until the face has passed. During this period (0.5–2 years) strong convergence occurs in the galleries and up to five reopenings are required. In the past, several attempts are made to avoid or reduce the problems related to this strong convergence. See [1] and [2]. Extremely strong arches were constructed and coal was extracted from between those arches to limit the pressure on them. In principle this method worked but it was much to laborious.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 30–September 2, 1989

Paper Number: ISRM-IS-1989-133

... 1.25 was modelled. This follows Sugawara et al. (1978) who concluded for a borehole cored with a thin walled bit, that the

**stress****distribution**in the basal region of the core is approximately constant for length to diameter ratios of 0.4 and greater (Figure 2) . For the case of a vertical borehole...
Abstract

ABSTRACT: Since the early 1960's, core discing has been extensively used as an indicator of large in situ stress magnitudes. Recently, field observations have shown that a good estimate of the principal stress directions can be obtained from the shape of the disc. To investigate the formation of the shape of these discs, a fully 3-D elastic boundary element analysis vas implemented, to calculate the stress and extension strain paths experienced by a core during unloading. It is concluded that core discing is a powerful in situ stress indicator, for regions of high horizontal in situ stresses. For the case of a vertical borehole, discing can be used to determine the direction and approximate ratio of the horizontal principal stresses. A measure of the inclination of a principal stress avay from the vertical can be gauged from any asymmetry of the disc. Thus discing can be used to check whether the vertical stress is a principal stress. 1.0 INTRODUCTION As the quest for new mineral and energy resources has progressed, be they of hydrocarbons, metals or geothermal energy, the need to search deeper within the earth's crust has become established. An increasing range of problems in Rock Mechanics require an understanding of the in situ stress state, and consequently a strong interest has developed into its nature and measurement at depth. At these deep levels, near surface stress measurement techniques become inapplicable, and techniques less direct in their measurement must be used. An assessment of the complete state of stress at depth can only be compiled from the results of a series of different measuring techniques or through the use of a priori information: often taking the form of assumptions. This paper explores the phenomenum of stress induced core discing and its potential to check one of the more widely made assumptions of in situ stress measurement namely, is the vertical stress a principal stress? It has for long been recognised that core discing is a sign of high stresses perpendicular to the borehole axis, and that the higher the stresses, the thinner the separation between adjacent discs (Obert & Stephenson,1965). With limited success several criteria have been proposed relating the magnitudes of the in situ stress to the strength of the intact rock material (Obert & Stephenson, 1965; Sugawara et al., 1978; Ganga Rao et al., 1979; Stacey, 1982). However as outlined by Barkovskii & Isaev (1979), these criteria yield only a very approximate measure of the stress state, since for numerous variables, such as the exact geometry of the drill kerf, no account is taken. 2.0 DISC SHAPES Principal stress orientations, as well as stress magnitudes can be determined from discing. Paillet and Kim (1987) describe 'saddle shaped' or 'potato crisp' shaped discs in basalt, interpretated as an indicator of anisotropic horizontal stresses. The axis of the peaks yield the direction of the minimum horizontal stress (Lehnhoff et al., 1982). Maury et al. (1988) in a review of discing, highlight the repetitivness in shape of discs.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 30–September 2, 1989

Paper Number: ISRM-IS-1989-078

... ~ # MWI I ~ . Ht VhNl 1 ~ + 11 I 1;>1 _I . . \ lo,, \ , ~ \ \ t : : :giJI\I'll - _- 'If+ + · AWII f f # , HIHNII f . . - vMiolIo'I I 1 MIt'I ~ I .. PosItiOnalthe~I~6: l00minlronlollhe'fK1ion Fig.7 Plots of principal stresses around the roadway, a),b),c):

**stress****distributions**as the roadway advances; e...
Abstract

ABSTRACT: A numerical case study was conducted to analyse the mechanism of large deformation of weak rocks around excavations. Typical results of in situ measurements are summarized. The numerical investigation using the proposed finite element model is described. Conclusions are drawn with descriptions of their practical implications in rock engineering. 1. INTRODUCTION Since 1978, a number of laboratory tests and in situ measurements have been conducted by British Coal and Nottingham University for analysis of rock behaviour in and around the Warwickshire Thick Seam at Coventry Colliery, U.K. (Mallory, 1982; Baxter,1987). The seam has been excavated using the longwall mining method at a depth of 750 m ~ 800 m. Although it is not at a significantly great depth, the weak rocks around the mining roadways and longwall faces have generally exhibited a large and time - dependent deformation, which is due to the relatively high in situ stress level comparing with its strength. In order to obtain a more complete knowledge of strata deformation mechanics, which mainly includes prediction of the extent of the strata movement, their interactions with support systems, and the effects of the progressive excavation and construction of the longwall face and gate roadways, a numerical study using the finite element method has been conducted based on the results of in situ measurements and laboratory tests. The in situ investigations involve the measurements of roadway closure, floor heave, pack load and closure, roof bed separation and support forces, etc. Typical results from the existing literature are summarised in the following section. The numerical investigation scheme is then described. Some of the computed results from the numerical analysis are presented in section 4 with comparison of the in situ measurements. At the end, several conclusions from this case study are drawn. 2. MEASURED STRATA DEFORMATIONS The thickness of the coal ceams at Coventry Colliery vary from 5.5 m ~ 7.3 m. The longwall face is about 3.5 m in height and 230 m in length. The Coal Seams are overlain by layers of mudstone and sandstone and below the coal floor are situated bands of weak seat earth mudstone. Coal was left in both floor and roof to help in stabilising the weak strata in the excavation areas. Fig.l illustrates a typical geological section of the thick seam which is abstracted from the general strata conditions in the colliery. The representative strength parameters of the strata are also shown in the figure. The roadway structure details and typical in situ measurements are shown in Fig. 2. The results obtained from in situ measurements show that vertical roadway closure was mud greater than the horizontal closure. In some of the sections, the maximum vertical closure was up to 2.5 m. Fig. 3 and Fig. 4 show plots of roadway deformation versus distance of face advance. The pack load and deformation versus face advance are shown in Fig. 5. A number of other measurement data are also available, such as the roof bed separation, change of support leg angle, leg penetration and arch support load, etc.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 30–September 2, 1989

Paper Number: ISRM-IS-1989-121

... Upstream Oil & Gas variation borehole drilling analytical solution injection reservoir geomechanics calculation borehole fracture pressure plastic strain Reservoir Characterization Simulation cohesion low strength material

**stress****distribution**criterion equation Fracturation...
Abstract

Abstract: Analytical and numerical methods are proposed in order to determine stresses distribution around borehole for axisymetric problems in the case of low strength materials. Calculation of these distributions leads to the determination of hydraulic fracture-pressure when the "in-situ" stresses conditions vary. 1-INTRODUCTION A comparative study of different models of rocks behaviour around a borehole for low resistance materials is proposed in this paper. Many authors assumed the apparition of a ring shaped plastified zone around the borehole for such materials. The work of Westergaard [1940], Gnirk [1972], Horsud (1981) who found analytical solutions for stresses around a borehole in a poorly consolidated material, can be quoted here. These previous works allowed to find a Stability criterion for the hole or to estimate the fracture pressure caused by an injection. The latter case is given special attention here, in order to describe the influence of "in-situ" stresses on fracture pressure for materials with different~ behaviours. This study can then be situated with in the context of the "leak of test", one of the main objectives of which is the estimation of the "in-situ" stresses. II-MODEL-HYPOTHESES The model is axisymetric (figure 2.1). The "in-situ" stresses before drilling are total axial stress σzo and total horizontal Stress σ n. The pore pressure We define: – Ri borehole radius – Ro external radius. the infinite where undisturbed. – H height of model - z depth 111- ANALYTICAL RESOLUTION MOHR-COULOMB CRITERION For low resistance material the Mohr-Coulomb model of soil plasticity is frequently used. Horsud et al [1981] [1982] have found analytical solution to modelize fluid extraction from the borehole or fluid injection until fracturation. Different steps are needed to simulate fracturation: Simulation of "borehole drilling" from initial conditions, the pore pressure being uniform. For the axisymetric model, the influence of the "in-situ" stresses can be described by the influence of the stresses ratio Ko. It is assumed that during a 'leak of test" the first drop in pressure place when P(Ri)= Pl, so it important to test the influence of ko this pressure independantly of the other parameters (cohesion, …). In order to find analytical solutions, the following must be assumed: – the flowrate is constant around the borehole (this condition may be too limitative to simulate a leak of test because a sharp increase in pressure can be observed during this test). – the loading (injection or fluid extraction) cannot be applied step by step. The equality σ θ = σ z is typical of this stresses distribution. If r = Ri the effective stress σ' θ is equal to Co; this shows that for a low cohesion (when neglecting σ T) the fracture condition is virtually verified before injection. The previous comments about stresses before injection leads us to contest some aspects about the analytical resolution (especially the effects of Ko). The calculation of stresses assuming a given sheme at fracturation (cases FRAC I, II or III) certainly causes the important variations of P1 with Ko shown above.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 31–September 3, 1986

Paper Number: ISRM-IS-1986-017

... ABSTRACT: Determination of the state of stress in a jointed and fractured medium is complicated by the spatial heterogeneity of the

**stress****distribution**. In a discontinuous rock mass this is related to the current state of geologic loading, and the stress path defined by its geologic history...
Abstract

ABSTRACT: Determination of the state of stress in a jointed and fractured medium is complicated by the spatial heterogeneity of the stress distribution. In a discontinuous rock mass this is related to the current state of geologic loading, and the stress path defined by its geologic history. The stress path develops over geologic time from a variety of physical, chemical and mechanical changes, including processes such as fracture, and slip and separation on planes of weakness. The results of a stress measurement exercise, using a 1.81 m diameter borehole, are presented. They confirm the role of geologic structure in determining stress distribution in jointed rock. Computational analysis of generic models of jointed rock masses has been used to develop guidelines for the preferred number and location of stress measurement sites in such a medium, which can provide a reliable estimate of the average field stresses. RESUME: La détermination de l''état de sollicitation d''un milieu fissuré et fracturé est compliquée par l''hétérogénéité spatiale de la distribution des contraintes. Dans le cas d''une masse rocheuses discontinue, celle-ci est relative à l''état courant de chargement géologique ainsi qu''à la trajectoire de contraintes définie par son histoire géologique. La trajectoire des contraintes se développe au cours du temps géologique d''une diversité de changements de nature physique, chimique et mécanique, y compris des procédés tels que la fracture, le''écoulement et la séparation suivant les plans de faiblesse. On présente les résultats d''un essai de mesure des contraintes en employant un trou de forage de 1.81 m de diamètre. Ils confirment le rôle de la structure géologique dans la détermination de la distribution des contraintes dans les masses rocheuses fissurées. On a fait appel à l''analyse de calcul de modèles génériques de masses rocheuses fissurées pour obtenir des indications du nombre préféré et de la location des sites pour la mesure des contraintes dans un tel milieu qui puissent aboutir à une appréciation sure des contraintes de champ. ZUSAMMENFASSUNG: Die Bestimmung des Spannungszustandes bei einem geklüfteten und rissig gewordenen Gesteinssystem wird durch die räumliche Heterogenität der Spannungsverteilung kompliziert. Bei einem unterbrockenen Gesteinssystem ist diese mir dem augenblicklichen geologischen Beladungszustand. sowie mit der von ihrer geologischen Vorgeschichte bedingten Spannungsrichtung verbunden. Die Spannungsrichtung entwickelt sich im Laufe der geologischen Zeit aus einer Vielzahl physischer, chemischer und mechanischer Veränderungen, einschliesslich Vorgänge wie Bruch sowie Fliessrutschung und Trennung an Schwachflächen. Die Ergebnisse eines Spannungsmessvorhabens unter Verwendung eines Bohrlochs mit einem Durchmesser von 1.81 m werden angegeben. Sie bestätigen die Rolle der geologischen Struktur bei der Bestimmung der Spannungsverteilung en einem geklüfteten Gesteinssystem. Rechneranalyse von allgemeinen Modellen geklüfteter Gesteinssysteme ist zur Entwicklung von Richtlinien betrefflich der vorzuziehenden Anzahl sowie Stellen der Spannungsmesspunkte bei einem solchen System, die eine sichere Auswertung der durchschnittlichen Feldspannungen liefern können, verwendet worden. 1. INTRODUCTION In the design of underground excavations, reliable determination of the virgin state of stress is usually regarded as an essential component of site characterisation. Significant improvements have been made, in recent years, in instruments and techniques for determination of the average state of stress in a small volume of rock. The volume sampled using conventional borehole devices is typically 0.001 m, while the zone of influence of even a moderated sized excavation may exceed 100,000 m. Thus, the point determinations of the virgin state of stress will be adequate for design of the excavation only if the natural stress distribution in the medium is reasonably homogeneous throughout the zone of influence of the excavation. It is inferred that the stress distribution in rock masses, particularly those that are fractured, faulted and jointed, is not homogeneous. This is concluded from the knowledge that, in its geologic history, a rock mass may be subject to episodes of tectonic and gravitational loading, fracturing, unloading, heating, cooling, water infusion, drainage and drying. Each of these physicochemical, thermal and mechanical processes may generate a highly heterogeneous state in the medium. It may also be inferred that the most recent geologic processes will be predominant in determining the current stress distribution. For the near-surface, these processes are erosion, current tectonism, fracture, and slip and separation on discontinuities. It is notable that there has been comparatively little effort devoted to understanding the relation between structure and state of stress in a rock mass. Price (1966) asserted a relation between orientation of the major principal stress, and the orientation of the acute bisector of the dihedral angle defined by conjugate faults. Jamison and Cook (1980) examined limits on the state of stress in a rock mass defined by rock material strength and rock structure defined by faulting. Apparently, there has been comparatively little effort devoted to determining the stress distribution in a regularly jointed medium. Model studies of discontinuous media, such as those reported by Chappell (1974), support the notion of a heterogeneous stress distribution in a discontinuous medium.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 31–September 3, 1986

Paper Number: ISRM-IS-1986-001

... on the rising portion of the load-deformation curve. 2. IN-SITU

**STRESS****DISTRIBUTION**IN ROCK MASSES Rock engineering problems are much less well defined than those described "0 Cl a ..J ( Deformation (a) Following Load Situation A "0 Cl a ..J H Deformation (b) Soft (AE) and Stiff lAG / AH) Loading...
Abstract

ABSTRACT: This paper comments generally on the inherent difficulties of interpreting in-situ stress determinations. These difficulties arise primarily from geological complexity and the localized significance of individual measurements. Frequently the stress state information is sought as input for excavation design studies. In such cases, greater emphasis should be placed on assessing the effect of in-situ stresses, and stress variations, in combination with several other factors (rock strength, excavation geometry,...) which influence design. Observation of the overall response of the rock mass to the combined effect of these variables is then more important than the specific determination of stress state. RESUME: Ce rapport offre des remarques en general sur les difficultes inherents d''interpreter les determinations des contraintes ''in-situ'' dans les roches. Ces difficultes arrivent primitivement a cause du complexite geologique, et le caractere ponctuel des essais individuels. Frequemment, on se sert de l''information qui concerne des contraintes pour dessiner les excavations. Dans tels cas, plus d''effort doit etre attacher au jugement des effets des contraintes in-situ, aux variations des contraintes de point a point, en combinaison avec des autres facteurs (la resistance des roches, la geometrie de l''excavation...)qui peuvent influencer le dessin. Les mesures de la reponse en total du massif rocheux aux effets integrals de ces variables sont encore plus important que la me sure specifique de la contrainte in-situ. ZUSAMMENFASSUNG: Dieser Beitrag ist uber die innliegenden Schwierigkeiten in der Ausdeutung des Messungen der "in-situ" Spannungen. Diese Schwierigkeiten kommen erstens von der Geologischen Complexitat und von der lokaler Wichtigkeit der einzelnen Messungen. Die Information an den Spannungszustand wird oft gesucht als Eingabe zu den Aushohlung Entwurf Studien. In diesen Fallen sollte ein grosseres Gewicht gelegt sein an die Auswertung der "in-situ" Spannungen und an die Spannungs wechseln, welche, in Kombination mit anderen Einflussen (Gebingsfestigkeit, Form der Hohle u.s.w) den Entwurf beinflussen. Die Besichtigung der uberall Reaktion der Gebirgsmasse zu dem kombinierten Effekt dieser Variablen ist dann wichtiger als die spezifische Messungen des Spannungszustandes. 1. INTRODUCTION Study of the deformation and failure of rock masses has relied traditionally on concepts of continuum mechanics. These concepts are powerful and can provide valuable insights for rock engineering. It is important to appreciate, however, that continuum mechanics, and engineering design procedures based on continuum mechanics, developed within the context of a concern for engineering materials and/or situations quite different from those often encountered in rock engineering. Care must be taken, therefore, to ensure that these design procedures are not applied inappropriately to rock. In many classical design problems (e.g. design of buildings, bridges, pressure vessels, air frames...) the applied loads are known and remain constant independent of the displacements (so called ''following loads/). The energy available to propagate rupture continuously exceeds the energy that can be stored or absorbed within the structure (Fig. l(a). In this case, onset of rupture, (at load F a ) is synonymous with collapse. It is important, therefore, to ensure that at no point in the structure does the applied load (or stress state) exceed the resistance (i.e. strength) of the structural material. Classical stress analysis is possible; the structural material adequately satisfies the requirements of homogeneity, continuity, and elasticity. The dimensions and locations of all components are well defined. Classical stress analysis is possible; the structural material adequately satisfies the requirements of homogeneity, continuity, and elasticity. The dimensions and locations of all components are well defined. Material properties (deformability, anisotropy, strength) can be determined reliably from laboratory tests on specimens of a size comparable to the prototype structure (or at least to components of it), and taken from the same material; strength variability can be determined and, together with other uncertainties in the analysis, can be accounted for by an appropriate "safety factor" in the analysis, thus assuring that loads in the structure are everywhere well below the limit (F a ) i.e., everywhere on the rising portion of the load-deformation curve. 2. IN-SITU STRESS DISTRIBUTION IN ROCK MASSES Rock engineering problems are much less well defined than those described above. The rock mass has usually been subjected to numerous epochs of loading throughout geological time, producing fracture and faulting of systems of different orientations in space, resulting in a 3-dimensional network of interconnected blocks. The scale of fractures or joints is such that direct laboratory testing of a "representative volume" of the rock mass is impossible. Groundwater influences the pressure distribution in the rock mass. Undulating topography will also affect both vertical (gravitational) and lateral load distributions. In some cases, (e.g. dam foundations, rock slopes) the applied loads have a "following" character. Here it is important to ensure that discontinuities are not so loaded (both by solid and fluid forces) that alone or in combination (e.g. as a wedge or connected planes) they begin to slide and, hence, collapse.

Proceedings Papers

Paper presented at the ISRM International Symposium, August 31–September 3, 1986

Paper Number: ISRM-IS-1986-044

.... This allows comparisons to be made between a known perturbance at the rock mass scale with a measured disturbance at a localized scale. This testing strategy well applies to study the complex problem of

**stress****distributions**in discontinuous rock masses. A series of tests of this category have recently...
Abstract

ABSTRACT: The in-situ response of an 8 m3 block of jointed biotitic gneiss to uniaxial and biaxial boundary loading was monitored using two types of borehole gauges; the USBM Borehole Deformation Gauge and the LUT Triaxial Strain Cell. From the displacements and strains recorded by the gauges as loads were applied, point wise stresses were calculated assuming both isotropic and anisotropic rock behaviour. Independent of instrument type, the results obtained are characterized by large variations between the stress magnitudes measured in different parts of the block. The measured stress directions are however in close agreement with the directions of the applied boundary loads. RÉSUMÉ: Le comportement d''un bloc de gneiss à biotite soumis, in-situ, à une charge limite uniaxial et biaxial a été observé en utilisant deux types de jauges pour trous de mines: la jauge USBM de déformation de trous de mines et la cellule LUT de mesure de contrainte triaxiale. A partir des déplacements et déformations enregistrés par les jauges au cours de l''application des charges, on a calculé ponctuellement les contraintes internes subies, en se basant sur la double hypothèse d''un comportement isotropique et anisotropique de la roche. Indépendamment du type d''instrument utilisé, les résultats obtenus se caractérisent par des variations considérables entre les intensités de contrainte mesurées en différents points du bloc. Les directions des Contraintes mesurées sont en corrélation étroite avec l''importance des charges limite appliquées. ZUSAMMENFASSUNG: Das Verhalten eines 8 m 3 zerklüfteten Biotitgneis-Blockes vor Ort bei uniaxialer und biaxialer Grenzzonenbelastung wurde mit zwei Arten von Bohrlock- Messgeräten - dem USBM Bohrloch-Deformationsmesser und der LUT-Messzelle für triaxiale Dehnung - gemessen. Aus den Verschiebungen und Dehnungen, die bei Angringung von Lasten von den Messgeräter ermittelt wurden, berechnete man punktweise Spannungen unter der Annahme von sowohl isotropischem als.auch anisotropischem Gesteinsverhalten. Unabhängig von dem jeweiligen Messgerättyp zeichnen sich die erhaltenen Ergebnisse durch grosse Unterschiede zwischen den in verschiedenen Teilen des Blockes festgestellten Spannungsgrössen aus. Die gemessenen Stressrichtungen stimmen gut mit den Richtungen der angebrachten Grenzzonenbelastungen überein. INTRODUCTION Block tests have recently become an important method for determining the mechanical characteristics of rock masses for modeling of planned nuclear waste storage repositories. An important advantage that block tests have over laboratory and borehole tests is that a relatively large, in-situ volume of rock is placed under known boundary conditions. This allows comparisons to be made between a known perturbance at the rock mass scale with a measured disturbance at a localized scale. This testing strategy well applies to study the complex problem of stress distributions in discontinuous rock masses. A series of tests of this category have recently been completed in a load controlled two meter cube of Precambrian gneiss. The primary objective of the tests was to study the distribution of stress in the jointed rock mass constituting the block, when subjecting it to known boundary loadings. Two types of instrument, both based on point deformation measurements in boreholes, were used to monitor the response of the block to loading. One was the USBM Borehole Deformation Gauge (BDG), and the other was the triaxial strain cell used by the Lulea University of Technology (LUT-Gauge). An additional test objective was to compare the results produced by these two instruments. 2 TEST BLOCK DESCRIPTION The testing took place at the Colorado School of Mines'' Experimental Mine in Idaho Springs, Colorado, USA. The block itself is located in the floor of the mine in an underground research laboratory developed for the Office of Crystalline Repository Development (OCRD). The surface of the block is at floor grade while the bottom is continuous with the surrounding rock mass. To minimize the effect of these conditions it was decided that all measurements be made at the block midplane. 2.5 meter deep vertical slots on the four sides define the block perimeter. In each vertical Slot, grouted flat jacks existed providing a means of applying normal stresses of magnitudes up to about 5 MPa to the block boundaries. The rock comprising the block contains fractures on several scales. Three major fractures were evident on the block''s surface as shown in Figure 1. A number of vertical boreholes were drilled in the block as part of an earlier study (Hardin et al., 1983). Both EX (38 mm) and NX (76 mm) boreholes were drilled that provide access to the block interior. Their locations and designations are shown in Figure 1. Additional fracture data was available from TV-logs of these boreholes (Sour, 1985) indicating that the major fractures dip vertically and are continuous throughout the test block. Also evident from these logs were randomly dispersed smaller fractures. 3 DETERMINATION OF MECHANICAL PROPERTIES For each gauge location, detailed definition of the rock deformability was necessary. Unfortunately the cores from the block drill holes were unavailable for those tests.

Proceedings Papers

Paper presented at the ISRM International Symposium, May 26–28, 1982

Paper Number: ISRM-IS-1982-057

... & Gas topographic model total stress pressure shaft nyset steggje project river water pressure

**stress****distribution**tunnel INVESTIGATIONS FOR A 1000METRE HEAD UNLINED PRESSURE SHAFT AT THE NYSET/STEGGJE PROJECT, NORWAY Untersuchungen fur einen nicht ausgekleideten Druckschacht mit 1000 Meter...
Abstract

SUMMARY: The paper describes pre-investigations for a 1000-metre head unlined pressure shaft at the Nyset-Steggje Hydro-Power Project in Western Norway. The field investigations performed have included a thorough engineering geological mapping of rock types, joint patterns and zones of weakness. Rock samples have been collected for laboratory investigations of rock mechanics properties. The rocks in the shaft area consist of gneiss and granite, only slightly jointed and with few zones of weakness. The drillability and wear investigations indicate medium to good drillability and medium to low wear. To locate the shaft, a finite element analysis of the rock stresses have been performed. The main condition imposed is that the minimum in situ compressive stresses must be higher than the water pressure at any point along the shaft. Because of complicated topography a simplified topographic model has been established. The planned shaft has a location oblique in relation to the valley side. The calculations are therefore performed by means of 3 two-dimensional models representing vertical sections cutting the shaft at critical points. In situ stresses will be measured. If these differ appreciably from the calculated stresses, a re-evaluation of the shaft will have to be undertaken. ZUSAMMENFASSUNG Die Voruntersuchungen fuer eine unverbaute Druckschacht von 1000 meter statischer Druckhöhe bei der Wasserkraftanlage Nyset-Steggje in West-Norwegen werden beschrieben. Gebirgsarten, Kluftscharen und zerstörte Zonen wurden im Felde ingenieurgeologisch kartiert. Ausgewahlte Felsproben wurden auf felsmechanischen Eigenschaften im Laboratorium untersucht. Die Gebirgsarten im Gebiet der Druckschact bestehen aus Gneiss und Granit, wenig zerklueftet und fast ohne zerstörte Zonen. Spezielle Laboruntersuchungen indikieren dass mittlere Bohrbarkeit und mittlerer bis geringer Verschleiss der Bohrwerkzeuge zu erwarten sind. Eine endliche Element Analyse der Felsspannungen wurde ausgefuehrt urn eine sichere Lage der Druckschacht zu finden. Eine grundlegende Bedingung ist dass die kleinste in situ auftretende Druckspannung in irgend einem Punkt entlang der Schacht kleiner sein muss als der dort Auftretende Wasserdruck. Die topographischen Verhaltnisse sind recht kompliziert weshalb ein vereinfachter topographischer Model verwendet wurde. Die geplante Schacht hat zur Talseite eine schrage Orientierung. Die Berechnungen wurden mit Hilfe von drei zweidimensionalen Modellen durchgefuehrt. Diese Modelle reprasentieren Vertikalschnitte durch die Schacht in kritischen Punkten. Die Spannungen in situ werden gemessen werden. Falls die so gemessenen Spannungen erheblich von den errechneten Spannungen abweichen werden, muessen bezueglich der Schacht unternommen werden. RESUME: L''article dećrit les reconnaissances preliminaires menees pour le projet hydroenergetique de Nyset-Steggje, dans la Norvège de I''ouest. Le projet comporte un puits non-revêtu destine à des pressions de 1000 mètres d'eau. Les reconnaissances effectueea en chantier ont incIus une cartographie des conditions geólogiques, formation rocheuse, plans de fissuration et zones de faiblesse. Des ećhantillons de roches ont ete preleves pour fins d''essais des proprietes, mecaniques en laboratoire. Dans les environs du puits, la formation rocheuse consiste de gneiss et de granite, avec peu de fissures et peu de zones de faiblesse. Les etudes de la forabilite et de l''index d''usure de la tête de forage indiquent une forabilite moyenne à bonne, et une usure moyenne à faible. A fin de determiner la position optimum du puits, une analyse par elements finis des contraintes dans Le rocher a ete faite. La critŕere principal impose requiert que la contrainte minimale in situ soit superieure à la pression maximale de l''eau sur les parois due puits, Les conditions topographiques complexes au site ont necessite le recours à un modèle topographique simplifie, Le puits propose est localise sur une oblique relativement à la pente de la vallee, Les calculs ont ete faits au moyen de trois modeles bi-dimensionnels que representent des sections verticales intersectant le puits à divers points critiques. INTRODUCTION Årdal og Sunndal Verk, one of the main producers of aluminium in Norway, is planning a new power project at Årdal in Western Norway. The main part of the project area is rugged mountain terrain, about 1,000–1,200 metres above sea level. Figure 1 shows how the Nyset River and Steggje River are regulated. Water from the lake Berdalsvatnet in the northern part of the area will be taken through a 12 kilometre long tunnel to the reservoir. The Steggje River and a tributary river are also taken into the tunnel on the way to the reservoir. In the reservoir area a 43 metre high earth fill dam with a bitumate membrane will be constructed. The maximum water level here will be 980.3 metres above sea level.

Proceedings Papers

Paper presented at the ISRM International Symposium, May 26–28, 1982

Paper Number: ISRM-IS-1982-080

... gradient and form an angle with it. Fluid Dynamics History water pressure excavation seepage force Upstream Oil & Gas calculation characteristic tensile stress

**stress****distribution**tunnel stress boundary condition lining increment flow in porous media seepage potential field water...
Abstract

SUMMARY: The influence of history of water load on tunnel stresses is discussed from the point that Water load on tunnel is seepage body force, corresponding calculation method is proposed and an engineering project designed by this method is given as an example. ZUSAMMENFASSUNG: Die Wasserbelastung eines Tunnels ist eine Feldkraft. Von diesem Standpunkt aus wird der Einfluss der Geschichte der wassrbelastung auf die Spannungen des Tunnels diskutiert. Eine entsprechende Berechnungsmethode wird vorgeschlagen. Eine nach dieser Methode konstruiertes Projekt wird als Beispiel angegeben. RESUME: La charge d''eau provenant du tunnel constitue la force volumetrique. Le texte discute, a partir de ce point de vue, l''histoire de la charge d''eau ainsi que son influence a l''egard de la pression du tunnel. On a pr''e sen t.e la methode de calcul correspondante et donne des exemples de travaux concus d''apres cette methode. INTRODUCTION The water load is generally considered as an inchangeable load acting on the boundaries of the lining in tunnel design. This simplification being unable to reflect the actual condition of water load on tunnel often leads to a conservative or unsafe design. Tunnel is a structure in rock mass with concrete lining as usual. Both rock mass and concrete are porous media. Water Percolating through these media forms a potenb1al field H(x 1 , x 2 , x 3 ) of seepage body forces, which act at every point of the field with three components: (Figure in full paper) From this point of view it implies that the process of tunnelling and lining will change the boundary conditions. Seepage potential field will be changed according to the change of boundary conditions even though the water head remains unchanged. Consequently the water load also varies in pace with various stages during tunnel construction so it has its own loading history. This is another important characteristic of water load acting on tunnel [1]. SEEPAGE FORCE AS THE WATER LOAD ON TUNNEL The permeability factor of intact rock is very small, being in order of 10 −7 - 10- 10 cm/s, but permeability factor of rock mass is considerably greater (about 103–106 times) than that of intact rock [2] [3]. This remarkable difference is caused by joints, cracks and fractures in rock mass with a width of 1 mm or sometimes greater than 10 mm. Fractures in rock mass connected with each other will form seepage passages for water. Fractures usually appear in groups. Each group of fractures has almost the same orientation, so the rock mass shows evidently the anisotropic permeability. As the permeability, of fractures is much greater than that of intact rock, we can approximately assume that the intact rock is impervious. Supposing the seepage flow in fracture is evenly distributed throughout the rock mass as a whole, we get a model of anisotropic homogeneous permeable medium. In general, the velocity vector at a point in space of anisotropic seepage potential field does not coincide with potential gradient and form an angle with it.

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-128

... the experimental resul ts of deformat ion velocity distribution around underground Openings of circular cross section. A se- ries of in situ test data were used to estimate the deformation velocity and

**stress****distribution**in rock sal t mass as a visco:elasto:plastic rock. The distribution od...
Abstract

In order to understand the earth pressure phenomena it is very important to investigate the distribution of stress and deformation around underground openings. Process of deformations, in general, is influenced by different mechanical properties of rock surroundings of the underground openings. Process of deformation Changes the stress conditions in rock massif. In this work, we want to analyze the experimental results of deformation velocity distribution around underground Openings of circular cross section. A series of in situ test data were used to estimate the deformation velocity and stress distribution in rock salt mass as a visco:elasto:plastic rock. The distribution od deformation velocity, presented with isotache linies, in different massif depths around circular openings are presented in the three different time periods; from 0 to 60 days,60 to 180 days and for 180 to 1000 days. An initial stage of stress around circular opening in a rock salt is also presented. some other results of investigations in the Tušanj rock salt mine, which are of interest in this cause, are presented in this paper, as well. Underground openings in competent rock, in general, sustain the elastic deformations on the frame in the moment of underground excavation. The further deformations on the opening frame and dip in massif, depend on rheological properties of surrounding rocks and on the state of strees around the openings. It is possible to predict by the laboratory investigation the elasto-plasic-viscous behaviour of rock if the stresses around the opening does not upset the 70 to 80%< of the rock strength. After a centain period of time, because of the rheological deformations of masif, the redistribution of stresses around the underground openings occurs, until the equilibrium state of the stress is not restituted. In the begining of the underground opening exising, radial stress from the zero values on the frame rise diper in masif, fig. 1. Tangential stress decrease from the maximum values on the frame, until both stresses become one value, H, see, fig. 1.a. The state of stress at any point of the massif depend on the ratio of undeground opening diametar, Ro, and the distance to the examined point, R. As the difference in state of stress in different points in masif occurs, radial deformations of different elementar volume of massif have different values. In general, time displacements of rock massif around unsupported underground opening in competent rock has direction to the central axis of the opening, fig. 1.g. In this work we want to present the migration of the area of deformation around the underground openings during a very long period of time. (Figure in full paper) Obtained results for Tušanj rock salt are presented in figure 2. Constant m becomes negative and is dependent on scale effect height to width ratio} on the samples. For height to width ratio, h:b=1, m=-14.6; h:b=Z, m=-8.6; h:b=3, m=-7.9; h:b=4,m=-7.9. It can be seen in the fig. 2 that strength of rock salt become greater as greater sample volume is, with constant scale effect (height to width ratio of the samples)

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-119

... Kuznetsov nonlinear deformative neighbourhood nonlinearity characteristic

**stress****distribution**mathematical model rock mass logic & formal reasoning Upstream Oil & Gas crack closure Collector property Poisson vo 0 permeability slonim 1979 media Proceedings of the International...
Abstract

The dependenoe of deformative properties and permeability on stress Conditions should be taken into account in a considerable number of problems in mining, underground engineering, liquid and gas extraction and repressuring. For this purpose, a mathematical representation of a nonlinear cracked permeable rock subjected to combined stress is proposed. This mathematical model is due to physical principles of cracked rock behavior (Brady 1970, Salganik 1973, Kuznetsov & Krigman 1978, Slonim 1979) and it is based on following assumptions: The anisotropy and nonlinearity of elastic properties are caused by cracks. The cracks are presented by fine ellipsoids of rotation (nearly penny-shaped cracks) interconnected by m1crocanals. The cracks may be closed Without slipping or opened in accordance with their size, pore pressure and the state of stress of the media. The interaction between cracks or identical order of size is negligible. The permeability is conditioned by cracks forming a filtrative pore space. The proposed model is completely described by following system of equations: (Equations in full paper) Here 6 ik ' ε ik ' u i are stresses, strains and displacements of media; ε 0 ik is a contribution of the elastic matrix into total strain of media; E o and ע o are Young's modulus and Poisson's ratio of the elastic matrix; p is density of media; X i are mass forces, r is radius of a crack, while 6° n is the normal stress closing the crack; n 1 , n 2 , n 3 are the coordinates of normal to crack (n 1 .sin8cosΨ, n 2 =sin8sinΨ,n 3 =cos8);Ψ,8 are spherical coordinates in an absolute system; P(r,Ψ,8t6 0 ) is density of distribution of sizes, directions and closing stresses of cracks. The active porosity is determined by relation α is relative gap of a crack (the ratio of gap to diameter). In elastic case, when the pore pressure is equal to p. When the cracks are distributed isotropicaly, formuli (8) characterize some orthotropic media with axes of orthotropic coinciding with main axes of the tensor of active stresses. The relations between the increments of stresses and strains in an orthotropic elastic media are known to be Therefore the effective diformative characteristics of regarded media can be calculated by comparison of right parts of expressions (8) and (10). The first term in (8) is defined by (4) and the second one is determined considering (7). It should be noted that the non-isotropical character of crack closure under non-homogeneous stress conditions may cause a considerable anisotropy of filtration. In the process of drainage accompanied by lowering of pore pressure, the load on the solid matrix of rook mass increases. a part of cracks is beeng closed and the values of HI and H2 decrease in accordance with (15). Formuli (4), (7), (8) with (14) show that in this case the cracked rock mass is homogeneous and isotropic when the depth is less then HI or more then H 2 . and it is vertically inhomogeneous and transversely isotropic within the range of depth from H 1 up to H 2 . The effective characteristics of the rock mass are determined by (4)((7)–(10) considering (14)–(15):

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-135

... excavation construction materials analysis model sequence plastic deformation Upstream Oil & Gas floor heave crosscut rock mass underhand cut fracture clayey ore timber stiffness matrix artifitial roof material stress analysis

**stress****distribution**deformation Proceedings of the...
Abstract

INTRODUCTION The 'Kuroko' is an unique complex ore consiting of Cu, Pb and Zn sulphide containing gold and silver. The surrounding rocks of Kuroko deposits are generally clayey, soft, and sticky and the underground rock pressure is 'quick and heavy'. Therefore, in the Kuroko mines the 'underhand cut & fill mining method with artifitial cemented roof' is usually adopted to attain the complete are extraction and to prevent the surface subsidence. Recently, new technique called the 'preburied cap timber technique' (Ohtsuki,1975) as been incorporated into the abovementioned mining method. This method is illustrated in Fig. 1, and outlined as follows. After a slice has been mined-out, wooden timbers are placed' about 1 meter apart parallel to a crosscut, 15% cement mortar is poured onto a floor about 60 cm thickness, and the rest of the mined space is filled by 3% cement mortar. When the underneath slice is mined, these pre-buried timbers are supported by post timbers and act as cap timbers. This method has successfully strengthened the artifitial roof and resulted in the cost reduction and labor saving. (Figure in full paper) On the other hand, miners working underground experience (Chonan,1973), that the damage of the supports is most severe during mining the top slice but it reduces considerably as the mining down to the lower slices. The purpose of this research investigation is to explain the various rock pressure phenomena and to clarify the effect of the filling in this method by an elasto-plastic stress analysis. MECHANICAL PROPERTIES OF CLAYEY ORE, ARTIFITIAL ROOF, AND FILLING MATERIAL Uni-axial compression, confined compression, and Brazilian tests were preformed on the clayey ore, artifitial roof material (consisting of tailing sand, volcanic ash, and 15% portland cement), and filling material (consisting of tailing sand, volcanic ash, and 3% portland cement). During the sample preparation and laboratory tests, sufficient care was taken to keep the water content of the samples as same as the field condition. The test results are tabulated in Table 1. (Table in full paper) ELASTO-PLASTIC STRESS ANALYSIS AND TREATMENT OF TENSILE FRACTURES In Fig. 2 the stress-strain curves of the clayey ore, artifitial roof material, and filling material obtained by uni-axial compression tests are shown. As is obvious from this figure, non-linear behaviour accompanied with 'strain softening' is remarkble; therefore, it is considered necessary to develop an elasto-plastic stress analysis method in order to explain the various phenomena in these soft and weak rock mass. For this purpose, firstly, the constitutive equation suitable to these soft and weak rock materials has been derived from the mathematical theory of plasticity based on the 'extended von Mises' yield criterion' and the 'associated flow rule'. Secondly, a new computational procedure using FEM has been developed. An iterative method ('stress transfer' method) has been incorporated for solving the material non-linear problems such as the elasto-plasticity including the strain softening characteristics (Yamatomi, 1979). In this FEM program, 'tensile fracturing' is also considered. If the maximum principal stress (σ 1 ) exceeds the tensile strength of an element,

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-120

... joint

**stress****distributions**are illustrated in Fig. 9. An arrow means a resultant stress acting on the lower or the left-hand side rock. Using DEM, Voegele has pointed out that the ground arch forms along the line 6–7-8–9-10–11–1213–14, 2) the roof arch forms along the line 2–3-4–5, 3) rock I moves...
Abstract

INTRODUCTION Numerical methods such as FEM based on continuum mechanics do not seem to be able to analyze the behavior of discontinuous rock masses adequately nor effectively. Weak regions such as joints and faults dominate their behavioral features. Recently some attempts have been made to solve this problem. TIWY have been successfully applied to various kinds of practical problems. However, they have some weak points at the same time. This paper proposes a new effective modeling technique based on the assumption that a discontinuous rock mass consists of rigid blocks and joints. RIGID-BODY JOINT-ELEMENT METHOD (RIM) Four current methods for analyzing the behavior of discontinua are summarized up in Table I. Fig. 1 shows examples of current methods in modeling a voussoir arch abutment. A new modeling technique has been developed to avoid the weak points and to incorporate the strong points of the current methods described in Table I (Asai 1981). Fig. 2 shows an example in this technique named rigid-body jointelement method (RIM). The method based on the combination of Goodman's joint element and Cundall's distinct rigid element. This hybridization brings us more strong points than either of the two methods. Their weak points have vanished. The features of this method are summarized in Table 2. As Shown in Fig. 2, the three kinds of elements (URJS, STRE, RBE) represent the whole system with arbitrarily shaped rocks by superposing the element stiffness matrices in just the same way as FEM. Fig. 3 shows the concept of URJS. This unit has two block Points. Each block point is located at the vertex opposite to each other. A block point has 3 degrees of freedom in a two-dimensional field. According to the notation shown in Fig. 3. the equilibrium equation for the two triangular rigid elements is: (Equation in full paper) APPUCATIONS A computer simulation model is illustrated in Fig. 8. The model has almost the same geometry as Voegele's (Voegele 1978). The model in Fig. 8 has the properties: density 2.6 t/m 3 , joint thickness: 0.02 m, joint stiffness: ks= 100 MN/m 3 , k n = 200 MN/m 3 . We applied RIM to this model in two cases. The simulations were carried out within the limitations: the material is linear and elastic, the displacements are small enough, the gravity IS turned on after excavation. Case 1(gravity and excavation).- The joint stress distributions are illustrated in Fig. 9. An arrow means a resultant stress acting on the lower or the left-hand side rock. Using DEM, Voegele has pointed out that the ground arch forms along the line 6–7-8–9-10–11–1213–14, 2) the roof arch forms along the line 2–3-4–5, 3) rock I moves into the excavation and then the joint opens at A. Fig. 9 leads to the following: I) compressive joint stresses are produced along the line 6–7-8–9-10- 11–12–13–14 and the line 2-C-4–5 instead of 2–3-4–5, but the two lines are not clearly distingushed from each other,

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-130

...Proceedings of the International Symposium on Weak Rock / Tokyo /21-24 September 1981 Effects of existing tunnel on the

**stress****distribution**of new tunnel lining T.lTo & M.HISATAKE Osaka University, Japan 1 INTRODUCTION When a new tunnel is constructed near an eXisting tunnel in weak rocks which...
Abstract

INTRODUCTION When a new tunnel is constructed near an existing tunnel in weak rocks which have time dependent characteristics, earth pressures on both tunnel linings increase With time and are different from those on a Single tunnel lining. Then, in order not to damage the both linings, safety distance between the tunnels should be known in advance. The authors made clear the effects of new tunnel construction on the stresses of the existing tunnel lining (Ito & Hisatake 1981a). But, a method to estimate earth pressure on the new tunnel lining has not been established, because it is difficult in analysis to take into account such facts that the earth pressure on the new lining is affected by time dependency of rocks, construction sequences of the both tunnels and earth pressure on the existing lining before the excavation of the new tunnel. In this paper, the earth pressure and Stresses of the new tunnel lining constructed near and parallel to the existing tunnel in the viscoelastic ground are analyzed by Integral Equation Method considering the above mentioned facts. Results obtained by this analysis are compared with those of model tests, and the stresses in the new lining and the safety distance are shown. ANALYSIS OF EARTH PRESSURE Stresses released on new tunnel boundary When the new tunnel T 2 is excavated after the construction of the existing lining L 1 , Stress vector P 2 on new tunnel boundary S 2 , which will be released at the new tunnel excavation, is affected both by the excavation of the existing tunnel T 1 and by earth pressure P 1 acting on the lining LI (Fig.l). Then P 2 has to be determined by considering construction sequences of the tunnel T 1 ' If the lining L 1 ' which is.assumed not to be deformed by earth pressures, is constructed at the time to after the excavation of the tunnel T 1 , PI increases with time and its component P l jis expressed by the following equations (Ito & Hisatake 198Ib), (Equation in full paper) where t i is the time after the construction of the lining LI,Φ(t i )is a creep function concerning shear deformation, P l jis the component of the stress vector released on SI at the excavation of the tunnel T 1 , and Poisson's ratio V of the ground is treated to be independent of time. And * indicates the following Laplace transform, and L − 1 shows Laplace inversion. Then, if stress change Δσjkon the assumed tunnel boundary S 2 caused by the excavation of the tunnel T 1 can be calculated, the component of the stress vector P 2 jon S 2 after construction of the lining LI, which is released at the excavation of the tunnel T 2 ,is determined by the following equation considering equation (1), (Equation in full paper) (Figure in full paper) Method of experiments First, a circular tunnel with diameter l5cm is excavated in the model ground of consolidated clay which is placed in the experimental device (80cm×90cm×30cm) and on the ground vertical pressure l6kN/m' less than consolidated pressure is applied in advance.

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-111

... Reservoir Characterization discrete analysis Takeuchi Deformation Characteristic jointed rock media lateral sl ip excavation settlement stress transfer mechanism consideration

**stress****distribution**fissured foundation photoelastic experiment experiment transfer mechanism...
Abstract

INTRODUCTION Generally speaking, actual foundation is inhomogeneous in mechanical properties and discontinuous in structure, and therefore it is desperately needed to make clear the characteristics of jointed rock media having such discontinuity. A great number of studies have been made on the jointed rock media, which include the studies of the shearing strength or deformation characteristics, the biaxial or triaxial tests on jointed rock, and the studies of the stress transfer mechanism of jointed rock media. The analysis of such jointed rock media is, however, still incomplete and these studies are not beyond qualitative discussion. The method of numerical analysis for the foundation having the discontinuous properties are generally classified into the following two groups. The finite element method with the use of joint elements. Discrete method where rigid elements are considered. The Goodman, Tayler εBrekke's method of the joint elements (1968) is only useful when the discontinuous surface is already known, and it is not possible to study the slip failure of foundation structures generally. Kawamoto & Takeda(l978) proposed a similar method where crack elements are introduced. Their method, however, can be deduced from the Goodman et al 's jointed element method. In contrast to these method, Cundall (1971) developed a method in which the foundation is idealized as an aggregate of completely discrete rigid elements, and he showed effectiveness in the analysis of a toppling failure of rocks. But the application is 1imited to analysisof a very shallow part of foundations. Independently in 1976 Kawai proposed a family of new discrete models which are called as the Rigid Body Spring Model (abbreviated as RBSM) by basing on the experimental evidance of sol ids under the ultimate state of loading. These models consists of rigid bodies and two types of connection springs, one of which resists the dilational motion, while the other the shear deformation. He conducted verification studies of his models by solving a series of collapse problems of the beam, plate, shell and three dimensional structures. They were proved to be very effective for the analysis of the materials having the yield criterion of Mohr-Coulomb's type because only the normal and shearing stresses are necessary to consider on the element boundary. Takeuchi (1980) attempted application of the RBSM models to no-tension analysis of the granular media which was originally proposed by Zienkiewicz (1968) and later by adding a scheme for checking on the contact of adjoining elements, he modified his program of the no-tension analysis and he called it the method of tension-crack analysis. Combined use of this method with the conventional elasto-plastic analysis, however, has not been attempted yet. In this paper, therefore, considering the effect of re-contact as well as separation of individual rock elements, the elasto-plastic analysis was carried out by basing on the intial stress method by Zienkiewicz, Valliappan & King (1969).

Proceedings Papers

Paper presented at the ISRM International Symposium, September 21–24, 1981

Paper Number: ISRM-IS-1981-171

... dowel effect rock bolt plane

**stress****distribution**Jointy Rock Upstream Oil & Gas tight hole model experiment safety factor prestress Po displacement rock foundation dependency loose hole strength experiment mild steel brass piano wire INTRODUCTION Rock bolts have...
Abstract

INTRODUCTION Rock bolts have been used widely for reinforcing relaxed zones around tunnels or caverns, and proved to be very effective. many researchers have carried out experimental or theoretical studies on rock bolting (Yamamoto 1978, Bjurstrom 1974 and others), and its mechanism of reinforcing has been becoming disclosed gradually. This paper describes some results obtained by model experiments concerning rock bolting in jointy rocks. In the experiments, special attention was paid to bonded or unbonded rock bolting, as well as the amount of prestress, setting angle and kinds of materials, etc. MODEL EXPERIMENTS ON ROCK BOLTING IN JOINTY ROCKS Outline of experiments A test piece simulating a rock joint consist of a pair of concrete blocks BLI and BL2 (Fig. 1). The size of a block is 7.5 cm thick and a contact plane AB (7.5 cm × 7.5 cm) of the two blocks is considered to be a rock joint in experiments. Each block has a bolt-hole of 2 mm (tight hole) or 40 mm (loose hole) in diameter. Rock bolts are made of piano wire, mild steel or brass, and the diameter of them is 2 mm. In case of a tight hole, there exists no allowance between a bolt-hole and a rock bolt, the condition of which would be a similar one of a bonded type bolt. In the other case of a loose hole, an emphasized condition of an unbonded type bolt would be produced. The procedure of an experiment is as follows. After giving prestress p o to a rock bolt by a screw jack, a load is applied by a universal testing machine, which causes a sliding load T on a rock joint AB. Increasing the load T gradually, the blocks BLI and BL2 move relatively in opposite direction each other and yield sliding displacement δ. The tensile force P of a rock bolt becomes larger and larger due to a sliding displacement and finally a break off of a rock bolt occurs. Forces T and P are measured with load cells and δ with a differential transducer. Recording of these values are made by a 2 pen-type X-Y plotter. Cases of experiments A friction resistance of a rock joint AB was measured first, and the value of 39 degrees of friction angle ф was obtained. The uniaxial compressive strength of blocks was 544 kgf/cm 2 . ExperLmer.r.swere carried out for 6 cases of setting angle θ (see Fig. 1) of a rock bolt (0, 5, 10, 15, 20 and 30 degrees), and for 2 cases of prestress P o of a rock bolt (40 and 60 % of a tensile strength of a rock bolt material). Three kinds of materials of a rock bolt were used, i.e., piano wire, mild steel and brass, and the tensile strengths P t of them were 589 kgf, 243 kgf and 165 kgf respectively. A diameter of a bolt-hole was 2 mm or 40 mm as mentioned before. (Figure in full paper)