The three dimensional elasto-viscoplastic composite element method is formulated in this paper for rock masses reinforced by a fully-grouted bolt. If a bolt segment penetrates a finite element representing the rock mass, then a composite element is formed including five sub-elements corresponding to the rock material, the grout material, the bolt material, the rock-grout interface and the bolt-grout interface. The displacements in each sub-element are interpolated from the corresponding nodal displacements of the composite element. By the virtual work principle the governing equation for the solution of the nodal displacements can be formulated. The elasto-viscoplastic characteristics of the materials are considered in the formulation. The new model can be incorporated into the conventional finite element analysis grid, in which several composite elements have fully grouted bolts embedded. In this way the mesh generation of large scale bolted rock structures becomes convenient and feasible. The model has been implemented in a FEM program, and a comparative study between the numerical analysis and a pull out field test has been carried out, from which the validity and the robustness of the new model are justified.

Extensive efforts have been made to gain a better understanding of the failure behaviour of rocks and rock-like materials, but crack propagation and failure processes under compressive-shear loading have not yet been comprehensively investigated. To address this area of research, the peak shear strengths (τ) and failure processes of specimens with multiple joints are studied by lab testing and particle flow code (PFC2D). Four types of failure modes are observed: (a) shear failure through a plane (Mode-I), (b) intact shear failure (Mode-II), (c) oblique shear crack connection failure (Mode-III), and (d) stepped path failure (Mode-IV). The failure mode gradually transformed to Mode-III as α (joint inclination angle) increases from 0° to 90° in the specimens. In addition, with increasing joint distance (d) in the specimens, the failure mode changes to Mode-II. As the non-overlapping length between joints (c) in the specimens increases, the failure mode changes to Mode-IV. The joint geometry has a major influence on the shear strength of the jointed specimens. The peak shear strength of specimens with different joint inclination angles is obtained when α = 45°. Additionally, the peak shear strength increases as the joint distance (d) and non-overlapping length (c) increase.

This paper presents the use of the trial load method and the block element method with elastoviscoplastic discontinuities for analysis of arch dams. The arch dam is considered as an arch-cantilever system and the foundation as a block element system. With the displacement compatibility condition at the contact surface of the dam and the foundation (including abutment), the governing equations of the arch dam and foundation are established. These methods are used for the analysis of the double curvature arch dam with complex geology conditions of the Xiaowan Hydroelectric Project in China. The deformation and stress states in both the dam body and the foundation are determined. Furthermore, the stability safety factors of the foundation and the abutment are calculated at the same time, which allows for an optimal design of the arch dam considering the strength, the deformation and the stability of the dam and foundation.

A new constitutive criterion, relating stress and displacements, is proposed to model the shear resistance of joints under constant normal load conditions. It is based on an empirical description of the surface, and on the results from more than 50 constant-normal-load direct-shear tests performed on replicas of tensile joints and on induced tensile fractures for seven rock types. This constitutive model is able to describe experimental shear tests conducted in the laboratory. Moreover, the parameters required in the model can be easily measured through standard laboratory tests. The proposed criterion was also used to estimate the joint roughness coefficient (JRC) value. The predicting values were successfully correlated with JRC values obtained by back analysis of shear tests.

The choice of a general criterion to determine the shear strength of rough rock joints is a topic that has been investigated for many years. The major problem is how to measure and then to express the roughness with a number (e.g., joint roughness coefficient) or a mathematical expression in order to introduce the morphology of the joint into a shear strength criterion. In the present research a large number of surfaces have been digitised and reconstructed using a triangulation algorithm. This approach results in a discretisation of the joint surface into a finite number of triangles, whose geometric orientations have been calculated. Furthermore, during shear tests it was observed that the common characteristic among all the contact areas is that they are located in the steepest zones facing the shear direction. Based on this observations and using the triangulated surface data, it is possible to describe the variation of the potential contact area versus the apparent dip angle with the expression Aθ*=A0[(θmax*−θ*)/θmax*]C, where A0 is the maximum possible contact area, θmax* is the maximum apparent dip angle in the shear direction, and C is a “roughness” parameter, calculated using a best-fit regression function, which characterises the distribution of the apparent dip angles over the surface. The close agreement between analytical curves and measured data therefore suggests the possibility of defining the influence of roughness on shear strength by the simple knowledge of A0, C and θmax*. Based on the samples studied here, the values of these parameters capture the evolution of the surface during shearing. Moreover, they tend to be characteristic for specific rock types, indicating that it might be possible to determine ranges for each rock type based on laboratory measurements on representative samples.

Note: Roches Reference LMR-CONF-2002-006 Record created on 2006-11-09, modified on 2016-08-08

This paper presents an elasto-viscoplastic block element method and its application to the deformation and stability study of arch dam abutment slopes. The paper is composed of two parts. The first part concerns the numerical methods used in the analysis, which includes the identification of the rock blocky system. the algorithm of unconfined seepage flow in discontinuity network taking the grout curtain and drainage curtain into account, and the elasto-viscoplastic block element method as well. In the second part a complicated arch dam abutment slope is studied, from which the seepage flow, the deformation, and the safety factor of the abutment slope are obtained. Based on the analysis suggestions about the seepage control and stabilization measures are made.

In this paper a methodology of the feedback analysis on the mechanical parameters and stability of the slope excavated in rock masses is presented. The method is based on the combination of the elasto-viscoplastic finite element method and the complex shape minimum method. Special attention has been paid to the simulation of the variation of parameters because of the blasting damage and the stress relief relaxation, and the de-coupling of parameters as well. The slope excavated in the granite rock masses situated at the ship lock of the Three Gorges Project is studied, from which the mechanical parameters of the rock masses and the deformation, as well as the stability of the rock slope are forecasted in time as the going on of the slope excavation.

In Switzerland, there is concern that sliding along joints under dams could lead to stability problems. As part of a research project funded by the Swiss Federal Office for Water and Geology, more than fifty constant-normal-load direct-shear tests have been performed on induced tensile fractures for seven rock types. Damage zones are evident on all of the sheared surfaces. There is evidence of both crushing and breaking of surface asperities. Damage is relatively sparse, and the location of the damaged zones is strongly related to geometrical features. However, the relationships between surface roughness, stress distribution, and damage are complicated and difficult to study, in part, because the boundary conditions governing the mechanical behavior change continuously during shearing. One of the primary objectives of this work is to better understand the micromechanical behavior of joints under shear loads, including the creation of damage zones. This requires understanding the relationships between material properties, surface geometry, contact area, stress distribution, and the creation of damage during shearing. A methodology for predicting damage during shearing has been developed based on analysis of maps of the joint surfaces obtained before and after shearing using a three-dimensional optical system. The surface data is analyzed to identify the areas on the joint surfaces most likely to be in contact during shearing; i.e. areas with positive slope with respect to the shear direction. Local gradients are also taken into account in predicting those areas of the joint surfaces most likely to be damaged during shearing. The damage predicted is compared to the damage mapped on laboratory test specimens.

Note: Roches Reference LMR-CONF-2000-008 Record created on 2006-11-09, modified on 2016-08-08

A series of railway tunnels will be dug through the European Alps at depths exceeding 2000 m over long stretches. The prevailing high stresses are expected to cause rock burst, large deformations and creep, depending on the nature of the rock. In the first part of the paper the theoretical relations between support pressure and tunnel wall displacements are briefly discussed. Emphasis is given to the post-failure behaviour of the rock mass and its influences on the tunnelling conditions. Frequently encountered strain softening rock types are shown to exhibit a strong dependence of the tunnel stability on the softening rate, which itself varies with the applied confinement pressure. Also discussed are results of laboratory tests and field observations which yield a critical deformation value for a given rock type. Beyond this value, the required support pressures are shown to increase sharply. Systematic monitoring of the rock deformations due to tunnelling can help to define the most adequate support measures and to improve the input values for static calculations by back analyses. Deep tunnels require support types able to control the tunnel wall displacements efficiently. They should oppose significant support pressures from incipient deformations up to large displacements. Various constructive solutions are discussed for-drill & blast as well as for TBM excavation. The last section briefly addresses time-dependent tunnel deformation and their influence on the time of placing the final lining.

In this paper are presented the studies using two different approaches on the behavior of a tunnel face reinforced by fiber-glass inclusions. On one hand, the face stability conditions are investigated using a three-dimensional reduced-scale physical model. The preliminary tests performed show the positive effect of sub-horizontal bolts on the tunnel face stability. On the other hand, an analytical model using homogenization approach combined with the hypothesis of spherical symmetry is developed which helps to better understand the phenomena involved and allows quick first-estimates of displacements and assessment of the failure zone. This simplified approach leads to a simple and efficient preliminary design method, very useful for tunnel designers.

Rock bolting is the most effective and also the most economical means of supporting excavations in rock. Various types of bolts, both tensioned and untensioned, are used today and an understanding of the way in which these bolts work is essential for an optimal, safe and economical use. The experience accumulated on this subject gives the know-how for the bolt reinforcement calculation and execution, but it does not explain the mechanical behaviour of the bolted rock joint. The object of this paper is to analyse the results obtained from large scale (1:1) laboratory tests of bolt-reinforced rock with fully grouted rods and hollow tubes. The experimental data are compared with 3D finite element simulations, looking for the global behaviour of jointed rock, reinforced by two types of bolts.

The construction of shallow tunnels frequently causes settlement problems. In addition, the stability of the tunnel face and wall may become critical when the strength of the ground is low with respect to the natural stresses. Ground inclusions have been used with success to improve the tunnelling conditions. For a long time, rock bolts have been installed around the tunnel wall. More recently, various types of inclusions such as glass fibre reinforced resin (GFR) bolts or jet piles, and canopies of pre-cut concrete elements or pipe roofs have been placed in or around the face respectively. Practical and theoretical aspects of the various types of ground inclusions are discussed in this paper.

In this paper a distinct bolt element model is proposed based on elasto-viscoplastic theory. The bolt and grout are looked as a kind of equivalent material whose constitutive equation is formulated on a rheological model, and the interface between bolt-grout material and rock is considered.

The increasing use of TBMs requires improved ground investigation. Pilot galleries can be used for a large scale ground investigation and have other beneficial effects for the main tunnel excavation. Examples of successful pilot galleries and of setbacks are given. The stability of the tunnel face influences strongly the selection of an adequate tunnelling method. The assessment of the stability at the tunnel face and behind and, when required, its improvement are discussed.