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Go ISRM-ARMS2-2001-119

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

Based on the field observation of the abutment pressure, the cracking process of coal mass is stimulated on the real triaxial compress machine, and the whole stress-strain curve is obtained. The theory of fractal dimension and damage mechanics is applied to describe the evolution of fissure system quantificationally, and the relation between the compression strength and the fractal dimension is obtained. The results have guidance meaning towards engineering, and new progress of cracking mechanism of fissured coal mass is made.

The typical stress-strain curve of the coal sample is shown in Fig, I, which consists of four parts. In the concave part of AB, the strain rate is higher than the stress rate, for the protogenic fissures in the coal are compacted. The approximately straight-line part of BC indicates the elastic deformation of coal sample. In the stress-hardening stage of CD, the strain rate increases quickly, new fissures grow, extend and link up, and the volume of sample expanses. D is the strength limitation.

ISRM-ARMS2-2001-048

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.35)

Scale effect is a well known concept in rock mechanics. The impact of scale on rock strength has been the subject of a considerable volume of theoretical work (e.g. Bazant et al, 1991; Kooijiman et al, 1991; Salem et al, 1989; Ringstad et al, 1993; Papamichos & van de Hoek, 1995). Most of these approaches predict same form of quasi logarithmic drop-off in strength with increasing characteristic volume. In the context of hollow cylinder tests, this can be visualised as a drop-off in strength with increasing internal hole diameter, under lest conditions where failure is induced within a limited volume in immediate proximity to the hole wall. For the test configurations summarized in Figure 1, tensile failure at the hole wall (for the splitting test) or tensile/shear failure (for the collapse tests) in proximity to the hole wall fall into this category. The experimental programs outlined below were undertaken to study the nature of scale effect for both types of tests, with a view to providing a basis for extrapolation of laboratory data to field scale. In each case the experimental data is matched to a theoretical prediction, as well as being defined by a statistical line of best fit, as a basis for extrapolation.

An experimental program to investigate the nature of scale effect during splitting tests was undertaken on samples that were loaded axially (to inhibit the formation of transverse fractures during internal pressurisation) and pressurised internally. Pressurisation to failure was undertaken with water at a constant pressurisation rate (3.5Mpa/min). In each case an unstable fracture was formed at failure, with instantaneous pressure drop, making it easy to identify a fracture initiation pressure. Two different sample geometries were employed, one with

ISRM-ARMS2-2001-045

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

collapse, collapse test, diameter, effect, extrapolation, failure, field, geometry, hole, hollow cylinder, hydraulic fracturing, laboratory, Reservoir Characterization, reservoir description and dynamics, reservoir geomechanics, rock, sample, sample geometry, sandstone, scale, scale effect, strength, tensile, test, Upstream Oil & Gas, well completion, Wellbore Design, wellbore integrity

SPE Disciplines:

The Strength of rock masses can be estimated by several methods. such as: back calculation of well documented case histories, numerical analysis of jointed rock masses and laboratory tests on models of jointed rock masses. It is difficult to obtain a large assortment of well-documented case histories. Numerical methods can be used to analyze several cases at low cost, but there is a need to evaluate the capability of these methods to simulate the real behavior of jointed rock masses. Numerical methods can be checked against well-documented case histories and against model tests. Model tests, although more time consuming than numerical tests, may shed some light on new failure modes, not yet disclosed by numerical codes. A research program was funded by FONDECYT (The Chilean Foundation for Science and Technology) in order to evaluate the strength of large-scale rock mass models with discontinuous joints. This paper describes the design of the laboratory facility that will be used to carry out the model tests.

I. 1 Basic requirements. Although it would be ideal to perform triaxial tests, the complexity of the loading system is such that it becomes more practical to perform biaxial tests. The sample has to be thin in order to minimize the influence of stresses perpendicular to the loading plane, approaching a situation of plane stresses. On the other hand, the sample must have a certain minimum thickness in order to prevent buckling. The next question is how to load the sample. The following considerations apply:

• The loading system should apply a uniform stress on the external races or the sample

•The loads should be perpendicular to the face, with no shear stresses at the contact. Contact shear stresses can result in a strength increase of up to 20%.

•The loading system must follow the deformations of the sample, it should allow the free deformation of the loading boundary.

•The loading system should be as stiff as possible, in order to observe the post peak behavior. During the design stage, an effort was made to satisfy the first three restrictions.

ISRM-ARMS2-2001-053

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

analysis, design, frame, friction, hydraulic jack, loading, loading frame, loading plate, loading system, management and information, model, mould, plate, Reservoir Characterization, reservoir description and dynamics, sample, steel, strength, stress, surface, system, test, testing, Upstream Oil & Gas

SPE Disciplines:

In-situ stress is internal stress existing in the underground rocks. During the development of oil and gas fields, the reservoir consolidation and expansion resulting from the in-situ stress will lead to the changing of the porosity and percolation. In-situ stress is an important factor to the design of the flooding networks. At the same time, study of in-situ stress play important roles in many aspects, such as the hydraulic fracturing design, the extension laws of hydraulic fractures, the sand production, and the formation slide creep resulting from earthquakes caused by water injections. During drillings, the measurement of in-situ stresses also play important roles in some problems, such as well bore stability, formation pressure gradient, the control for directional drilling trace, and casing distortion in the rheological formation. As a method to analyze the rock stress states, the acoustic emission measurement is based on Kaiser effect, which is an abnormal phenomenon appearing when the material is repeatedly loaded. The Kaiser effect shows that the rock memorizes its stress history. Firstly, when cores from the fields are compressed, acoustic emission signals will be received at the same time. When the loads on the samples are small, the acoustic emission frequencies will be low. However, once the loads attain some value, the acoustic emission will increase abruptly. The load value at that moment is equal to the maximal normal stress of the samples. Usually a vertical core and three horizontal cores are required for rests since rocks are in a three dimensional stress state. Three principal in-situ stresses will be given by experimental results. However, this is not the case for the inclined cores, since the coordinate system of the well axis does not coincide with the coordinate system of the in-situ stress principle direction any longer. As a result, a new method is needed to solve the problem. The method proposed in this paper has been used to determine the in-situ stresses at great depth by acoustic emission technique on the cores of an inclined well. The method is realized by using coordinate system transformations and solving nonlinear equations.

ISRM-ARMS2-2001-051

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

acoustic emission, acoustic emission technique, axis, coordinate, coordinate system, coordinate system transformation, core, Drilling, field, in-situ stress, in-situ stress determination, Kaiser effect, method, Reservoir Characterization, reservoir description and dynamics, reservoir geomechanics, rock, transformation, Upstream Oil & Gas, well

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)

ISRM-ARMS2-2001-071

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

analysis, application, Artificial Intelligence, boundary, Clement, complex reservoir, equation, fractured rock, fractured rock mass, head, hydraulic fracturing, infiltration, model, permeability, program, reservoir description and dynamics, seepage, Simulation, surface infiltration, Upstream Oil & Gas, water, well completion

SPE Disciplines:

When large openings are created in rock masses, their stability is typically evaluated by comparing the induced stress state to a properly formulated failure criterion. The parameters included in the criterion are usually determined from both laboratory properties and in situ rock mass characteristics, combined to define the large scale material behavior. In this paper, the authors use a recently developed multiaxial (3D) failure criterion with a new scale effect function to analyze the stability of two underground excavations in hard rocks. To do so, a simple numerical code based on an elastic-brittle (plasticity) type model is used in combination with the proposed criterion and scale effect function. Comparison of the calculated results and field observations shows a fairly good agreement between predicted and actual responses.

ISRM-ARMS2-2001-055

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

analysis, Aubertin, continuity, criterion, equation, failure, Lac du Bonnet granite, Martin, material, MPa, multiaxial, multiaxial failure criterion, Reservoir Characterization, reservoir description and dynamics, reservoir geomechanics, rock, rock mass, stability analysis, strength, stress, structural geology, Upstream Oil & Gas

Country:

- North America > Canada (0.93)
- North America > United States (0.69)

Many instabilities of underground cavernsare ascribed to influence of faults located nearby. Brekke, and Selmer-olsen (1966) concluded in a survey of factors causing failures of underground excavations in Norway, that faults are often a major cause of failures. Mikula (1993) also made the same conclusion after investigating the forms of failures in open stope mining in Australia. Especially in the recent years, many reports on fault-induced problems regarding the stability of underground caverns have been published. It is obvious, that the fault-related instabilities of under- ground excavations have become a crucial issue that engineers and researchers are facing in the current time. However, owing to the fault existing, the rock mass behavior is rather discontinuous, non-linear and non-elastic. It is almost impossible to derive a theoretical solution to survey the stability of underground caverns considering the Faults. The numerical simulated method is thus the only effective way to investigate the effects of faults on the underground cavern stability. In order to establish an economical and reliable support system, a better understanding of the fault-induced behavior of underground excavations regarding the stability is required . Based on this considerations, the main objective of this work aims to insight into the effects of faults on the stress and displacement redistribution around underground openings. In order to investigate into the influence of faults, a new “interface” element of modeling the fault and a two- dimensional explicit finite difference program (FLAC2D) were employed. Furthermore, the attention was focused on assessing the effects of the mechanical properties of the faults on the stability of the excavations considering the location and the dip ofthe fault. The results obtained can offer a preliminarily guidance to the support design.

ISRM-ARMS2-2001-118

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

analysis, Clement, condition, displacement, effect, excavation, fault, fault effect, Influence, interface, model, point, Reservoir Characterization, reservoir description and dynamics, shear, shear displacement, stability, stiffness, stress, stress distribution, tensile, Upstream Oil & Gas, Wellbore Design, wellbore integrity

SPE Disciplines:

The discrete element code, UDEC, is specially designed to model jointed rock mass and has been widely used to simulate tunneling, Deep excavations and rock slopes (Cundall 1980, Lemos 1987, Barton et al. 1994). A UDEC model is an assemblage of blocks and contacts, respectively, representing rock material and rock joints. The blocks are further subdivided into triangular finite difference elements to present the deformability of rock material. The rock joints may deform and fail and thus the UDEC provides A way to model possible separation of rock material blocks due to joint failure. Different from other continuity- based numerical methods, more attention should be paid in UDEC modeling to obtain reliable results. E.g., in situ stress is usually not uniform due to the existence of rock joints. Applying a uniform in situ stress to the model does not reach force equilibrium and a consolidation stage is required to avoid extra displacement being transferred to the tunnel excavation stage. To include major joints as more as possible in the model, the model size cannot be so large as other numerical methods do and the hybrid DEM/BEM scheme would be a very practical alternative to limit the model size rather than using traditional fixing condition at far field boundary. Since the UDEC is a 2-D oriented program, 3-D space cannot be physically involved but working face effect must be taken into account when applying shortcrete and rockbolts. This paper is to investigate the UDEC modeling of tunnel excavations and supports. Main considerations in the UDEC modeling are examined including setting of computational model, hybrid DEM/BEM scheme, tunnel excavation and supports, and judgement of tunnel stability.

ISRM-ARMS2-2001-076

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

SPE Disciplines:

Hehua, Zhu (Department of Geotechnical Engineering of Tongji University) | Jiangbin, Wu (Department of Geotechnical Engineering of Tongji University) | Maoyu, Cui (Department of Geotechnical Engineering of Tongji University) | Bin, Ye (Department of Geotechnical Engineering of Tongji University)

Construction is the most important phase for the stability of surrounding rock of tunnel. Therefore, researchers are interested in the influences of different approaches of tunnel construction on the stability of surrounding rock during excavation. The task of tunnel construction analysis is to studied the influences of construction procedures construction techniques and construction rate on internal force of support or lining. The first one who studied the mechanics of tunnel construction is Pro. Wittke( 1976). Liu Huaihuan( 1983) suggested the influences of tunnel construction analyzed using stress releasing coefficient in planar strain analysis. The analysis of structural stress during construction is also used to find the best procedures of construction or large underground structures. In order to obtain the stress of a certain section in a single tunnel, theoretically, the stress increment of every step of excavation should be accumulated from the beginning of excavation to the end of construction. All the time researchers have been trying to find out a simple and fine way to analyze the influences of construction. At first, people considered it as planar problem and by using releasing coefficient of radial displacement to take account of the change of “Virtual supporting force" in radial direction (Yu Xuefu et al. 1983, Zhang Wugong et al. 1990, Pant 1982a.b). Later the method using two and a half dimensional method to analyze three dimensional problem was developed, but it was still refined by initial assumptions. Someone used "step-by-step forward analysis" and "coupled FEM~BEM" to simulate the state of the surrounding rock around advance face and these kinds of methods can be well employed in the case that the axial direction of tunnel disagrees with the direction of principal earth stress (Sun Jun & Zhu Hehua 1993, Ohnishi et al, 1982, Swobooda et al. 1986). However the "air-element" employed in this method cannot simulate the excavation steps of non-linear problem. Some people did much research on the "space lime effect" during excavating by the use of "Generalized virtual supporting force method” and boundary element method (Zhu Hehua & Yang Linde 1994, Zhu Hehua 1991, Zhu Hehua et al. 1991) . At present, there are few studies on three-dimensional simulation of the whole process of tunnel construction. Moussa& Wager (1997) studied the effects of construction speed on the mechanical behaviors of NATM tunnel by the use of three- dimensional FEM. They took account of the construction factors which can be reference to other three-dimensional analysis of tunnel construction.

Shotcrete and rockbolts are broadly used in NATM tunnels as supports.

ISRM-ARMS2-2001-120

ISRM International Symposium - 2nd Asian Rock Mechanics Symposium

Thank you!