M.H. Leite

Determination of unconfined compressive strength and Young's modulus of porous materials by indentation tests

The formation of a compacted zone under the indenter seems to be the major factor controlling the indentation process in porous rocks. In the case of very porous materials, where the pore structure fails and deformation (by structural collapse) proceeds with almost no increase in the applied load and with very limited damage to the surrounding material, no chipping is observed. The extent of the compacted zone is controlled by the porosity of the material and by the strength of its porous structure. This paper presents an interpretation model developed by the authors to obtain the uniaxial compressive strength of porous materials from the results of indentation tests. It is based on the model proposed by Wilson et al. (Int. J. Mech. Sci., 17, 1975, 457) for the interpretation of indentation tests on compressible foams and on an estimation by the authors of the extent of the compacted zone under the indenter. The results of indentation tests can also be used to obtain the Youngs modulus of the material with a model proposed by Gill et al. (Proceedings of the 13th Canadian Symposium on Rock Mechanics, 1980, 1103). Uniaxial compression and indentation tests have been performed on artificial porous materials showing porosities varying between 44 and 68%. The uniaxial compressive strength values obtained from both types of test show a very good agreement. For the Youngs modulus, the values obtained from the two types of test are different but the variation of the moduli with porosity is the same. Finally, a parameter called permanent penetration modulus is proposed as a means of characterizing the uniaxial compressive strength of porous materials.

An integrated approach to rock stress measurement in anisotropic non-linear elastic rock

Abstract A mathematical model that allows the simultaneous introduction of non-linear elasticity and transverse isotropy for the interpretation of doorstopper stress measurements is presented, along with the description of a field testing procedure that gives the parameters required in this model. Results from stress measurement simulations performed in the laboratory on a Barre granite block and on rock salt cylinders are presented in order to validate the proposed methodology. A sensitivity analysis of aniseffects of anisotropy and nonlinearity using the simulation results is also performed and shows the influence of both phenomena on calculated stress intensities and orietations.

Stress measurements in soft rocks

The objective of this paper is to demonstrate that accurate and reliable in situ stress measurements can be performed in soft rocks. A quick overview of the mechanical behaviour of soft rocks is presented. After reviewing stress measurement techniques that have been used in soft rocks, the modified doorstopper technique is presented and the advantages it has over other techniques in soft rocks are underlined. Results from laboratory simulations in controlled conditions show that the technique is reliable and accurate. It is then shown, through field applications in a potash mine in Brazil, in an underwater tunnel in shales in Canada and in an exploratory drift in molassic rocks of the French Alps, how the technique yields results that can sometimes be confirmed by field observations.

The RPR method for the Doorstopper technique : four or six stress components from one or two boreholes

Abstract The importance of continuosly recording strain (or displacement) recover during stress relief when conducting in situ stress measurements has been recognized by many authors as a means of assessing the quality of a measurement. This paper describes an important new use for these strain recovery curves, as four components of the stress tensor can now be obtained from one modified doorstopper stress measurement using a parameter called the recovered to peak strain invariant ratio (RPR). Modifications made to the standard doorstopper cell and to the standard field procedure are briefly described. These allow continuous recording of strain recovery while stress is being relieved, as well as the monitoring of temperature at the rock-cell interface. Then, a description of the finite element modelling allowing the production of curves relating the far field stress components in a plane parallel to the measurement plane to the stress component normal to this plane is given. Validation of the model is achieved by laboratory stress measurement simulations. Finally, comments on preliminary results of this method applied to the borehole deformation gauge stress measurement technique are made.

A stress calculation model for the 3D borehole slotter

A stress calculation model is proposed for a new stress measurement device called the 3D borehole slotter. This instrument allows the calculation of the 3D in situ stress tensor in a single borehole without the need for a drill on the site at the time of the measurement and without bonding strain gauges at the borehole wall. The 3D borehole slotter requires cutting half moon shaped slots with different orientations at a borehole wall using a small diamond impregnated blade and monitoring the strains which are relieved normal to the slot. This paper shows that combining six slots cut on the borehole wall with one to three slots cut parallel to the borehole axis and five to three cut with a 45° angle, allows the calculation of the in situ stress tensor. The optimal slot combination is determined and 3D finite element analyses are used to evaluate the degree of stress relief normal to the slot at the strain sensor location and also to estimate the strains induced parallel to the slot by this local stress relief. The results obtained from these analyses are then used directly in the stress calculation model.

Experimental and numerical evaluation of stress redistribution in thick-walled rocksalt cylinders

Abstract Whenever an opening is excavated in soft rocks showing elastic–visco-plastic behaviour, like rocksalt and potash, the stress state prevailing soon after the excavation will not remain constant in time but will eventually evolve from the initial state, σ o , generally considered to be elastic, to a state of stress that can be considered to be stationary for all practical purposes, σ ss . The knowledge of the stress state prevailing around the excavation some time after its completion is essential for many short-term applications such as the interpretation of in situ tests based on the cavity expansion principle and of stress measurements based on overcoring techniques. Since no analytical solutions are available for the evaluation of this stress redistribution process, numerical analyses are often used to study this problem. The results obtained from such analyses depend on many factors but mainly, the kind of creep law formulation and related parameters. This paper presents an experimental methodology to quantify the amount of stress redistribution occurring in a thick-walled cylinder under a given set of conditions. This experimental methodology does not require any assumption regarding the creep behaviour of the material. As a consequence, it allows the validation of numerical analyses by comparing the amount of stress redistribution obtained experimentally and that obtained from these numerical analyses.

Designing mine pillars with the convergence—confinement method

Abstract This paper shows how the convergence—confinement method can be extended to predict the axial stresses in pillars in uniform and non-uniform multiple pillar arrays. The conventional graphical method, which allows the dimensions of single pillar and uniform two-pillar arrays, is first extended to uniform arrays consisting of three and four pillars and then to non-uniform arrays consisting of two pillars. As graphical solutions are not feasible when the number of pillars in uniform arrays is greater than four or when the number of pillars in non-uniform arrays is greater than two, an algorithm which makes use of the pillar reaction curves is proposed for predicting the axial stress acting on each pillar of the array. Some of the axial pillar stresses predicted with the proposed extensions of the convergence—confinement method are compared to those obtained with other prediction methods. Features of the proposed algorithm are briefly discussed.

Determination of creep parameters of rock salt by means of an In Situ sharp cone test

Abstract A new simple field testing technique, the sharp cone test (SCT), for the determination of short-term creep parameters of rock salt is described. The interpretation model, which is based on a transient creep power law, is presented. The influence of the interpretation model hypotheses is assessed by a finite element analysis. Creep parameters for artificial rock salt obtained both from sharp cone and triaxial compression creep tests are presented and compared.

STRESS MEASUREMENTS IN CONCRETE STRUCTURES WITH MODIFIED DOORSTOPPER TECHNIQUE

This paper describes the use of a modified doorstopper technique for measuring both absolute and relative stresses in concrete structures. The equipment, field procedure, and stress calculation model are presented and are followed by the results of some stress measurement campaigns in concrete structures. The poly-technique stressmeter, a stressmeter designed for monitoring the very low stress variations in concrete structures such as dams, is also described. Results from lab tests and a 7-month field monitoring period show the stability and sensitivity attainable with the new stressmeter. The stress calculation model for both absolute and relative stresses requires no hypotheses concerning the principal stress orientation or magnitude and can take into account the heterogeneous, anisotropic, and eventually nonlinear behavior of concrete found at the measurement scale.

Deformability of Rock-Like Materials Using a Sharp Cone Test

This paper presents a new idea for a field testing technique, the sharp cone test (SCT), for the determination of certain deformability parameters of rock-like materials. The test is described, and the theoretical background required for its interpretation is given. The influence of the interpretation model hypotheses is assessed by a finite element analysis. The results of sharp cone tests in materials of known linear elastic and viscoelastic behaviors are presented to validate the interpretation model.