Erling Nordlund

Study of Rock Joints Under Cyclic Loading Conditions

SummaryA conceptual model for the behaviour of rock joints during cyclic shear and under constant normal stresses was proposed according to results from shear tests with 50 concrete replicas of rock joints. The shear strength and deformability of joint samples were found to be both anisotropic and stress dependent. Based on these experimental results, a two-dimensional constitutive model was developed for rock joints undergoing monotonic or cyclic loading sequences. The joint model was formulated in the framework of non-associated plasticity, coupled with empirical relations representing the surface roughness degradation, appearance of peak and residual shear stresses, different rates of dilatancy and contraction, variable normal stiffness with normal deformation, and dependence of shear strength and deformability on the normal stress. The second law of thermodynamics was represented by an inequality and used to restrict the values of some of the material parameters in the joint model. The new joint model was implemented into a two-dimensional Distinct Element Method Code, UDEC, and its predictions agreed well with some well-known test results.

Experimental verification of the Kaiser effect in rocks

SummaryThe Kaiser effect is a measure of damage developed in rock. It was investigated using 61 core specimens of eight types of rocks under uniaxial cyclic loading. The effects of the delay time between subsequent loading cycles and the holding time on the Kaiser effect were examined in granite specimens. The characteristics of AE occurring during unloading and holding were also studied. The experimental results showed that most of the rocks, with the exception of some iron ores, showed an obvious Kaiser effect before the load was very near the level of the strength. The delay and holding times did not strongly influence the Kaiser effect. Breakdown of the Kaiser effect was expressed by the felicity ratio that may be taken as a measure of the quality of rock. The onset of the continuously increasing AE in the first loading cycle can be taken as a measure of the damage in rock. The mechanism of the Kaiser effect was studied with the aid of a micromechanical model. An expression for the damage surface in the stress space was derived from the model. The theoretical expression was compared with the results of the Kaiser effect tests, which showed a satisfactory consistence.

An experimental study on the anisotropy and stress-dependency of the strength and deformability of rock joints

The anisotropy and stress-dependency of the strength and deformability of rock joints were investigated experimentally through shear tests of concrete replicas of natural rock joints. 50 concrete r ...

The stress-strain behaviour of rock material related to fracture under compression

Fracture is the main reason for the non-linear behaviour of hard rocks. The fracture mechanics of rock is studied in this article by analysis of the fracture process under compression. A constitutive model that describes the relationship between the macro deformation of rock and the micro fracture within rock is developed. The propagation of microcracks, the non-linearity of deformation, the loading-and-unloading hysteresis and the variation of the apparent Youngs modulus and Poissons ratio are studied using the developed model. The model simulations demonstrate that: (1) the fracture toughness, initial crack length, crack density, and Youngs modulus are four crucially important parameters that affect the deformation behaviour of rock; (2) the elastic parameters (E and v) of the rock matrix should be measured in triaxial tests. If they are measured in uniaxial tests, the upper straight unloading portion of the stress-strain curve is suggested to be used for the purpose, unless the closure effect of open cracks will be included in the estimations. In addition (3), the slope of the reloading stress-strain curve is a measure of the damage in material.

Deformation of brittle rocks under compression - with particular reference to microcracks

A constitutive model for brittle rocks has been developed based on the analysis of microcracks. The macroscopic deformation of a rock is decomposed into three components: response of the rock matri ...

Comparison Between Stress and Strain Quantities of the Failure–Deformation Process of Fennoscandian Hard Rocks Using Geological Information

The aim of this paper was to compare the stress and strain quantities that are related to the failure–deformation process of hard rock. The data used here was obtained from laboratory uniaxial compression tests performed on different types of Fennoscandian hard rocks. The failure–deformation process quantities were compared at each deformation stage and for each single specimen. Moreover, geological information such as the rock origin process and the rock characteristics of the specimens were studied and linked to the stress and strain quantities. The purpose was to investigate the influence of the rock origin process and rock characteristics on these quantities. The main results of this study showed that the normalized crack damage lateral strain (ε3cd/ε3p) and the volumetric strain (εcrv−ci and εv−cd) quantities were strongly affected by the grain size. The normalized and volumetric quantities are weakly dependent on the mineral composition.

Large Scale Dynamic Testing of Rock Support System at Kiirunavaara Underground Mine

A series of five large scale dynamic tests were conducted at the LKAB Kiirunavaara mine using explosives to generate the dynamic load on the support system. This was done with the aim of developing a testing methodology for in situ testing of ground support. Furthermore, the response of the installed rock support system to strong dynamic loading was evaluated. The tests included ground motion measurements, fracture investigation, ground and support motion imaging, as well as deformation measurements. The results indicated that the relation between the burden and the used amount of explosive had a vital role in either reducing or involving the effect of the detonation gases in the test results. In addition, the type of explosive which was used in the tests had a great impact on minimising the gas expansion effects. Higher peak particle velocities were measured compared to those of similar large scale tests carried out in other countries. However, the level of induced damage was limited to a fractured zone behind the support system and propagation of cracks in the shotcrete. Measured peak particle velocities were used to calculate the kinetic energy transmitted to the fractured zone of the test wall. The energy absorption by the Swellex, reinforced shotcrete and weld mesh was estimated by measuring the elongation/deflection of the support elements and relating these measurements to previously conducted laboratory tests. The comparison of maximum estimated energy absorbed by support system with the maximum estimated kinetic energy indicated that as the support system is still functional, the energy is partly reflected back to the surrounding rock. The results of the measurements in Tests 1, 2, 4 and 5 are presented in this paper and the methodology used to design the tests is discussed.

Assessment of damage in rock using the Kaiser effect of acoustic emission

The aim of our study is to explore the possibility of using the Kaiser effect method to assess damage in rock induced by blasting. Cores were drilled from a hard rock tunnel. The specimens prepared ...

The effect of thrust on the performance of percussive rock drills

Abstract The performance of two different percussive rock drills has been studied theoretically by simulating the drilling process. The two drills studied correspond to an idealized drill and a more complex, prototype drill. The efficiency, the maximum stress (maximum tensile stress) and the minimum stress (maximum compressive stress) have been determined during the simulations. Also, the stability of the drill string with respect to buckling has been estimated as a function of a constant applied thrust. Two cases of applied thrust were simulated. They correspond to a constant applied thrust and an applied thrust with time dependent harmonic variations. The results show, as expected, that for low frequency variations in thrust, the performance of the rock drill is almost the same as for a constant applied thrust. In practical drilling the applied thrust is in general almost constant during drilling in homogeneous rocks. It was therefore assumed in the remainder of this study that the applied thrust can be treated as a constant force. The results obtained for a constant applied thrust may be summarized as follows: the magnitude of applied thrust selected will be a compromise between a number of contrasting requirements such as: (i) high efficiency, (ii) low stress levels in the drill string, (iii) stability of the drill string with respect to buckling, and (iv) small wear of the drill bit.

Numerical analyses of the hangingwall failure due to sublevel caving: study case

The sublevel caving used in Kiirunavaara mine induces failure and subsidence of the hangingwall. Two sections of the mine were studied by means of numerical analyses. Numerical models were developed using finite element and discrete element codes. The former was applied to calculate the location of new failure surfaces in the hangingwall and to estimate the break angle when mining advances downwards. The latter was used to analyse the displacement path of the caved rock during draw and to determine its effect on the stability of the hangingwall and footwall. The models were calibrated using displacement monitoring data. The finite element analyses indicated that the break angle is almost constant for deeper mining levels but may change if the geometry of the orebody changes. The discrete element model showed the formation of a stationary zone along the footwall that reduces the magnitude of the shear forces during draw, increasing its stability.