Showing papers in "Rock Mechanics and Rock Engineering in 1989"

Longitudinal waves propagation in a limestone: The relationship between velocity and density

Abstract: In the following paper a general relationship between velocity and density in a limestone is established. Statistically significant differences may be found between the equations concerning each one of the four rock types defined on the basis of variations in composition and texture. There is a clear relation between the highest velocities and the existence of pressure-solution evidences. The later are an indication of changes in texture induced by tectonics.

Influence of stress-dependent elastic moduli on stresses and strains around axisymmetric boreholes

Abstract: Porous or clastic rocks often have elastic moduli which are not constant but increase with increasing minor principal stress. The use of classical constant modulus linear elasticity in these cases can lead to erroneous predictions of the deformations and of the initiation and extent of failure around underground excavations. To illustrate these effects, solutions are developed for axisymmetric excavations in infinite media having power law and exponential variations of elastic modulus with minor principal stress. The maximum stress concentrations do not occur at the excavation boundaries and are less than the constant value of 2.0 given by constant modulus elasticity. When modified slightly to allow for test boundary conditions, the theory gives predictions that are consistent with aspects of the results obtained in hydrostatic compression tests on thick walled cylinders of three sedimentary rocks.

The effective failure modes and stability of slopes in rock mass with two discontinuity sets

Abstract: The stability of rock slopes in discontinuous rock mass associated with the construction of power plants, highways and open pits is always of paramount importance during the lifetime of these structures. The likely forms of instabilities observed in the excavation of rock slopes and some mathematical methods for the stability analyses are well documented in literature. Since most of the mathematical approaches used are based on the limiting-equilibrium concept, there seems a need to check the validity of these approaches under some controlled conditions. In this paper, the authors describe methods for the stability of a blocky column and discontinuous rock slopes derived on the basis of dynamic equilibrium equations and compare the results calculated according to the developed method with those of experiments on model blocky columns and model slopes in the laboratory. Test results confirm that the limiting equilibrium approache is valid and an effective way of dealing with the stability problems in discontinuous rock mass as long as the likely forms of instability are properly treated in these approaches.

Measurement of rock mass deformation with grouted coaxial antenna cables

Abstract: Techniques presented herein show how reflected voltage pulses from coaxial antenna cable grouted in rock masses can be employed to quantify the type and magnitude of rock mass deformation. This measurement is similar to that obtained from a combined full profile extensometer (to measure local extension) and inclinometer (to measure local shearing). Rock mass movements deform the grouted cable, which locally changes cable capacitance and thereby the reflected wave form of the voltage pulse. Thus, by monitoring changes in these reflection signatures, it is possible to monitor rock mass deformation. This paper presents laboratory measurements necessary to quantitatively interpret the reflected voltage signatures. Cables were sheared and extended to correlate measured cable deformation with reflected voltage signals. Laboratory testing included development of grout mixtures with optimum properties for field installation and performance of a TDR (Time Domain Reflectometry) monitoring system. Finally, the interpretive techniques developed through laboratory measurements were applied to previously collected field data to extract hitherto unrealized information.

A force-indentation model for brittle rocks

Abstract: A force-deformation model is developed for localized loading of brittle rocks that depicts the action of a percussive drill bit. This model consists of a series of successive cycles each of which is composed of a crushing and a chipping phase. Crushing involves the fragmentation or comminution of individual grains or grain clusters and is manifested by an increase in the applied force, while chipping is associated with the ejection of crushed material accompanied by a reduction in the load. This work represents the foundation for the analysis of the response of a brittle rock to the action of a jackhammer.

Floor design in underground coal mines

Abstract: Floor failure and excessive heave in underground coal mines can jeopardize the stability of the whole structure, including the roof and pillars, due to differential settlements and redistribution of stress concentrations. Besides, floor failure is detrimental to haulageway operation and can lead to unacceptable conditions of high deformation. Thus, the design of any underground opening must consider roof/pillar and floor as one structural system.

Time dependent strain recovery of cores from the KTB — Deep drill hole

Abstract: The time dependent strain recovery is investigated for drill cores of two rock types from the German Continental Deep Drilling Project (KTB). The strains are measured with inductive displacement transducers in the core axis and in three different radial directions under constant conditions of temperature and moisture. It turns out that the process of time dependent strain recovery is closely connected with the emission of acoustic signals. A non-foliated, fine grained lamprophyre (depth: 2231.50 m) and a strongly foliated gneiss (depth: 2415.90 m) show a distinctly different relaxation behaviour. The relaxation times for both, deformation and acoustic emissions, are approximately the same and 57 hours for the lamprophyre and 28 hours for the gneiss. In the case of the viscoelastically isotropic lamprophyre principal in situ stresses have been calculated.

Friction on a granite to granite interface

Abstract: Direct shear tests were conducted on a granite to granite interface for the purpose of tracing the evolution of frictional resistance as the initially smooth and polished surface wears during continuing shearing displacement. At the moment when sliding on the freshly manufactured interface starts (first slip), the friction angle is very low, between 15° and 20°, but then it increases with displacement rapidly without reaching a peak in the first test (maximum displacement less than 25 mm). Upon repeated shearing of the same surface (without re-finishing), this process of “displacement-strengthening” continues until a total accumulated displacement of about one half of one meter is reached. At this point, the angle of friction would typically be between 42° and 44°. Once the residual surface was established, the effect of time-related parameters, the duration of stationary contact under constant normal and shear load and variable displacement rate, were investigated. The frictional resistance is shown to increase with decreasing displacement rate and to increase with the duration of stationary contact. At a normal load greater than about one half of one MPa, shear displacement on a residual surface proceeds in an unstable, stick-slip manner. The change from stable to unstable sliding, that can be effected by either decreasing the sliding velocity or increasing the normal load, is not instantaneous, but occurs over a finite amount of displacement.

Deformation and stability of an elasto-plastic softening pillar

Abstract: A model of rock pillar or coal seam is considered assuming linear elastic behaviour before reaching the maximum strength and post-peak behaviour characterized by the residual strength. The deformation and stress across the pillar height are assumed to be uniform and the interaction with overlying rock strata is treated assuming beam model of the strata. The elasto-plastic stress distribution within pillar and the onset of instability occurring for the critical opening span are determined. Comparison with a solution for a simplified “spring” model of pillar is also presented.

Computation of the seismic stability of rock wedges

Abstract: Newmark's concept of computing the permanent displacement under seismic loads has been combined with the conventional limit equilibrium analysis to compute the displacements of a rock wedge. The rock wedge formed by the intersecting planes may or may not have a tension crack in the upper slope surface. As the static analysis of a rock wedge is available from the literature, only the seismic problem is treated theoretically in more details.

Momentum and energy processes during jackhammer operation

Abstract: An analytical model has been developed that describes the motion of the bit of a pneumatic jackhammer and the forces exerted at its tip during penetration of a target. This model assumes that the energy is transmitted in the form of a one-dimensional longitudinal wave without inclusion of the effect of lateral inertia. The piston and the bit can be of arbitrary shape, but are approximated here by uniform stepped sections.

In situ test of a borehole extensometer

Abstract: Stresses in the Earth's crust are known to represent a key element of geodynamical processes at various spatial and temporal scales. Static stresses within lithospheric plates can be estimated on the basis of direct in situ measurements in boreholes or near the surface. Earthquake focal mechanisms yield the same type of information by sampling at greater depths. The quantification of geological deformations happens to be the principal tool to determine possible changes in time of the tectonic stress pattern. These changes can however be recorded only qualitatively and on the geological time scale. In seismic regions it is clear that the regional stress field varies on a time scale defined by the recurrence of earthquakes, say several tens of years. Our goal was to design a robust and cheap instrument which could continuously monitor stress changes at depths large enough to reduce meteorological influences.

The theory of the recoverable linear strain sensor and its application in geomechanics

Abstract: After introducing the new linear strain sensors and the theory behind their modus operandi, the paper discusses in general the performance characteristics as well as the potential value of their application in geomechanics. Application features and effects of application environments are then outlined and examples of practical applications are given.