| |
|
|
|
|
|
Deformacje i pękanie skał w warunkach laboratoryjnych |
|
|
|
|
Deformacje i pękanie skał w warunkach laboratoryjnych Authors: J. Gustkiewicz, A. Nowakowski
Laboratory testing of mechanical properties of materials are still the major source of information about the mechanical behaviour of rock under loading. Laboratory tests enable the repeatability of the testing process conditions in the context of preparation of samples, test equipment, the method of load application or the method of measuring of relevant physical quantities. It is generally assumed that the laboratory conditions ought to reproduce as closely as possible the real life conditions and the laws governing the rock mass behaviour under those particular conditions can be then sought. In the Laboratory of Rock Deformations of the Strata Mechanics Research Institute the expression "laboratory testing of mechanical behaviour of rocks" covers a wide range of laboratory experiments, both destructive and non-destructive. The fundamental tests involve uni- and three-axial compression test, supported by other tests when necessary. These tests are either performed at the Strata Mechanics Research Institute or in other research centres in Poland and abroad, as a part of cooperation programmes (AGH-UST, Cracow, Institute of Geonics of the Academy of Sciences in the Czech Republic, Ostrava, Laboratory of Geophysics of the Earth Interior and Tectonophysics CNRS, Grenoble). Laboratory tests of deformations of cohesive media in the Strata Mechanics Research Institute were at first performed not on rocks, but on loose media. In the late ,1960s the research programmes were undertaken to find an answer to the question how the measured deformation should be related to the actual spot the measurement was taken and the length of the base line. These experiments, known as measurement of strain fluctuations, were then utilised in the studies of the random character of deformations, being the consequence of the random character of the structure of the medium. Results were compiled in the papers by Kuśmierczyk and Żurawska (1970), Klein and Kuśmierczyk (1972a, 1972b) and Klein(973a, l973b). These studies were crowned witib the doctor's dissertation by Ginter Klein, published in 1975 (Klein1975). The standard deviation of fluctuations of rock mass deformations was expressed as the experimental function of the sensor length and the average deformation of the sample, yielding the linear dimension of the representative sample volume. These issues are covered in more detail in the work by Gustkiewicz (1975c, 1985a, 1985b) and Gustkiewicz et al. (1975). In 1976 the research work was focused on finding the measuring equipment to analyse rock samples in precisely those conditions which prevail at the given depth. The Karman chamber was found most adequate for that purpose. Inside the chamber a cylindrical samples is compressed by hydrostatic pressure (ambient pressure) from the outside, at the same time it might be also compressed in the axial direction with the use of a press piston. With an eye to engineer such a device, the cooperation began with the High Pressure Research Centre of the Polish Academy of Sciences in Celestynow, near Otwock. The research work on the device known as GTA-10 was crowned with success and a fully operative prototype was engineered in 1978 and then patented in 1983 (Wysocki and Gustkiewicz, 1983). The equipment enabled the testing of cylindrical rock samples in die ambient and porous pressure range up to 400 Mpa. The maximal force of axial load was 1500 kN. The sample could be heated up to 450K. The operating principles of this device and the first results are provided in the work by Dhigosz et al. (1981a, 1981b). The GTA-10 in its present-day form is shown in Fig 1. The scope of tests to be performed using the GTA-10 is very wide. The equipment was used to investigate the effects ofhydrostatic and porous pressure on the strength limit of rocks and corresponding values ofYoung modulus and Poisson ratio. The changes of rock volume due to hydrostatic pressure and their consequences for the mechanical properties of rocks were explored, too in the contacts with sorbing and non-sorbing liquids. The potentials for further research in that field are still wide and research work is now underway. The relationship between the longitudinal and lateral deformation of the sample at the strength limit and the applied pressure was explored and the criterion was established that determines the two characteristic pressures: transition pressure between brittle cracking and ductile flow and the pressure at which the rock becomes fully ductile (Gustkiewicz 1985c). Further tests performed on rocks in three-axial state of stress in the presence of non-sorbing (naphtha, N sub 2) and sorbing (H sub 2 O, CO sub 2) pore liquids provided most valuable information about the relationship between the effective pressure and rock behaviour after the limit strength is exceeded (brittle, ductile). Furthermore, the changes in thelimit strength and other material constant were quantified, depending on the applied pore liquid (Gustkiewicz 1985d, 1990). A singular case of three-axial test is the compression test where the variations of rock sample volume depending on the hydrostatic pressure and measured and registered. These experiments provided vital information about the behaviour of the core and pore space in high pressure conditions. On that basis characteristic points were defined on the compressibility curve: limit pressure of cracking, consolidation pressure, which mark the beginnings or ends of major changes in the rock structure (Gustkiewicz, 1989a). In 1993 the Laboratory obtained new equipment: a testing machine INSTRON 8500 Rock Testing System (Fig 2). It was manufactured by INSTRON Company but tailored to the specific needs of the Strata Mechanics Research Institute. It met all the conditions requisite for testing brittle materials (rocks). The press frame has high rigidity (10 MN x 10 sup -1) and the pressing force is considerable (5000 kN), hence the limit strength can be exceeded even in the case of large samples. A digital control system with the feedback loop configuration enabled us to gradually vary thepressing force and longitudinal or circumferential deformation during the experiment. Tests performed on the INSTRON machine supported the analysis of full stress-strain curves, with the special focus on post-destructive regions. The results of those experiments (Nowakowski, Walasz-czyk 1999) were further utilised in mathematical modelling of room and pillar mining, which was the subject matter of the doctor's dissertation presented in 1999 by Andrzej Nowakowski. The main theses of the dissertation were then published by Nowakowski and Walaszczyk (2000,2001). The impacts of uni-axial test conditions on the shape of post-critical section of the sample stress and longitudinal deformation were subjected to research investigation, too. In their work Kortas and Nowakowski (2002) made an attempt to explain the phenomena previously reported by Wawersik (1968), which were observed during the uni-axial compression tests. The full list of major achievements of the Laboratory: Methods of deformation measurements, testing methodology -control of uniform sample loading during the uni-axial compression test by determining the bending moment as the function of loading, such conditions of the sample mounting in the press are sought that the bending moment be minimised; -the relationship is investigated between the shape of the post-critical section of the stress-strain curve obtained from the uni-axial compression test and dimension (slenderness) of the sample and the way it is mounted in the press (friction force between the sample bases and press boards); -application of resistance strain gauges to the studies of rock samples subjected to hydrostatic pressures up to 400 MPa; -the patented method of measuring lateral and longitudinal deformations of a sample using induction sensors, under the hydrostatic pressure up to 400 MPa, the method might be applied in measurements of elastic or ductile deformations. Fluctuations of deformations -analysis of deformation fluctuations as a random function for the loose media and rock samples; -determining the probabilistic characteristics of fluctuations, particularly the standard deviation of fluctuations as an experimental function of the mean deviation and the length of the base line; the function might be further utilised to establish the effective length of the base line. Rocks under high pressures - research methods and rock properties -in cooperation with other research units, we designed, engineered and were granted the patent rights to the original equipment for testing deformations and cracking of rock samples saturated with sorbing and non-sorbing liquids, hydrostatic pressures up to 400 MPa, the axial force 1500 kN; -Relationships were found experimentally between relationship between the longitudinal and lateral deformation of the sample at the strength limit and the applied pressure; transition pressure between brittle cracking and ductile flow and the pressure at which the rock becomes fully ductile were defined as characteristic points on the curve; -It was shown experimentally that when the closing pressure for rock cracking or the consolidation pressure are exceeded, the inclination of the strength limit vs pressure curve might be enhanced (consolidation point); -The synoptic description of deformative and mechanical parameters of rocks was completed; -Relationships were highlighted between the effective pressure and the changes of mechanical properties of rocks; experiments were performed for sorbing and non-sorbing liquids, taking into account liquids and gases; -The impacts of the pressure of absorbed/adsorbed CO2 on its transition pressure and coal ductility were highlighted; -It was demonstrated that compressing rock samples under a sufficiently high pressure brings about a change in their properties; -In the case of a sample with fixed dimensions and loaded by a constant force two opposite effects are reported: stress increase due to swelling and relaxation; -The relationship between the pore structure and the effects of pore pressure on the limit strength and corresponding strains is shown experimentally in the conditions of three-axial state of stress.
|
|
|
|
|
Advertisement |
 |
|
|
2011