RU2339816C1 - Method of determining long-term strength of mine rocks - Google Patents

Abstract

FIELD: mining engineering.

SUBSTANCE: invention relates to mining industry and can be used for determining a long-term strength limit of mine rocks by samples. A method determining a long-term strength of mine rocks is based on a regularity of changes in a damping time Δt_i of the acoustic emission at a gradually increasing loading of a sample. A mine rock sample is subjected to a graduated axial loading σ(t) in a uniform compression chamber at a constant lateral strain. The sample is kept at a specified known value of the axial stress on each loading phase until the parameters of the acoustic emission impulse flow are stable. Damping time Δt_i of acoustic emission activity N_Σi is determined on each loading phase of the sample. Difference of duration Δt_i-Δt_i-1 of the acoustic emission damping time is determined for each of its following and preceding loading phase. When the specified difference value is positive, an axial stress value of the preceding phase is recorded which is taken for the long-term strength limit

of mine rocks.

EFFECT: enhanced accuracy of determining a long-term strength limit of mine rocks.

2 dwg

Description

The invention relates to mining and is intended to determine the long-term strength of the rocks on the samples.

A known method for determining the long-term strength of rocks is that the rock sample is loaded with axial and constant lateral pressure until the creep strain begins to develop, the lateral pressure on the sample is increased stepwise to a certain value and maintained constant, while the axial pressure is increased to the next the moment of the beginning of the development of creep deformation, the limit values of the principal stresses corresponding to this moment are taken equal to the axial and lateral pressures, measured during loading these pressure and their corresponding deformations are revealed, which are used to judge the long-term strength of the rock [1].

The disadvantage of this method is the low accuracy of determining long-term strength. This is due to the fact that the onset of the development of creep deformations is determined by the maximum volumetric deformation of the seal, determined by the corresponding formulas through the measured longitudinal and transverse deformations of the sample. In turn, the structural heterogeneity of the rock leads to the fact that the longitudinal and transverse strains, estimated in local areas of the sample, do not objectively reflect its volumetric deformation.

The closest in technical essence to the present invention is a method for determining the long-term strength of rocks, which consists in a stepwise increase in the axial stress of a rock sample, measuring the parameters of the flow of acoustic emission pulses, holding the sample at a fixed known value of the axial stress at each stage until the stabilization of the pulse flow parameters acoustic emission [2].

In the specified prototype method, the first local maximum of acoustic emission activity is revealed, which determines the long-term strength of rocks.

The disadvantage of this method is the low accuracy of determining the ultimate strength of rocks. This is due to the fact that the maximum activity of acoustic emission is significantly affected by random factors, for example, such as structural heterogeneity and defective rocks. As a result, the first local maximum can occur at a voltage that does not correspond to the desired ultimate strength, i.e., the latter will be determined with an error.

This application solves the problem of increasing the accuracy of determining the ultimate strength of rocks.

To solve the problem in a method for determining the long-term strength of rocks, which consists in a stepwise increase in the axial stress of a rock sample, measuring the parameters of the flow of acoustic emission pulses, holding the sample at a fixed known value of the axial stress at each stage until the parameters of the flow of acoustic emission pulses are stabilized, loading the sample is carried out under constant lateral compression, at each stage of loading of the sample, a time interval attenuation of the acoustic emission activity therein, and then determine the difference of durations of time slots attenuation of the acoustic emission of each subsequent sample and the preceding stages of loading and with a positive value of this difference is fixed amount of axial tension preceding stage, which was taken as the limit of rupture strength of rocks.

The proposed method is based on the fact that the tensile strength of long-term strength corresponds to the state of maximum compaction of the rock. In the process of a stepwise increase in axial stress in the sample, when approaching the magnitude of the stress of the ultimate strength, the number of defects arising and increasing in size decreases. Since these defects are sources of acoustic emission, the latter also decreases. Under conditions of comprehensive compression, the acoustic emission decay time interval also decreases to a steady state. When the stresses exceed the long-term strength, an increase in the volume of the sample occurs, its decompression due to an increase in defectiveness, leading to an increase in acoustic emission, and, as established experimentally, an increase in the time interval for attenuation of acoustic emission. To eliminate the effect on the measurement results of cracks breaking the sample into blocks and generated from the action of internal stresses when removing the rock sample from great depths, the tests are carried out with lateral compression of the sample. This allows the change in the time interval of attenuation of acoustic emission to manifest itself most clearly.

во времени, а также график изменения во времени разницы Δt_i-Δt_i-1 длительностей временных интервалов затухания акустической эмиссии каждой последующей ступени нагружения образца и предшествующей ей ступени нагружения образца.The method for determining the long-term strength of rocks is illustrated in Figs. 1 and 2, where Fig. 1 shows one of the possible schemes of a device for implementing the method, and Fig. 2 shows graphs of a stepwise increasing axial stress of a rock sample σ (t) and acoustic activity emissions

in time, as well as a graph of the time variation of the difference Δt _i -Δt _{i-1 of the} durations of the time intervals of attenuation of acoustic emission of each subsequent stage of loading of the sample and the previous stage of loading of the sample.

The implementation diagram of the method is presented in figure 1. It contains a sample 1 placed in a fluid-filled inner cavity 2 of the compression chamber 3. Sample 1, with its upper end, contacts the piston 4 of the controlled axial pressure source 5, which is connected to the axial pressure meter 6. The internal cavity 2 is hydraulically connected to the constant side source 7 pressure. Acoustic emission receiving transducer 8 is mounted on the piston 4 and is connected to the acoustic emission activity meter 9. To prevent liquid from entering the pores and cracks of the sample 1, a thin insulating shell 10 made, for example, of rubber, is placed on its surface.

The method for determining the long-term strength of rocks is as follows.

The rock sample 1 is installed in the chamber 2 (see Fig. 1) and loaded with a stepwise increasing axial stress σ _o (t) (see Fig. 2) at constant lateral pressure σ _b . The magnitude of the voltage increment of one stage is selected in the range from 2 to 10% of the breaking load, determined from preliminary measurements.

при скачкообразном увеличении осевых напряжений σ_о (t) на каждой ступени во времени увеличивается, а на участке заданной нагрузки уменьшается до установившегося значения. При возрастании осевых напряжений в образце от первой ступени к ступени, на которой достигается состояние максимального уплотнения горной породы в образце, соответствующие промежутки времени затухания активности акустической эмиссии Δt₁, Δt₂, Δt₃ и т.д. будут уменьшаться до величины Δt_i (фиг.2), после чего из-за начавшегося разуплотнения горной породы будет происходить его увеличение. При этом разницы Δt_i-Δt_i-1 между временным промежутком каждой последующей ступени Δt_i и временным промежутком Δt_i-1 предшествующей ей ступени будут отрицательными. Появление положительного знака разницы Δt_i+1-Δt_i будет свидетельствовать о переходе горной породы из состояния максимального уплотнения к состоянию разуплотнения, для которого в силу меньшего сжатия горной породы характерно более длительное затухание процесса образования новых, прорастания существующих трещин и, соответственно, больший временной интервал затухания активности акустической эмиссии. После появления положительного знака упомянутой разницы фиксируют величину осевого напряжения предшествующей ступени, по которой определяют напряжение предела длительной прочности горных пород

.Acoustic Emission Activity

with a spasmodic increase in axial stresses, σ _о (t) at each stage increases in time, and at a given load, decreases to a steady-state value. With an increase in axial stresses in the sample from the first stage to the stage at which the state of maximum rock compaction in the sample is achieved, the corresponding time intervals of the attenuation of acoustic emission activity are Δt ₁ , Δt ₂ , Δt ₃ , etc. will decrease to the value Δt _i (Fig.2), after which, due to the beginning of unconsolidation of the rock will increase. In this case, the differences Δt _i -Δt _i-1 between the time interval of each subsequent stage Δt _i and the time interval Δt _{i-1 of the} previous stage will be negative. The appearance of a positive sign of the difference Δt _{i + 1} -Δt _i will indicate the transition of the rock from the state of maximum compaction to the state of decompression, which, due to the smaller compression of the rock, is characterized by a longer attenuation of the process of formation of new, germination of existing cracks and, accordingly, a longer time interval of attenuation of acoustic emission activity. After the appearance of a positive sign of the mentioned difference, the axial stress value of the previous stage is fixed, by which the stress of the long-term strength of rocks is determined

.

Thus, the proposed method allows to solve the problem of more accurately determining the long-term strength of the rocks in comparison with the prototype method.

Information sources

1. USSR author's certificate No. 1479846, cl. E21C 39/00, publ. 05/15/89, bull. Number 18.

2. USSR author's certificate No. 1809053, MKI E21C 39/00, publ. 04/15/93, bull. No. 14.

Claims (1)

The method for determining the long-term strength of rocks, which consists in a stepwise increase in the axial stress of a rock sample, measuring the parameters of the flow of acoustic emission pulses, holding the sample at a fixed known value of axial stress at each stage until the parameters of the flow of acoustic emission pulses are stabilized, characterized in that the loading of the sample carried out under conditions of constant lateral compression, at each stage of loading the sample determine the time interval of attenuation a acoustic emission in it, then the difference in the lengths of the time intervals of acoustic emission attenuation for each subsequent and previous stages of loading of the sample is determined, and if the difference is positive, the axial stress of the previous stage is fixed, which is taken as the long-term strength of rocks.

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