Experimental And Theoretical Research Of Mechanical And Acoustic Emission Characteristics Of Marble Under Different Loading And Unloading Conditions

With China’s sustained and rapid development of economy and construction,hydropower, mining, energy, transportation and other areas that require theconstruction of deep underground engineering, whose design, construction, operationand stability, etc. are directly dependent on the rock deformation, strength and othercharacteristics, also increases. The rock mass structure, the complexity and variabilityof conditions and environments of the rock and construction factors cause rockdeformation and failure. In the research of mechanical properties of rock in theloading conditions, many experts and scholars have done a lot of work, established arelatively complete set of experimental, theoretical analysis methods, which havebeen successfully applied to the actual project, and came to a series of research results.However, due to the differences of mechanical properties of rocks between theunloading conditions and loading conditions, the current analysis methods of theloading of rock are sometimes inconsistent with engineering practice, leading to someengineering accidents. With the test method consistent with the actual excavation, thispaper analyzes the deformation, destruction and other mechanical properties of themarble under unloading and loading conditions from macro side, thus providing atheoretical basis for better research of rock under unloading conditions. This papermakes the following findings:(1) Analysis of mechanical properties of rocks under different unloading initialconfining pressure and under different unloading rates through the loading andunloading test of marbles in different stress path. The results show that axial strainplays a dominant role in loading stage, while ring strain plays a dominant role inunloading stage. In the test conditions of pre-peak axial pressure and unloadingconfining pressure, with the increase of the initial unloading confining pressure, thepeak strength of the rock sample also increases, but is still lower than that of thetriaxial rock samples in corresponding confining pressure. At different unloading ratesand under different initial confining pressure, the time rock samples take to break is also different. The greater the initial unloading confining pressure is, the moreunlikely the sample is to break. Under the same initial unloading confining pressure,the faster the unloading is, the more prone the rock sample is to be damaged. In thestress control mode, failure characteristics of rock samples are divided into three types:the main shear, conjugate shear, splitting plus shear failure; in deformation controlmode, they are divided into: the main shear failure and shear failure plus splitting.Failure characteristics of rock samples are related to the initial unloading confiningpressure and the unloading rate of rock samples. Under the same initial confiningpressure, failure angle becomes big with the increase of the unloading rate; at thesame unloading rate, failure angle becomes small with the decrease of the initialunloading confining pressure.(2) Through the analysis, which is conducted with the acoustic emissionmonitoring system, of the acoustic emission characteristics of the process ofdeformation and failure of rock, we can draw the following conclusions. In theuniaxial test, triaxial testing and axial compression and unloading confining pressuretest before the failure of the rock sample, ringing count rate has a significantincreasing trend; however, after the destruction, with the stress drop, acousticemission activity weakens. The whole tests have experienced5stages of a quietperiod, active period, extremely active period, active period, quiet period. In theloading phase, acoustic emission activity of conventional triaxial tests is the samewith that of unloading confining pressure test. From the start of unloading confiningpressure to the end of the trial, acoustic emission activity of unloading confiningpressure test is more intense. The maximum ring count rate of unloading confiningpressure test is far higher than that of the conventional triaxial test, indicatingunloading confining pressure test is more destructive. In unloading confining pressuretest under the control of deformation, the maximum ring count rate increases with theincrease of initial unloading confining pressure in the same unloading rate at the sameunloading rate and under different unloading confining pressure, which verifies that atthe same unloading rate and under high confining pressure, rock failure needs moreenergy and rock is more difficult to destroy. (3) Applying the experimental data obtained in this experiment to the differentstrength criteria of the unloading of rock, we can see that: internal friction angle jand cohesion c, obtained from Mohr-Coulomb strength criterion and theMogi-Coulomb strength criterion, are almost the same, but the sum of squareddeviations is a big difference, so Mogi-Coulomb criterion is better than theMohr-Coulomb criterion. The internal friction angle j obtained from D-P criteria isless than that obtained from Mohr-Coulomb criterion, while the cohesion c of D-Pcriteria is greater than that of Coulomb criterion. Square deviation obtained from D-Pcriterion is less than that obtained from Mohr-Coulomb criterion, but is greater thanthat of Mogi-Coulomb criterion. Comparing the actual measured destruction angleand cohesion c with those obtained from the criteria, we can see the error ofMohr-Coulomb criterion and the Mogi-Coulomb criterion is smaller than that of D-Pcriterion. According to the principle of minimum sum of squared deviations,Mogi-Coulomb criterion is more applicable to the experiment of marble study.