Mechanical Characteristics Of Deeply Buried Marble And Its Technical Application

With the development of the deep underground engineering in21th century, almost all deeply buried tunnels, nuclear wastes as well as exploitation of deep geoheat involve the deep rock mass mechanics. Since1980s, many underground research laboratories have been established in America, Canada, Sweden, Japan, Switzerland, and achieved much significant results in monitoring techniques, theoretical research and correlatable numerical analyses on hard rocks under high crustal stress. Among them, it was very impressive for the study of lac du bonnet granite in Canada by URL.The methods of research on deeply buried marble can refer to the previous route of the granit byURL, which could include four aspects as follows:1) Strength feature. Besides traditional peak strength and residual strength, researchers have proposed a crack initiation strength and a damage strength in order to characterize the state of cracks. In general, the threshold value of majority strength of microscale intact rocks can be acquired by tests, while the threshold values for macroscale rock mass can only be obtained using numerical simulation. Recently, it is very efficient to solve the macroscale rock mass in terms of the PFC-based SRM.2) Excavation damage zone (EDZ). Due to the stress redistribution, disturbance of blasting and changes of temperature and humidity, a EDZ will form after excavation in the deeply buried tunnel face. It is very important to evaluate stability, percolation and diffusion mechanics and support measures of surrounding rocks by understanding the region, degree and evolution of EDZ.3) Reformed experimental technique. Although the true triaxial testing machine is now popular in many laboratories, the test of hard rocks has more requirements including simultaneously monitoring the acoustic emission in compression, changes of wave velocity, and permeability, or simulating at varied temperature and quickly recording seadata from tests.4) Large-scale in-situ test design and implement.The room experiments only solve the mechanic investigation of small-scale block rocks,instead for macroscale rock mass, it is necessary to design and carry out the in-situ test.In this work, room testing and description methods of mechanics to deeply buried marble are studied. The former covers the test of varied damage thresholds and post failure behaviors of marble samples. The results are compared to that in the case of granite, which enrich the research achievement of the hard rocks. The latter explores the description of PFC methods in mechanic behaviors of marble, namely describing the features of excavation damage, rock mass strength and post failures by means of PFC. This study broadens the application of PFC in strength and damage fields of hard rocks. Compared with traditional continuum methods, PFC has higher accuracy in prediction of damage thickness to hard rocks, showing a vast importance to support design.Due to a specific shift characteristics of brittleness-ductility plasticity with the confining pressure for marble in Jinping, it determines that large-scale deformation behaviors can not occur in the same way in common soft rocks, also it is not the pure brittle failures. Thus the thickness of breakage might not be deep. The breakage thickness of surrounding rock predicted by PFC includes the influence of excavation and stress redistribution and is close to the maximum extent. The resultant may provide a guiding significance for support design.Main work and points of innovation:(1)10groups of room single-axial compression tests of deeply buried marble are accomplished, and the crack initiation strength, damage strength and peak strength of marble are determined according to stress-strain curves.(2)12groups of normal tri-axial tests of deeply buried marble are completed, and the peak strength and residual strength are detected. These reveal a brittleness-ductility shift characteristic for marble with increasing confining pressure, and a idea plasticity response under higher confining pressures.(3) Cundall’s BDP (Brittle-Ductile-Plastic model) can be used to describe the shift of brittleness-ductile-plastic under varied confining pressures, and the breakage depth of Jinpin III level marble mass is predicted.(4) Features of the shift between brittleness-ductile-plastic of Marble are described by PFC method, and the micro-parameters is studied based on the Bond Particle Mode.(5) On the base of reasonable description of the shift between brittleness and ductile for marble, a numerical simulation based on PFC is developed to directly analyze the excavation breakage zone of surrounding marble mass.(6) According to the obtained method of predicting breakage zone, seven groups of surrounding marble samples from water tunnel with various depth in Jinping are calculated. The results indicate that the calculated value is in agreement with the measured value.(7) A perspective to the future work is made through analyzing the current progress in room tests, field tests, theoretical investingation and numerical simulation.