Study On Rock Macroscopicand Mesoscopic Failure Mechanism Under Unloading Conditions And Designing And Calculating Methods Of Underground Engineering

In my essay, the developing of my thinking is just based on two subjects. First,failure mechanism of rock under unloading stress paths, especially rock macroscopicand mesoscopic failure mechanism have been studied for complex loading andunloading processes caused by engineering excavation, especially unloading process,including the evolution law of energy during different unloading processes, theevolution law and quantitative characteristics of AE during rock unloading failureprocesses, numerical simulation of rock unloading failure process based on particle flowcode. Then, using numerical limit method, rock classification and designing andcalculating methods of rock underground engineering have been discussed for practicalproblems in designing and calculating methods of rock underground engineering,including improving basic quality index in rock classification, the influence of span onsurrounding rock classification, the shear strength of concreter lining, designing andcalculating methods in rock underground engineering.The following achievements have been obtained:(1) Based on strain energy principle, energy evolution rules during the unloadingdamage process of marble under different unloading stress paths have been studied withprogramming method, as follows: axial energy curve during damage process changesnon-linearly, transforms from slow growth to rapid growth, then to slow growth andfinally to release. The total energy curve transforms from slow growth to rapid growth,then to slow decrease and finally to release. The main influence of different unloadingstress paths on energy evolution law during failure process focuses on yield andweakening stage. More energy are consumed by rock as confining pressure increasingas well as increasing rate of axial energy curve significant increases. The lowerunloading rate is, the higher increasing rate of axial energy is, more obvious transitionof axial energy curve at unloading point is; The increasing rate of axial energy increaseswhile unloading stress level is closed to bearing capacity peak, negative growing rate oftotal energy curve is higher.(2) The evolution law of acoustic emission characteristics has been analyzedduring failure processes under different unloading conditions, further quantized withprogramming method based on fractal theory, as follows: AE count rate maximizes at failure point during different failure processes and there is a relatively tranquil period ofAE before AE count rate reaching to maximum. The higher confining pressure is, theshorter relatively tranquil period endures, the higher maximum of acoustic count ratebecomes. The higher unloading rate is, the higher count rate of relatively tranquil periodis, the shorter time of duration is. When unloading point is in plastic stage, there will bea shorter relatively tranquil period before failure. Fractal dimension of AE is lowerbefore failure, stress path with maximum of fractal dimension is loading axialcompression and unloading confining pressure, path with smallest is conventionaltriaxial loading path.(3) Numerical simulations of rock loading and unloading failure process are carriedout by modifying FISH based on particle flow code, powerfully compensatemacroscopical lab tests from the microscopic view, as follows: numerical simulations ofcomplex unloading tests can be done effectively by designing different unloading stresspath. Relationship between stress path and mesoscopic energy such as friction energy,kinetic energy, bond energy and strain energy, relationship between mesoscopic cracksand failure precursors, and process of generating, propagating and penetratingmesoscopic cracks are studied, as follows: There is quite complicated nonlinearrelationship between meso-structure parameters and macroscopic strength parametersby simulating different unloading tests. Confining pressure has great influence onfriction energy caused by sliding friction between particles, further changes failuremode. The higher unloading rate is, the more cracks in rock interior developinadequately, the less bond energy is. Failure process is caused by combination effect ofboth compression failure and tensile-shear failure. Tensile-shear failure develops withdevelopment of compression failure, there is a concentrating trend of tensile-shearfailure along main shear surface.(4) The damage process of unloading rock has been studied from the microscopicview, the results of the examples show that analysis of unloading surrounding rockisfeasible, as follows: Mogi-Coulomb rule in classical failure rules is relatively moresuitable for experimental analysis under loading axial stress and unloading confiningpressure path; microcracks of rock can generally be assumed to be elliptic which candescribe more exactly strength criterion under unloading stress path from the point ofview of micromechanics and analytic function of single connectivity domain; thestress-strain relationship has been established during unloading failure process based on complex stress state of cracks of unit.(5) The assumption of surrounding rock classification about rock undergroundengineering has been put forward based on standard for engineering classification ofrock masses and the stability analysis and design theory of surrounding rock ofunderground engineering, as follows: first, qualitative characteristics of surroundingrock are improved and tweaked in table of surrounding rock classification. Forcorresponding to basic quality BQ values of different levels surrounding rock areadjusted for qualitative classifications corresponding to quantitative classifications.Then, surrounding rock classification ideas are put forward, benchmarked against rockmass quality and engineering span, to reflect the impact of span on stability ofsurrounding rock. Sub-classifications of station tunnel and running tunnel are provided,basic quality indicators of sub-classification are quantified, combined with characters ofunderground engineering. Quantitative indicators of rock self-stability are reflected bysafety factor and physico-mechanical indices of rock are obtained by the way ofinversion. By researching physico-mechanical indices and surrounding rockclassification of Chongqing Metro1#and6#, compared to national standard, theclassification method can get the following main conclusions: sandstone classificationof running tunnel promotes from III and IV to II and III, about a third of sandymudstone classification of running tunnel promotes from IV to III, about80%ofsandstone classification of station tunnel promotes from IV to III.(6) Designing and calculating methods of rock underground engineering have beendeveloped and perfected using numerical limit method, then reasonable designing andcalculating parameters and calculation method about lining have been discussed, asfollows: first, a new method of concrete shear strength is presented combing shear testsand uniaxial compressive tests under the present condition of laboratory, to determineshear strength indexes c、φ of several different strength grades of concrete; then, therelationship between shear strength and compressive strength of concrete has beendiscussed, stability of shear strength test results is validated and standard value anddesign value of concrete strength are taken usually into shear strength based onmohr-coulomb theory and numerical method. Some parameters such as loading burstsize, standard of dividing line, safety factors of rock about stress release and computingmethod of safety factor of surrounding rock and lining structure are determined withexamples.Finally, calculated results of Chongqing Metro and Qingdao Metro show that thickness of lining may be decreased in the III level surrounding rock; thickness orstrength of initial support should properly increase to ensure construction safety.