Experimental Study On Fatigue Characteristics Of Karst Areas Limestone And Engineering Application

It is known that the research on fatigue mechanical characteristics of rock under different loading condition is relatively lagging behind in institute foreign and domestic; a systematic research about the fatigue properties and damage evolution laws of karst limestone is absence in particular. For analyzing the long-term stability of surrounding rock between railway tunnel and karst cave, relying on the project of National Natural Science Foundation, a systematic research which take Yichang Wanzhou railway tunnel limestone as research subject was conducted by using combining methods of literature survey, laboratory testing, theoretical analysis and numerical simulation in this paper. The strain rate effect of limestone within static category was studied systematically; fatigue mechanical properties of limestone in different load conditions was explored; multi-factors fatigue life prediction models and fatigue damage evolution model were established and were applied in elastoplastic numerical calculations in this article. For the analysis of surrounding rock long-time stability when taking rock fatigue damage into account in karst tunnel, important basic information and research ideas for future reference were provided by those research results. The main research conclusions are as follows:(1) In the range of static loading rate, limestone has strong rate sensitivity. Tests show that the peak strength of limestone increases obviously with the rise of loading rate, and the relationship between peak strength and logarithm of strain rate is linear. However peak strain shows decreasing trend with the increase of strain rate. By analyzing the rupture process of limestone, it is concluded that there is little effect on unstable crack expansion phase while greater impacts on the stable crack expansion when increasing strain rate. The higher the strain rate, the later rock sample reaches the crack propagation stage (plastic phase) which means the proportion of plastic stage in the whole stress-strain curve decrease and that of elastic stage increase correspondingly.(2) The effect of tests procedure on the tests results of Schmidt hammer was studied, and a tests procedure of Schmidt hammer with smaller tests disturbance and more accurate was proposed. The accuracy for predicting limestone uniaxial compression strength (UCS) by using ultrasonic, Schmidt rebound and ultrasonic-rebound combined methods, three non-destructive estimation tests, was comparatively studied. Basted on the results, a non-destructive method for estimating the strength of standard samples was established. And, according to tests data, a two-parameter exponential model for predicting UCS of limestone was proposed(3) Fatigue mechanical tests under various loading conditions were carried out by using Yiwan railway limestone. The conclusion is that the fatigue life of limestone was trading to prolong linearly in the semi-logarithmic coordinate with the decrease of upper limit stress; the greater stress amplitude the shorter fatigue life of rock which means stress amplitude play a key role on fatigue life; rock fatigue "threshold value" is not only closely associated with rock types, and increased with the raise of loading rate; in the range of0.5-3Hz, fatigue life of limestone advanced with the growth of loading frequency, and they met a linear relationship in double logarithmic coordinates. According to the tests, the S-N curve based, taking into account the impact of supper limit stress and stress amplitude life estimation models as well as a multi-factors fatigue life estimation model based on loading frequency and supper limit stress were estimated.(4) The limestone fatigue deformation underwent initial deformation stage and stable development stage as well as acceleration phase three different stages. Contrasting fatigue terminal deformation and control deformation of static whole stress-strain curve, it is found that they were very close. It is hardly affected by changing loading frequency to the fatigue axial terminal deformation of limestone which means’the limit deformation law when approaching to fatigue failure’untouched by the load frequency. An average strain increment based fatigue life calculation model was proposed according to the development rules of limestone fatigue deformation. Only a limited number of cyclic compression tests and uniaxial compression tests were required to determine the model parameters, which greatly reduced the fatigue tests inputs.(5) Nonlinear deformation lagged effects of rock was explored and its cause was analyzed. Five common hysteresis loop shapes of cyclic loading and unloading deformation curves were summarized and the causes of those various forms were discussed also.(6) Uniaxial static compression experiment, cyclic dynamic tests and ultrasonic tests were adopted to research the relationship among static, quasi-dynamic and dynamic elastic moduli of limestone.(7) Fatigue tests results showed that the decay of limestone dynamic elastic modulus went through three different stages with the conduction of cyclic loading. Thus, a fatigue damage development model of limestone was established based on the residual stiffness. And, a set of calculation method which can consider dynamic fatigue damage of surrounding rock and suit for application in rock engineering was proposed by combining with multivariate estimation model of limestone fatigue life. The numerical programming producer of this method was achieved by utilizing a secondary development of FLAC3D software. And, it was applied to a dynamic simulation calculation of Luzhuba2nd tunnel under the action of earthquake disaster.