Study On Triaxial Test Of Rock Under Loading Or Unloading Conditions And Its Application

Unloading failure of engineering rock mass caused by excavation is a common phenomenon and been paid more and more attention. The mechanical properties of rock mass under unloading conditions are essentially different from that under loading conditions, which been researched by many kinds of methods. However, there are many problems need further research because of the invisibility of rock mass and the complexity of unloading problems.75percent of the land rock is sedimentary rock and sandstone is a typical sedimentary, which is widely distributed on the ground. Mechanical properties (including deformation features, strength criterion, constitutive relation and mechanical parameters, etc) of sandstone in Yi Chang under loading or unloading conditions are researched by tri-axial test in this dissertation. The main results are as follows:(1)30MPa is lower than the inversion confining pressure of selected rock samples, which is the highest initial confining pressure. Therefore, the loading and unloading failure of rock samples show the feature of brittle failure and the feature of median grained sandstone samples are more obvious than that of fine-median grained sandstone samples. For the median grained sandstone samples, when the unloading rate is low, the loading and unloading failure modes transform from tensile-shear failure to compression-shear failure with the increase of the initial confining pressure and the latter one is more general, while tensile-shear failure is more general when the unloading rate is high. For fine-median grained sandstone samples, the loading and unloading failure modes mainly are compression-shear failure. Fracture planes all run through the sedimentary weak planes and are not affected by them.(2) The higher the initial confining pressure is, the lower the confining pressure difference and axial pressure difference are, which illustrates that unloading failure are more likely occur in higher initial confining pressure and more attentions are need to paid in high stress especially in unloading failure problems. Rock samples show more brittleness in larger unloading rate. The stress difference changes little with the increase of unloading rate, while the confining pressure difference increases with the increase of unloading rate, and time difference between the starting point and ending point of unloading process shows the trend of decrease.(3) The peak strength of sandstone in unloading failure is higher than that in loading failure when initial confining pressure is low, while the result is just opposite when initial confining pressure is high, and the ratio of variation values is generally less than10%.(4) Based on tri-axial tests of median grained sandstone with parallel and vertical sedimentary planes, the orthotropy of strength feature, deformation regularity and the mechanical parameters is studied. The test results show that in the same confining pressure, the strength values of rock mass with vertical sedimentary planes are larger than those with parallel sedimentary planes. The strength difference between the rock with parallel and vertical sedimentary planes decreases gradually with the increase of confining pressure, which means the decrease of the anisotropy. The deformation parameters and shear strength parameters of the rock mass differ under unloading and loading conditions, which is largely related to the anisotropy of rock mass.(5) The mechanical parameters of rock samples will be degraded during the redistribution of stress in unloading process and the degradation is a dynamic process. The relation between mechanical parameters of sandstones in Yi Chang and the unloading degree of confining pressure is deducted based on tri-axial tests.Test results show that the deformation modulus of the rock samples decreases in the unloading process with the decrease of the confining pressure, which decreases sharply with increase of the unloading quantity of the confining pressure. The axial stress decreases rapidly to residual strength when unloading failure occurs. Meanwhile, the deformation modulus also decreases sharply, which is only14-30%of the values in tri-axial compression failure. Unloading of confining pressure affects shear strength parameters (cohesion c, internal friction angle φ) obviously, and cohesion c is more sensitive to the change of confining pressure.The mechanical parameters of rock mass(sandstone) in numerical simulation of excavation and unloading can be adjusted as follows:When unloading ratio is less than30%, the deformation modulus E decreases1-7%, the internal friction angle φ increases0.1-4%and the cohesion c decreases0.2-1.2%. When unloading ratio is30-50%, E decreases3-17%,φ increases1.4-8%and c decreases3-4%. When unloading ratio is50-80%, E decreases8-31%,φ increases3.8-7%and c decreases7-17%. When unloading ratio is80-90%, E decreases15-38%,φ increases5-18%and c decreases6-24%. When unloading ratio is90-100%, E decreases29-61%,φ increases9-28%and c decreases24-45%.Take upper limit values in higher initial stress, larger unloading rate and with larger deformation modulus in tri-axial compression failure. Conversely, take lower limit values.(6) Based on the test results, the strength criterion of sandstone in Yi Chang under unloading condition is researched, and the elastic-brittle-plastic constitutive model which is suitable for the unloading rock mass is deduced in this dissertation.(7) Based on summarizing documents, the definition of excavation unloading areas is improved, and it is explained as:"There is adjustment of stress in rock mass within certain areas which is caused by slope excavation. And new mechanical equilibrium state is built. The adjustment of stress causes to decrease of bearing capacity of rock mass, degradation of macro mechanics parameters and decrease of stability of rock mass in certain areas, which is called excavation unloading zone".(8) Excavation unloading zones are divided according to analysis of the first, third principal stress, and stress vertical to the excavation surface calculated by FLAC software. Based on that, point safety factor method is put forward based on Mohr-Coulomb criterion to divide excavation unloading zone, which adopts the decrease percentage of rock mass point safety factor to quantization the division of excavation unloading zones. Combine the consideration of point safety factor method with stress analysis to divide unloading zones in slope excavation.