The In-situ Stress Distribution Within Alpine Slope And Its Influence On Engineering Stability Of Jointed Rock Mass

Alpine valley area with special geological and geomorphological features is a long-term complex geological processes and the river surface weathering. This causes the original stress field redistribution and adjustment of alpine valley area, and formed the current special stress field distribution. In addition, owing to the complex geological structure, river valley area of modern tectonic activity is strong, and joint fracture is active in the slope rock mass. Owing to the existence of the joints and fissures, rock mass showed a strong heterogeneity, discontinuity and anisotropy, this impact the rock mass’s properties of strength and deformation. Underground engineering in the mountain valley, will inevitably cause deformation of rock mass, then, the release of in-situ stress and adjust, affect the stability of the joint. Therefore, the underground projects built in the mountain valley, we should take into consideration the stress field distribution and the stability of the joints, then to analyze the mechanics characteristics of rock mass engineering.This paper used the finite difference software FLAC-3D to research the stress field distribution characteristics in mountain. Considering the influence of topography and geomorphology, gravity and tectonic stress, weathering denudation on the mountain valley to the the stress field:(1) Different mountain slope will bring massive influence on the distribution of initial field stress. Through numerical analysis, using the different mountain slopes to research the initial in-situ stress’s value and distribution under the condition of the gravity and different horizontal pressure coefficient. Get the slope effect exist on the slope surface from a horizontal distance, and with the increasing of mountain slopes and tectonic stress, the effect is more obvious. And points out that the construction of the underground engineering in this area, we can’t simply take the upper weight of the mountain as the in-situ stress.(2) On the basis of the first step, considering the river incised, established the form of "V" symmetrical distribution of different slope valley models, in the meantime, set up four levels of different weathering zone in the valley surface, to simulated the effects of the unloading. Get the in-situ stress distribution characteristics in the valley of weathering denudation. Take the numerical results compared with the in-situ stress, verified the valley "hump" type distribution characteristics of ground stress field. At the same time, take the numerical results in the slope of 45 degree and 2.25 horizontal tectonic stress compared with the in-situ stress in Er-Tan project. The results are extremely similar, this verify the numerical simulation is good.Secondly, this paper introduces how to use the model test and numerical method to study the strength and deformation of the joint rock mass characteristics. Through the joint specimen with three fissure Angles of 30°,45°,60°and whole specimen’s uniaxial compression in the indoor experiment, research the existence of joint how to influence the rock mass’s strength and deformation characteristics And combined with the field investigation of fissures distribution of rock mass, using Monte Carlo method, to generate the random joint numerical specimen, using the random load numerical test to study the fissure distribution and variability of mechanics parameters how to influence the strength of joint rock, the size of REV are also studied.Thirdly, this paper puts forward a second envelope line of rock mass strength criterion, to meet the REV size, the numerical simulation of the load under different confining pressure test specimens, using the stress strain curve obtained from the experiment to get the secondary envelope of the specimens, as joint rock strength criterion, which laid a foundation for calculation of engineering examples.Finally, combined the equivalent mechanical parameters of joint rock mass is obtained by REV and the distribution characteristics of stress field in the mountain valley area together, studied the distribution characteristics of stress field in the mountain valley how to influence the stability of joint rock mass engineering. In V region and alpine mountain valley in different elevation distance from slope foot, three different caverns are excavated respectively, using the joint equivalent mechanical parameters of rock mass for numerical analysis, comparing the surrounding rock stability condition, get the following conclusion:(1) The horizontal in-situ stress is the important factor affecting the stability of joint rock mass. In general, with the increase of the horizontal ground stress, after the cavern excavation, the fracture zone increases also. But when the cavern in the depths of the mountain, vertical become the direction of maximum principal stress, with the increase of the horizontal ground stress, the maximum principal stress and minimum principal stress’s difference reduces, it is good for the stability of cavern surrounding rock after excavation, the fracture zone will be decreased with the increase of the horizontal ground stress.(2) The slope effects are important factors which influence the stability of joint rock mass. Excavation nearer the cavity from the slope toe, the greater of the volume of the fracture zone, adverse to the joints’ stability of surrounding rock.Also illustrates the calculation of in-situ stress of conventional practices-using the upper rock mass directly buried depth h as initial vertical geo-stress, within a certain distance within the mountain close to slope surface is wrong.(3) In the valley, cavern excavation should try to avoid stress peak area and the increased stress area. Elevated stress area is usually exists in the weathering zone, despite the low level of stress in the area, but as a result of rock mass weathering by the outside world, rock mass mechanics parameters is low, not conducive to the stability of surrounding rock. And the peak stress area with the highest elevation stress level, maximum principal stress and minimum principal stress difference is the biggest, extremely adverse to the stability of cavern surrounding rock By contrast, in the original rock stress zone, the maximum principal stress and minimum principal stress difference between small, it is good for stability of surrounding rock after excavation.(4) In the valley, the lower the position that the cavern excavation, the more stability of surrounding rock mass.The influence of the reason is that the bottom of the valley have high stress concentration -- "high stress package ". In 0 m elevation, due to the distance close to the bottom of the valley, near the slope toe itself is seen as a stress concentration area, therefore, stress level is higher, is not conducive to the stability of surrounding rock; With the increase of elevation, the influence of "high stress package" also gradually decreases, and stress level is gradually reduced, thus, after cavern excavation, fracture zone also gradually reduced.