Reaserch On Unloading Effects Of Deep-Incised Valley Under High Geostress And Engineering Environment After Earthquake Disturbance

The country emphasizes that put speeding construction of traffic facilities on the priority position during the second ten years of the western development. As the important transportation corridor of the proposed railway into Tibet through the east margin of Tibetan Plateau, deep-incised valleys in southwest China have geological environment characteristics of high geostress-strong unloading and earthquake disturbance. Therefore, reasearch on the engineering geological environment of the valley zone is significant for the reasonable layout planning, rapid consturction and secure operation of the quasi circuits.Research methods, main contents and main conclusions in this paper are as follows:(1) In order to research the unloading effects of the deep-inscised caused by the geological evolution background of high geostress-strong unloading, established the concepualization model of deep-incised unloading valley by ABAQUS finite element soft. Simulated the forming process of the deep-incised valley with the river undercutting in the high geostress by loading a triangular distribution highland stress with depth as the initial boundary conditions and the mothed of unloading from top to bottom. Compared unloading effects including plasticity yield zone, shear stress, as well as tensile stress near the slope surface of three highland stress-unloading valleys and the gravity stress-unloading valley, and used "shear stress playing coefficient " on the basis of Mohr-Coulomb strength theory to estimate quantifiationally the depth of slope rock mass shear failure under uloading. Drawed the conclusion that the valley unloading effect is stronger under high geostress based on the above analysis. While, the result from the elevation analysis show that the unloading effect in the low elevation zone is more significant. Railways consturction in high geostress-unloading valley may face kinds of geologic hazards such as landside, rock mass collapse and rockburst and so on which need various measures to prevent and control. Meanwhile, raising blindly the line height in order to avoid abominable geological conditions in the low elevation will lead to the extending of line development and make the construction of bridges through the valley more difficult. Thus, railways planning in this kind of zone needs consider by combination of many factors.(2) According to research the influence on the engineering geological environment of deep-incised valley under high geostress caused by earthquake disturbance, applied ABAQUS dynamic finite element technology for simulation of valley seismic reponses on the basis of saving the unloading effects. Show the development process of the plasticity zone of the slope rock mass by using the time-integration. The time-domain analysis result shows that by constract of the acceleration on the point inside the slope, it amplifies in some degree on the slope surface point. During the same earthquake, the horizontal vibration of the slope surface point in the high geostress-unloading valley is more intensive than the gravity stress-unloading valley broadly. The result form comparing the stress-strain field of the high geostres-unloading valley before and after the earthquake means that the earthquake disturbance makes the accumulation of stress and strain inside the supergene structure of the slope side release rapidly to lead to a new round of deformation and failure of rock mass on the slope side and the geological conditions in the low-middle elevation to cotinue to worsen allover, then transforms the valley stress-strain field to the negative direction to force the railway construction after the earthquake to face more severe engineering geological environment. Therefore, it is put forward that the earthquake histories should be taked into account for line planning in southwest deep-incised valley besides the background of high geostress-strong. During the railways construction in the high geostress-unloading valley after the earthquake, more secure line layout planning should be formulated, and abundant pre-arranged plans and intensive control measures for various of geological disasters should be taked.(3) Established the numerical simulation model based on layue river valley along the Sichuan-Tibet road.Analysis the slope stability relatively systematically by the strength reduction method of FEM.The result means that both of the high geostress-strong unloading and earthquake had significant influences on the slope stability. The excavation of the proposed circuit in the valley zone is liable to touch off slope failsure. The amount of excavation shoud be controlled strictly and take enough measures to prevent and dispose the slope failsure.