The Evolution And Catastrophic Mechanism Of Talus Slopes In Alpine Areas

Talus slopes consist of homogeneous rock granules with unique physical and mechanical characteristics of loose structure, high compressibility, and low cohesiveness. Due to its poor stability, frequent recurrence, and high burstiness, talus slopes have great impacts on the construction and operation of highway, railway and other infrastructure in alpine areas, even playing a crucial role in some regions. Talus slope are mainly distributed in alpine gorges and areas with strong earthquake activities, with characteristics of wide distribution, high recurrence frequency and diversity of chain reactions. They tend to evolve into a series of chain disasters, posing long-term hazards to the human engineering activities. Meanwhile, they can also bring many ecological environmental problems for our country in establishing the green channel concept. It is obvious that with massive infrastructure construction, studies on the evolution and catastrophic mechanism of talus slopes will not only develop practical guidance for geohazard prevention of mountainous engineering facilities such as highways, but also enrich the theoritical research on geohazards.This study selected Tianshan Highway and China-Pakistan Highway as typical case studies of talus slopes in alpine regions. The distribution rules and the structural characteristics of talus slopes in both highways were ascertained on the basis of field investigations, geological and environmental data, literature review and so on. Then, considering freeze-thaw cycles, ice wedging and other special outside dynamic effect in alpine areas, indoor tests were employed to study the micro transformation and macro cracking mechanism of talus caused by cyclic freezing and thawing and ice wedging. Using repeated 3D laser scan on some fixed locations, we measured and calculated the fragment production rate of exposed bedrock. Afterwards, through comprehensive analysis of triggers of talus slopes failures and physical simulations and numerical simulations, the failure mode, catastrophic mechanism and deposition characteristics of talus slopes under rainfall or snow melting and rockfall impacts were verified. The chain disaster effect of chain reaction of slopes after failures was analyzed, and the formula for debris flow characteristic parameters computing was put forward.To sum up, this dissertation takes Tianshan Highway and China-Pakistan highway as typical case of study for talus slopes in alpine regions, and systematically discusses the evolution process and catastrophic effect of talus slopes, providing the basic catastrophe pattern and design parameters for engineering management of this special type of slopes. The main results are as follows:(1) Beginning with the distribution and geological background of talus slopes along Tianshan Highway and China-Pakistan Highway, the spatial distribution rules and linear density characteristics of talus slopes were attained through the detailed investigation along the highways. According to its characteristics combined with literature review, the reclassification of talus slope was developed, verifying talus slopes without clay and talus slopes with clay as two additional types of talus slopes. The typical characteristics of talus slopes without clay included relatively big difference in grain size and obvious gravity separation. And they were mainly composed of gravel and breccia, and some were composed of coarse and medium sand without clay, characterized as loose and little cohesive force etc. "Dual structure" was found in colluviums of talus slopes with clay. The slope deposition consists of incohesive external layer made up of gravels and bottom layer with certain cohesive force. The forming process of talus slopes without clay could be summarized as high stripping, accumulation migration and slip. As for talus slopes with clay, the process could be summarized as high stripping, accumulation migration, rock decomposition caused by freeze-thaw, leaching consolidation, creep etc.(2)Through detailed and intensive study on rock micro crack propagation under freeze-thaw cycles, it was found that its early strength of three types of rocks(granite, phyllite, sandstone) tended to increase rapidly and the late strength tended to decrease gradually under the three kinds of operating condition(natural, saturated with normal temperature, and saturated with high temperature). It is possible in the early freeze-thaw period, that migration of mineral components into micro crack space and the crack shuts up afterwards contributes to the increase of rock strength, while the decrease of late strength may due to partial connection of micro cracks which is shut in the late freeze-thaw period after several cycles.(3) On the basis of the degradation characteristics of rock mass structural plane in alpine areas, through frost wedging tests of rocks with artificial cracks, it was revealed that the deformation process of rock mass could be divided to frozen shrinkage stage, frost heaving stage and thawing shrinkage stage. The micro strain of cracks at the top and bottom of rocks during freeze-thaw cycles increases with the growth of number of freeze-thaw cycles. The micro strain of cracks at the top of the rock sample was about ten times of the ones at the bottom of rock samples. The quantitive relationship between micro strain of top crack with the same crack depth(y) and freeze-thaw cycles(x) was y=5.2288x2-160.9x+2484.1.(4) For the analysis of the slope formation rate, in-situ repeated 3D laser scan were applying to four kinds of geotechnical slopes along Tianshan Highway and China-Pakistan Highway such as granite, sand slate, phyllite and valley debris. The annual fragment production rates were estimated: 0.0601(m3/?·a)for debris slopes, 0.0448(m3/?·a)for phyllite slopes, 0.0361(m3/?·a)for sand slate slopes, 0.0146(m3/?·a)for granite slopes. It was found that fragment production rate in summer and autumn was higher than one in spring and winter.(5) Based on the mechanism analysis for talus slope initiation induced by rainfall or snow melting, combined with field investigations and physical simulation, four initiation patterns was put forward, including fluctuation-type, S-type, S-pulse-type, pulse-type. The quantitative relation y=-6×10-5x2+0.0072 x between the scour depth(y) and rainfall intensity(x), and y=-3×10-7x3+3×10-5x2-0.0004 x between sand flow rate(y) and rainfall intensity(x) were established. The talus slopes without clay tended to advance gradually in superficial layer, while talus slopes with clay tended to advance gradually in superficial layer and fail in middle to deep layer.(6) For the failure mechanism of talus slopes induced by rockfall impacts, both physical and numerical simulation were employed, the influences of rockfall energy and impact frequency on slope stability were simulated. A power function with power exponent as two between failure mass of talus slopes and rockfall mass was established. It was also found that when the impact frequency changed from 5s to 3s, failure mass of talus slope increased by about 16.4%, while impact frequency increases from 3 s to 1 s, the failure mass increased to 74.5%. Thereby, we put forward the high frequency continuous impact effect.(7) Considering the peculiarity of granule mixtures as debris flow source material in alpine regions, we proposed the four stages of hazard evolution, including inoculation, catastrophe, development and circulation, revealing the motion phases of the subsequent debris flow, including water flow containing sand?dilute debris flow?viscous debris flow surge?viscous debris flow?viscous debris flow surge?dilute debris flow?water flow containing sand. The surge was characterized with big “head”, flat and short “body”, thin and long “tail” and so on. The basic characteristic parameters computing formula for debris flow along Tianshan Highway and China-Pakistan Highway was established.