| Landslide disaster is one of the major geological disasters threatening the safety of human life and property,and it is especially severe in the Three Gorges Reservoir area of China.Therefore,it is of great practical significance to study the mechanism of landslide deformation.Generally speaking,the shear strength parameters of rock and soil are obtained by field sampling,indoor or in-situ tests(direct shear,triaxial,etc.)to analyze the stability of landslide,and the safety factor of landslide is further calculated.Due to the limitation of site conditions,in-situ test can only be carried out on the sliding mass,but cannot be carried out effectively on the sliding zone.Sliding zone is the key structure to control the launch and movement of landslide,with the soil structure and shear strength usually quite different from that of sliding mass.Therefore,in-situ test on sliding zone can theoretically reflect a more real state of landslide stability.At present,creep test is the only way to study the long-term deformation and strength properties of rock and soil.For landslides that have not yet occurred or are experiencing slow deformation,in-situ creep test can effectively investigate the deformation characteristics of sliding zone soil,thus providing a more reasonable basis for landslide stability evaluation and prediction.The Huangtupo landslide is the largest landslide in the Three Gorges Reservoir area.According to the monitoring data,the landslide is in a slow deformation process for a long time(the surface displacement in front of the landslide is about 30 mm/year);the burial depth of the sliding zone is about 50-70 m.Due to the influence of long-term vertical load,the structure of the sliding zone is compact,which is significantly distinct from the remolded sample.Based on the above characteristics,in-situ creep tests were carried out on the sliding zone soil of No.1 Landslide(the largest deformation sub-landslide of the Huangtupo landslide).Based on in-situ creep test of sliding zone soil,the mechanism of deformation characteristics of in-situ sliding zone soil is further explored in this thesis.By comparing the large-scale direct shear test with the in-situ test,the strength difference between the remolded sample and the undisturbed sample is found,that is,the compressibility of the short-term consolidated soil conducted in the laboratory test is much greater than that of the long-term consolidated soil in the natural state,and the difference of shear strength parameters caused by this may bring great errors to the stability evaluation of landslide.Two sets of shear strength parameters are set,one is considering the influence of structural capacity,the other is not considering the influence of it.The simulation of landslide launch input with the two groups of parameters are carried out respectively.The comparison of numerical tests proves the necessity of considering soil structural capacity,which provides a real and reliable basis for landslide stability evaluation.The main research contents and achievements are as follows:(1)By carrying out the in-situ triaxial creep test,the deformation characteristics of the sliding zone soil are obtained;after applying various levels of axial compression,creep continues and the specimens do not undergo significant damage,showing attenuation creep behavior;in all aspects of the specimens,the deformation of the 35°(horizontal orientation)surface is the most obvious,showing characters of orientation along with other surfaces,that is,the deformation of the 35°surface is proceeds along the sliding direction of the landslide,which is closely related to the cracks and the orientation of the rock arrangement in the specimen.It is known from the test that the compressibility of the undisturbed sliding zone soil is very low,which is attributed to the high rock content(54.3%)and the long history of consolidation.The compact structure formed in the soil mass has not been destroyed.In order to explore the deformation characteristics of undisturbed soils,the constitutive model of sliding zone soils is constructed,and the relationship between stress-strain-time is obtained by considering the time effect,which is then verified by fitting test data.(2)In view of soil structure,the reasons of low compressibility and high strength of undisturbed soil are explained respectively from the micro and micro levels.In order to further study the difference of shear strength between undisturbed and remolded soils,large-scale direct shear tests of sliding zone soils were carried out.Compression tests of remolded soils and undisturbed soils are carried out.The results show that the compressibility of remolded soils is much greater than that of undisturbed soils under the same consolidation stress.Two groups of direct shear tests of over-consolidated soil and isobaric consolidated soil were carried out,respectively 800 kPa consolidation stress for the over-consolidation group and 200 kPa,400 kPa,600 kPa and 800 kPa consolidation stress for the isobaric consolidation group.The test results show that the cohesion of over-consolidated soil is obviously greater than that of normal consolidated soil,which comes from the recovery of structural force.(3)According to the above two sets of shear strength parameters,the numerical simulation of the Huangtupo No.1 landslide is carried out to analyze and verify.In order to analyze the stability and failure process of landslide,it is necessary to measure the residual strength of sliding zone soil,thus the undrained ring shear test is carried out.The ICL-2 ring shear apparatus developed by Kyoto University in Japan was used to carry out undrained shear test.The purpose of the test is to accurately obtain the residual strength of landslide under undrained conditions and provide reasonable parameters for numerical simulation.(4)Two sets of parameters,i.e.the parameters of sliding zone soil considering structural capacity and those obtained normally from indoor direct shear test,are input into LS-RAPID software to simulate the launch and movement process of landslide,and the pore-water pressure ratio(r_u)is used as the external dynamic factor to trigger landslide.The results show that for the first set of parameters,the leading edge of the landslide will be destroyed and failure gradually spread to the whole landslide when r_u rises to 0.28;for the second set of parameters,the simulation results show that the landslide is in an unstable state just at the beginning,and large-scale damage has occurred before the pore water pressure is applied.Results drew from this thesis show that the characteristics of undisturbed soils,i.e.the structural capacity of soils,should be fully considered in the study of sliding zone soils.On this basis,the stability evaluation and prediction of landslide will be more scientific.At the same time,due to the limitation of test conditions,only one in-situ test has been carried out in this thesis,and the in-situ test should be further promoted,which will be more conducive to the scientific research and engineering treatment of landslides.The main innovations of this thesis are as follows:(1)Based on the tunnel site of the Huangtupo landslide in Badong County,the in-situ creep test of sliding zone soil was carried out,and the long-term deformation characteristics of sliding zone soil under in-situ stress are innovatively studied.Based on the test results,the causes of low compressibility of sliding zone soil are put forward,and the effect of structural capacity of undisturbed soil on its deformation is analyzed.The constitutive model of stress-strain-time sliding zone soil is established,and the undisturbed sliding zone soil is revealed.(2)The concept of soil structural capacity is put forward,and two groups of specimens are set up.One group restores the structural capacity,the other group does not consider the structural force,then the indoor direct shear test is used to test its shear strength.The influence of structural forces on shear strength parameters is revealed through the comparison of tested shear strength.(3)Employing the LS-RAPID,a professional landslide simulation software,two sets of parameters are simulated.The software is sensitive to the input of shear strength parameters,and can accurately analyze whether the landslide launches due to the different input parameters.According to the results of numerical simulation,it is revealed that evaluating whether landslides occur or not should take full account of the structural characteristics of the sliding zone,that is,the structural capacity of the sliding zone soil. |
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