| The scientific evaluation and effective prevention of reservoir landslides are very urgent to ensure the safe operation of large-scale water conservancy projects.As one of the main control measures for landslides,stabilizing piles have been widely used in reservoir landslide control projects.Under the operation of the reservoir,the cyclical fluctuation of the reservoir water level will cause the periodic variation of the seepage field of the landslide.Which in turn causes the multi-field dynamic response of the stress field,displacement field and seepage field.This changes the state of the landslide and affects its stability and deformation evolution process.For landslides that have been implanted with stabilizing piles,the cyclical fluctuation of the reservoir water level will also affect the stabilizing effect of the piles,and influence the landslide prevention project.Therefore,it is urgent to study the multi-field characteristics and evolution mechanism of landslide-stabilizing piles system.This study should be based on the study of reservoir landslide deformation evolution and multi-field characteristics.These would provide a basis for landslide treatment and long-term stability analysis.At present,some engineering examples in the Three Gorges reservoir area show that,under the cyclical fluctuation of the reservoir water level,the landslides with stabilizing piles will still move.The treated landslide generally deformed in the middle and front parts,which leads to a significant reduction of the front resistance of stabilizing piles,and affects the prevention and control effect of the stabilizing pile.The Majiagou landslide in the Three Gorges reservoir area deformed since the reservoir was first impounded in 2003,with obvious cracks appeared at the trailing edge.After the engineering piles were implemented in 2007,the landslide did not stop.In order find the failure reason of the stabilizing pile of the Majiagou landslide and obtain the deformation rule of the landslide-stabilizing pile system,the Majiagou landslide in the Three Gorges reservoir area was seleted as a research object.Based on field test and numerical simulation,the related research on the multi-field characteristics and deformation evolution mechanism of the Majiagou landslide-stabilizing pile system was carried out.The main research results are as follows:（1）The spatial position distribution and motion characteristics of the three-layer sliding surface of the Majiagou landslide are determined.The shallow sliding surface S1is located at the base of the landslide,and its deformation is relatively small.The deep sliding surface S3 developed in the bedrock is located at the weak interlayer of the purple-red mudstone in the deep part of the landslide,which is the main sliding surface of the Majiagou landslide.The speed and shear deformation are the largest.The slip surface S2developed between the two sliding surfaces of the landslide is also located in the weak interlayer in the bedrock.Based on the reservoir impounding process and the landslide deformation process,it is determined that the induced factors of the deep slip surface S3and S2 are the reservoir water impound to the corresponding slip surface elevation,so that the soft layer in the landslide is soaked and softened by the groundwater level.（2）According to the monitoring results of landslide displacement,the deformation rules of Majiagou landslide are following:Firstly,the rate of change of landslide surface displacement shows fast and slow alternate step change characteristics;secondly,the research shows that the landslide deformation rate is mainly a dynamic response process to the reservoir water level decline.The landslide deformation response lags behind the reservoir water drop effect,and the lag time is about two months;the responses to the reservoir water decline of different parts of the landslide are not the same,the more near to the reservoir,the greater response of the landslide deformation to the reservoir decline;thirdly,the landslide displacement value show obvious differences in space and time.The deformation of back edge is lower than the landslide front and the response time is lag about two months.（3）Based on the field test and multi-field monitoring of the landslide-stabilizing pile system,the multi-field response characteristics of the system were revealed.The multi-field monitoring of the system includes two aspects:landslide and test piles.The landslide monitoring includes surface displacement monitoring,deep displacement monitoring and fiber monitoring.The test piles are monitored by the axial force of the reinforcing bars,the concrete strain monitoring of the pile body,and the soil pressure gauge of the pile.The analysis of the monitoring results shows that:First,the seepage field of the landslide has a dynamic response to the periodic fluctuation of the reservoir water level.The performance of the landslide front is basically synchronous with the reservoir water level,and there is about 1 to 2 months lag between the fluctuation of the groundwater level in the middle parts and the reservoir water level.Secondly,there is a partial anti-sliding effect on the soil around the pile.But under the cyclical reservoir water level drop and rainfall,the test pile is not enough to withstand the cyclic load.Thirdly,the monitoring results of stress,strain and bending moment of the test piles are consistent,and the piles are bent and deformed toward front at the sliding surfaces S2 and S3,and the piles are bent backward at the sliding surface S1.Synchronous deformation occurs with the sliding body of the piles.Fourthly,the deformation evolution of the landslide-stabilizing pile system has dynamic response characteristics to the landslide seepage field.During the reservoir drawdown,the landslide deformation and the deformation curve of the test piles with time both present"step-type"growth.（4）Based on the improved support vector machine algorithm and the finite element method,the coefficients of the Majiagou landslide were inverted.The uniform sample design method is used to design the training sample parameters of the landslide permeability coefficient.The saturated-unsaturated seepage field finite element calculation software is used to generate the learning samples.The improved support vector machine is used to establish the mapping relationship between the permeability coefficient and the water head,and the landslide groundwater level monitoring is adopted.Data intelligent inversion of landslide permeability coefficient,in which the permeability coefficient of gravel soil is 1.17×10-2cm/s,the permeability coefficient of fractured rock mass is 6.43×10-4cm/s,and the permeability coefficient of bedrock is 5.72×10-5cm/s.The inversion value is brought into the finite element model to analyze the landslide transient seepage.The results show that the hydrodynamic pressure value and the landslide affected range are greater when the reservoir water falls.（5）Based on the fluid-solid coupling numerical simulation method,two external factors（reservoir water level and rainfall）were considered.Multi-field characteristics and evolution mechanism of the Majiagou landslide under the condition of no piles,implanted piles and implanted test piles were studied.The results show that:First,the reservoir water level fluctuation directly leads to the dynamic fluctuation of the landslide groundwater level,leading to the redistribution of landslide stress and the response of displacement field.Second,the different parts of the Majiagou landslide have different responses to reservoir water level fluctuations and rainfall.The front part of the landslide is mainly affected by the drawdown of the reservoir,but the rear part of the landslide is more sensitive to the rainfall.Thirdly,the engineering piles have a certain partial slip effect on the sliding surface S1.However,the engineering piles increase displacement of the front part of the landslide due to the increase in body weight compared to the in situ soil.Fourthly,there are local resistance for the landslide by test piles.But after the reservoir water level drops,the deformation of the front of the landslide is intensified,the resistance in front of the piles reduced,leading to co-deformed of the test piles and the landslide.Fifthly,the drainage measure for the reservoir landslide under the reservoir operation was proposed.This method was verified by numerical simulation method.Calculation results showed that the drainage measure for Majiagou landslide has a good control effect on the front and middle portion of the landslide. |
