| Landslide is one of the most common geological hazards in the world,and it is extremely destructive and has a wide range of impacts.China has a vast territory,a large population,diverse topography and geomorphology,and complex geological structures.Since ancient times,China has been a country with frequent slope geological disasters.Due to the rapid development of country's economic construction and the demand for a livable earth,engineering construction and the safety of life and property will be severely affected by landslides.Soil arching effect is defined as the stress transformation between deformable soil or rock and adjacent non-deformable parts.The previous research mainly focused on the soil arching between the slope stabilizing piles,while neglecting the important role of the slope internal arching effect.Many observations of landslide deformation and slope failure reflect the role of soil arching in the stability of landslide slopes.Therefore,it is urgent to conduct a systematic study on the role of soil arching in slope deformation and slope failure,and provide a scientific basis for engineering practice based on this.From the perspective of slope deformation and evolution,this thesis summarises the morphological characteristics of typical arching-type slopes and necking-type slopes and establishes the geomechanical models of the two type slopes.According to a combination of physical model tests,numerical simulation,and theoretical analysis,the changes of various physical fields in the slope models were observed and summarised.This study focuses on revealing the deformation and failure mechanism of arching-type slopes and necking-type slopes based on the soil arching effect.Finally,the stability analysis of the two types of slopes is applied to engineering cases.The main research content and results are summarised as follows:(1)Build simplified geomechanical models of arching-type slope and necking-type slope.From the aspects of topography and geomorphology,geomaterial properties,and geological structure conditions,the formation conditions and triggering factors of the two types of slopes are analysed and summarised as well as the deformation and failure process of two types of slopes according to the data collection of typical arching-type slopes and necking-type slopes.Based on the slope morphological characteristics,geomaterial composition,triggering factors,etc.,simplified geomechanical models of arching-type slope and necking-type slope were established.For the geomechanical model of an arching-type slope,the four main characteristics are summarised as follows:(1)there is non-uniform or relative dis-placement between the yielding and unyielding parts of slope;(2)the unyielding part is supported;(3)the locally yielding part is due to the loss of support in the slope;(4)the slope lies on a bedding plane with lower friction.With regard to the formation and failure process of soil arching,the formation and evolution process of the necking-type slope can be divided into the initial stage,the compaction stage,and the failure stage.In different deformation stages,the strength of geomaterials and the overall displacement strain of the front edge of the slope body also have different characteristics.(2)Self-designed multi-smartphone measurement system and multi-field monitoring system in the physical model test.Based on the structure-from-motion technique in computer vision,a multi-smartphone measurement system(MSM)was designed and developed to monitor the three-dimensional morphological deformation of landslides in model tests.The system mainly includes nine mobile phones,a USB expansion slot,control computer,group control software and data analysis and processing software.By testing different types of landslide models in static and dynamic tests,the measurement accuracy of the system is 1.1 mm,which can meet the requirements in slope model tests.Compared with traditional three-dimensional morphological measurement methods,such as 3D laser scanners,this system has the characteristics of short-period monitoring,low equipment prices,and easy construction and development.Together with the multismartphone measurement system,a multi-field monitoring system for slope model test was developed.The multi-field monitoring hardware part uses nine mobile phones,a digital camera,a high-speed camera,an infrared camera module,and a deep flexible Inclinometer,an optical fibre sensing system and seven earth pressure cells,and a data logger.The multi-field monitoring system includes the monitoring of "point","line",and "area" information.Among them,"area" information includes surface temperature,surface displacement,surface strain,surface velocity and three-dimensional shape monitoring,"line" information includes deep displacement and deep strain monitoring,and "point information" includes soil stress monitoring.(3)The self-developed multi-field monitoring system method is used to study the deformation and failure characteristics of the arching-type slope.The displacement field,strain field,velocity field,temperature field,and stress field information of the archingtype model slope in the process of excavation were monitored and recorded.The deformation characteristics of the arching-type slope under the soil arching effect were revealed.(1)The field information obtained through the various sensors or techniques successfully records the complete process of slope failure due to excavation and provides a qualitative interpretation of the failure evolution process.(2)Displacement field monitoring demonstrates that relative vertical displacement mainly occurs between the arch-shaped yielding area and two stationary side parts.Three stages of slope deformation are identified based on the maximum displacement of the slope,namely,initial deformation stage,uniform deformation stage,and accelerated deformation stage.(3)Based on the recorded strain field,the triangular slip plane in the slope gradually expands and extends during excavation,which sustains the increased unsupported part and maintains the slope stable.With the observation of the changes in the stress field,the direction changes in the major and minor principal stresses suggest that self-weight transformation loading occurs from the lower middle area to the two sides.Based on monitoring of the displacement,strain,and stress fields,the evolution process of the arching effect in the slope under excavation is detected.The deformation characteristics of the slope related to arching failure are also demonstrated through an arch ring.(4)In regard to the measurement of the slope velocity during failure,failure of the arch-shaped area is initiated on the right side and expands across the whole area.The slope failure time can be effectively predicted by applying the inverse velocity method.In addition,by monitoring the temperature field and 3D deformation,the post-failure behaviour includes cracks and failure volume of the slope.(4)Arching-type slopes under different angles and different relative densities were carried out,and the mechanism of deformation and failure of the slopes was further summarised.(1)In summary,the yielding area,including the arch-shaped large deformation area and upper small deformation area with the boundary of the arch ring,and two unyielding sides were clearly observed and considered features of arching-type slope deformation related to the arching effect.According to the calculation of lateral resistance and the total resistance,the arching effect plays an important role in understanding the deformation behaviour and failure mechanism of arching-type slopes.With regard to the process of final failure,toe failure takes place first,and sliding failures occur successively in the central area following tension cracks.Finally,bulking failure appears on the lower slope.(2)The deformation of the arching-type slope was caused mainly by the loss of the support and increasing self-weight,which were affected by the slope angle and relative density.The deformation process of the slope can be divided into a constant stage and an acceleration stage.From the displacement curve,the relative density determines the slope of the curve in the acceleration stage,and the slope angle is related to the switching point between the constant stage and acceleration stage.A large relative density slope tends to result in shear strain in the lower part of the yielding area.In addition,with the decrease in the slope angle,the excavation widths for the stress redistribution inside of the slopes tend to increase.(3)In arching failures,slopes with higher relative densities can generate larger initial kinetic energy due to the large failure volumes and accumulated potential energy,which may cause serious consequences.(5)Carried out the numerical simulation and theoretical analysis of the arching-type slope,and quantitatively evaluated the stability of the arching-type slopes.Through the discrete element numerical software PFC3 D,a numerical arching-type slope is established,and numerical experiments are carried out.The numerical model has an acceptable agreement with the observation of the physical model test in the displacement during the excavation.It also provides the declination of the contact force inside the slope,which helps better understanding the role of the arching effect in the slope.Based on the free surface analysis and the Mohr-Coulomb yield criterion,the static equilibrium equation of arching-type slopes is established according to the continuum theory.With the boundary conditions obtained on the unstressed surface,the critical excavation width and stable arch curve of the three-dimensional arching-type slopes are obtained.According to the excavation width and the critical excavation width,the stability coefficient Fs of the arching-type slopes is established to evaluate the slope stability.By theoretically deriving formulas,the stability of the arching-type slopes mainly depends on the slope excavation width,boundary conditions,and the properties of the geomaterials.The boundary condition is mainly the shear strength of the sliding surface,and the properties of rock and soil include the bulk density,friction angle and cohesion of the geomaterials.The results of the five model tests are compared and verified with the derived theoretical analysis.The derived critical excavation width and stable arch curve are suitable for analysing slope model tests under various conditions.Therefore,it can be further used for the stability analysis of arching-type slopes at sites.(6)The mechanisms of deformation and failure of the arching-type slope at different stages are investigated by physical model tests.The rear loading method is used to simulate the deformation characteristics of the necking-type slope in the compression stage.Through digital image analysis,the larger displacement occurred in the lower central area,while the displacement on both sides of the slope was relatively small,especially at the front edge of the slope.In the necking-type slope,the non-yielding part is the two wings,and the yielding part is the entire slope.Due to the arching effect,the downhill load is transferred to the wings on both sides.Through the monitoring of the stress at the two flanks,the stress at the slope front increases sharply under the rear loading.The stress change during the loading process may be due to the combined effect of compression and shear between the slope body and the two flanks.With regard to the monitoring of deformation and stress,the results of the model test have verified the behaviours of necking-type slope at sites during the compaction stage.The failure process of the necking-type slope in the failure stage is carried out by using the physical model test with a tilting table.Through high-speed camera monitoring,the gradual arching failure process is observed in the necking-type slopes with different compaction degrees and different front edge widths.In addition,according to the record of the uplift angle of the tilting table at the whole final,it is concluded that the necking-type slope with narrow front width,small flank angle,and high relative density is more stable.(7)The numerical simulation and theoretical analysis of the necking-type slope were carried out,and the stability of the necking-type slope was quantitatively evaluated.Numerical simulation studies of the necking-type slope under rear loading show that archshaped contact bonds appear at the slope front due to the support and hindrance of the two wings,resulting in stress concentration at the front end of the slope,which also verifies the arching effect in the compaction stage of the slope.Based on the minimum size of the hopper opening in the hopper theory,the equilibrium equation of the differential area in the slope front of the necking-type slope is established,and the critical front width of the three-dimensional necking-type slope is obtained.The factor of safety of the slope is further obtained,and the stability analysis of the necking-type slope is carried out.According to the formula established by theoretical analysis,the stability of the necking-type slope mainly depends on the slope boundary conditions and the properties of the geomaterials.The boundary conditions are the width of the slope front,the flank angles,and the shear strength of the sliding surface.The properties of the geomaterials include the bulk density and unconfined compressive strength.By comparing the final failure angle and theoretical failure angle of the seven tilting table tests,the theoretical equation basically accords with the experimental observation results.Therefore,it can be further used for the stability analysis of the necking-type slope at sites.(8)A slope in a hypothetical slope and Jiangjiapo slope are selected as the typical arching-type slope and necking-type slope,respectively.The stability of the two slopes is evaluated on the basis of the collection and summation of the geological engineering conditions of the study area.Through the formula for arching-type slope,the safety factor of the arching-type slope is obtained as 1.7 without considering the effect of rain,while under rainfall and seepage conditions,the safety factor is reduced to 1.1.The slope is in a safe state and is close to failure.Therefore,the continued excavation process needs to be considered more carefully.If it is planned to further excavate at the toe of the slope,appropriate protection methods can be used,such as filling compacted geomaterials at the toe of the excavated slope or using multiple interval excavation methods.Through the formula for necking-type slope,the safety factor of the Jiangjiapo slope is obtained less than one under non-rainfall conditions,and the analysis result is not consistent with the actual situation.This is probably due to the deviation of the strength of the geomaterials caused by the on-site measurement.In the compression stage of the necking-type slope,the strength of the geomaterial at the slope front near the two flanks will be much greater than the strength of the rock and soil in other areas.Generally,the derived equation for the analysis of necking-type slope needs to be further verified and demonstrated through more field cases.The innovative points in this thesis are listed as follows:(1)Two geomechanical models for arching-type slope and necking-type slope were built according to the typical slopes at sites.From the view of formation and breakage of arching effect,the evaluation process of necking-type slope can be divided into three stages,namely,initial stage,compression stage,and failure stage.(2)A multi-smartphone measurement system and a multi-field monitoring system were designed.The recording of multi-field information including displacement field,velocity field,stress field,temperature field,strain field during deformation and failure can provide better understanding of the model slope behaviours from various perspectives.(3)The deformation behaviour of arching-type slope and necking-type slope confirm that the arching effect play an important role in the slope stability of the two type slopes with the observation of the relative displacement between the yield area and unyielding area.Through the physical modelling,the deformation characteristics and failure mechanism of the arching-type slope and necking-type slope were revealed.Analytical solutions for the maximum excavation width of arching-type slope and critical front width of necking-type slope are derived based on free surface problems and hopper theory,respectively.The theoretical analysis is verified from physical model tests and can applied in slopes at sites. |
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