Study On Loess Slope Deformation And Failure Based On Undisturbed Loess Centrifuge Experiments

The loess regions generally have more mountains and less lands,so a lot of projects are built adjacent to the loess slopes.The change of original slope shape by natural erosion or artificial cutting will break the equilibrium state of internal deformation and stress distribution of slope body,thus affecting the stability of slope.At present,the slope type design and stability evaluation methods of loess high slopes are mainly based on engineering experience analogy methods,supplemented by limit equilibrium analysis methods,which fails to consider the synergistic change processes of slope deformation and stress.Due to the high cost and difficulty of in-situ displacement and stress fields monitoring in high slopes,the conventional studies on internal deformation and stress distribution of soil slopes mainly rely on numerical simulation methods or physical model tests with reduced scale.Most of the relevant numerical simulations were based on the homogeneous soil mathematical models,so the particularity of undisturbed loess structure had not been considered.Because the loess is fragile and easy to crack,it is difficult to excavate and process large loess samples without damage and the success rate is low.In the past,the remolded loess models were mostly used to simulate the undisturbed loess slopes in the reduced scale physical model tests,especially in the centrifuge tests.Although a few centrifuge tests attempted to use small to medium scale undisturbed loess slope models,the relevant studies did not monitor the distribution of deformation and stress fields in slope overall.The aeolian original loess has unique structure and deformation characteristics,and the deformation and failure modes of the undisturbed loess are different from those of the homogeneous remolded loess slope.Therefore,the actual distribution and evolution characteristics of deformation and stress fields as well as failure development mechanism of undisturbed loess slope are still not clear.It is of great practical significance to continue to deepen relevant researches,which can provide theoretical basis for loess slope stability evaluation,slope type design,deformation monitoring and early warning,and slope cutting and reinforcement.In this study,the excavation and processing methods of large size undisturbed loess sample were optimized.Four samples of 1m×1m×1m（weighing about 2 tons）of undisturbed Q₃ Malan Loess were excavated from typical loess strata in the loess hilly and gully region of western Shanxi Province.The basic physical and mechanical indexes of undisturbed soil samples were tested.Based on the survey data of slopes in Loess Plateau,three representative slope types with combinations of different slope angles and different morphology were selected.At the same time,in order to ensure the reliable implementation of the tests,the preparation methods of large size undisturbed loess model were explored,and the simulation tests of undisturbed loess slopes were carried out relying on a large centrifuge system.Two groups of parallel tests were set for each slope type to monitor the vertical and horizontal stresses respectively.A total of 6 centrifugation tests were carried out for 6 groups of models.For each group of models,a piece of undisturbed loess model and a piece of remolded loess slope model with the same soil density were set side by side.Three interrelated centrifugal loading conditions were designed for different slope types,and the loess slopes with multi-stage equivalent slope height between5-84m were simulated.A multi-dimensional comprehensive monitoring system consisting of deformation monitoring system,visual monitoring system and stress monitoring system was upgraded and organized.Based on the visual monitoring system,the deformation and failure modes of the undisturbed loess slopes and the remolded loess slopes were compared.PIV technology was used to restore the two-dimensional displacement fields of the undisturbed loess slopes with different equivalent slope heights.Based on the monitoring data of the stress fields and interpolation method,the two-dimensional stress fields sequences of the undisturbed loess slope were generated.The main conclusions are as follows:（1）Differences between the undisturbed loess slopes and the remolded loess slopes are clarified:The deformation of the undisturbed loess slopes are mainly manifest as lateral expansion of soil within a certain range of slope surfaces,and the differential settlement of slope tops are not obvious;For the remolded loess slopes,the closer the slope tops are to the free faces,the greater the settlements are,and the slopes are subjected to a wide range of combined deformations,and the toes move forward are more significant than those of the undisturbed loess slopes.Before the overall failure of the undisturbed loess slope,there are few transverse cracks（along the slope strike）at the tops of the slopes,and all the cracks are downward,while the slope surface cracks are few and all the cracks are also downward,and the relative shifting of soil on both sides of the cracks are not obvious.There are many cracks on the tops and slope surfaces of the remolded loess slopes,which are extend inclined in the slopes,and relative shifting of soil on both sides of the cracks are obvious.For the steep slope,before the overall failure of undisturbed loess slope,several local failures occur on the slope surface successively,and finally thrust load caused landslide happened with the upper part of slope moves first,and the back wall of the slide surface is steep.The remolded loess slope with the same slope type fails earlier than the undisturbed loess slope,and it is a retrogressive landslide with the foot sliding first and the middle and upper part sliding after,and the back wall of the sliding surface is inclined.（2）Distribution and evolution pattern of displacement-stress fields of undisturbed loess slope are revealed:With the unloading of free face,horizontal tensile strain of soil first occurs in the slope toe area,which causes the lower slope to move forward.Meanwhile,the horizontal stress relaxation zone is formed in the slope with a wider influence range than the tensile strain zone,and with a distribution shape that is wide at the bottom and narrow at the top.With the increase of slope height,the horizontal tensile strain zone,plastic deformation zone and stress relaxation zone migrate or expand from the slope toe area to the back and top of the slope.The stress concentration anti-slide zone gradually forms near the slope toe due to the resistance of slope bottom soil.The plastic deformation in the gentle slope develops evenly and slowly,which delays the stress concentration and overall failure of the slope.Slope by steps can significantly weaken the continuity of strain and stress distribution in the slope,and enhance the anti-slide effect of slope toe area.When the slope becomes steep,the slope surface movement decreases,the stress of soil near the slope is concentrated,which leads to local failures in the slope surface,then the stress field in the slope changes significantly,forming the whole landslide.（3）Characteristics of burstiness and high-velocity motion of undisturbed loess slope failure are confirmed:There are often no obvious precursors before the overall failure of undisturbed loess slope.In particular,the deformation and stress redistribution reaction of the upper slope is lagging behind,and the surface deformation is not significant.The displacement monitoring shows that the displacement acceleration of slope shoulder and slope surface is not obvious before the overall failure of slope.Therefore,the overall failure of loess slope is often abrupt.The steeper the slope is,the smaller the slope surface deformation is before the failure,and the more difficult it is to carry out warning according to deformation monitoring.When the slope occurs overall failure,the sliding surface produces approximate brittle cut-through in a very short time.The back wall of the sliding surface is steep and has little friction,the center of gravity of the sliding mass is high.The sliding mass disintegrates rapidly when sliding,and the resistance between the sliding surface becomes rolling friction and decreases.The shear outlet is usually higher than the slope toe due to the influence of the slide resistance zone,the landing process of the slide mass is delayed,and the lower part of the slide bed is relatively gentle,so the horizontal separation speed is fast,then it is easy to form high-speed and long-distance landslide.（4）Development mechanism of deformation and failure of the undisturbed loess slope based on the vertiloess structure model is elucidated:The vertiloess structure formed by the vertical arrangement of strong and weak structural units in the aeolian original loess leads to the deformation and failure mode of the undisturbed loess slope is different from that of the homogeneous soil slope.Based on the theory of vertiloess structure,the vertical strong structure units in loess are generalized into the hidden soil columns which can skew,bend or be cut off horizontally.The deformation and failure model of undisturbed loess slope which considering the vertiloess structure is established.This model reflects the structural and mechanical anisotropy of the undisturbed loess caused by the vertiloess structure,and explains the deformation characteristics of the undisturbed loess slope,which is dominated by horizontal deformation and the differential settlement of the slope top is not significant.Based on the established model and observed deformation and failure laws,the development mechanism of deformation and failure of undisturbed loess slope is explained,which is firstly the lower part of the slope body gradually moves forward（slow change）,then the upper part of the slope body lost support of basis and toppled and split（abrupt change）,thereupon the remaining slide surface was sheared through（secondary abrupt change）.（5）Suggestions for the application of undisturbed loess slope engineering are put forward.In terms of stability analysis and slope type design,attention should be paid to the stress relaxation affected zone distribution characteristics which is wide at the bottom and narrow at the top of slope.It should be noted that the potential sliding surface is steep at the upper part and gentle at the lower part,and the upper part is ripped and the lower part is sheared manly.Models reflecting the anisotropy of undisturbed loess should be adopted for deformation calculation.In terms of monitoring and warning,the horizontal displacement monitoring of slope toe and slope waist should be emphasized according to the characteristics of antecedent deformation in slope toe area,supplemented by monitoring measures of slope internal stress.In terms of stability enhancement and reinforcement,attention should be paid to slope foot fixation and waist strengthening,and support measures should be timely or in advance layered follow-up to control the deformation development and stress relaxation of slope body beforehand.