Residual Strength Variation Of Debris Landslide Slip Soils Under Different Shear Rates

The study of shear strength of slip soils is one of the key parts of the stability evaluation and prediction of landslide. Recent researches about the slip soils focus mainly on the geological characteristics, structure, composition, physical and mechanical properties, but studies on the dynamic strength of slip soil and its variation in the process of landslide deformation, especially based on the landslide deformation condition are still very rare. The actual strength of slip soils varies as external factors such as water content and stress conditions differ. Moreover, experiment results are not only influenced by method adopted, but also related to experiment conditions and shear rates. Therefore, computation of strength of slip soils based on actual landslide sliding phenomenon, for example, displacement and development of crack, is most objective and accurate. And experimental study using ring shear will help reveal the sliding mechanism and treatment measure. Establishing the dynamic soil strength under landslide deformation stage for landslide stability evaluation and prediction has fundamentally important theoretical meaning and application value as well.This paper took the reserch on the features of the slip soil geological occurrence and the shear strain and the stress varation in the process of the debris landslide deformation relying on the Three Gorges Reservoir landslide professional monitoring points. On the base of it, the slip soil residual strength was studied in the condition of different water contents and shearing rates by ring shearing test. And the critical test techneque was obtained though the above reserches. At last, a criterion of slip soils residual strength based on Bingham model is also established.The research and its fruits follow below.(1) Deformation phase and stability analysis of the typical gravel soils landslides in the Three Gorges Reservoir Area. Take a resurrection of gravel soils and old landslide in the Three Gorges Reservoir Area as an example, according to the development process of the signs of the landslide macroscopic displacement, this paper concludes that the landslide generated a whole resurrection and the overall slope is in the slow creep-the isokinetic creep stage under the influence of135m water level of the second stage water storage and prolonged rainfall in the Three Gorges Reservoir Area. The middle and the front of the sliding body is in a constant velocity deformation stage, and the landslide leading edge has accelerated deformation characteristics. The landslide stability changes at the same time the Landslide deformation stages change. Landslide resurrection and the mechanism of deformation increasing is:High-water impoundment led to a substantial uplift of the groundwater level at the middle and the front part of the landslide, the rock and soil effective stress of the Landslide Resistance sliding segment decreases, the strength of slip soil decreases, the ability of sliding resistance decrease, Landslide generated a whole resurrection and the surface macro-displacement and deformation is significant. Because the transient saturated zone appeared on the landslide surface during prolonged rainfall conditions, combined with the condition that the groundwater level uplift in the middle and rear part of the landslide, the weight at the middle and rear part of the sliding body increases, the sliding force increases, the deformation and displacement increase and the landslide stability reduce. The impact factors of landslide deformation are that the high water level of the Three Gorges reservoir water storage+rainfall and its library water fluctuations.(2)The study of the landslide shears strain and the stress characteristics during deformation. This paper established the displacement field and the strain field of landslide by using the displacement monitoring information of the deep part of landslide. Displacement and strain field analysis shows that landslide shear strain is more concentrated at the sliding zone and the strain concentration is more significant at the front edge than at the middle and rear part of the landslide, which has the deformation characteristics call passage type. And because landslide strain rate, a single month strain increment, is significant, slip soils is in the plastic deformation or after the destruction stage, the strength of the sliding zone is its residual strength. In the plastic yielding stage sliding zone soil residual strength is its suffered shear stress which is constantly changing. Increment of shear stress not only depends on outside incentives also depends on the shear rate, and after each uninstall to stop, slip soils will form a new strength, which means that Slip soils embodied in a dynamic residual strength and the strength properties of sliding zone soil in the destruction stage requires a further pilot study. At the same time, through the comparison between the shear strain and monitoring holes failure to display of the S1, S2, S3deep displacement monitoring hole, This paper found that:when S1, S2, S3monitoring holes failure, the monitoring maximum displacement is between100～300mm, the maximum shear strain between0.15～0.26. The reason of the failure of monitoring holes is that the shear strain is too large and the monitoring holes deformed, which resulting in displacement monitoring instruments cannot be decentralized to the bottom of the monitoring holes, thus unable to continually get the deformation information. The main factors affecting the measurement range is the strain value of the monitoring holes and has nothing to do with the monitoring displacement value.(3) Carry on the tests of the residual strength of the slip soil with the different shearing rates. A series of exploration tests showed that the key control technology and the relevant test parameters of such trials are the selected shear rate and normal stress, controlling of the moisture content. The shearing rate had significant influence on the expected results; If the shearing rate was small, the variation of the residual strength would be not caused by the shearing rate. The proper moisture content before the test was the insurance of the study for the slip soil residual strengths with different shearing rates. High moisture content and high shearing rate would lead leakage and soil squeezing phenomenon. The value of the normal stress had a certain impact on the peak intensity. The displacement-time consolidation curve showed that the consolidation displacements of the different water content specimens had different variation processes with consolidation pressure and time before shearing. The greater the water content, the slower the consolidation displacement rate, and the larger the consolidation displacement in the same time. In the shearing process, the vertical displacement was related to the pre-equipped water content and the shearing rate. The slip soil residual strength shearing tests with different rates showed that there were some differences in the residual strength of the soil with different shearing rates. The residual strength increased with the increasing of shearing rates. When the water content were more than or equal to16percentage, the shear surface was not obvious and the residual strength was stable during the test. When the moisture content was of12percentages, the shear surface was obvious and the residual strength was fluctuant.(4)Carry on the study of the residual strength criterions of the slip soil with different shearing rates. Different shearing rates tests showed that the residual strength of the slip soil became large with the shearing rate increasing in the shearing rates range, which indicated that the slip soil had viscous nature, and it was an elastic, plastic and viscous media. Bingham model can analyze the dynamic strength of the slip soil on different deformation states. This model divided the strength into viscous strength and elastic-plastic strength. The viscous strength is reflected by the viscous nature of the slip soils, controlled by the deformation rate, which was a dynamic strength. The elastic-plastic strength is reflected by the slip soil’s anti-deformation ability, and existed in the whole deformation stage, was affected by the moisture content, which was also a dynamic strength. The analysis of the different shearing rates results by the viscous-plastic strength criterion indicated that there was an apparent linear relationship between the residual strength and the shearing rate. These results conformed to the Bingham model. Meanwhile, the slip soil’s viscosity coefficient became large with the increasing normal stress, which indicated that the viscous property was an instinct nature with relative to the soil structure. When the test normal stress was between100and400kPa, the sample’s viscosity coefficient was between0.35and1.55kPa·min. When the shear rate was less than0.1mm per minute, experiment sliding zone soil the variation of the residual strength was limited. Therefore, under the conditions of the slow shearing test whose shearing rate was less than0.02mm per minute in conventional shearing test the viscous strength value was small.