Time-dependent Deformation And Long-term Stability Of Landslide & Stabilizing Piles System Under Water Level Fluctuation

Periodic variation of the seepage field changes the stress state of landslide under water level fluctuation. Owing to the rheology and long-term strength of soil and rock, the deformation of landslides and respond of stabilizing structures are all time-dependent. It is vital to investigate into time-dependent deformation and long-term stability of landslide-stabilizing piles system, allowing for widespread use of stabilizing piles in landslide control. The importance of the study is of practical significance to landslide control and optimal design especially.A lot of engineering practice and experimental investigation have manifested that landslide shows clear time-dependent characteristic under the effect of inner and exterior geologic process while its evolution process is mainly affected by the external dynamic conditions and internal rock and soil mechanics property. On the one hand, the stress field of landslide is changing all the time because of the influence of earthquake, rain, rise and fall of reservoir water level and other environmental forces thus to affect the time-dependent deformation and long-term stability; on the other hand, the deformation and strength features of rock and soil itself have distinct time effect, its stress-strain relation exerts rheological feature, and the strength parameters show degradation under the effect of environmental forces, so the strength features of landslide rock and soil also affects the evolution process of landslide. The changing of the stress field and deformation field of landslides will certainly leads to the dynamic characteristic of the internal force and deformation response of the structure of landslide prevention and control system. Slide-resistant pile is one of the most widely used prevention measures in resisting landslides, the loading and resistance of the pile show aging properties under the water level fluctuation, leading to the persistently changing of its inner force and deformation, and the dynamic response of slide-resistant pile affects the deformation evolution and global stability of the landslide after treatment.This dissertation presents the research on the time-dependent deformation and long-term stability of the landslide-stabilizing piles system under the condition of reservoir water fluctuation, choosing the typical landslide in Three Gorges Reservoir Region as the research object, analyzing the evolution characteristics of landslide deformation based on the field monitoring data, acquiring the rheological model of landslide rock and soil via large-scale indoor experimental study, inversing rheological parameters of rock and soil via monitoring data, finally studying the time-dependent deformation and long-term stability of the landslide-stabilizing piles system by the numerical simulation method. Time effect of the external dynamic process and internal rock and soil mechanics property is considered in this paper at the same time, embroidering the evolution law on the reservoir landslide, studying the deformation characteristics and stability variation of the landslide on the reinforcement conditions by the anti-slide piles. The conclusion of the research is significant for the control efficiency evaluation and optimum design of the anti-slide pile control engineering.It is an expansive issue in the landslide hazard research that the Theological behavior of the rock-soil mass is considered in the landslide deformation evolution and long-term stability. This issue relates to numerous aspects like the in-site monitoring date analysis, rock-soil mass rheological laboratorial test, rheological model establishment, intelligent parameter inversion, numerical simulation, etc. The paper presents a systematic research on this issue, combining the geotechnical investigation, in-site monitoring, laboratorial test, theoretical study, numerical simulation, etc. The study and achievements are mainly as follows.(1) Based on the geotechnical investigation and in-site monitoring data, the process and features of the landslide deformation evolution are analyzed. The research indicates that the Ma-Jia-Gou landslide is characterized as a typical advancing landslide. This landslide is of three characteristics. First, the deformation of the landslide surface presents alternate step changes in rate. Second, the landslide displacement value and the displacement mutational time apparently differ from each other; compared with the front part of landslide, the back part responses weaker and slower. Third, when the deformation experiences a steady rate, the landslide is in a slow constant advancing state of deformation, representing the features of creeping. Through studying the monitoring data curves of the deformation rate versus each influence factor, the factors influencing the landslide deformation evolution are analyzed both qualitatively and quantitatively. The study illustrates that the rainfall intensity has rare impact on the landslide deformation evolution, while the evolution of deformational rate is a process of responding the decline of reservoir water level. The landslide deformation occurs behind the fall of water level in the reservoir 65 days on average. Different sections of the landslide show respective extent of response to the water level decline. Specially, for the relation between the deformation and water level falling rate, the gray related degree of the front section overwhelms that of the back in the landslide.(2) By the means of large-scale three axial test and Shear rheological test, the creeping deformational rule of the coarse grain soil and the embedding section rock in the landslide are studied respectively. The result indicates that the sliding body soil shows distinct damping characteristics of the creeping feature, for the large content of giant grains, such as gravels; under any levels of the horizontal deviator stress, the ultimate creeping rate tends to zero. In the section of sliding interface, the soil is more prone to deform with large clay particle content; the creep lifespan consists of the damping creep and the steady creep successively, and the creep speed in the steady stage is proportion to the horizontal deviator stress. In the section of embedding, the shear rheological test curve of the mealy sandy mudstone, suggests that the rock has experienced the three typical stages of creep, which are the damping creep stage, steady creep stage and accelerating creep stage. Adopting the methods of the isochronous curve and the acceleration curve, the long-term strength of the landslide soil and rock are verified. The long-term cohesive forces of the sliding body soil, sliding interface soil and the rock, decline 19.5%,38.2% and 31.96% separately; meanwhile the long-term friction angles of the three decline 24.8%,22.4% and 1.31% separately.(3) Specific to the different creep characteristics of rock and soil, different rheological models are picked to study the rheological behaviors of the sliding body soil, sliding interface soil and the embedding rock, namely the three-parameter model, the Burgers model and the CVIS model. Based on the laboratorial creep test data, the result consolidates the fitness of the three chosen models, after the theoretical analysis and the curve fitting of the rheological model parameters under different confining pressures (normal stresses) and different stress conditions. The response surface based rheological parameter reversion method is proposed. The target response and corresponding reversion parameters are achieved through theoretical analyzing. The reversion parameter span is verified by the curve fitting of the creep test data on rock and soil. The three-dimensional numerical simulation scheme is designed by Box-Behnken method and the central composite sampling method. On the basis of the displacement date monitored in the numerical model, the response surface function model is established between the landslide deformation responses and the rheological parameters. Eventually, the goal of landslide rock and soil mess parameter identification is accomplished in different rheological models.(4) Upon the features of seepage field and the rheological characteristics of landslide rock and soil, the landslide deformation regularity in the stage of creeping is researched on the condition of reservoir water effect, before and after the establishment of landslide-stabilizing pile. The short-term landslide deformation is divided into four stages without the landslide-stabilizing pile, namely the instant unloading deformation stage, the stress modification deformation stage, the sliding interface creep initial stage and the entirety creep stage. The long-term landslide deformation is divided into two stages under the continuous fall of water level, which are the damping creep stage and the steady creep stage. When the landslide-stabilizing pile is established, the sliding body shows distinct difference from that before the establishment. In the front of the piles, the displacement in four stages has no changes, comparing with no pile established; whereas the sliding body behind the pile shows a decline in the total displacement, and so is the ratio of the instant deformation and the displacement in the stress modifying stage. The phenomenon indicates that the sliding body behind the pile deforms as creeping occurs in the soil, and has rare relation with the external dynamic force effect.(5) Based on the dividing stages of time-dependent deformation, dynamic process of earth pressure against stabilizing pile and rock resistance against fixed end of the pile is studied, and a model is proposed to generalize the distribution of earth pressure and rock resistance in various stages. In transient and strain-adjustment deformation stages, earth pressures against the front and the back of the fixed end are both triangle-distributed. Nevertheless, rock resistances against the front and the back of the pile are trapezoid-distributed and parabola-distributed respectively. In the start stage of creep, earth pressures against the front and the back of pile distribute as convex and concave parabolas, and rock resistances distribution present the similar shapes with different values. In the complete creep stage, there is no pressure against the front but concave parabola-distributed earth pressure against the back of the pile. Rock resistance against the front changes to triangle-distribution, and rock resistance against the top of the back changes to zero, but distributes as convex parabola against the bottom of the back.(6) Combining the developing process of landslide, evolution characteristics of horizontal and vertical soil arches of landslide and stress arch of stabilizing pile are studied, and the coordination laws between strain and stress arches and deformation stages are revealed. Horizontal soil arch develops from frictional soil arch predominantly to frictional soil arch and end support arch together, and ends with end support arch. Positions of maximum strength of vertical soil arch between piles changes in different stages. In the start stage of creep, position of maximum strength is close to surface of landslide. In the complete creep stage, position of maximum strength changes to the center of loaded segment. Soil arching effects of earth pressure against loaded segment and rock resistance against the front and back of fixed end are positively evident with the increase of landslide deformation. Nevertheless, they present different trends with the increase of depth, and arching effect of earth pressure is increasingly apparent, which is opposite to the effect of rock resistance against the front of fixed end. Arching effect of rock resistance against the back of fixed end first increases and then decreases.(7) Comprehensive strength reduction method, considering rheology character, is used to study deformation characteristics of landside in failure stage, before and after it is reinforced by stabilizing piles. The correlations between information fields, including displacement, plastic zone and shear strain rate are also analyzed. Before it is reinforced, failure process of landslide is divided to three phases. In the primary accelerating phase, its acceleration remains a small steady value. Plastic zone forms gradually in the front of landslide, and spreads toward trailing edge along sliding surface. In the quick accelerating phase, its acceleration increases quickly, and a turning point appears in the speed-curve. Whilst plastic zone forms in slide mass gradually. In the rapid accelerating phase, speed-curve is almost perpendicular to time-axis, and plastic zones in sliding surface and slide mass transfix completely. After it is reinforced by stabilizing piles, landslide changes to rapid accelerating phase immediately as it is unstable in the failure process of landslide-piles system. Rapid deformation mechanism is studied through the analysis of failure process. Plastic zones in sliding surface and slide mass transfix because of low reduction factor in the front of landslide, and failure occurs in the subordinated slide mass. With the successive deterioration of soil strength, plastic zone in the front develops toward the back, and plastic zone behind forms in the sliding surface and slide mass behind stabilizing piles. Nevertheless, driving force of landslide is in the tolerance of stabilizing piles. Deformation of slide mass behind piles remains small in consequence that plastic zones ahead and behind piles are not transfixed. With the further deterioration of soil strength, driving force increases constantly, causing the failure of stabilizing piles. Plastic zones ahead and behind piles transfix immediately, and the deformation of slide mass behind piles increase rapidly without a slow accelerating phase.(8) Stabilities of the landslide are evaluated before and after it is reinforced by stabilizing piles, according to the displacement-time curve. Before it is reinforced, hydrostatic unloading and hydrodynamic pressure, induced by water fluctuation, have different influences on every part of landslide. Long-term stability of three subordinated slide masses are 1.038,1.042 and 1.043 respectively. All subordinated slide masses failure caused by decline of water level, and the values of decline are 12m,27m and 30m respectively. Water head differences in two water-influenced subordinated slide masses are 4.7m and 4.5m respectively. After it is reinforced by stabilizing piles, the stability coefficient of slide mass behind piles is 1.24. Whilst reduction cohesions of slide mass soil sliding surface soils are 11.2kPa and 24.6kPa, and reduced internal friction angles are 22.26°and 17.01°espectively. They are all larger than long-term cohesions and internal friction angles, showing that pile failure occurs before soil strength deteriorated to long-term value. In consequence, landslides reinforced by piles of traditional statistic design will be unstable in decades.In this dissertation, the main innovation points are as follows:(1) The study for rheological property of slip mass, slip bands and the bedrock of the fixed end of the anti-slide piles is analyzed. The rheological models, parameters and the long-term strength value are determined. The creep law on coarse-grained soil and silty mudstone are studied separately through large-scale three triaxial rheological test and shear rheological experiment, and the long term strength of soil mass and rock mass are determined separately by isochronous curve method and the acceleration curve method. Based on the different creep properties between landslide rock and soil, we selected three-parameter model, burgers model and CVIS model to describe the creep behavior of slip bands, slip mass and the bedrock. Based on the field monitoring data, the rheological models and parameters are determined for the analysis of landslide-stabilizing piles system with the rheological parameters of landslide rock and soil via the method of response surface curves.(2) Considering the rheological property of the rock and soil, the time-dependent deformation of landslide-stabilizing piles system is studied, and the deformation evolutionary process and the law of anti-slide pile dynamic response on creep phase of landslide under the action of reservoir water are revealed. The evolution law of groundwater seepage field on the landslide under the condition of reservoir water fluctuation is analyzed. Short term and long term evolution stage of landslide deformation are respectively studied under the action of single water level decreasing range or continuous water level drop by the numerical simulation method combining the rheological models and parameters of the rock and soil. Based on the different stage of landslide deformation, the effect law of soil pressure on the loaded section and the rock resistance on the embedded section of the stabilizing pile are studied, and the dynamic change law of the stress on the anti-slide piles and the interaction process of the landslide-stabilizing piles system are revealed.(3) Based on the evolution characteristics of deformation as the impacted stage of landslide instability, this paper studies on the changing rule of the landslide stability before and after anti-slide pile applied. The strength reduction method considering rheological effect of rock and soil is used to contrast the changing rule of deformation characteristic values under the impacted stage of landslide instability, such as surface displacement, plastic zone, strain rate, etc. And as instability criterion, the displacement curve is used to analyze the long-term stability of landslide before anti-slide pile reinforcing; Landslide-stabilizing piles system has been regarded as research object, strength reduction is respectively used for landslide zone soil, embedded section of rock and rock and soil system. Changing rules have been studied on long-term stability of landslide under different strength reduction, and propose the stabilizing evaluation method combining strength reduction value and long-term strength values of soil.