Study On Earthquake Induced Behaviors Of Slopes Covered By Deposits

Slopes covered by deposits are widely distributed in China's south-western mountainous areas, thus, studies for earthquake induced behaviors of slopes covered by deposits play an important role for both national economic construction and protection of human life and property. Due to othernesses in geological geneses, geological environments and evolution histories of different slope bodies, and also influenced with a good many uncertainties such as earthquakes, rainfalls and artificial activities, slopes covered by deposits in different regions have their own characteristics; in terms of engineering researches, because of the similar landforms and geologic features of the slopes covered by deposits in the same region, there are commonnesses in deformation laws and stability conditions of the slope bodies. From macro perspectives, such a slope could be considered as a strong coupling system that combined deposits and bedrock, responses of the slope to earthquake loads are influenced with behaviors of the two coupling domains and their interactions. On the other hand, deposits could be viewed as a composite material of multi-phases, mechanical property of deposits depends upon factors such as soil-rock volume ratio, and shape and gradation of rubbles, besides, it is influenced by patterns, rates and levels of loads. Based on above mentioned characteristics of slopes covered by deposits, five subjects were studied in this dissertation as follows:1) According to topographic features and regional environments in Sichuan province, characteristic factors interrelated with stabilities of slopes covered by deposits were induced to be the following7indicators:(1) thickness of deposits;(2) height of slope;(3) grade of slope;(4) lithologic association;(5) groundwater condition;(6) average annual precipitation;(7) earthquake effect. And on the basis of gathered information for68slopes covered by deposits of highway in Sichuan province, typical reinforcement schemes are classified into6types:(1) retaining wall;(2) anti-sliding pile;(3) rockbolt;(4) synthetical schemes containing anchor cable;(5) anti-sliding pile+retaining wall;(6) rockbolt+retaining wall. Then, statistical relations between each typical reinforcement schemes and above mentioned characteristic factors were thoroughly discussed.2) In order to further explore aging deformation characteristics of slopes covered by deposits under earthquake loads, a slope covered by deposits of Panzhihua-Tianfang motor way at exit of the Tianfang end of Wangjiangling tunnel is taken as the study object, which is about9.7km from the epicenter of8.30Panzhihua6.1earthquake in2008. On the basis of the continuous borehole inclination monitoring results, coseismic responses and subsequent aging deformation of the slope covered by deposits were investigated. The results show that, the maximum values of coseismic longitudinal displacement for each inclinometer borehole are26to28mm, and the coseismic displacement decreases gradually along with depth. After aging deformation of the slope body near inclinometer boreholes J3and J5reaching the peak value300days after8.30Panzhihua earthquake, it tended to stable condition gradually; But, for inclinometer borehole J1, because of influences of comprehensive factors, its longitudinal displacement was still in the adjusting state430days after8.30Panzhihua earthquake. It is therefore concluded that, by the end of the rainy season in2009, the slope covered by deposits was globally stable.3) Coseismic response rules of the slope covered by deposits were then further studied with the2-D fast Lagrangian finite difference method numerically, and effects of amplitudes and spectrum characteristics of the input horizontal accelerations were also investigated. Simulation results reveal that:(1). Both the cumulative plastic deformation and globe stability condition of the deposits are very sensitive to the amplitude especially the peak value of the input horizontal acceleration;(2). Evolution rules of horizontal displacements of the deposits depend to a great extent on the intrinsic attributes of the earthquake loads;(3). After continuous loadings of the Loma Prieta horizontal acceleration with peak value of0.49m/s2and duration of20s, the coseismic relative longitudinal displacements corresponding to inclinometer borehole J3in the numerical simulation are quite close to the field monitoring results in both the magnitude and distribution pattern.4) Considering of the differences in deformation laws of the deposits and bedrock under earthquake actions, a coupling numerical method that bridging both the finite difference method and discrete element method is introduced in this dissertation. After the applicability of quasi-static analysis of the coupling method being verified with a case study of a shoulder retaining wall acted by soil backfills, the introduced model was extended from quasi-static to dynamic analysis to study deformation rule and dynamic characteristic of the slope covered by deposits subjected to earthquake loads. In the proposed model, the bedrock and deposits are respectively described with the plane-strain finite difference meshes and a collection of parallel bonded rigid disks of DEM. Smooth transition across the continuous and discontinuous domains is obtained by imposing the compatibility condition and equilibrium condition along the interfaces, and the same time step was chosen for the two coupling domains in the iteration procedure. In the course of computation, after the static equilibrium under gravity loads being obtained, dynamic calculation would begin under excitation of horizontal seismic accelerations inputted from the continuum model bottom, and free-field horizontal seismic accelerations inputted from the continuum model bottom, and free-field boundaries were adopted for lateral grids of bedrock meshes to eliminate the radiation damping effect in dynamic calculation. Through above mentioned calculation procedures, dynamic analyses of the slope covered by deposits were conducted, and effects of the relevant parameters such as spectrum characteristics, duration and amplitude of input horizontal acceleration and parallel-bonded strength of the deposits were further investigated. Dynamic coupling numerical results show that, unlike the traditional analysis means based on the continuum theory, with the introduced dynamic coupling method in this dissertation, the whole processes of catastrophes such as meso-damage, multi-stage sliding and surface collapse of the deposits in the earthquake could be studied systematically.5) Three groups of geotechnical centrifuge model tests were conducted to further study the deformation development laws of slope covered by deposits after earthquake actions. Slope bodies were generalized into fabricated soil (mixed with weathered granite soil and sandy soil) models, and influence depth of earthquake to the slope was taken as10m, then, effects of loose degrees of the earthquake loosed slope body on aging deformation and long-term stability of the slope were investigated. The following conclusions could be drawn from analyses:(1). With development of damage extents of the earthquake loosed slope body, adjustment extents of the stress/strain fields in the short term after the earthquake increased accordingly;(2). As compaction coefficients of the earthquake loosed slope body vary from0.90to0.80, the main adjusting time of the slope's displacement field after the earthquake lies within0.7-1.1years;(3). In the tests, because of the gentle gradient of the slope (35.7°), aging displacments of the slope bodies are domindated with the vertical consolidation.