Study of the seismic performance of hybrid A-frame micropile/MSE (mechanically stabilized earth) wall

April 1, 2017, 12:00 am

≫ Next: Model predictions for behaviors of sand-nonplastic-fines mixturesusing equivalent-skeleton void-ratio state index

≪ Previous: Modeling of strength and deformation of overconsolidated clays based on bounding surface plasticity

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Abstract

The Hybrid A-Frame Micropile/MSE (mechanically stabilized earth) Wall suitable for mountain roadways is put forward in this study: a pair of vertical and inclined micropiles goes through the backfill region of a highway MSE Wall from the road surface and are then anchored into the foundation. The pile cap and grade beam are placed on the pile tops, and then a road barrier is connected to the grade beam by connecting pieces. The MSE wall’s global stability, local stability and impact resistance of the road barrier can be enhanced simultaneously by this design. In order to validate the serviceability of the hybrid A-frame micropile/MSE wall and the reliability of the numerical method, scale model tests and a corresponding numerical simulation were conducted. Then, the seismic performance of the MSE walls before and after reinforcement with micropiles was studied comparatively through numerical methods. The results indicate that the hybrid A-frame micropile/MSE wall can effectively control earthquake-induced deformation, differential settlement at the road surface, bearing pressure on the bottom and acceleration by means of a rigid-soft combination of micropiles and MSE. The accumulated displacement under earthquakes with amplitude of 0.1‒0.5 g is reduced by 36.3%‒46.5%, and the acceleration amplification factor on the top of the wall is reduced by 13.4%, 15.7% and 19.3% based on 0.1, 0.3 and 0.5 g input earthquake loading, respectively. In addition, the earthquake-induced failure mode of the MSE wall in steep terrain is the sliding of the MSE region along the backslope, while the micropiles effectively control the sliding trend. The maximum earthquake-induced pile bending moment is in the interface between MSE and slope foundation, so it is necessary to strengthen the reinforcement of the pile body in the interface. Hence, it is proven that the hybrid A-frame micropile/MSE wall system has good seismic performance.

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Model predictions for behaviors of sand-nonplastic-fines mixturesusing equivalent-skeleton void-ratio state index

May 16, 2017, 12:00 am

≫ Next: New lightweight geomaterials: Biocemented sand mixed with expanded polystyrene beads

≪ Previous: Study of the seismic performance of hybrid A-frame micropile/MSE (mechanically stabilized earth) wall

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It is a challenge to suggest a constitutive model for describing the stress-strain behavior of sand-fines mixtures due to that these granular mixtures exhibited very complex behaviors at different densities, pressures and fines contents. In this study, an elastoplastic constitutive model within the framework of the bounding surface plasticity and critical state theories was proposed for sand-nonplastic-fines mixtures by using the concept of the equivalent-skeleton void ratio and equivalent-skeleton void-ratio state index. The proposed model with a set of material constants calibrated from a few tests could be used to model the fines-dependent and state-dependent behaviors of the sand-nonplastic-fines mixture including the strain-softening and volumetric-expansion behaviors in the drained triaxial compression tests, and also the effects of fines content on the critical state lines in both the deviatoric stress versus mean effective stress and the void ratio versus mean effective stress planes.

New lightweight geomaterials: Biocemented sand mixed with expanded polystyrene beads

May 22, 2017, 12:00 am

≫ Next: Evolution of particle breakage and volumetric deformation of binary granular soils under impact load

≪ Previous: Model predictions for behaviors of sand-nonplastic-fines mixturesusing equivalent-skeleton void-ratio state index

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Evolution of particle breakage and volumetric deformation of binary granular soils under impact load

August 16, 2017, 12:00 am

≫ Next: Experimental Investigation on the Movement of Soil and Piles in Transparent Granular Soils

≪ Previous: New lightweight geomaterials: Biocemented sand mixed with expanded polystyrene beads

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Abstract

Binary granular soils, mixtures of carbonate sands and nonplastic fines, are widely used for constructions of foundation, airport and embankment in island and coast. Impact load (e.g., sea wave, aircraft landing, pile driving and dynamic compaction during foundation, et al.) is frequently exerted to the mixtures. It is therefore of extreme importance to investigate the evolutions of particle size distribution, particle breakage and volumetric deformation of the mixtures under impact load due to that the grains of carbonate sands are easily to be crushed, which may significant affect its mechanical behavior. Three mixtures (i.e., 100% carbonate plus 0% fines (by dry weight), 90% carbonate plus 10% fines and 80% carbonate plus 20% fines) were prepared to analyze the effect of fines content on particle breakage and volumetric deformation under impact load. It was observed that a unique fractal grading could be obtained for all the mixtures when the blow number was large enough (\(N>20,000\)). The void ratio of the mixtures converged to be a constant ultimate value as the mixture reached the fractal state. The volumetric strain and relative particle breakage with respect to the blow number could be described by hyperbolic functions, indicating that the volumetric strain and relative particle breakage progressively increased to ultimate values with increasing the blow number.

Experimental Investigation on the Movement of Soil and Piles in Transparent Granular Soils

August 23, 2017, 12:00 am

≫ Next: Biocementation of calcareous sand using soluble calcium derived from calcareous sand

≪ Previous: Evolution of particle breakage and volumetric deformation of binary granular soils under impact load

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In order to visually observe the displacement characteristics of internal soil, model tests on the penetration of six flat-ended piles in synthetic transparent soil were carried out. The granular-soil particles were registered by laser speckles, thus the penetration process of the piles was visualized. The movement of pile-soil interaction was determined by comparing the images before and after penetrations. Based on the test results, the vertical and horizontal displacement of the granular caused by a sequence of jacked piles was discussed. The sheltering effect of the adjacent piles during the penetration of the follow-up pile was analyzed. The responses of the granular soils during the penetration of piles were discussed, and test data was compared with DEM analyses. The pile heave at different stages of the penetration and the ground uplift at different positions were evaluated and compared.

Biocementation of calcareous sand using soluble calcium derived from calcareous sand

August 25, 2017, 12:00 am

≫ Next: Macro-meso effects of gradation and particle morphology on the compressibility characteristics of calcareous sand

≪ Previous: Experimental Investigation on the Movement of Soil and Piles in Transparent Granular Soils

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A soil improvement method based on a microbially induced carbonate precipitation (MICP) process has been developed in recent years. In this method, calcium carbonate is precipitated in-situ to act as a cementing agency. Calcium chloride is normally used as the calcium source for the MICP process. The use of calcium chloride causes two problems. The first is chloride is corrosive to concrete, and the second is the cost of calcium chloride is relatively high. An improvement to this method is to use other alternative calcium sources. A method to produce soluble calcium using calcium rich calcareous sand and use it as a calcium source for the MICP process to improve the properties of soil has been proposed in this paper. A comparative study between the effect of MICP treatment using soluble calcium produced from calcareous sand and that using calcium chloride with the same concentration of calcium was carried out. The results from both series of tests showed that with increasing amounts of cementation solutions, the strength and stiffness of the treated calcareous sand increased and the permeability decreased. The scanning electron microscopy (SEM) and X-ray diffraction analyses revealed that the aragonite crystals with an acicular mineral morphology were formed when the soluble calcium was used, whereas the calcite crystals with a rhombohedral mineral morphology were formed when calcium chloride was used. This study also shows that it is feasible to treat calcareous sand using a MICP method with soluble calcium produced from calcareous sand.

Macro-meso effects of gradation and particle morphology on the compressibility characteristics of calcareous sand

September 21, 2017, 12:00 am

≫ Next: Particle breakage and deformation of carbonate sands with wide range of densities during compression loading process

≪ Previous: Biocementation of calcareous sand using soluble calcium derived from calcareous sand

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In order to fully develop the South China Sea, a large number of reclamation projects using calcareous sand have been carried out in this area recently. A deep understanding of the physical and mechanical properties of calcareous sand is of critical importance. Therefore, the calcareous sand near a certain reef of the South China Sea is used in this study to investigate the effect of three-dimensional (3-D) particle morphology and gradation on the compressibility characteristics of calcareous sand. This paper proposes a 3-D mesoscope observation method to obtain the average 3-D angularity parameter S_d and 3-D aspect ratio T_d of calcareous sand with different particle sizes. It is found that the morphology of coarse particles (diameter: 5 ~ 1 mm) is significantly multi-angular, while the morphologies of middle particles (1 ~ 0.25 mm) are mostly dendritic and schistic. Compared to the 3-D S_d of quartz sand, the calcareous sand’s particle morphology is much more irregular and multi-angular, which makes it easy for the calcareous sand to form large pores and, thus, be more compressible. In order to systematically study the effect of gradation on the calcareous sand’s compressibility characteristics, a number of compression tests on calcareous sand with different gradations are taken. The influential mechanism is then discussed by analyzing the test results from a mesoscopic viewpoint. It is found that changing the coarse fraction content is the most efficient way to reduce the compressibility of the calcareous sand. That is because of the coarse fraction’s high angularity, which makes the skeleton-bearing capacity of the calcareous sand sensitive to the change of coarse fraction content. An empirical formula is proposed to evaluate the compressibility of the calcareous sand with different coarse fraction contents.

Particle breakage and deformation of carbonate sands with wide range of densities during compression loading process

October 1, 2017, 12:00 am

≫ Next: Influence of stress anisotropy on the cylindrical cavity expansion in undrained elastic-perfectly plastic soil

≪ Previous: Macro-meso effects of gradation and particle morphology on the compressibility characteristics of calcareous sand

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In this technical note, evolutions of the particle size distribution, particle breakage, volume deformation and input work of carbonate sands with varying relative densities were investigated through performing a series of one-dimensional compression tests. Loading stress levels ranged from 0.1 to 3.2 MPa. It was found that the initial relative density could greatly affect the magnitude of particle size distribution, particle breakage, volume deformation and input work. Particularly, it was observed that the specimen at a lower relative density underwent much more particle breakage than that at a higher relative density. This could be attributed to the change of the coordination number with the initial density. However, a unique linear relationship between the particle breakage and input work per volume could be obtained, which is independent of the initial relative density.

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Influence of stress anisotropy on the cylindrical cavity expansion in undrained elastic-perfectly plastic soil

October 20, 2017, 12:00 am

≫ Next: Experimental and Numerical Analysis of XCC Pile-Geogrid Foundation for Existing Expressway Under Traffic Load

≪ Previous: Particle breakage and deformation of carbonate sands with wide range of densities during compression loading process

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This work presents an analysis of the influence of stress anisotropy on cylindrical cavity expansions in an undrained elastic-perfectly plastic soil. This problem was formulated by assuming a large strain in both the elastic and plastic zones around the cavity and a plain strain condition during the cavity expansion process. The solutions for the limit pressure, stress, and excess pore pressure were obtained by introducing the anisotropic initial stress coefficient K₀ into the conventional cylindrical cavity expansion method. The proposed solutions were then used to interpret the piezocone penetration test, and the suitability of the solutions was verified by comparing the prediction with the piezocone penetration test data. Subsequently, parametric studies were carried out to investigate the influence of stress anisotropy on the stress, excess pores pressure distributions around an expanding cylindrical cavity, and limit pressure. The results show that the proposed cylindrical cavity expansion method under stress anisotropy is suitable and can be used to investigate the piezocone cone test. The present work improves upon the conventional theoretical framework of cavity expansion and can be applied to the determination of the stresses around axially loaded piles and around in-situ testing devices such as penetrometers.

Experimental and Numerical Analysis of XCC Pile-Geogrid Foundation for Existing Expressway Under Traffic Load

November 11, 2017, 12:00 am

≫ Next: Model tests on XCC-piled embankment under dynamic train load of high-speed railways

≪ Previous: Influence of stress anisotropy on the cylindrical cavity expansion in undrained elastic-perfectly plastic soil

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A series of dynamic large-scale model tests and three-dimensional finite element analyses for XCC pile composite foundation are conducted to investigate the dynamic behavior and the settlement of XCC pile composite foundation of existing expressway under traffic load. The test and FE results are presented in the variation of dynamic stress, distributions of skin friction, deviator stress, and the settlement of XCC pile composite foundation. The test results reveal the transfer mechanism of dynamic stress, and a linear relationship between the transferred stress and traffic load is found. Also, XCC piles can improve the stability of composite foundation because of lower neutral point and less sensibility to the traffic load. The distribution characteristics of deviator stress in the horizontal and vertical direction have been found by the numerical simulation. A modified model for predicting the traffic-load-induced settlement of XCC pile composite foundation is proposed. The asymmetric settlement of XCC pile composite foundation is revealed.

Model tests on XCC-piled embankment under dynamic train load of high-speed railways

July 1, 2018, 12:00 am

≫ Next: Study on Low-Strength Biocemented Sands Using a Temperature-Controlled MICP (Microbially Induced Calcite Precipitation) Method

≪ Previous: Experimental and Numerical Analysis of XCC Pile-Geogrid Foundation for Existing Expressway Under Traffic Load

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Piled embankments, which offer many advantages, are increasingly popular in construction of high-speed railways in China. Although the performance of piled embankment under static loading is well-known, the behavior under the dynamic train load of a high-speed railway is not yet understood. In light of this, a heavily instrumented piled embankment model was set up, and a model test was carried out, in which a servo-hydraulic actuator outputting M-shaped waves was adopted to simulate the process of a running train. Earth pressure, settlement, strain in the geogrid and pile and excess pore water pressure were measured. The results show that the soil arching height under the dynamic train load of a high-speed railway is shorter than under static loading. The growth trend for accumulated settlement slowed down after long-term vibration although there was still a tendency for it to increase. Accumulated geogrid strain has an increasing tendency after long-term vibration. The closer the embankment edge, the greater the geogrid strain over the subsoil. Strains in the pile were smaller under dynamic train loads, and their distribution was different from that under static loading. At the same elevation, excess pore water pressure under the track slab was greater than that under the embankment shoulder.

Study on Low-Strength Biocemented Sands Using a Temperature-Controlled MICP (Microbially Induced Calcite Precipitation) Method

January 1, 2019, 12:00 am

≫ Next: DEM Investigation of Fracture Characteristic of Calcareous Sand Particles Under Dynamic Compression

≪ Previous: Model tests on XCC-piled embankment under dynamic train load of high-speed railways

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MICP (Microbially Induced Calcite Precipitation) technique is recently known as a new research area in geotechnical engineering. This technique provides a more environmentally way to enhancing the soil strength and stiffness by the MICP process in the soil pores. However, the spatial uniformity of MICP in the treated sands, which determines the effectiveness of MICP technique, remains a challenging issue even in the laboratory tests, especially for low-strength biocemented sands. Noting that the MICP process could be greatly inhibited under low temperatures before the homogeneous conditions of MICP reactions is achieved in sands, a temperature-controlled MICP method is proposed in this paper to improve the MICP uniformity in low-strength biocemented sands. A series of temperature-controlled MICP tests are made and the results are compared with the MICP tests under a constant temperature.

DEM Investigation of Fracture Characteristic of Calcareous Sand Particles Under Dynamic Compression

January 1, 2018, 12:00 am

≫ Next: Thermo-mechanical behavior of energy pile under different climatic conditions

≪ Previous: Study on Low-Strength Biocemented Sands Using a Temperature-Controlled MICP (Microbially Induced Calcite Precipitation) Method

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Calcareous sand as one kind of fragile geotechnical materials, however it is often under dynamic loading in engineering. In this study, discrete elements method (DEM) simulation was used to investigate the fracture characteristics of single and contacting spherical calcareous sand particles under dynamic compression. Calcareous Sand particle was simulated by cluster made up of elemental ball. The DEM models included two kinds of particle arrangements with one and five particles bounded in a cylindrical wall, and the dynamic compression loading were applied by a drop hammer made of clumps. The numerical simulation results of single particle under dynamic compression included stress-strain relation and Weibull distribution were coincide with the experimental results. For five particles numerical model, grain size distribution was obtained for all five cluster to differentiating the degree of breakage of particles in different positions. The transfer of force between particles was also exposed through the observation of the force chain evolution during compression.

Thermo-mechanical behavior of energy pile under different climatic conditions

November 2, 2018, 12:00 am

≫ Next: Particle breakage and energy dissipation of carbonate sands under quasi-static and dynamic compression

≪ Previous: DEM Investigation of Fracture Characteristic of Calcareous Sand Particles Under Dynamic Compression

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The thermal–mechanical behavior of the energy pile under three kinds of climatic conditions was investigated in this study. A small-scale floating energy pile and a small-scale end-bearing energy pile, which were embedded in normally consolidated clay, were employed. The energy piles were subjected to cyclic heating/cooling, heating/recovery and cooling/recovery to simulate the energy pile work in the regions of warm/cold balanced climate, warm-dominated climate and cold-dominated climate, respectively. The thermal response and the mechanical response of the energy pile under different climatic conditions, as well as the different response between the floating energy pile and the end-bearing energy pile, were analyzed and discussed comprehensively. The results show that the thermo-mechanical performance of energy pile depends on the types of climatic conditions, and the behavior of the floating energy pile is different from the end-bearing pile. Larger irreversible displacement could be induced by thermal cycles for the floating energy pile compared to the end-bearing energy pile, while irreversible tip resistance could be induced for end-bearing energy pile. Under warm/cold balanced climate, the largest irreversible tip resistance and pile displacement could be induced for end-bearing energy pile and floating energy pile, respectively, and the smallest thermally induced irreversible displacement was observed when the energy pile was under cold-dominated climate.

Particle breakage and energy dissipation of carbonate sands under quasi-static and dynamic compression

April 1, 2019, 12:00 am

≫ Next: Effect of particle shape of glass beads on the strength and deformation of cemented sands

≪ Previous: Thermo-mechanical behavior of energy pile under different climatic conditions

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Quasi-static and dynamic compression tests are conducted on carbonate sand using a Material Testing System and a modified split Hopkinson pressure bar, respectively. The particle size distributions (PSDs) of carbonate sand before and after loading are measured via laser diffractometry. The stress–strain curves demonstrate that the carbonate sand investigated in this study exhibits strain rate effects. The stress–strain curves show slightly different features for quasi-static and dynamic loading conditions. The particle breakage extent, which is quantified from the PSDs of the samples before and after loading, is investigated at different stress levels and input energy values. The breakage efficiency under the quasi-static loading condition is higher than that under the dynamic loading condition. As a result, the particle breakage extent is higher under the quasi-static loading condition than under the dynamic loading condition at the same stress level. Furthermore, the particle breakage modes are highly dependent on stress. The breakage modes under the dynamic loading condition change from attrition and abrasion at low stress levels, resulting in the appearance of plateaus in the grading curves, to fracture at high stress levels, resulting in the disappearance of plateaus in the grading curves.

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Effect of particle shape of glass beads on the strength and deformation of cemented sands

June 11, 2019, 12:00 am

≫ Next: Limit lateral resistance of XCC pile group in undrained soil

≪ Previous: Particle breakage and energy dissipation of carbonate sands under quasi-static and dynamic compression

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Few studies have focused on the influence of particle shape on the mechanical properties of cemented sand. To address this lack of information, this study investigated the influence of the cement content and particle shape on the strength and deformation of cemented sand based on a series of unconfined compression tests of mixed specimens. Cemented specimens were prepared with cement contents ranging from 4 to 8% and different mixing ratios of angular glass beads (AGBs) and rounded glass beads (RGBs). A shape parameter (overall regularity \( O_{\text{R}} \)) was proposed to quantitatively evaluate the shape of the particles. Mixed specimens were examined using scanning electron microscopy (SEM) analysis to illustrate the properties of the bonds with different mixing ratios of AGBs and RGBs. The test results indicated that the strength and stiffness increased as the cement content increased and the \( O_{\text{R}} \) decreased. The trend of particle shape on the strength and stiffness was found to be independent of the cement content. The SEM images showed that the effective cementation area between angular particles is larger than that between rounded particles and that between angular and rounded particles, which resulted in increased strength and stiffness of the cemented sand.

Limit lateral resistance of XCC pile group in undrained soil

July 12, 2019, 12:00 am

≫ Next: Sedimentary characteristics of coral mud in the South China Sea during the reclamation process

≪ Previous: Effect of particle shape of glass beads on the strength and deformation of cemented sands

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This paper investigates the issue of laterally loaded X-section cast-in-place concrete (XCC) pile group in undrained soil and, in particular, evaluates limit lateral resistance (LLR) through finite element limit analysis. Four types of XCC pile groups that are commonly used in practice are selected for discussion here. Both the upper- and lower-bound numerical results for the LLR of the XCC pile groups are presented. The influence of loading direction, pile group shape, and XCC pile cross-sectional shape is evaluated with a series of parametric studies. Design equations are subsequently proposed for computing the LLR of four types of XCC pile groups in undrained soil.

Sedimentary characteristics of coral mud in the South China Sea during the reclamation process

October 1, 2019, 12:00 am

≫ Next: Probabilistic Risk Assessment of unsaturated Slope Failure Considering Spatial Variability of Hydraulic Parameters

≪ Previous: Limit lateral resistance of XCC pile group in undrained soil

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Coral mud is a form of calcareous ooze composed of fine-grained coral debris distributed as interlayers due to particle sorting during the formation of reclamation reefs in the South China Sea. To analyze the effect of coral mud on the stability of reef foundations formed by dredger fill, in this paper, the effects of initial concentrations, initial settlement heights, and salinity levels on the sedimentary characteristics of coral mud are discussed based on laboratory simulation tests. Comparative tests of marine sedimentary soil with similar particle compositions and of terrestrial silty clay with similar plastic indexes were carried out, and the sedimentation velocity characteristics of coral mud were identified at the microscopic level. The experimental results show that reducing initial concentrations and initial settlement heights are conducive to the coral mud filling process and that a medium salt content has no significant effect. The sedimentation rate of coral mud is roughly 3–7 times that of common soil, and the time spent in the isokinetic sedimentation stage is only 1/3 that of common soil. Microscopic studies show that the settling velocity of clay in water is proportional to the sphericity of flocs. According to our experimental results, the relationship between the sedimentation velocity and initial concentrations of coral mud is obtained and can be used to guide the design of similar calcareous soil projects across the globe and of dredger filling projects in particular.

Probabilistic Risk Assessment of unsaturated Slope Failure Considering Spatial Variability of Hydraulic Parameters

December 1, 2019, 12:00 am

≫ Next: Probabilistic stability analysis of earth dam slope under transient seepage using multivariate adaptive regression splines

≪ Previous: Sedimentary characteristics of coral mud in the South China Sea during the reclamation process

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Probabilistic risk assessment of slope failure evaluates the slope safety in a quantitative manner, which considers the failure probability and failure consequence simultaneously. However, risk assessment of unsaturated slope accounting for spatially variable soil-water characteristic curve (SWCC) model parameter and saturated hydraulic conductivity has been rarely reported. A probabilistic risk assessment approach is proposed in current study for rationally quantifying the unsaturated slope failure risk with the aid of Monte Carlo (MC) simulation. The SEEP/W and SLOPE/W modules contained in Geostudio software are applied to carry out deterministic analysis, where factor of safety (FS) of the unsaturated slope is calculated by Morgenstern–Price method. The spatially variable hydraulic parameters are characterized by their respective random fields that are transferred from the random void ratio field in this study, rather than generating them separately. The proposed approach is subsequently employed to an unsaturated slope example for exploring the influences of spatially variable void ratio. Results show that the unsaturated slope failure risk is considerably affected by the spatially variable void ratio, and the single exponential autocorrelation function (ACF) popularized in geotechnical engineering tends to underestimate the failure risk in the unsaturated slope risk assessment.

Probabilistic stability analysis of earth dam slope under transient seepage using multivariate adaptive regression splines

January 30, 2020, 12:00 am

≫ Next: A p – y curve model for laterally loaded XCC pile in soft clay

≪ Previous: Probabilistic Risk Assessment of unsaturated Slope Failure Considering Spatial Variability of Hydraulic Parameters

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Earth dams are widespread throughout the world and their safety has gained increasing concern from geotechnical engineering societies. Although probabilistic stability analysis approach has been widely applied to the safety assessment of geotechnical structures, few studies have been performed to investigate the effects of water level fluctuations on earth dam slope stability considering uncertainties of soil parameters. This study proposes an efficient probabilistic stability analysis approach by integrating a soft computing algorithm of multivariate adaptive regression splines (MARS). The calibration of a MARS model generally requires a large number of training samples, which are obtained from repeated runs of deterministic seepage and slope stability analyses using the GeoStudio software. Based on the established MARS model, the earth dam slope failure probability can be conveniently evaluated. As an illustration, the proposed approach is applied to the probabilistic stability analysis of Ashigong earth dam under transient seepage. The effects of the uncertainties of soil parameters and water level fluctuation velocity on the earth dam slope failure probability are explored systematically. Results show that the MARS-based probabilistic stability analysis approach evaluates the earth slope failure probability with satisfactory accuracy and efficiency. The earth dam slope failure probability is significantly affected by the water level fluctuation velocity and the coefficient of variation of the effective friction angle.