Piled Embankments Research Articles

Simplified analytical solution for stress concentration ratio of piled embankments incorporating pile\u2013soil interaction

Piled embankments have been extensively used for high-speed rail over soft soils because of their effectiveness in minimizing differential settlement and shortening the construction period. Stress concentration ratio, defined as the ratio of vertical stress carried by pile heads (or pile caps if applicable) to that by adjacent soils, is a fundamental parameter in the design of piled embankments. In view of the complicated load transfer mechanism in the framework of embankment system, this paper presents a simplified analytical solution for the stress concentration ratio of rigid pile-supported embankments. In the derivation, the effects of cushion stiffness, pile–soil interaction, and pile penetration behavior are considered and examined. A modified linearly elastic-perfectly plastic model was used to analyze the mechanical response of a rigid pile–soil system. The analytical model was verified against field data and the results of numerical simulations from the literature. According to the proposed method, the skin friction distribution, pile–soil relative displacement, location of neural point, and differential settlement between the pile head (or cap) and adjacent soils can be determined. This work serves as a fast algorithm for initial and reasonable approximation of stress concentration ratio on the design aspects of piled embankments.

Numerical Investigation of Soil Arching in Dense Sand

Soil arching is relevant in various engineering applications particularly in cases where there is relative movement between soil and structure. When a portion of the soil mass yields, redistribution of stresses take place between the yielding mass and its surrounding. Several trapdoor experiments have been performed with different improvements and extensions over the past few decades to study soil arching. Numerical modeling of a trapdoor mechanism is performed in this study and the suitability of Hardening Soil constitutive model over the Mohr-Coulomb model has been studied. In the present study, extensive parametric studies were conducted to examine the extent of the zone of influence and zone of yielding. The zone of influence, beyond which there is no influence of soil arching, is bounded by straight lines to form a trapezoidal shape. The dimensions of the influence zone depend on the fill height (H) and trapdoor width (B). The two families of failure surfaces, internal and external, are described and their categorization with respect to the H/B ratio is well explained. Furthermore, the zone of yielding and the region enclosed within limits of influence and failure planes are classified as active and passive zones, considering the dilation and compression in the direction of trapdoor displacement. The equilibrium of stress transfer in soil arching that remained as an unjustified assumption until now is systematically brought out in the present study. The present study also highlights salient characteristics of the plane of equivalent settlement and critical height. The present study throws some light on the need for further understanding of arching in various field applications, such as piled embankments, tunnels, culverts, buried pipelines, excavations supported by contiguous pile walls, and slopes stabilized by a row of piles.

Stress Distribution in Basal Geogrid Reinforced Pile-Supported Embankments Under Seismic Loads

Basal geosynthetic reinforced pile-supported embankments are proven as the more appropriate ground improvement technique for constructing embankments for roads over very soft clay deposits and approach roads or embankments to bridges. Numerous experimental and analytical works are available on the soil arching phenomenon of geosynthetic reinforced piled embankments subjected to static loading conditions. This study attempts to evaluate the stress distribution and soil arching in geosynthetic reinforced pile-supported embankments subjected to seismic excitations. Time-history analysis has been performed on the basal geogrid reinforced pile-supported embankments by varying the height of embankment and tensile modulus of geogrid. Analyses of results show that for α (the ratio of height of embankment to pile centre to centre spacing) less than or equal to 4.5, a geogrid tensile modulus of 3000 kN/m is sufficient to withstand vertical stresses due to earthquakes. And for the considered embankment height and pile diameter when α nearly equal to 4.5, differential settlements are very less irrespective of seismic excitations.

Lessons from Full-Scale Experiments on Piled Embankments with Different Geosynthetic Reinforcements

Geosynthetic-reinforced piled embankments (GRPEs) are very complex structures with three differently behaving inclusions; i.e., piles, geosynthetics and soils. Very careful experiments are needed to investigate the behaviors of these GRPEs. In this study, the results of full-scale experiments conducted on GRPEs with three different geosynthetic reinforcements (GRs) and on an unreinforced piled embankment (PE) are presented. In the experiments, the vertical load transported towards the model piles and the vertical displacements of the soft foam layer underneath the reinforced embankment were measured and compared with the calculation results obtained by the concentric arches model used in the Netherlands. The vertical loads measured in the unreinforced PE and one GRPE agree well with the theoretical values. However, those measured in the two other GRPEs were significantly lower than the theoretically predicted values. It was concluded that these discrepancies were caused by two weak points in the execution of the experiments: inappropriate GR fixating at the sides of the model in combination with significant unexpected compaction in the soft foam layer that originated from the compaction procedure. These factors should be considered carefully in future experimental research to attain a better understanding of the behaviors of GRPEs.

Improved Analytical Soil Arching Model for the Design of Piled Embankments

Abstract Because analytical soil arching models are fundamental for the design of piled embankments, numerous soil arching models were proposed for this purpose. However, the results obtained from ...

Behaviour of piled embankment with multi-interaction arching model

The main focus of this paper is to present a multi-interaction model under a simplified form for calculating load transfer to piles and geosynthetic strain of piled embankments. The multi-interaction model is established by considering the integrated load transfer mechanisms and all the interactions between the elements of a geosynthetic-reinforced and pile-supported embankment, including fill soils, geosynthetic, platform, subsoil and pile. The proposed method is based on the combination of the tensioned membrane theory for geosynthetics, soil arching theory for fill soils, friction interaction model and, subsoil support. Thereby it allows providing a suitable design approach that enables a realistic and suitable approximation in designing soil–geosynthetic systems. The proposed method can be applied to both cohesive and non-cohesive fill soils. Using this method, the influence of the subsoil consolidation degree, subsoil ultimate bearing capacity, fill shear strength and platform, are investigated considering the behaviour and deformation of piled embankments. The result of the proposed method is compared to field measurements and to five other design standards to study its validity. The results showed a good agreement between the proposed method with measured data and are better than other design methods as well as two international design standards EBGEO and BS8006.

Geosynthetic reinforced piled embankment modeling using discrete and continuum approaches

Understanding the load transfer mechanism can support engineers having more economical design of geosynthetic reinforced piled embankments. This study aims to investigate the load transfer mechanisms by two different numerical methods including the Discrete Element Method (DEM) and the Finite Difference Method (FDM). The DEM model adopts (a) discrete particles to simulate the micro-structure of the granular materials and (b) coupled discrete element – finite element method (DEM-FEM) to capture the interaction between granular materials and geotextiles. On the other hand, the FDM model uses an advanced constitutive soil model considering the hardening and softening behaviour of the granular materials. The numerical results show that the geotextiles can only contribute to the vertical loading resistance in cases where the soils between piles are soft enough. In terms of design, an optimum value of the geotextile tensile stiffness can be found considering the load, the soft soil stiffness and the thickness of the embankment. Both the DEM and the FDM show that a high geotextile tensile stiffness is not required since an extra stiffness will slightly contribute to the efficiency of the geosynthetic reinforced piled embankments. Nevertheless, both models are useful to optimize the design of geosynthetic reinforced piled embankments.

Numerical stability analysis of piled embankments reinforced with ground beams

Rigid piles are extensively used to support high-speed railway embankments resting on soft ground, and ground beams are structural elements at ground level to connect adjacent pile caps to improve the stability of pile-supported embankments. In this study, a validated three-dimensional numerical model of piled embankment was developed based on a well-documented centrifuge test, where the Mohr–Coulomb tension crack model was adopted for capturing the brittle failure behavior of concrete piles. The results showed that concrete piles are subject to a serious bending failure within the clay layer when the overlying embankment collapses. By introducing the ground beams in conjunction with rigid piles, the resultant displacement of the improved area was markedly decreased and the pile failure mode was changed. The combination of concrete piles and ground beams results in a synergistic effect. According to the mechanical response of individual piles, the foundation soil was divided into the extension-bending, bending, bending-compression, and compression zones to elucidate the reinforcement mechanism related to ground beams. A proper arrangement of ground beams was ultimately obtained based on the localized slip resistance performance of the improved ground. Overall, this research contributes to the knowledge of piled embankment performance evaluation and may further extend the practical application of ground beams.

Quantitative analysis of shallow unreinforced and reinforced piled embankments with different heights subject to cyclic loads: Experimental study

Reinforced piled embankment is an important technique which is widely used for construction on soft ground due to its benefits in reducing settlement, construction time and cost effectiveness. Most of the existing research has considered the response of deep embankments subjected to a uniform surcharge static load. Limited studies have examined the effect of cyclic loading or the influence of varying the height of the embankment. In this study, the effect of applying cyclic loading on a particular area of unreinforced and reinforced embankments of different heights was experimentally investigated and analysed. Twelve main experiments were undertaken to investigate the effects of monotonic and cyclic loading conditions, the number of reinforcement layers (n) and increasing the mean value and amplitude of the loading cycles on the behaviour of embankments of different heights. The experimental results showed that the transfer of loads to the piles was increased during the monotonic loading stage but at a lower rate with increasing the embankment height. Under cyclic loading conditions, arching of the soil was adversely affected during the initial stages of cyclic loading regardless of the embankment height. However, by increasing the number of cycles, arching of the soil started to regain some degree of improvement. Including layers of reinforcement increased the magnitude of the loads transferred on piles. However, the improvement was more obvious for a thinner embankment. Finally, the surface settlement increased with raising the embankment height but decreased when the number of reinforcement layers was increased, while the extent of heaving outside the loaded area was reduced with increasing both the embankment height and the number of reinforcement layers.

Centrifuge Model Test on the Performance of Geogrid-Reinforced and Pile-Supported Embankment Over Soft Soil

We conducted a centrifugal model test to investigate the deformation and failure mode of a geogrid-reinforced and pile-supported (GRPS) embankment. The embankment filling was simulated using the incremental acceleration method, and the surcharge was subsequently applied using an inflatable airbag on the embankment. By installing different types of instruments, the displacement of the soil, the axial force and bending moment of the piles, and the earth pressure and pore water pressure of the foundation were monitored. The test results revealed that the embankment tended to slide in a circular-arc manner from shallow to deep, and the stress distribution of the piles varied with the locations. The stress concentration ratio of the embankment was higher, which indicates a more efficient load transmission. Additionally, the excess pore water pressure in the GRPS embankment was lower compared with that in conventional piled embankments.

Load-deformation of piled embankments considering geosynthetic membrane effect and interface friction

The use of high-strength geosynthetics to enhance the load transfer mechanism onto columns is an established and increasingly popular technique in geosynthetic-reinforced and column-supported embankments. The main focus of this paper is to extend the existing models that describe the membrane action and soil arching with skin friction along the geosynthetic. This extension was undertaken to identify the parameters that affect the tension in the geosynthetic and assess the effect of geosynthetics on the load transfer. A general expression for the increase in load-bearing capacity due to the membrane action based on strict equilibrium conditions is also shown. The geosynthetic deformation is described assuming both circular and parabolic deformation shapes. These two deformation shapes do not result in a significantly different membrane effect. Therefore, the choice of deformation parameter is more important than the choice of deformation shape. The new method using both deformation models was combined with the Concentric Arches model of Van Eekelen and co-workers, and compared with the measurements and numerical results. A reasonable consistency is found. For the considered cases, the skin friction along the geosynthetic reduces the maximum geosynthetic deflection to 2.5 to 5.3%. This reduction becomes more important when the embankment is higher.

Analytical solution for geogrid-reinforced piled embankments under traffic loads

This paper describes a simplified numerical model for geogrid-reinforced piled embankments considering dynamic loads. Based on this model, the vertical stress acting on the geogrid is investigated comprehensively and is found to exhibit an inverse trapezoidal distribution. An analytical solution for the geogrid-reinforced piled embankment under a static load is then derived for vertical equilibrium and an assumed inverse trapezoidal load distribution on the geogrid. This is extended by considering an additional vertical stress induced by the dynamic loads, based on the Boussinesq equation. However, the characteristics of the traffic loads (i.e. frequency and load numbers) are ignored. The analytical solution is then validated against three case studies and parametric numerical studies under various geometries (pile spacing and embankment height), geogrid stiffnesses and embankment friction angles. These parametric studies show that the geogrid tension increases with the pile spacing, embankment height and geogrid stiffness but decreases with the friction angle of the embankment. The pile spacing is found to be the most influential of all the factors examined in this paper, increasing from 2.0 m to 3.0 m when the geogrid tension is increased by approximately 248%.

Geosynthetic-reinforced pile-embankments: numerical, analytical and centrifuge modelling

This paper presents numerical and analytical models to complement experimental data obtained from 40 centrifuge tests of geosynthetic-reinforced piled embankments, 12 of which had a surcharge applied at the embankment surface. The main parameters of study were pile diameter, pile spacing, embankment height and geosynthetic stiffness. The tests measured the force transferred to the piles, the embankment surface settlements and the maximum geosynthetic reinforcement deflections below the embankment under conditions without any support from the underlying soil. Maximum geosynthetic deflections measured longitudinally between piles were then compared with values predicted by European design guidelines. A 3D numerical model, initially validated by centrifuge tests, was then used to compute those geosynthetic tensile forces not measured in the reduced-scale physical models. Numerical values of geosynthetic tensile forces were also compared with corresponding values predicted by European guidelines.

Load transfer and deformation of geogrid-reinforced piled embankments: field measurement

The collected field data for a geosynthetic-reinforced pile-supported structure (GRPS) in Hamburg is studied to identify the effect of environmental changes on the load transfer process over the structure lifetime. The prediction models in three design codes are applied to estimate the loads and strains in different structural components of the embankment. The results show that the models fail to accurately estimate the geogrid reinforcement deflection and overestimate the loads on the pile foundation in this project.

Deformations in trapdoor tests and piled embankments

Fill deformation and surface settlement can be induced by differential settlement at the bottom of the fill in piled embankments. The deformation patterns and the relationship between the surface settlement and the differential settlement at the bottom of the fill have not been well investigated. Two-dimensional single-trapdoor, twin-trapdoor, and multi-trapdoor tests, including four tests with geosynthetic reinforcement, were conducted using elliptical steel rods as an analog to soil. The deformation pattern and influence regions in the single-trapdoor tests were evaluated using the measured deformations. The fill deformations in the trapdoor tests followed the Gaussian distribution. The superposition results of these Gaussian distribution curves of the single-trapdoor tests were compared with the measured deformations in the twin-trapdoor and multi-trapdoor tests. The differences between the measured and calculated results indicate that additional interaction occurred between adjacent trapdoors. The deformation shapes of the fill at the bottom of the geosynthetic-reinforced and unreinforced test sections were different. However, the settlement pattern at the elevation level above 1.5 times the clear spacing of pile caps followed the same Gaussian distribution curve, if the volumetric settlement was the same. A method for predicting the surface settlement of 2D piled embankments is then presented.

Centrifuge model studies on the settlement response of geosynthetic piled embankments

The paper aims at comparing the deformation behaviour of unreinforced embankments, basal reinforced embankments and geosynthetic reinforced embankments with floating and end bearing piles. The floating piles were terminated within the soft foundation soil, while the end bearing piles transfer the loads to a hard stratum. Four series of centrifuge model tests were conducted at gravity g = 40, using the 4.5 m radius large beam centrifuge facility at the Indian Institute of Technology Bombay. The tests were performed at constant pile spacing, and the embankment was constructed using an in-flight sand hopper. The deformations were measured using Linear Variable Differential Transformers (LVDTs). A digital image analysis technique was employed to arrive at the surface displacements, displacement vectors, and displacement contours. The geogrid reinforced embankments resting on end bearing piles and floating piles showed 88% and 44% lesser settlements respectively, compared to the unreinforced embankments. The geogrid reinforcement helps to distribute the load uniformly. A deep-seated slope failure was observed for unreinforced embankments. The piled embankments proved to be a practical solution for construction without prolonged waiting periods for pre-consolidation of the foundation soil.

Analysis of unreinforced and reinforced shallow piled embankments under cyclic loading

Reinforced piled embankments are being increasingly used for construction over soft ground due to their efficiency in reducing potential settlement, speed of construction, and associated cost. Most of the previous studies focused on developing understanding of the behaviour of thick embankments that are loaded with a static surcharge load. Data on the behaviour of shallow piled embankments under cyclic loadings are scarce. In this study, an experimental programme was undertaken using a fully instrumented testing rig to generate data and improve our understanding of the behaviour of unreinforced and reinforced shallow piled embankments subject to monotonic and cyclic loadings that were applied over a predetermined area of the embankment. The experimental results showed that collapse of soil arching is instantaneous and occurs during the first few cycles of load. However, strength was regained, and recovery of the arching effect was observable during further stages of cyclic loadings due to densification of the embankment material and deformation of the soft subsoil. Inclusion of reinforcement layers was found to enhance the performance of load transfer mechanisms by concentrating stresses on pile caps. The results clearly showed a significant reduction in surface settlement, soft subsoil settlement and heaving with increasing numbers of reinforcement layers.

3D modeling of geosynthetic-reinforced pile-supported embankment under cyclic loading

Geosynthetic-reinforced pile-supported (GRPS) embankments are an attractive soft soil improvement technique. They are broadly used in infrastructure projects. Most previous studies focused on investigating reinforced piled embankments under static loading. There is a paucity of studies on structure behavior under cyclic loading. In the study, three-dimensional numerical modeling using the finite element method (FEM) was performed with the objective of better understanding the cyclic behavior of GRPS embankments in terms of load transfer mechanisms and settlements. An advanced constitutive model based on the hypoplasticity concept was employed for the embankment fill to simulate the cumulative strain during each loading cycle. Parametric studies related to the embankment height, traffic loading parameters (amplitude and frequency), and number of geosynthetic reinforcement layers are presented. The numerical results obtained indicate that the presence of a geosynthetic reinforcement decelerates the reduction in the arching effect within the embankment fill and decreases the cumulative settlements. The rate of the cumulative settlements decreases with the number of load cycles. The number of geosynthetic reinforcement layers does not appear to influence the soil arching and cumulative settlements.

“Soil arching” for piled embankments: insights from stress redistribution behaviour of DEM modelling

For piled embankments, it is widely recognised that the majority of embankment load can be transferred to the piles through the “soil arching” mobilised in the embankment. To date, no consensus has been reached on the shape and composition of the “soil arching”. In light of this, a total of 131 2D trapdoor-like discrete element method models were conducted in this study to present the soil arching effect, stress state and deformation behaviours of the piled embankments. Then, in-depth discussions on the composition and mobilisation mechanism of “soil arching” were performed. The horizontal stress concentration caused by the lateral extrusion of embankment fill was considered to be the composition of the “soil arching”, while the vertical stress redistribution is only the result of load transfer induced by the “soil arching”. Afterwards, the criterion that comprehensively considers the redistribution behaviours of the vertical and horizontal stresses was proposed to determine the feature parameters of “soil arching”.

Simplified method for estimating vertical stress-settlement responses of piled embankments on soft soils

Soil arching development patterns of piled embankment have been comprehensively investigated by performing multi-trapdoor tests in many previous studies. However, the application of these findings is inevitably limited as the subsoil-pile system under the embankment is generally not explicitly taken into account. In this study, an analytical method was employed to evaluate the compressibility of the subsoil-pile system. The compressibility analysis results were used together with the analytical models for soil arching development obtained from multi-trapdoor tests to estimate the vertical stress-settlement response of the subsoil-pile-embankment system. A parametric study was conducted to investigate the effect of the following influence factors: friction angle of embankment fill, embankment height, coefficient of skin friction, pile spacing, pile cap width, and pile to subsoil modulus ratio on the vertical stress-settlement response. Based on the parametric study, correlations were developed between the influence factors and the vertical stress-settlement responses of the subsoil-pile-embankment system. The developed method was favorably examined by a set of field test results in the literature.