Piled Embankments Research Articles

Large-scale Field Tests of the Performance of Geogrid-reinforced Piled Embankment over Soft Soil

When constructing superstructures on soft soils, geogrid-reinforced embankments and pile-supported embankments are used widely to improve the soft foundation and prevent issues including excessive settlements and large lateral displacements, so it is crucial to understand and evaluate their performance systematically. This study focuses on a case of piled embankments for a motor-racing circuit under construction in China. Two large-scale field tests of pile-supported embankments with and without reinforcement were carried out to investigate the effect of geosynthetic reinforcement on the improvement of embankments. During long-term monitoring, instruments in the test sites measured earth pressures, settlements, lateral displacements, and pore water pressures, and the performances of the reinforced or unreinforced embankments were examined. Test results from the two test sites under similar loading are compared, and it is concluded that the geosynthetic-reinforcement considerably influences the improvement of load transfer and diminishment of total and differential settlements. The lateral restraint provided by the geogrids also reduced the lateral soil displacement and the influence of subsoil depth. The smaller excessive pore water pressures in the site with reinforcement were associated with the enhanced load transfer due to the geogrids. This study provides an important reference for the design and construction of this circuit and other related projects.

Dynamic response analysis of piled embankment and train derailment evaluation subjected to seismic and high-speed train loads

Based on a train-rail-piled embankment three-dimensional finite element model, the vibration displacement, acceleration and frequency spectrum of the rail and subgrade caused by combined seismic and high-speed train loads was investigated. On this basis, performance of piled embankment under combined seismic and high-speed train loads was analyzed. Numerical examples indicate that the seismic load dominates the ground displacement and the moving train load has negligible effects on the ground displacement. However, the moving train load has a significant effect on the rail vibration displacement during earthquakes. The rail and subgrade vibration displacement of piled embankment under combined seismic and moving train loads is obviously lower than that of ordinary subgrade, and its dominant frequency tends to transfer from high to middle and low frequency compared with ordinary subgrade, indicating that pile embankment has a good vibration reduction effect. Moreover, the derailment coefficient and lateral displacement of piled embankment is obviously decreased, which provides important guidance that pile embankment has significant vibration reduction effect and thus it can effectively prevent derailment or overturning during earthquakes.

Bearing Capacity of Foundation and Soil Arching in Rigid Floating Piled Embankments: Numerical Study

The definition of rigid floating piles in engineering applications remains ambiguous. This paper develops a numerical model of piled embankments and the modeling method is verified through engineering cases. Utilizing this model, the critical bearing capacity of the foundation for rigid floating piles is firstly determined to be approximately 150 kPa. Subsequent parametric studies show that soil arching in a rigid floating piled embankment begins to occur when the ratio of the embankment height to the clear pile spacing H/(s − a) ≥ 1.6. However, plastic failure does not occur in rigid floating piled embankments due to the weaker foundation, indicating that soil arching cannot fully develop. Finally, factors of the embankment fill properties are examined and it is shown that the large friction angle and high cohesion greatly enhance soil arching and reduce settlement in the embankment. The vertical stress above the subsoil decreased by 34.9% with an increase in the friction angle from 20° to 40°. Also shown is that the increase in the cohesion from 1 kPa to 7 kPa reduced the settlement on the top of and the base of embankment by approximately 72% and 58%, respectively. It has also been found that rigid floating piles rely on end resistance and skin friction to sustain the superimposed load.

DESIGN OF BASAL REINFORCED EMBANKMENTS SUPPORTED BY GEOTHERMAL ENERGY PILES

TThis paper compares the optimal design of embankments reinforced at the base with geosynthetics and supported by geothermal piles and non-geothermal piles using optimization. An optimization model was created to complete this analysis. The model is based on the fabrication cost of the structure, which consists of various materials such as geosynthetics, gravel fill, concrete and steel reinforcement. It was subjected to the limit state and serviceability conditions according to the Eurocode design rules and recommendations for the analysis of conventional and geothermal piles, taking into account geotechnical, structural and settlement constraints. To consider all limit states for embankment along with the piles and geosynthetic, the most important analyses were performed: Stresses in the geosynthetic reinforcement, bearing resistance of the pile, group resistance of the pile, pile group extent, vertical load transfer, lateral sliding, strain/stiffness of the geosynthetic reinforcement, double shear capacity/punching, and the geometric constraints recommended based on the design experience gained from the past project. The optimization model developed in this paper was applied to an example with selected design parameters to demonstrate the usefulness of the optimization model. The results show that the correct determination of the structure geometry, pile dimensions and geosynthetics leads to a significant cost reduction. The construction of geothermal piles for piled embankments support is very economical due to the low vertical load on the piles, if there is a possibility of using energy in the surrounding buildings.

Surrogate-assisted uncertainty modeling of embankment settlement

The structural optimization of basal reinforced piled embankments is usually conducted by examining design alternatives while ignoring the inherent variability of soil properties and studying only a limited number of structural variables. As an alternative, this paper proposes a hybrid modeling framework to introduce soil property uncertainty into embankment settlement calculations. This is important because settlement is critical in the serviceability assessments considered during structural optimization. The proposed framework consists of uncertainty modeling, finite element method, surrogate modeling, and probabilistic analysis. More specifically, a neural network with Monte Carlo dropout that accounts for uncertainty is employed to correlate the soil properties which affect the long-term performance of embankments over soft clay. Next, a coupled finite element analysis is performed using two constitutive soil parameters generated by the neural network to predict post-construction settlements. Combining the finite element (input source) with a surrogate model (data-driven approximation) yields substantial settlement outcomes for structure evaluations. A case study is then used to validate the effectiveness and applicability of this framework. Finally, an exhaustive search approach is used to design a cost-effective improved ground within ultimate and serviceability limit state constraints. Pareto front is computed using a logistic function at different settlement reliability levels.

Three-Dimensional Analysis of Load Transfer Mechanism for Deep Cement Mixing Piled Embankment under Static and Cyclic Load

Piles have been widely used to improve the bearing capacity of the soft foundation. The existing research obtains significant findings on the load transfer mechanism for rigid piled embankments. However, limited studies have been focused on the deep cement mixing (DCM) piled embankment. To grasp the load transfer characteristics of DCM piled embankments, a three-dimensional numerical simulation was conducted in this study, which was validated by the measurements from the field case. It was found that the effect of soil arching was reduced compared with the rigid piled embankment. This induced approximately 61.5% larger vertical stress transferred to the subsoil surface and approximately 83–150% larger settlement of the embankment in DCM piled foundation system. To further understand the working mechanism of this system, the factors which influence the load transfer mechanism were investigated. It is found that the area replacement ratio is the most influential factor affecting the settlement at the top of the embankment, whereas the elastic modulus of the DCM pile influences most the vertical stress and the earth pressure coefficient. The cyclic load with vehicle speeds of 90 km/h will lead to approximately 34% growth of embankment settlement and about 11% reduction in the maximum earth pressure coefficient. Based on the numerical simulation results, the analytical equation of the normalized vertical stress acting on the subsoil surface for the DCM piled foundation was proposed and validated by two field cases, with the difference in the range of 13.8~16.7%.

Load Transfer in Geosynthetic-Reinforced Piled Embankments with a Triangular Arrangement of Piles

A geosynthetic-reinforced piled embankment situated along a newly constructed high-speed rail line was fully instrumented and monitored to explore soil arching in an equilateral-triangular arrangement of piles with circular cap configuration. During construction, the settlement and vertical stress on the pile caps and subsurface at ground level and the tensile strain in the geogrid were recorded. A modified analytical arching model is derived from the established concentric arches (CA) model to elucidate the load transfer mechanism, specifically for a triangular array of piles. The general framework assumes that the embankment load, not resting on the subsurface beneath the arches, is transferred outward along the arches and distributed uniformly over the improved ground. Compared to the conventional CA model, the proposed model gives a lower proportion of load carried by piles, more in line with field measurements. Given the friction and adhesion between soil and geosynthetics, a novel quartic equation is applied to characterize the tensioned membrane effect of geosynthetic reinforcement based on Pham’s circular and parabolic models. The stiffness variation of the biaxial geogrid was critically assessed for the triangular pile arrangement, and its implications for the design were revealed. Finally, the pile efficacy and critical arch height were determined from different analytical methods and checked against field observations, demonstrating the potential of the proposed model.

Numerical analysis of dynamic behavior of piled embankment under train loading

Piled embankments, which have many advantages, have been widely employed in the construction of railways over soft soil. However, the load transfer mechanism and deformation characteristic induced by train loading, which has great importance for piled embankment design, are not well known and deserve more research attention. In this paper, a three-dimensional numerical model of a piled embankment subjected to train loading is proposed and calibrated by test results, which considers the dynamic behavior of soft clays by a simplified dynamic constitutive model based on bounding surface theory that has been successfully encoded into the ABAQUS. Based on the proposed numerical model, the development of soil arching and accumulative deformation in a piled embankment subjected to train loading is investigated. The load transfer mechanism and deformation characteristics of a piled embankment in terms of the embankment height, velocity of the train, pile spacing and wheel static load are analyzed. The research results can provide some reference for the design of piled embankments under train loading.

Ground Reaction of Lightly Overconsolidated Subsoil in Reinforced Piled Embankment under Cyclic Loads

Subsoil support is generally ignored in the design of reinforced piled embankments, resulting in a very conservative design for settlement control. This design philosophy may lead to an unnecessary increase in construction costs, especially for embankments constructed over subsoil of medium and high compressibility (i.e., compression index of subsoil larger than 0.2). This paper presents the ground reaction of lightly overconsolidated subsoil in a reinforced piled embankment subjected to cyclic loads for the purpose of investigating the general behavior of lightly overconsolidated subsoil, and it promotes the sustainable development of piled embankment technology. The ground reaction of subsoil under both static and cyclic loads was comprehensively analyzed in terms of settlement and incremental vertical stress, which exhibited approximately the same profile. However, the settlement of lightly overconsolidated subsoil under a cyclic load was 23% larger than that under a static load. A parametric study was then performed under cyclic loads, and the results showed that the vertical stress carried by the subsoil was the most sensitive to the pile spacing amongst all the parameters considered in this paper. The analysis demonstrated an approximately 88% increase in stress when spacing was enlarged from 2.0 to 3.0 m. Finally, a modified analytical method for the ground reaction of lightly overconsolidated subsoil under cyclic loads was presented, and it showed reasonable agreement with the numerical simulation, particularly for relatively low geogrid stiffnesses, low embankment height (<6.5 m), and small pile spacing (e.g., 2–3 m center to center).

Ground improvement techniques applied to very soft clays: state of knowledge and recent advances

Soft ground improvement techniques have evolved substantially in Brazil in recent years. However, their application in soft and very soft clays requires a good understanding of the fundamentals of ground improvement techniques suited to the problem as well as the actual field behavior when implemented on a real scale. This paper describes some of the most widely used ground improvement techniques in the context of very soft clays in Brazil. The techniques described in the paper use prefabricated vertical drains (PVD) such as vacuum preloading; or combine PVD and rigid inclusion, such as CPR grouting; or are purely column-like elements such as piled embankments (including those executed with the deep mixing technique, DSM); or combine column-like elements with the drainage function, such as stone columns and geosynthetic encased columns; or use cementitious binders such as shallow soil mixing. The paper reference condition is a soft clay foundation in which no strengthening is implemented, such as, an embankment with basal reinforcement or soft clay with vertical prefabricated drains, or the use of vacuum preloading to speed up the consolidation rate. The applications of the ground improvement techniques are illustrated by case histories, numerical analyses, or physical models. Different types of measurements are used to evaluate the performance of each technique, including settlements, horizontal displacements, excess pore pressures, embankment applied stresses, stress concentration factors, and clay strength following the ground treatment. The settlement improvement factor β, the ratio between the settlements for untreated and treated conditions, is shown to be a suitable parameter to assess the degree of improvement imposed in the soft foundation by ranking the various methods in increased order of strengthening effect.

Membrane Effect of Geogrid Reinforcement for Low Highway Piled Embankment under Moving Vehicle Loads

In this paper, the membrane effect of geogrid reinforcement was investigated based on numerical simulation to understand the serviceability and deformation of highway piled embankments under moving vehicle loads. The membrane effect of geogrid reinforcement in low embankments (i.e., the ratio of embankment height to pile spacing is less than 1.5) was clearly emphasized. It has been found that the maximum settlement of geogrid occurs in the central area between the piles, and the maximum tension was concentrated at the corner of the pile cap. Due to the attenuation of the soil arching effect under moving dynamic loads and the punching mechanism, the settlement and tension of the geogrid increased considerably by approximately 35% and 23% compared to those under static loads. A parametric study was also achieved, and the results presented that the geogrid reinforcement tension increased by increasing the pile spacing, embankment height and geogrid stiffness, vehicle wheel load and vehicle velocity. It was also found that the reinforcement tension was most sensitive to the pile spacing among all the parameters considered in this paper, whose magnitude increased by approximately 104% as the pile spacing increased from 2.0 m to 2.5 m under dynamic loads.

Field measurements in a partly submerged woven geotextile-reinforced pile-supported embankment

The geosynthetic reinforcement in a pile-supported (GRPS) embankment can be designed using the CUR226 (2016) design guideline. This design guideline adopted the Concentric Arches model (Van Eekelen et al, 2013, 2015). The validated application of this model is limited to GRPS embankments whose geometry and materials meet the conditions of the field cases and experiments used for the validation of the Concentric Arches model. This means for example that: the whole embankment should be located above the groundwater table and that at least one geogrid layer should be used as reinforcement. If these requirements are not met, additional measurements are requested by CUR226 to demonstrate that the system comes within the framework of the guideline. This paper presents and discusses field measurements that were therefore conducted in a partially submerged GRPS embankment. The embankment was reinforced with two layers woven geotextile and no geogrid. It was concluded that the Concentric Arches model matches the measured geotextile strains very well, so that we may conclude that the CUR226 is also applicable for geotextile-reinforced piled embankment without geogrids. Furthermore, an increasing groundwater level seems to result in a reduction of the geotextile strain. It is recommended to conduct more research to further analyse the influence of water in GRPS embankments.

Soil arching effect in reinforced piled embankment for motor-racing circuit: Field tests and numerical analysis

When constructing superstructures on soft soils, severe issues including excessive settlements, bearing failures, and large lateral displacements arise and cause challenges for geotechnical engineers. Reinforced piled embankments are being used increasingly for various types of engineering construction such as highway widening and roadway construction, thereby making it crucial to comprehend and evaluate systematically the effect of soil arching, which has a significant influence on the performance of such embankments. This study focuses on two large-scale field tests of the piled embankments for an international motor-racing circuit in China, and the effect of the load transfer platform formed by geosynthetic reinforcement and gravel layer is investigated. Field monitoring data of soil pressures and settlements of pile caps and soils are reported and analyzed, and the test results show that soil arching acts significantly to transfer the load to the pile caps with an increase in loading from the embankment. Geogrids within the gravel layer can not only improve soil arching (reflected by higher stress concentration ratio) but also diminish differential settlements between pile caps and the ground surface. A numerical test considering different influencing factors is also conducted, thereby enabling both studying the effects of varying the field parameters and comparing with the insitu measurements. A proposed numerical model of the reinforced piled embankment is also shown to work well when compared with the results of the field tests. This study enhances the comprehension of soil–pile–geogrid interactions and guides similar projects in the future.

An evaluation of reinforcement mechanical damages in geosynthetic reinforced piled embankments

The use of geosynthetic reinforcement in piled embankments over soft soils is an effective solution for the reduction of settlements and to increase the embankment stability. The most efficient position for the reinforcement layer is on the pile cap or head. However, a direct contact of the reinforcement with sharp edges may damage it, compromising its efficiency to transfer loads to the piles. This paper investigates the possibility of mechanical damages in geosynthetic reinforcements on pile caps by large scale laboratory tests. Tests with and without pieces of nonwoven geotextile protective layer between the caps and the reinforcements were executed. Wide strip tensile tests were performed on exhumed reinforcement specimens after the tests to assess tensile strength and stiffness variations. A statistical analysis of the results shows reductions in tensile strength of unprotected reinforcement layers of up to 28%. A mechanical damage index is introduced and its correlation with calculated reduction factors is investigated. The use of a piece of a thick geotextile layer to protect the reinforcement against mechanical damage can be effective. However, the geotextile product must be properly specified and installed with due care.

Analytical model for the design of piled embankments considering cohesive soils

Geosynthetic-reinforced and pile-supported (GRPS) systems have already proven their good performance in supporting embankments constructed over soft soil. The load transfer mechanism in GRPS embankments depends on the complex interaction between the soil in place, the structural elements and the embankment's soil type (cohesive or cohesionless). However, the cohesion influence of the embankment soil has not been well investigated as it is often not considered in the design of such systems. The main aim of this study is to present an analytical model for GRPS embankments that combines several phenomena such as the concentric arches model in cohesive fill soils, the hyperbolic model for the isochrone geogrid curve, and subsoil's consolidation. Three-dimensional numerical analyses are also conducted to evaluate the embankment soil cohesion influence on the soil arching. Both the numerical and analytical results agree that the cohesive embankment fills strengthen the soil arching effect and increase the efficacy if compared with cohesionless embankment fills. A comparison of the analytical model with measured data and other design methods for full-scale field tests proved the proposed model efficiency. The proposed analytical model therefore can be applicable for GRPS embankments with cohesive and non-cohesive fill soils.

A Design Chart for the Analysis of the Maximum Strain of Reinforcement in GRPEs Considering the Arching and Stress History of the Subsoil

Geogrid-reinforced piled embankments (GRPEs) provide an economical and effective way to construct highways and railways on soft soil foundations. This paper proposed a new design method for GRPEs. The method was based on the soil arching and tensioned-membrane effects, the bearing capacity of the subsoil was considered as well. The originality of the proposed method lies in considering the stress history of the subsoil, and different over-consolidation ratios (OCRs) were used in calculating the settlement of subsoil. This design method, initially, established the vertical equilibrium of the unit body between the pile caps immediately above the subsoil. After that, the design charts were produced by solving the overall equilibrium equation from which engineers can intuitively obtain the maximum strain of reinforcement, and the tensile force can be used in the ultimate limit state analyses. The design method was then validated by three case studies, which showed good reliability with the maximum error being less than 18%. Parameter study results indicated that the maximum strain of reinforcement for the under-consolidated soil was 80–120% larger than that for normally consolidated soil and more than four times greater than that for over-consolidated soil.

Monitoring of an instrumented geosynthetic-reinforced piled embankment with a triangular pile configuration

ABSTRACT A comprehensive monitoring programme was undertaken at a geosynthetic-reinforced piled embankment over soft ground to better understand its time-dependent performance. The programme involved continuous monitoring of earth pressure, axial force in piles, settlement of structural elements, lateral movement of subsoil, and geogrid tensile strain of the instrumented system. Earth pressure measurements evidenced the mobilization of soil arching; pile efficacies were derived from established analytical methods and assessed against observed data for piles with circular cap configuration arranged in a triangular pattern, with the Concentric Arches model giving the best predictions. The development of axial load in piles can be properly captured by a regression equation with analytical approach. Penetration and compression behaviours of structural elements were evaluated by settlement measurement on piles and soils at ground level relative to pile toe or substratum. The proposed approach in obtaining tensile force of geogrid was verified with the measured differential-settlement data.

A simplified model for the analysis of piled embankments considering arching and subsoil consolidation

An analytical model is presented for the design of geosynthetic-reinforced and pile-supported (GRPS) embankments in this paper. The originality of the proposed solution lies in the fact that it allows considering the influence of the subsoil consolidation on the soil arching and geosynthetic strain. A nonlinear function is implemented to describe the subsoil behavior with the consolidation process in a closed-form solution. A simplified approach is then presented to link the arching development with the subsoil consolidation. The arching theory is combined with the tensioned membrane theory and the soil-structure interaction mechanisms to provide a simple and suitable design approach that enables a realistic approximation for designing soil–geosynthetic systems. The analytical model is capable of performing an ultimate and serviceability limit state design of GRPS embankments. While current methods cannot fully address the important effects of the subsoil consolidation, the analytical results suggested that arching and differential settlements increase with an increase of the subsoil consolidation degree. The analytical model is compared to field measurements and five other design standards for several full-scale field tests to study its validity. The results showed a satisfactory agreement between the proposed model and measured data, and generally better results are obtained as compared with other design methods.

Soil Arching of Piled Embankment in Equal Settlement Pattern: A Discrete Element Analysis

Soil arching, which occurs in the piled embankments, plays an important role in stress redistribution between the relatively soft subsoil and the stiffer piles. The formation of the soil arching depends on the differential settlement of the embankment fill above the pile and the subsoil. The soil arching effect is barely investigated in the literature from the perspective of differential settlement of piles and soils. Based on the discrete element method (DEM), this paper develops a classic trapdoor test model to investigate the differential settlement in piled embankment during the downward movement of the trapdoor, and to explore the formation mechanism of soil arching in equal settlement pattern by changing the width of the pile cap and the height of the embankment. Due to symmetry, only one section of the laboratory test model is simulated herein. It was found that the soil arching formed under the equal settlement pattern remained unchanged after a certain degree of development, and the height of the equal settlement did not change at 0.7(s-a), where s is the pile spacing, and a is the width of the pile cap. The height of the embankment (H) and the width of the pile cap (a) have a significant influence on the formation of the equal settlement pattern when the width of the trapdoor is kept constant. Both the decrease in “H” and the increase in “a” facilitate the differential settlement of the soil between the piles and the pile-soil, enabling the slip surface to develop upward gradually, thereby hindering the formation of the equal settlement pattern.

Towards Sustainable Soil Stabilization in Peatlands: Secondary Raw Materials as an Alternative

Implementation of construction works on weak (e.g., compressible, collapsible, expansive) soils such as peatlands often is limited by logistics of equipment and shortage of available and applicable materials. If preloading or floating roads on geogrid reinforcement or piled embankments cannot be implemented, then soil stabilization is needed. Sustainable soil stabilization in an environmentally friendly way is recommended instead of applying known conventional methods such as pure cementing or excavation and a single replacement of soils. Substitution of conventional material (cement) and primary raw material (lime) with secondary raw material (waste and byproducts from industries) corresponds to the Sustainable Development Goals set by the United Nations, preserves resources, saves energy, and reduces greenhouse gas emissions. Besides traditional material usage, soil stabilization is achievable through various secondary raw materials (listed according to their groups and subgroups): 1. thermally treated waste products: 1.1. ashes from agriculture production; 1.2. ashes from energy production; 1.3. ashes from various manufacturing; 1.4. ashes from waste processing; 1.5. high carbon content pyrolysis products; 2. untreated waste and new products made from secondary raw materials: 2.1. waste from municipal waste biological treatment and landfills; 2.2. waste from industries; 3. new products made from secondary raw materials: 3.1. composite materials. Efficient solutions in environmental engineering may eliminate excessive amounts of waste and support innovation in the circular economy for sustainable future.