Pile-supported Embankments Research Articles

Evolution of the Soil Arching Effect in a Pile-Supported Embankment Considering the Influence of Particle Breakage

Evolution of the Soil Arching Effect in a Pile-Supported Embankment Considering the Influence of Particle Breakage

Performance Evaluation of Design Methods for Geosynthetic-Reinforced Pile-Supported Embankments

Geosynthetic-reinforced pile-supported (GRPS) embankments are widely used in soft soil regions to support roadways. Various design methods for GRPS embankments have been developed; however, engineering experience and recent research have shown that the performance of these design methods varies case by case. This paper systematically evaluates the performance of several empirical GRPS embankment design methods, which include the BS8006 method, the Nordic method, the EBGEO method, the FHWA method, and the CUR226 method. A preliminary assessment of these methods using three field case studies confirms that their performances differ from each other and from field measurements, the differences sometimes being quite significant. To enable a more systematic evaluation of the design methods, a validated finite element model (FEM) was first used to conduct a comprehensive parametric study considering typical soft soil conditions and various geometrical parameters, the results of which served as a baseline for the evaluation of the design methods. The efficacy, stress concentration ratio, maximum differential settlement, and maximum reinforcement tension were used as indicators for the method evaluation. The results show that, overall, the CUR226 method outperforms other design methods. The percentage difference between the CUR226 method and FEM can be as small as 5%, and the maximum value is 130%. It was also found that the Nordic method does not apply to small embankment heights, and that the BS8006 method provides a large overestimation of reinforcement tension when pile spacing is large (>4 ft) for all embankment heights considered.

Investigation on the long-term strain of geogrid in GRPS embankment under cyclic loading

Geogrid is a horizontal reinforcement material that is used in geosynthetic-reinforced pile-supported (GRPS) embankments in freeways with soft soil. Nevertheless, the long-term deformation mechanics of geogrid in the GRPS embankment under cyclic loading is not clear enough. This paper presents a laboratory model study of a GRPS embankment to study the long-term geogrid strain under cyclic loading. The effects of parameters such as number of load cycles, load frequency, geogrid layer numbers, and length of supporting piles on the long-term strain of geogrid were investigated. The results showed that geogrid strain increased with the number of load cycles, mainly developed in the first 10 000 load cycles and increased slowly in the later period. When the frequency was increased from 1 to 5 Hz, geogrid strain increased dramatically. When compared with the condition of a single geogrid layer, the upper layer of two geogrid layers caused a less geogrid strain. Compared with long piles and short piles GRPS embankments, a sufficient membrane effect of geogrid was found for long-short piles GRPS embankments. Finally, the 95% prediction interval proposed in this paper could be expressed using a logarithmic function, providing a theoretical foundation for future engineering applications.

Proposed Method for the Design of Geosynthetic-Reinforced Pile-Supported (GRPS) Embankments

Soft soils with unfavorable properties can be improved using various ground-improvement methods. Among these methods, geosynthetic-reinforced pile-supported (GRPS) embankments are considered a reliable option for challenging ground conditions and time-bound projects. Nevertheless, the intricate load transfer mechanism of the GRPS embankment presents challenges due to the multiple interactions among its components. To overcome the limitations of current design methods that do not fully account for all interactions, a simplified design method has been developed for GRPS embankments. This method uses numerical analysis to predict pile load efficiency and geosynthetic tension. In this study, a validated model of the GRPS embankment, which incorporates certain simplifications for design purposes, was adopted. Based on this simplified model, a database of load efficiency and geosynthetic tension was collected to derive the design equations. The design method employed six parameters, namely, pile cap width, pile spacing, embankment height, oedometric modulus of the subsoil, geosynthetic stiffness, and embankment fill unit weight. The design process utilized Plaxis 3D and Curve Expert software. The results showed reasonable agreement between the findings of the proposed design method and the field measurements of eight case studies.

Geosynthetic-reinforced and pile-supported embankments: theoretical discussion of finite difference numerical analyses results

Piled foundations are commonly employed to reduce settlements of artificial earth embankments on soft soil strata and geosynthetic reinforcements are installed at the embankment base to increase pile spacing and reduce construction costs. Despite the well-documented effectiveness of this technique, the mechanical processes, developing during the construction in the different elements constituting the ‘geo-structure’, are not fully understood and the design approaches are based on very simplified assumptions. They disregard the deformability of the various elements constituting the system and cannot be employed to estimate settlements. With the aim of introducing a displacement-based design approach to optimise the use of reinforcements and piles, in this article, the mechanical response of the system during the embankment construction is studied by means of large displacement non-linear finite difference numerical analyses, in which the geosynthetic reinforcement is modelled as an elastic membrane. The arching effect developing within the embankment body is described and the evolution of the process zone, where shear strains localise, is discussed. The global system response is described in terms of (i) average, (ii) differential settlements at the top of the embankment and (iii) maximum tensile force within the geosynthetic reinforcement.

Recent advances in subgrade engineering for high-speed railway

Abstract In the last decade, the design and construction technologies of subgrade in high-speed railway (HSR) developed significantly. This article reviews corresponding development in five aspects, including mechanical properties of fill materials, dynamic performance of subgrade, foundation treatment, retaining structure, and smart construction technologies. It shows that for unbonded granular materials, it is acceptable to use static strength for subgrade design, but for clayey soil it would be more appropriate to base on shakedown theory. The mechanism for lime modified clay has been thoroughly reviewed, and the effect of lime content, curing age, and curing conditions on the behavior of lime-treated clay is discussed. The dynamic response of subgrade, especially the long-term deformation and dynamic stability analysis are important to understanding the behavior of HSR subgrade. The effect of track types, operation speed, etc. on the dynamic response of subgrade are reviewed first. Then, the prediction methods, influencing factors, and corresponding issue for long-term deformation of subgrade are presented, followed by the methods used for dynamic stability analysis. Three types of foundation treatment methods, including geosynthetic-reinforced pile-supported (GRPS) embankment, pile-raft structure, and pile-plate structure are reviewed for the corresponding load transmission mechanism, and application scenario. The static and dynamic behavior of four types of retaining structures are presented, including cantilever retaining wall, geosynthetic reinforced soil retaining wall, anchored retaining structure, and retaining wall reinforced by soil nailing. Finally, a series of new technologies correlated to smart construction are introduced, relating to the survey, design, construction, detection, and management of subgrade.

The performance of T-shaped deep mixed soil cement column-supported embankments on soft ground

T-shape deep mixing (TDM) pile is a new method for soft ground improvement, which has an enlarged head at shallow depth, the pile shape analogous to the letter “T”. In this paper, three-dimensional numerical simulation is used to investigate the load transfer mechanism, failure pattern, and settlement behavior of TDM pile-supported embankments by dividing TDM pile into two parts: the enlarged head part and the pile body part and using the concrete model to simulate the TDM piles. The results reveal that the stress arches exist both in the embankment fill and the soil under the flange, and stress concentration will occur at the variable section of TDM piles, resulting in plastic yielding of the pile body, which in turn leads to load transfer to the soil, and additional stresses in the soil will cause additional compression of soft soil between TDM piles. Parameter analysis indicates that the load transfer, plastic zone, and post-construction settlement are related to the height of the enlarged head (H) and pile spacing (S), while pile length (L) only plays an important role in the post-construction settlement.

Multi-objective optimization of geosynthetic-reinforced and pile-supported embankments

The design of geosynthetic-reinforced and pile-supported (GRPS) embankments is traditionally optimized by searching for the most cost-effective solution among several workable candidates. The candidates are usually based on experiences of engineers, and the real optimal design could be therefore missed. This paper intends to address the above-mentioned issue by systematically optimizing the design of GRPS embankments considering simultaneously the cost and the safety in the entire design space. It is thus a multi-objective optimization (MOO) problem that differs from the studies only focusing on minimizing the construction cost. A practical MOO procedure is proposed in this paper, and it is applied to an illustrative GRPS embankment case. A set of nondominated optimal designs (Pareto front) are obtained at first, allowing an informed design decision. Then, four candidates located on the Pareto front are highlighted. Each of them represents an attractive design: the safest, the least-cost, the best trade-off (knee point) considering the two objectives, and the cheapest one for a target safety requirement. Finally, the optimal design can be selected from these four candidates depending on specific project purposes. For the case study, the knee point design leads to improvements in both the two defined objectives (i.e., decreased cost and increased safety) compared to the initial design, showing great benefits of performing a MOO analysis. By using the procedure, the optimal designs are also efficiently determined for the cases of different embankment heights.

Physical model study of pile type effect on long-term settlement of geosynthetic-reinforced pile-supported embankment under traffic loading

In coastal area, the long-term settlement of high-way embankment is seriously concerned. Geosynthetic-reinforced pile-supported (GRPS) embankments have been widely adopted for the construction over soft soils to solve the large settlement problem. However, the long-term performance of GRPS embankment under traffic loadings is not fully understood. In this study, a series of physical model tests were conducted. Different pile types (rigid pile, rigid-flexible pile, flexible pile) and geosynthetic layers (single-layer and double-layer) were comparatively investigated. Two stages of long-term cyclic loadings were applied to simulate the traffic loadings. Based on the results, it is found that the model test with rigid pile has the least settlement, whereas the model test with flexible pile experiences the largest settlement when subjected to both 1 Hz and 2 Hz cyclic loadings. In the view of long-term settlement control, the rigid pile is the best choice for the GRPS system. The applied cyclic loadings have the lowest effect for the pile strain distribution in rigid pile model but much higher effects in both flexible pile and the rigid-flexible pile models, which demonstrates that the traffic loading after the construction of GRPS embankments may leads to unneglected stress redistributions. The findings of this study would provide the reference to consider the traffic loading effects on different pile types of GRPS embankment.

Integrated Analytical Model for Characterizing Stress Distribution of Geosynthetic-Reinforced and Pile-Supported Embankments

Integrated Analytical Model for Characterizing Stress Distribution of Geosynthetic-Reinforced and Pile-Supported Embankments

A parametric study of embankment supported by geosynthetic reinforced load transfer platform and timber piles tip on sand

This paper presents the results of finite element parametric study on the performance of pile-supported embankment system built over soft soil utilizing geosynthetic-reinforced load transfer platform (GLTP), where timber piles tipped on sand. The studied parameters included thickness of top soft layer, lateral extent of GLTP, embankment height (H), pile spacing, GLTP configuration, and different soil profiles. Three performance criteria were considered: maximum geosynthetic tensile strain, settlement, and slope stability. The results show that extending GLTP to 1.5H for low embankment load and full extent of 2.0H for medium and high embankment loads satisfies slope stability. The required number of geosynthetic layers varies from 3 layers for low and intermediate embankment loads to 7 layers for high embankment load with large pile spacing. Increasing the thickness of top soft layer results in increasing the strains along geosynthetics. However, increasing the number of geosynthetic layers and thickness of GLTP results in lower strains along geosynthetics but have little effect on settlement. Increasing 2 geosynthetic layers results in 15–20 % reduction in geosynthetic strains, but <2 % reduction in settlement. Meanwhile, increasing GLTP thickness by 50 % results in 10 % reduction in settlement, and 20–25 % reduction in geosynthetic strains.

Effect of geogrid reinforcement on the load transfer in pile-supported embankment under cyclic loading

This paper investigated effects of geogrid reinforcement on the load transfer in pile-supported embankment under cyclic loading using self-moving trapdoor tests. In the self-moving trapdoor test setup, the trapdoor between two stationary portions (which were equivalent to the piles) was supported by compression springs to simulate the subsoil. Quartz sand and a biaxial geogrid were used as the test fill and reinforcement material, respectively. Tests results show that soil arching above the geogrid reinforcement and load transfer to the stationary portions (caused by the soil arching and tensioned membrane effect) experienced a process of “relatively enhancing - relatively degrading” with an increase in the number of cycles and maintained similar degrees within each complete cycle of cyclic loading. Moreover, the inclusion of geogrid reinforcement reduced the mobilization of soil arching, but increased the degree of load transfer to the stationary portions. In addition, cyclic loading with a higher frequency tended to mobilize more soil arching and induce a higher degree of load transfer to stationary portions. Also observed was that a higher frequency cyclic loading tended to decelerate the degradation of load transfer to stationary portions and caused less surface settlement, which indicating increased load-carrying capacity of pile-supported embankment.

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.

Numerical Study of Bearing Capacity of the Pile-Supported Embankments for the Flexible Floating, Rigid Floating and End-Bearing Piles

Floating pile-supported embankment involves more complex load transfer mechanisms, and there are no clear uniform guidelines available for its design. The concepts of flexible floating, rigid floating and end-bearing pile-supported embankments were proposed in this paper. Based on three typical field cases, their pile-soil interactions and soil arching effect were examined using the three-dimensional finite element method. Due to symmetry, only a half-width embankment model was simulated here. It has been found that the flexible floating piles carry the load mainly relying on the skin friction, but end-bearing piles rely on the pile tip resistance. The rigid floating piles were somehow in between. The earth pressure coefficient (K) in the end-bearing pile-supported embankment reached a maximum of 3.28, greater than Rankine passive values of the earth pressure coefficient (KP), in which the soil arching was fully developed. The K in the embankment with rigid floating pile reached 2.21, where soil arching might be partially formed. At the bottom of the flexible floating pile-supported embankment, the K tended to equal the Rankine active values of the earth pressure coefficient (Ka), and thus soil arching was insignificant. It has also been found that using rigid floating piles might significantly improve the bearing capacity of the embankments and was cost-effective for deep soft soil areas.

Stability Analysis of Pile-Supported Embankments over Soft Clay Considering Soil Failure between Piles Based on Upper Bound Theorem

Most analytical methods used to analyze the stability of pile-supported embankments are based on shear and bending failure of piles. However, the centrifuge test and practical engineering show that rigid pile-supported embankments have a failure mode of soil sliding around piles. Therefore, the stability analysis method of the pile-supported embankment under the failure mode of the soil sliding around piles based on the upper bound theorem is given in this study. First, the failure mechanism is assumed to be a rigid body and slide along two logarithmic spiral surfaces in the embankment and soft soil. The rate of external work and internal energy dissipation of the failure mechanism are further obtained, wherein the internal energy dissipation rate of piles is obtained based on the limit state of soil sliding around piles. Secondly, according to the equation formed by the upper bound theorem, the optimization model of the safety factor function when the potential slip surface is passing through different pile rows is obtained. Then, the algorithm of solving the model is given and the overall safety factor of the pile-supported embankment is obtained. Compared with other methods, the advantage of this method is that it considers the influence of embankment soil property and the width perpendicular to the two-dimensional plane on the pile load and the influence of the soil internal friction angle on the slip surface. Finally, it can be concluded from parametric analysis that: with the increase of the horizontal distance from piles to the slope toe, the reinforcement effect of piles first increases and then decreases; with the decrease of the embankment height and the increase of the soft soil cohesion and internal friction angle, the overall safety factor of the embankment increases.

A simplified method for assessing the serviceability performance of geosynthetic reinforced and pile-supported embankment

A simplified method for assessing the serviceability performance of geosynthetic reinforced and pile-supported embankment is presented, where the subsoil consolidation is introduced remaining compatible with the development of soil arching and the reinforcement sag. A piecewise function of the ground reaction curve is developed and used to quantify the arching efficiency. The link between the arching evolution and the subsoil consolidation is then established through the load-carrying equilibrium in the area between piles together with the tensile membrane theory. The reaction of the subsoil is described using the 1-D consolidation theory where the stress history is considered. A parametric study is performed to demonstrate the serviceability performance of a geosynthetic reinforced and pile-supported embankment. The serviceability design of the geosynthetic reinforced and pile-supported embankment is achieved with the proposed method which offers an approach to estimate the time consumed and the subsoil settlement required to achieve a service state.

Optimized Design of Piled Embankment Using a Multi-Effect Coupling Model on a Coastal Highway

This study presents a multi-effect coupling model to optimize the design of a geosynthetic-reinforced pile-supported embankment (GRPSE) considering the coupling effects of soil arching, membranes, and pile–soil interaction on a coastal highway. The developed model could optimize the design of the GRPSE to fulfill the design and construction requirements at a relatively low project cost. This was achieved by adjusting the critical factors that govern the settlement of GRPSEs, such as pile spacing, tensile stiffness of geosynthetic reinforcement (GR), arrangement of piles, pile cap size, and cushion thickness. The model predictions were validated by a series of field tests using a range of geotechnical sensors. The results show that model predictions agreed with experimental measurements reasonably well. In addition, the results indicate that in comparison to a square arrangement of piles, a triangle net arrangement can decrease the differential settlement of pile soil. Furthermore, this study demonstrates that a change in the GR’s tensile stiffness has little impact on the settlement of GRPSEs. This study can help to improve the stability of roadbeds of coastal highways.

Seismic performance of near-fault geosynthetic-reinforced pile-supported embankment

The seismic performance of embankments is an important consideration for the design and construction of high-speed railways (HSRs) in near-fault areas. Incorporating geosynthetics into embankment soil can improve seismic resistance. However, the effect of different reinforcement methods on the seismic performance of embankments is not well understood. In this study, two 1 : 20 scaled embankment models (full-length-geosynthetic-reinforced pile-supported embankment (FRPE) and turn-back-geosynthetic-reinforced pile-supported embankment (TRPE)) were tested on a shaking table to compare their seismic performance and failure characteristics. The results show that under near-fault bidirectional seismic excitation, the pile foundations of both embankments exhibited bending deformation, with the largest bending moment in the middle of the pile body. The TRPE reduced the vertical dynamic response of the embankment slope but exhibited a more remarkable horizontal dynamic response than the FRPE. Furthermore, the embankment deformation and excess pore water pressure of the TRPE were generally larger than those of the FRPE. Nonetheless, the TRPE has potential application in practical engineering as it ensures earthquake resistance, with higher economic benefits.

Reinforcement load in geosynthetic-reinforced pile-supported model embankments

The stress conditions of geosynthetic reinforcements (GRs) are crucial in achieving the accurate serviceability design of geosynthetic-reinforced pile-supported (GRPS) embankments. However, the sensitivity of load distribution to the settlement process has been reported in geosynthetic-reinforced embankment overlying cavities. In this study, a three-dimensional model embankment was used to perform experiments and evaluate the load acting on the GR. A flexible pressure-mapping sensor was introduced to investigate the pressure distribution for two types of supporting conditions: partitioned displacement by multiple movable trapdoors and even trapdoor settlement underneath different subsoil materials. The results showed that the load on the GR was concentrated on the strip areas between adjacent pile heads along with the settlement. The measured load on the GR strip area was related to the settlement process and finally exhibited a U-shaped distribution after detachment from the support underneath. The soil arch height in the subgrade continuously increased with the settlement; meanwhile, the pile head load increased rapidly at first and then decreased slightly or remained stable depending on the foundation support stiffness. For both types of settlement behaviours, soil arching exhibited stress history-related characteristics that influence the load transfer in GRPS embankments.

A Field Study on the Arching Behavior of a Geogrid-Reinforced Floating Pile-Supported Embankment

Geogrid-reinforced floating pile-supported (GRFPS) embankments are widely used in areas with moderately compressed soil. Due to the interaction among foundation soil, suspended piles and geogrids, the characteristics of stress and deformation are very complicated. In this paper, a field filling test of GRFPS embankments with two pile lengths (L = 8 m, 15 m), S1 and S2, was carried out in areas with moderately compressed soil. The evolution of soil arching, deformation of soil, piles and geogrids during the construction and equilibrium stage of embankments were monitored. Results show that with the increasing of filling height, there were three stages for the evolution soil arching: no soil arching, formation of soil arching, and stabilization of soil arching. When the pile-soil settlement ratio η reached its minimum value, soil arching formed, and the differential settlement between pile and soil at the top of pile cap (Δδ) was around 1% of the net pile spacing (s-d, where s is the pile spacing, d is the diameter of pile cap). The corresponding critical filling heights (Hcri) were 1.4 (s - d) and 3(s - d) for S1 and S2, respectively. The strain of geogrid mainly developed during the initial stage of filling with the development of Δδ. When the filling height was larger than 5.2 m, the magnitude of soil arching was larger for the embankment with longer pile length (S2). Meanwhile, for S2, the soil arching coefficient Cc gradually transited from friction piles into the end-bearing piles with the increasing filing height of the embankment. The strain of the geogrid mainly occurs in the initial stage of filling (t < 75 d), mainly due to the differential settlement between the pile and soil. After that, the overall settlement of the composite foundation also slightly caused the strain of the geogrid. Finally, three states of soil arching effects of the GRFPS embankment and the evolution characteristics of each index under different states are proposed. This case study provides an enhanced understanding of the performance GRFPS embankment.