Elsevier

Engineering Geology

Volume 153, 8 February 2013, Pages 53-67
Engineering Geology

Analyses of a pile-supported embankment over soft clay: Full-scale experiment, analytical and numerical approaches

https://doi.org/10.1016/j.enggeo.2012.11.006Get rights and content

Abstract

The reinforcement of soils using rigid inclusions is a technique used to reduce settlements and to ensure the stability of an embankment built over soft soils. This technique reduces construction delays and is an economical and reliable solution, which has led to its widespread use. Thus, many design methods have been developed to assess the performance of these reinforced structures. These methods are mainly based on results from small scale models and numerical analyses. The reliability of these methods must be validated under in-situ conditions.

This paper presents an analytical and numerical study of full-size experiments at the Chelles test site (France). The work presented in this paper is part of the ASIRI French National Research Project. The experiment consisted of a 5-m-high embankment built over soft alluvial ground improved by rigid vertical piles. The embankment is divided into four zones that illustrate the influence of the piles and the geosynthetic reinforcements on the soil's behavior. The performance of the embankment support system is assessed by monitoring data (total stresses, horizontal and vertical displacements). Several in-situ and laboratory soil investigations were performed using two axially loaded test piles. These tests verified the geotechnical hypothesis used for the numerical model and defined the soil–pile interaction parameters.

Several analytical methods and numerical models were tested to assess the arching effect. Comparisons between the experimental data and these design methods are presented in terms of stress and the settlement efficacy of the improved system. The results show that these methods overestimate the stress efficacy but that the settlement efficacy is a reliable parameter to assess the overall performance of the rigid inclusion technique.

Highlights

► Monitoring of a piled supported embankment built over soft alluvial soil ► Several analytical and numerical models were tested to assess the arching effect. ► Comparisons between the experimental data and the design methods ► Settlement efficacy is a reliable parameter to assess the embankment performance.

Introduction

Embankments constructed over soft soils induce a significant load over a large area. The technique of reinforcing soil with columns has proven to be an interesting solution that prevents failure or excessive deformations of embankments (Alexiew and Vogel, 2002, Kempfert et al., 2004). This technique combines three components: (1) embankment material, (2) a load transfer platform (LTP), and (3) vertical elements extending from the LTP to the stiff substratum. Optional configurations can be made by adding geosynthetics inside the LTP or pile caps. The surface and embankment loads are partially transferred to the piles by arching that occurs in the granular material constituting the LTP. This causes homogenization and the reduction of surface settlements. Friction along the piles is also involved in the improvement mechanism, leading to a complex soil/structure interaction phenomenon (Jenck et al., 2005, Smith, 2005, Combarieu, 2008). Although this technique is widely used, the mechanisms involved are still poorly understood.

This paper presents an analytical and numerical study at the Chelles experimental test site in France that was carried out in 2007. This experiment was part of the ASIRI research project, which has the ultimate goal of developing guidelines for the use of vertical rigid piles in France (Simon, 2009). The purpose of this paper is to compare the predictions from several design methods to measurements made on a full size experiment in a pile-reinforced embankment to assess their pertinence for design.

Section snippets

Background

Many authors have been interested in the technique of reinforcing soil using columns. Their papers have mainly been concentrated on transferring loads to the pile head by the phenomenon of arching. Low et al. (1994), Zaeske (2001), Jenck et al. (2007) and Chen et al. (2008b) developed physical test models to assess the load distribution between piles and foundation soils. However, difficulties arise in reproducing the behavior of the reinforced soil at a small scale, leading most of the studies

General project information

The Chelles test site was described by Briançon et al. (2009) and Briançon and Simon (2012). Details on its construction, ground conditions, and experimental data may be obtained by consulting these references. In the present paper, only the information required to describe the numerical models and the analytical design is presented.

The experiment was built prior to a bridge construction project on compressive type alluvial soils. Soil data has been collected by geotechnical programs in the

Analytical methods

Several current design methods permit the calculation of the stress efficacy or the stress reduction ratio by assessing the soil arching between the piles. Most of the methods are formulated for a three dimensional problem (Combarieu, 1988, Russell et al., 2003, Chen et al., 2008a), as is necessary in real applications (Kempton et al., 1998). Other authors only considered a two-dimensional problem (Low et al., 1994). The stress efficacy E of the pile support is defined as the proportion of the

Numerical modeling

Finite difference numerical models were used to simulate the Chelles experiments. The objective was to precisely model the mechanical behavior of the piled-embankment. The results presented herein were obtained using FLAC3D (Itasca, 2009) from an elementary cell (CE) and a global model (MG) of the site. These models simulate the mechanical behavior of the reinforced subsoil by explicitly considering (i) the geotechnical characteristics of the soils and the interface, (ii) the behavior of the

Discussion

As shown by the results obtained from the different models, due to the 3D nature of the experimental site, only the 3D global model accurately reproduces the experimental behavior of the Chelles site. This model fit the experimental measurements while accounting for the complex mechanisms and interactions of the full-scale test.

The differences in stress efficacies between the experimental and numerical results in zone 2R might be caused by the assumptions of a continuum numerical model. The

Conclusion

A full-scale experiment was developed to study a soil reinforcement technique using vertical rigid piles. Compressible alluvial soils intended to support a high embankment were reinforced by concrete piles and geosynthetics. Numerous devices were used to monitor the evolution of the slope. Experience shows that this technique reduces settlements and improves the stability and the performance of structures. Analytical and numerical methods were used to predict the behavior of the embankment.

The

Acknowledgments

This work is part of a French National Research Project, ASIRI, aimed at formulating guidelines and recommendations for the design of soils reinforced by stiff vertical piles. The authors would like to thank the French National Project (ASIRI) for funding this research within the partnership between Fondasol, IREX, Keller, LCPC, EGIS, Socotec, and Tencate Geosynthetics. This work was made possible thanks to the financial support of Drast and RGCU and the “Conseil Général de Seine et Marne”, who

References (53)

  • F. Bourges et al.

    Calcul des efforts et des déplacements engendrés par des poussées latérales de sol sur les pieux

  • L. Briançon et al.

    Full-scale experiments of pile-supported earth platform under a concrete floor slab and an embankment

  • L. Briançon et al.

    Performance of pile-supported embankment over soft soil: full-scale experiment

    Journal of Geotechnical and Geoenvironmental Engineering

    (2012)
  • L. Briancon et al.

    Etat des connaissances: Amélioration des sols par inclusions rigides

  • L. Briançon et al.

    Full-scale and small-scale experiments of ground improvement by pile-supported earth platform

  • BS8006, British Standards

    Code of practice for strengthened/reinforced soils and other fills

  • BS8006-1, British Standards

    Code of Practice for Strengthened/Reinforced Soils and Other Fills

    (2010)
  • B. Carlsson

    Reinforced Soil, Principles for Calculation

    (1987)
  • R.P. Chen et al.

    A theoretical solution for pile-supported embankments on soft soils under one-dimensional compression

    Canadian Geotechnical Journal

    (2008)
  • Y.M. Chen et al.

    An experimental investigation of soil arching within basal reinforced and unreinforced piled embankments

    Geotextiles and Geomembranes

    (2008)
  • Chevalier, B., 2008. Etudes expérimentales et numériques des transferts de charges dans les matériaux granulaires....
  • O. Combarieu

    Effet d'accrochage et méthode d'évaluation du frottement négatif

    Bulletin de Liaison des Laboratoires des Ponts et Chaussées

    (1974)
  • O. Combarieu

    Amélioration des sols par inclusions rigides verticales. Application à l'édification des remblais sur des sols médiocres

    Revue Française de Géotechnique

    (1988)
  • O. Combarieu

    Remblai sur sol compressible et inclusions rigides. Amélioration de l'approche de dimensionnement

    Revue Française de Géotechnique

    (2008)
  • Fascicule n° 62 Titre V

    Règles techniques de conception et de calcul des fondations des ouvrages de génie civil

    (1993)
  • M.G. Filz et al.

    Net vertical loads on geosynthetic reinforcement in column-supported embankments

  • Cited by (0)

    View full text