Analyses of a pile-supported embankment over soft clay: Full-scale experiment, analytical and numerical approaches
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)
- et al.
3D finite element modelling of piles groups adjacent to surcharge loads
Computers and Geotechnics
(1996) The use of geosynthetics to improve the performance of foundations in civil engineering
- et al.
Pile foundation analysis and design using experimental data and 3-D numerical analysis
Computers and Geotechnics
(2009) - et al.
Soft ground improvement by vertical rigid piles. Two-dimensional physical modelling and comparison with current design methods
Soils and Foundations
(2005) - et al.
Discrete element modelling of a granular platform supported by piles in soft soil—validation on a small scale model test and comparison to a numerical analysis in a continuum
Computers and Geotechnics
(2009) - et al.
Use of lime and cement treated soils as pile supported load transfer platform
Engineering Geology
(2010) - et al.
Axisymmetric finite element analysis of pile loading test
Computers and Geotechnics
(2009) - et al.
Remblais ferroviaires renforcés sur pieux en Allemagne: Projets phares
Travaux
(2002) - et al.
Embankment supported on piles with biaxial geogrids
- et al.
Embankment support using geogrids with vibro concrete columns