Numerical stability analysis of piled embankments reinforced with ground beams

https://doi.org/10.1016/j.trgeo.2020.100427Get rights and content

Highlights

  • Ground beam was introduced and evaluated for stability improvement of piled embankments.

  • A validated 3D numerical model of piled embankments with ground beams was developed.

  • The Mohr-Coulomb tension crack model was used for capturing softening feature of concrete piles in tension.

  • Synergistic effect from the combination of ground beams and piles on embankment stability was justified.

  • Recommendations for the combination of concrete piles and ground beams were given.

Abstract

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.

Introduction

With the growing demand of modern transportation, a large number of high-speed rail lines [1] have been constructed in recent years. As a result, specifications regarding the post-construction settlement of railway subgrades are becoming more precise. The application of rigid piles, such as cast-in-place concrete piles [2], reinforced concrete piles, and pre-stressed concrete piles, emerged as a feasible solution to reinforce embankments over soft clay. Nevertheless, issues caused by the instability of embankments supported by rigid piles still occur frequently.

Compared with other types of piles such as stone columns [3], which enhance the shear resistance of the improved ground via confining pressure, the mechanical characteristics of deep mixed (DM) piles and rigid piles under an embankment are complicated [4], [5], [6], [7], [8], [9]. Under the combined action of the vertical load and lateral earth pressure, an intricate failure pattern, including compression, tension, shear, bending, inclination, or a combination of the above, occurs for DM and rigid piles [10]. Unlike flexible and semi-rigid piles, the plain concrete piles, which exhibit relatively high shear strength, compressive strength, and stiffness, have often been utilized for rapid embankment construction in China [12], [13]. A series of centrifuge tests conducted by Zheng et al. [11], [12], show that the initial bending failure takes place at the interface between the soft clay and bearing stratum, while the primary bending failure occurs within the upper region of a rigid pile, leading to failure of the overlying embankment. Moreover, Zheng et al. [13] found that the failure process of concrete piles is progressive rather than simultaneous under extreme conditions.

To increase the horizontal resistance of rigid piles, Nguyen et al. [14], [15] used a shallow mixing layer to fix and reinforce isolated DM piles, which could also reduce the settlement of a high embankment over soft ground [16], [17]. Other studies dealt with the influence of the surface improvement layer on the stability of DM columns under an embankment [6]. A ground beam (grade beam) is a component of building’s foundation that typically spans between pile caps to limit the lateral movement of piles, and its application has been extended to piled embankments in high-speed railway (Fig. 1). However, this novel application lacks sufficient research, which entails in-depth and systematic analyses of structural stability under an embankment load.

Numerical methods have been widely applied to analyze the mechanical responses of piles and the failure modes of embankments supported by different inclusions. Comodromos et al. [18] adopted a damage plasticity model to assess the influence of tension cracks under horizontal loading. Similarly, Larsson et al. [8] applied a damage plasticity model to analyze the tensile and shear softening behavior of lime-cement columns. Yapage et al. [19] developed a constitutive model in consideration of shear strain-softening characteristics to study the failure of deep cement mixing columns beneath an embankment. Additionally, Zheng et al. [13] proposed a constitutive model considering the brittle failure characteristics and frictional properties of concrete to reveal the progressive failure mechanism of an embankment supported by concrete piles. Besides, other types of constitutive models [20], [21] have been developed to describe the brittle failure behavior of concrete material. As such, a need has arisen for a simple yet accurate numerical method that can replicate the failure process of rigid piles composed of brittle material. Reinforced concrete pile is also a commonly used type of rigid columns, but it is not the focus of this study because it is mainly characterized by ductile deformation.

Due to the huge difference between the stiffness of concrete piles and soil, piles tend to bear most of the embankment load and horizontal earth pressure. Consequently, the stability of a piled embankment is primarily controlled by the mechanical behavior of the piles. In this paper, a finite difference method coded in the commercial software Fast Lagrangian Analysis of Continua (FLAC3D) [22] is utilized to investigate the mechanical performance and failure mechanism of concrete piles to evaluate piled embankment stabilization. A modified Mohr–Coulomb model, i.e., the Mohr–Coulomb tension crack model, is adopted to simulate the brittle behavior of concrete piles and is validated via a physical three-point bending flexural test and a simulation scenario reproducing a published centrifuge test [11]. Ground beams are included in the model to reveal their effect on improving the stability of piled embankments. The optimal layout of the novel earth structure is obtained by linking the pile heads at different locations with the ground beams.

Section snippets

Constitutive model of concrete pile

Concrete is known as a brittle construction material. By provoking an abrupt decline in pile stiffness, concrete cracking greatly influences the nonlinear ground response when the embankment pressure is high enough to cause tension cracks in a pile [18]. As a result, the behavior of a fractured concrete pile after failure should be carefully addressed to understand the failure mechanism of embankments supported by concrete piles.

The conventional Mohr–Coulomb model is classified as a linearly

Geometry and boundary condition

The numerical model was first established in accordance with an available centrifuge test for validation and parameter calibration. Zheng et al. [12] previously conducted a centrifuge test of an embankment supported by rigid piles. The centrifuge equipment used is briefly described below; more details can be found in the reference [28]. The model tests were performed in a stainless-steel model box with the following internal dimensions: 685 mm long by 350 mm wide and 475 mm high. A centrifugal

Ground beams as structural reinforcement

The numerical model was successfully validated in the previous sections. In this part, we investigated the system stability when ground beams were used in conjunction with rigid piles to support an embankment. To further elaborate the effects of ground beams on the overall stability of concrete piled supported embankments in a practical problem, a full scale 3D numerical model is established with its geometry rearranged to a height of 5 m, a crest width of 24 m, and a slope gradient of 1:1.5 as

Concluding remarks

In this paper, a validated numerical model was proposed to assess the impact of ground beams on the stability of a piled embankment, and recommendations for the ground beams arrangement were provided. The conclusions can be summarized as follows:

  • (1)

    The MohrT model, validated by both three-point bending test and centrifuge test, is capable of simulating the stress release and stress redistribution process of the pile group.

  • (2)

    The dominate failure mode of the embankment supported by isolated piles was

CRediT authorship contribution statement

Hongfei Ma: Methodology, Software, Formal analysis, Writing - original draft, Visualization. Qiang Luo: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition. Tengfei Wang: Validation, Conceptualization, Supervision, Writing - review & editing, Funding acquisition. Hao Jiang: Investigation, Writing - review & editing. Qingyuan Lu: Software, Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant numbers 41901073 and 51878560], China Postdoctoral Science Foundation [grant number 2019M663556], and Fundamental Research Funds for the Central Universities [grant number 2682020CX66].

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