Numerical study of the deformation performance and failure mechanisms of TDM pile-supported embankments

Abstract

The T-shaped deep cement mixing (TDM) pile is a technique newly used to support embankments built on soft clays. However, comprehensive studies considering the cost effectiveness have not yet been performed, and the TDM pile bending moment behavior under embankment loading is not fully understood. Hence, this paper aims to evaluate the performance of a TDM pile-supported embankment (TPSE) over soft foundation subsoils using 3D numerical modeling based on a case history of a conventional deep cement mixing (DCM) pile-supported embankment (DPSE). The performances of pile-embankment systems are investigated and discussed through several parameters, including the embankment center settlement, lateral movement at embankment toe, deflection and bending moment of the pile at the embankment shoulder, and embankment stability. First, to capture the thickness effect of the uppermost fill material layer, numerical analyses with different fill material thicknesses are examined. Subsequently, extensive parametric studies of TPSE are carried out to assess the TDM pile cap size and length influences. The results of the parametric studies are analyzed and compared to those obtained from DPSE systems with equivalent embankment loading and volume between the TDM and DCM piles. Finally, a preliminary guideline for evaluating the overall improvement performance of TPSE is proposed to determine the best TDM pile cap size for each pile length in comparison with the DPSE case. Some guidelines are also defined, and small TDM piles with thicker caps are suggested to improve pile-embankment systems.

Introduction

Embankments constructed on soft foundation subsoils are a challenging task for geotechnical engineers that often face problems such as excessive settlements and lateral displacements, low bearing capacities, and embankment slope instabilities. To solve such problems, concrete piles (e.g., [1], [2], [3], [4]), stone piles (e.g., [5], [6], [7]), deep cement mixing (DCM) piles (e.g., [8], [9], [10], [11]), and T-shaped deep cement mixing (TDM) piles (e.g., [12], [13]) are frequently used to improve the strength and stiffness of such subsoils before constructing embankments [14]. However, soil–cement mixing, especially for DCM piles, has been more broadly used than the other methods due to its rapid installation and cost-effectiveness [11], [12], [15], [16], [17], [18].

TDM piles, which are a new type of DCM pile, were recently introduced for use as TDM pile-supported embankments (TPSE, see Fig. 1a) instead of DCM pile-supported embankments (DPSE, see Fig. 1b) because of their lower deformation (both settlement and lateral movement of pile-embankment systems) than conventional DCM piles [12]. A TDM pile comprises two parts, including a TDM pile cap and pile body. The TDM pile cap with a smaller depth has a greater diameter than the pile body with a greater depth. In contrast, a DCM pile has a constant diameter throughout its length. Therefore, the area improvement ratio at a shallow depth of the TDM pile is greater than that of the DCM pile. The TDM pile acts as a stiffer element at the shallow depth to facilitate the embankment load concentrated on the TDM pile cap. This mechanism can increase the pile efficacy and reduce the vertical stress on the surrounding soil [19], [20]. Thus, TPSE provided lower total ground surface settlements and lateral movements than DPSE based on full-scale embankment tests by Liu et al. [12]. In practice, a particular machine, namely, foldable mixing blades [12], [21], [22], [23], is utilized for installing TDM piles rather than the conventional device to reduce the construction time. Moreover, due to the larger area improvement ratio of the TDM pile at shallow depths, the TDM piles can be installed at larger pile spacings than the DCM piles to obtain similar embankment performances [12]. Therefore, the number of piles and cement amounts used for TPSE are less than those for DPSE, which means a reduction in pile construction costs [12]. The primary function of using a TDM pile for supporting embankments is similar to the use of a single or multilayer geosynthetic reinforcement (e.g., [15]) or a shallow stabilization (e.g., [24]) placed on top of DCM pile-foundation systems (acting as a load transfer platform) to convey surcharge embankment loads on DCM piles. Therefore, the total settlement at the embankment base of TPSE without any reinforcement on the pile top could be smaller than that of DPSE [12]. As a result, TPSE can provide a significantly lower construction cost than the other methods, as described above (without considering the additional cost of geosynthetic reinforcements and the increased volume improvement ratios used for covering the whole soft soil surface).

In terms of the bearing capacity of a single TDM pile and composite foundation composed of TDM piles and soft clay under vertical loading based on 1-g laboratory tests [20], [21], [22], [25], full-scale field tests [21], [22], and numerical simulations [25], it is found that the use of TDM piles can provide an increase or similarity in the bearing capacity compared to the use of DCM piles. The bearing capacity behavior of TDM piles is importantly dependent on the enlarged TDM pile cap sizes and its pile unconfined compressive strength, strength of untreated soils, area improvement ratios and the different types of foundations above the piles (e.g., rigid and flexible foundations [26]). Comparative laboratory tests of TPSE and DPSE by Yi et al. [19] and Phutthananon et al. [13] showed that the TDM piles exhibit a higher pile efficacy and a lower settlement than the DCM piles. In addition to these laboratory tests, Yi et al. [27] conducted a numerical modeling of a full-scale TPSE and concluded that enlarging the TDM pile cap is a significant parameter that affects the soil settlements. Phutthananon et al. [13] also found that the use of TDM piles (same volume as DCM piles) beneath the embankment induced a smaller differential settlement between the TDM piles and surrounding soil. These piles also provided similar total settlements to DCM piles. Recently, Phutthananon et al. [28] implemented an optimization approach to determine the optimum pile dimensions and moduli for the best TPSE performance in terms of settlements and load transfer behavior. These authors concluded that considering TDM piles with an optimal configuration can provide lower differential settlements and a greater pile efficacy than DPSEs at an equivalent cost.

Previous studies were performed only on the bearing capacity and deformation (especially for the vertical responses) of TDM piles under embankment loads. TDM piles have proven to be an economical and effective method to prevent excessive deformations and enhance the load transfer mechanisms. However, one of the main drawbacks of the DCM pile technique is bending failure. This failure is often located at the embankment toe and has frequently been found and could be classified as the most critical collapse mechanism based on a series of centrifuge model tests and numerical analyses [8], [10], [29], [30], [31], [32]. Therefore, not only the factor of safety (FS), which defines the global failure system (${\text{FS}}_{\text{slope}}$) but also the FS dedicated to the bending failure of the piles (${\text{FS}}_{\text{bending}}$), is significant for designing TPSEs. Generally, for soft ground areas, fine-grained sediment is deposited to form a soft clay layer. The weathered crust layer is frequently found in many regions, such as Ariake Bay, Japan [29], [33], the eastern coast of China [12], [34], the eastern coast of Singapore [35], [36], the Bangkok Plain [37], [38], [39] and the London Plain [40], due to the precompression of upper portions of soft clay by desiccation and weathering. Desiccation and weathering affect the undrained shear strength and natural water content, causing apparent overconsolidation at shallow depths [40]. Weathered crust layers with thicknesses in the range of 1 to 3 m have been reported by several researchers (e.g., [12], [29], [33], [34], [39], [41], [42]). However, the weathered crust layer may not be formed in the newly reclaimed land, which has not been encountered with desiccation and weathering processes [43]. The existence of a thick weathered crust layer can have a considerable impact on the performance of pile-embankment systems, especially for preventing excessive settlement [44], whereas limited studies have investigated the effect of the uppermost weathered crust layer on the stability of pile-embankment systems (i.e., ${\text{FS}}_{\text{bending}}$ and ${\text{FS}}_{\text{slope}}$). In other words, it is highly desirable to perform a comprehensive study to evaluate the influence of the thickness of the weathered crust layer on the safety behavior of pile-embankment systems. However, the performance study of TPSEs has not yet been comprehensively investigated considering the important mechanisms and effects described above. Moreover, it seems that only a limited number of studies on the performance of TPSE exist in terms of the lateral displacements, induced bending moments within the TDM piles, slope stability and failure mechanism. In addition, the comparison of the deformation characteristics performance between TPSE and DPSE, as reported in Liu et al. [12], was based on different pile volumes. Consequently, the comparative performance of TPSE and DPSE becomes questionable.

This paper presents an investigation on the performance of TPSE based on 3D finite element models. This examination is based on a well-reported case history (DCM pile-supported highway embankment over soft soils in Thailand [10]). The performance of this kind of pile-embankment system is investigated and discussed in terms of settlements, lateral movements, pile deflections, bending moments, and embankment stability. To improve the understanding of the performance of such systems on the thickness effect of the uppermost weathered crust layer, several weathered crust thicknesses are first studied. Parametric analyses are performed to evaluate the cap size and length of the TDM pile influences in which both fully penetrating and floating piles are considered. The performance of TPSE with various pile cap sizes is then compared to DPSE considering the same volume between the TDM and DCM piles. Finally, the effects of both the TDM pile cap size and the length on the overall improvement performance of TPSE are assessed and used to recommend a proper TDM pile cap size for any pile length.