Numerical study of the deformation performance and failure mechanisms of TDM pile-supported embankments
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 () but also the FS dedicated to the bending failure of the piles (), 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., and ). 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.
Section snippets
Details of the selected case history
Jamsawang et al. [10] reported an interesting case history of a highway embankment built on a thick soft Bangkok clay improved with a DCM pile. This embankment was constructed for highway No. 3117 and is located in the Bangbo district, Samutprakan Province, Thailand. The subsoil profile consisted of a 3-m-thick fill material (or weathered crust) layer followed by an 11-m-thick soft clay, a 9-m-thick medium stiff clay and a 5-m-thick stiff clay layer. The groundwater level was approximately
Influence of the fill material thickness
A fill material layer is often found above the soft clay layer in many countries, such as Japan (i.e., [29], [33]), China (e.g., [12], [34]), Thailand (e.g., [37], [38], [39]), and Sweden (e.g., [41]). A minimum of a 1-m-thick fill material layer below the ground surface can probably be found. The presence of the fill material layer causes a reduction in the settlements and lateral movements of the pile-supported embankment due to its relatively higher stiffness than the soft clay below [44].
Parametric analyses of DPSE and TPSE under pile volume control
For the further performance assessment of DPSE and TPSE, two significant parameters, including the pile length of both DCM and TDM piles and the cap size of the TDM pile, were analyzed in the parametric analyses. Fully penetrating and floating pile types for both DCM and TDM piles are considered based on the improvement depth ratio () parameter, in which the fully penetrating pile ( = 1.0) is classified when the pile tip is placed on the stiffer soil layer (medium stiff clay layer for this
Preliminary guidelines for evaluating the improved performance of TPSE
As presented and discussed above, the performance of TPSE is highly dependent on the TDM pile cap size and pile length. Under the same pile volume, strength, and spacing between the TDM and DCM piles (equivalent pile cost), small settlements and lateral movements can be achieved in the case of DPSEs, while large embankment stabilities and high values of FS against bending failure (in some TDM pile shapes) are obtained from the case of TPSEs. To evaluate which TDM pile cap size (i.e., ) at
Conclusions
This study investigated the capabilities of TDM piles, which were used as pile-embankment systems built on soft foundation subsoil instead of conventional DCM piles. A series of 3D numerical models based on the selected reference DPSE case were performed as a parametric study of the effect of the TDM pile shape and length on the performance of TPSE. The embankment base settlements, lateral movements, pile deflections, pile bending moments, and slope stabilities of TPSEs were interpreted and
Ethical statement
Authors state that the research was conducted according to ethical standards.
CRediT authorship contribution statement
Chana Phutthananon: Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing. Pornkasem Jongpradist: Conceptualization, Funding acquisition. Daniel Dias: Supervision. Pitthaya Jamsawang: Conceptualization, Funding acquisition.
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.
Acknowledgments
This research was funded by King Mongkut's University of Technology North Bangkok (KMUTNB) under Contract No. KMUTNB-62-KNOW-14. The authors also extend their appreciation to the King Mongkut's University of Technology Thonburi (KMUTT) for providing the Postdoctoral Fellowship to the first author and with the National Research Council of Thailand (NRCT) through grant No. NRCT5- RSA63006 to the third author.
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