## 1 Introduction  
Pre-stressed high-strength concrete pipe piles (PHC pipe piles) are manufactured using pre-stressing technology and a centrifugal pipe molding process in specialized factories [1, 2]. These piles are autoclaved, cured, and then sunk into the ground as the foundations of soft-foundation embankments using either the hammering method or the static compression method [3, 4]. The advantages of PHC pipe piles include production in an assembly line, easy control of pile quality, fast construction, the high bearing capacity of a single pile, and environmental friendliness. PHC pipe piles are primarily used for the soft foundation treatments in high-grade highways, particularly for road sections with structural objects, bridge head transition sections, and embankments with high reclamation and thick layers of weak soil. Additionally, they are also suitable for road sections with a tight schedule and general soft foundation conditions [5, 6, 7, 8]. Numerous studies have been conducted on the bearing capacity and load transfer laws of PHC pipe piles [9, 10, 11]. These studies have included field tests, finite element simulation studies [12, 13], predictions of pile bearing capacities [14, 15], and construction reliability [16]. Despite these efforts, slip accidents still occur in actual projects, notably in regions like southwestern and southeastern China [5, 17], primarily due to insufficient horizontal bearing capacities [18, 19]. Common structural forms, such as monopile structural forms without a pile cap and monopile structural forms with a pile cap, provide limited horizontal restraints on composite roadbeds and lack sufficient lateral stability [20, 21]. The absence of connections between the piles and the reliance on soil constraints between piles and the bottom of the piles result in a weak overturning resistance. Additionally, different construction influences can contribute to the problem of the insufficient bearing capacity of composite roadbeds. Relevant scholars have explored the use of an integral structural form consisting of piles and tie beams in various practical projects. These projects include micropiles [22], rotary spray piles, and gated double-row piles [23]. The findings suggest that this form can promote uniform stress and deformation throughout the skidding-resistant system, while enhancing the overall stiffness and horizontal bearing capacity of the structures. [PERSON] et al. [24] conducted model tests and a theoretical analysis, revealing that combining pile cap beams with geotextiles can mitigate uneven settlement issues in embankments supported by piles with individual square pile caps. [PERSON] et al. [25] introduced a new composite structural form for pile caps with beams in practical engineering. This form incorporates multiple cross-floor beams connecting traditional pile caps (refer to Figure 1). Field monitoring demonstrated significant reductions of 57.1% in the lateral displacement of the roadbed, 44.8% in the total settlement between the piles and the surrounding soil, and 79.9% in the differential settlement. However, there is limited literature on the structural form and engineering applications of PHC pipe pile composite roadbeds with tie beams. To minimize the occurrence of such accidents, further research is necessary to investigate the structural form of PHC pipe pile composite roadbeds and their load carrying capacity. This is crucial because sliding damage to composite roadbeds can be sudden and often leads to a 'domino effect'. This paper focuses on investigating the impact of various structural forms and construction factors on the load-bearing capacity of PHC pipe pile composite roadbeds. The K31+100-K31+388 section of a highway in Guangdong, China, was studied as the actual  
Figure 1: Field construction drawings for a PHC pile-cap-beam-supported embankment. (Reproduced from ([PERSON] et al., 2022 [25])). engineering background and a composite roadbed model was established using Plaxis 2D finite element software (V22.01.00).