桩土应力比是浅埋岩软土地基中刚性桩复合地基设计的关键参数,但其取值规律及影响因素尚不明确。依托某混凝土闸坝工程,通过室内物理模型试验,系统研究单桩复合地基的承载特性与桩土应力比,分析垫层类型、桩受力情况、桩距及桩端持力层等因素的影响,获取桩土应力比随荷载变化规律及承载力对应的桩土应力比。结果表明:随荷载增加,摩擦桩桩土应力比先增后减,端承桩桩土应力比基本持续增大;水泥土垫层可显著提高复合地基承载力与桩土应力比,其最大桩土应力比约为砂砾石垫层的2倍;桩端持力层强度与桩间距对桩土应力比影响显著,可通过增大桩间距、调整桩端持力层来优化方案;端承桩复合地基在水泥土垫层下的承载力与砂砾石垫层接近,但桩土应力比更高,可采用复合嵌入式褥垫层以兼顾防渗与应力调节需求。研究成果可为浅埋岩软土刚性桩复合地基优化设计提供理论依据。

[Objective] Pile-soil stress ratio is a key parameter in the design of rigid pile composite foundations for shallow-buried rock and soft soil foundations, but the rules governing its value and influencing factors remain unclear. [Method] Based on a concrete sluice dam project, this study carried out 7 sets of scaled indoor physical model tests, systematically studied the bearing characteristics of soft soil single-pile composite foundations and the pile-soil stress ratio, analyzed the influences of factors such as cushion type, pile loading conditions, pile spacing and pile end bearing stratum, and obtained the variation trend of pile-soil stress ratio with load and the pile-soil stress ratio corresponding to bearing capacity. [Results] In the initial loading stage, the P-S curves of the tested single-pile composite foundations exhibited linear changes, and the soil under the bearing plate was in an elastic deformation state. With the increase of load, a sudden change in slope appeared in the P-S curve of the bearing pile at the lower end of the cement-soil cushion, and the characteristic value of foundation bearing capacity should be inferred according to the proportional limit. The P-S curves of friction piles under the cement-soil cushion and friction piles and end-bearing piles under the gravel cushion mainly exhibited a gradual change characteristic, and the characteristic value of foundation bearing capacity should be estimated according to the relative deformation value of 1% of the side length of the bearing plate. Under the two cushion conditions, the characteristic values of the bearing capacity of the single-pile composite foundation of end-bearing piles could meet the design requirements, while those of friction piles could not. The pile-soil stress ratio of end-bearing piles basically showed a monotonous increase with load, while that of friction piles showed an initial increase followed by a decrease. The maximum pile-soil stress ratio of end-bearing piles in the cement-soil cushion was 16.8, and that of friction piles was 13.7, with an increase of about 22.6% for end-bearing piles. The pile-soil stress ratio of end-bearing piles corresponding to the design bearing capacity of 290 kPa could be taken as 9.7, and that of friction piles could be taken as 8.1. Therefore, for shallow buried rock-soft soil foundations, rigid pile composite foundations should adopt end-bearing piles embedded in rock. The bearing capacity of end-bearing piles in the cement-soil cushion was close to that in the gravel cushion, but the pile-soil stress ratio decreased from 16.8 to 8.2, a decrease of about 51.2%, indicating that the gravel cushion had a better stress adjustment capacity. The pile end bearing stratum and pile spacing were key design parameters. When the pile spacing was adjusted from 1.4 m to 1.8 m, and the pile tip bearing stratum was adjusted from weakly weathered rock to strongly weathered rock, the pile-soil stress ratio decreased by 42.3%, but the bearing capacity still met the design requirements, which was more economical. [Conclusion] The cement-soil cushion significantly improves the bearing capacity of the composite foundation and the pile-soil stress ratio, and its maximum pile-soil stress ratio is about twice that of the gravel cushion. The strength of the pile end bearing stratum and the pile spacing have a significant influence on the pile-soil stress ratio, and the scheme can be optimized by increasing the pile spacing and adjusting the pile end bearing stratum. The bearing capacity of the end-bearing pile composite foundation under the cement-soil cushion is close to that under the gravel cushion, but the pile-soil stress ratio is higher. A composite embedded cushion layer can meet both seepage control and stress adjustment requirements. The research results can provide a theoretical basis for the optimal design of rigid pile composite foundations in shallow buried rock-soft soils.