变截面桩复合地基是一种新型地基加固方法，能有效满足实际工程对承载力与沉降控制的需求。通过室内模型试验与数值计算相结合的方法，研究变截面单桩与变截面桩复合地基承载特性。主要研究内容与结论如下：
（1）通过单桩室内模型试验，研究变截面桩桩身轴力、桩侧摩阻力、桩顶沉降和桩身不同深度荷载分担比，探究变径比对其工作性能的影响，并对其进行优化设计。结果表明：在相同荷载下截面桩的桩顶沉降明显小于等截面桩，当变径比为0.8时，沉降控制效果最佳，材料利用率最高。变截面位置土体对桩体具有支承作用，其变径处底面土体能承担部分桩顶荷载；而桩侧能够分担更多桩顶荷载，减轻桩端土体承载，当变径比在0.8 ~ 0.9时侧摩阻力发挥程度更高。
（2）基于相似理论设计变截面桩复合地基、土工格栅+变截面桩复合地基两组室内模型试验，对比分析复合地基沉降位移、桩身轴力、桩土应力比等。分析结果表明：变截面桩能够充分调动桩周土体承担桩顶荷载，中性点位于桩身变径处距桩顶0.18 m附近。土工格栅在荷载作用下能充分发挥其张拉膜效应，复合地基沉降量减少22.8%，桩土应力比提升幅度为30.56%，土工格栅能够大幅度改善复合地基承载性能。
（3）基于室内模型试验建立复合地基数值计算模型，在静荷载作用下，与室内模型试验结果进行对比验证，二者结果基本吻合。计算结果表明：在使用变截面桩加固地基后，最大沉降相较于等截面桩减少15.55%，桩土差异沉降得到有效控制，同时提高了地基整体承载力。
（4）基于上述研究，进一步分析桩间距、变径比、变截面位置、桩身模量对复合地基沉降、桩土间差异沉降等影响。研究表明：增大桩体变径比、变截面位置下移、减小桩间距以及增大桩身弹性模量，均能较好改善复合地基的承载特性，控制地基沉降变形。

The tapered pile composite foundation is a novel ground reinforcement method that effectively meets the real-world engineering requirements for load-bearing capacity and settlement control. This study investigates the load-bearing characteristics of both single tapered piles and tapered pile composite foundations through a combination of laboratory model test and numerical calculations. The main research contents and conclusions are as follows:
(1) Through single pile indoor model tests, the study investigated the axial force of variable cross-section piles, side friction, pile head settlement, and the load-sharing ratio at different depths of the pile. It explored the impact of the diameter ratio on its performance and optimized its design. The results show that under the same load, the pile head settlement of variable cross-section piles is significantly less than that of uniform cross-section piles. The optimal settlement control effect is achieved when the diameter ratio is 0.8, where the material utilization is the highest. The soil at the variable cross-section provides support to the pile, with the soil at the bottom of the changing diameter able to bear part of the load at the pile head; meanwhile, the pile sides can share more of the pile head load, reducing the load on the soil at the pile end. The side friction is most effectively utilized when the diameter ratio is between 0.8 and 0.9.

(2) Based on the theory of similarity, two sets of laboratory model test were designed: one with variable cross-section pile composite foundations, and the other combining geogrids with variable cross-section piles. Comparative analysis was conducted focusing on settlement displacement of the composite foundation, axial force in the piles, and the pile-soil stress ratio. The analysis indicates that variable cross-section piles effectively mobilize the surrounding soil to bear the load at the pile top, with the neutral point located about 0.18 meters from the pile top at the point of diameter change. Under load, the geogrid fully utilizes its tensile membrane effect, reducing the settlement of the composite foundation by 22.8% and increasing the pile-soil stress ratio by 30.56%. This demonstrates that geogrids significantly enhance the bearing capacity of the composite foundation.
(3) Based on indoor model tests, a numerical model of the composite foundation was established. Under static load, the model was validated by comparing its results with those of the indoor model tests, showing fundamental agreement. The calculations indicate that after reinforcing the foundation with variable cross-section piles, the maximum settlement was reduced by 15.55% compared to uniform cross-section piles. The differential settlement between the pile and the soil was effectively controlled, and the overall bearing capacity of the foundation was enhanced.

(4) Building on the aforementioned studies, further analysis was conducted to assess the impact of pile spacing, diameter ratio, variable cross-section positioning, and pile body modulus on composite foundation settlement and differential settlement between pile and soil. The research indicates that increasing the pile diameter ratio, lowering the position of the variable cross-section, reducing the spacing between piles, and increasing the modulus of elasticity of the pile body can all significantly enhance the bearing characteristics of the composite foundation and effectively control the deformation due to foundation settlement.