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摘要: 为了揭示湿陷性黄土地区刚-柔性桩复合地基的荷载传递机理, 开展了现场原型试验, 分析了桩身和桩间土的应力在不同荷载与深度下的变化规律; 通过与刚性单桩的对比, 总结了刚-柔性桩复合地基的桩土相互作用特点; 结合已有文献, 分析了湿陷性黄土地区刚-柔性桩复合地基与软土地区刚-柔性桩复合地基在力学表现上的差异。分析结果表明: 湿陷性黄土地区刚-柔性桩复合地基中柔性桩的主要作用是挤密桩间土, 消除其湿陷性, 试验场地处理后湿陷系数基本小于0.015;由于柔性桩的挤密作用, 桩间土的承载力得以充分发挥, 刚性桩的荷载传递能力得以增强; 软土地区柔性桩的荷载分担率一般大于桩间土, 由于黄土的承载力较高及柔性桩与桩间土的模量比小, 湿陷性黄土地区桩间土的荷载分担率稳定在26%左右, 远大于柔性桩的7%;复合地基中的刚性桩属于端承摩擦桩, 随着荷载增加, 刚性桩的荷载传递能力逐渐强化, 荷载分担率逐渐增加, 最终稳定在67%左右; 刚性桩荷载传递能力的增强并不利于刚-柔性桩复合地基承载能力的充分发挥, 在设计时需要充分考虑对纯摩擦桩有效桩长的影响, 以及对端承摩擦桩桩端土体承载能力的影响。Abstract: In order to investigate the loading transfer mechanism of composite foundation with rigid-flexible piles in collapsible loess area, a field prototype test was conducted. The stress variation rules of piles and soil among piles with different loads and depths were analyzed. Taking the behaviors of rigid single pile as comparison, the characteristic of pile-soil interaction in composite foundation with rigid-flexible piles was summarized. Combined with existing literatures, the difference of mechanical performance between the composite foundation with rigid-flexible piles in collapsible loess area and soft soil area was analyzed. Analysis result indicates that the flexible pile of composite foundation with rigid-flexible piles in collapsible loess area is mainly used to compact the soil among piles and eliminate the collapsibility of soil, and the collapsibility coefficients are basically less than 0.015 after the treatment in the test site. As a result of the compaction of flexible pile, the bearing capacity of soil among piles can be fully developed and the loading transfer ability of rigid pile can be enhanced. The load-sharing ratio of flexible pile is always larger than that of soil among piles in soft soil area. As a result of the high bearing capacity of loess and small modulus ratio between the flexible pile and soil among piles, the load-sharing ratio of soil among piles in collapsible loess area is stable at about 26%, much higher than 7% of flexible piles. The rigid pile in composite foundation pertains to an end bearing friction pile. As the loading increases, the loading transfer ability of rigid pile improves gradually, and the load sharing ratio increases and finally settles at 67%. The improvement of the loading transfer ability of rigid piles is against the development of the bearing capacity of composite foundation with rigid-flexible piles, of which effects on the valid length of pure friction pile should be taken into account in the designing process, and its effects on the bearing capacity of soil at the bottom of the pile of end bearing friction pile should be also been considered.
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图 3 承载板下土压力盒布置(单位: mm)
Figure 3. Layout of soil pressure cells under bearing plate (unit: mm)
图 4 A-A断面竖向测试元件分布(单位: mm)
Figure 4. Distribution of test components along vertical direction of A-A section (unit: mm)
图 7 复合地基中刚性桩轴力分布
Figure 7. Axial force distributions of rigid pile in composite foundation
图 9 复合地基中刚性桩桩侧摩阻力分布
Figure 9. Frictional resistance distributions of rigid pile in composite foundation
图 10 挤密前后地基土湿陷系数
Figure 10. Collapsibility coefficients of foundation soil before and after compaction
表 1 主要土层物理力学性质指标
Table 1. Physical and mechanical property indexes of main soil layers
地层编号 土质 平均厚度/m 指标统计值 含水率/% 密度/ (g·cm-3) 饱和度/% 孔隙比 压缩系数/MPa-1 压缩模量/MPa ②-1 黄土状粉土 11.5 平均值 10.4 1.52 28.6 0.973 0.31 8.66 标准值 11.5 1.50 31.7 1.002 0.36 7.53 ②-4 粉土 7.6 平均值 17.4 1.84 66.1 0.730 0.14 14.20 标准值 18.5 1.82 70.6 0.751 0.16 12.96 
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表 2 试验概况
Table 2. Presentations of tests
试验对象 试验点 土体挤密情况 桩长/m 桩直径/mm 砂垫层 承载板直径/mm 强度等级 测量仪器 刚性桩单桩 G 均已挤密且试验位置大致对称 8 400 无 400 C15 振弦式混凝土应力计 刚-柔性桩复合地基 刚性桩 F 有 1 654 C15 振弦式混凝土应力计 柔性桩 土压力盒
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表 3 桩间土应力与桩土应力比
Table 3. Stresses of soil among piles and pile-soil stress ratios
荷载/kN 111 204 297 391 485 578 672 桩间土应力均值/kPa 83 99 110 122 133 141 155 刚性桩-土应力比(SG/ST) 4.123 7.879 11.937 14.672 15.556 20.426 22.846 柔性桩-土应力比(SR/ST) 1.018 1.020 1.024 1.025 1.049 1.057 1.065 荷载/kN 765 859 952 1 045 1 138 1 231 1 324 桩间土应力均值/kPa 160 171 179 186 191 206 217 刚性桩-土应力比(SG/ST) 24.125 26.759 31.117 31.788 33.770 33.029 33.335 柔性桩-土应力比(SR/ST) 1.069 1.053 1.112 1.156 1.199 1.199 1.207
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