Experimental study on vertical tensile bearing mechanism of screw cast-in-place piles
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摘要: 通过室内模型试验和现场试验对螺杆灌注桩抗拔承载机理进行了研究，研究表明螺杆灌注桩作为抗拔桩优势较为明显。通过室内模型试验，采用图像处理技术直观地得到上拔荷载作用下螺杆桩和直杆桩的桩周土体位移场和上拔破坏模式，进而对其抗拔承载机理进行探讨，并采用现场试验进行验证。模型试验和现场试验表明，螺杆桩的单桩抗拔承载力高于同直径的等截面桩，螺纹段的抗拔侧阻力与等截面桩的侧阻力相比明显提高。对于螺纹段，极限抗拔侧阻力表现为周边土体的抗剪强度，破坏面为连续的拱形；对于直杆段，极限抗拔侧阻力表现为桩体与桩周土体的摩擦阻力，破坏面为桩周圆柱面。根据试验结果从工程实用的角度，引入抗拔折减系数给出螺杆灌注桩单桩抗拔极限承载力的表达式，并给出相关参数的取值建议。Abstract: The vertical tensile bearing capacity of the screw cast-in-place pile is studied through the indoor model tests and field tests, and it is shown that the screw cast-in-place pile is superior to be used as the uplift piles. Through the indoor model tests, the failure modes of the screw pile and the straight rod pile by uplift loads are obtained intuitively by using the image processing technology, then the tensile bearing mechanisms are discussed, and the field tests are used for verification. The model and field tests show that the tensile bearing capacity of the single screw pile is higher than that of the straight rod pile with the same diameter, and the tensile lateral resistance of the screw pile is obviously higher than that of the straight rod pile with the same diameter. For the screw section, the tensile ultimate lateral resistance is the shear strength of the surrounding soil, and the fracture surface is a continuous arch. For the straight rod section, the tensile ultimate lateral resistance is the friction resistance between the pile body and the soil around the pile, and the failure surface is the cylinder around the pile. According to the test results and from an engineering practical point of view, the expression for the tensile ultimate bearing capacity of a single screw cast-in-place pile is given by introducing the drawdown coefficient, and the values of the relevant parameters are suggested.
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图 4 模型桩抗拔极限承载力随相对密实度的变化曲线
Figure 4. Tensile bearing capacity-relative compactness curves of model piles
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图 5 螺杆桩相对于直杆桩抗拔极限承载力提高比例随相对密实度的变化曲线
Figure 5. Curves of increase ratio of tensile bearing capacity of model piles
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图 6 模型桩的平均抗拔侧阻力随桩顶位移的变化曲线
Figure 6. Curves of average tensile lateral resistance of model piles
表 1 模型桩参数
Table 1 Parameters of model piles
单位: mm 桩型 桩长 外径 内径 螺距 螺牙宽度 螺牙厚度 螺杆桩 400 40 30 50 5 2 直杆桩 400 40 — — — — 
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表 2 土性指标
Table 2 Property indices of soil
密实程度 相对密实度Dr 干密度/(g·cm-3) 内摩擦角φ/(°) 黏聚力c/kPa 松散 0.22 1.43 22.99 4.5 中密 0.40 1.50 23.34 5.0 密实 0.72 1.65 26.57 6.0
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表 3 模型试验抗拔极限承载力
Table 3 Tensile bearing capacities of model piles
相对密实度Dr 抗拔极限承载力/N 螺杆桩抗拔极限承载力相对直杆桩提高比例/% 螺杆桩 直杆桩 0.22 261.6 145.4 80.0 0.40 324.2 156.5 107.3 0.72 619.4 306.4 102.2
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表 4 模型试验误差分析
Table 4 Error analysis of model piles
相对密实度Dr 实测极限抗拔侧阻力/kPa 计算极限抗拔侧阻力/kPa 误差/% 螺杆桩 直杆桩 螺杆桩 直杆桩 螺杆桩 直杆桩 0.22 9.77 4.95 10.33 5.66 -5.4 -12.5 0.40 10.00 5.64 12.83 6.11 -22.0 -7.7 0.72 24.82 12.04 24.57 12.07 1.0 -0.2
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表 5 土性指标
Table 5 Property indices of soil
岩土名称 天然重度γ/
(kN·m-3)天然含水率
w/%孔隙比
e塑性指数
IP液性指数
IL压缩模量ES/
MPa压缩系数
a1~2/
MPa-1黏聚力c/kPa 内摩擦角φ/
(°)承载力特征值fak/kPa 素填土① 18.0 — — — — — — 5 8 — 粉质黏土② 19.0 32.7 0.903 13.1 0.62 3.28 0.62 20 11.4 80 粉质黏土③ 19.7 25.6 0.74 14.9 0.36 6.13 0.29 38 15.2 150
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表 6 试验桩的参数
Table 6 Specific parameters of test piles
桩号 桩长/m 直杆段桩长/m 螺纹段桩长/m 外径/
mm内径/
mm螺距/
mm螺牙高度/
mmLG1/ LG2/ LG3 6 2 4 600 540 500 30 CL1/CL2 6 6 — 600 — — —
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表 7 现场静载试验主要试验成果
Table 7 Results of field static load tests
静载试验类型 极限承载力/kN 分段平均侧阻力/
kPa抗拔折减系数 直杆段 螺纹段 抗压静载试验 LG1 1409 53.25 143.62 — CL1 988 58.13 — — 抗拔静载试验 LG2 800 24.77 102.57 0.71 CL2 600 49.39 — 0.85
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