Experimental study on difference of shear strength between gravel materials and actual vibroflotation piles
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摘要: 振冲桩法通过碎石等透水性材料对软弱地基进行置换挤密而形成复合地基,以往对该复合地基抗剪强度关注相对较少。由于碎石桩和桩周软土存在挤密和置换作用,碎石原料与振冲形成的实际桩体不论颗粒组成还是变形受力特性都存在差异,基于工程现场对成桩体的检测和取样,在室内开展土工试验,分析碎石原料与实际成桩体在物理力学参数方面的差异,试验表明：①实际成桩体中的细颗粒含量显著高于碎石原料,其主要来源于振冲施工过程中桩周软土的混（挤）入；②成桩体最大干密度高于碎石原料,与碎石原料级配不良及细颗粒混入有直接关系；③以现场实测压实度备样分别对碎石原料和两种桩间距成桩体开展大型三轴试验,成桩体整体强度较碎石原料降低约10%,考虑其在应力变形特性上的差异,若以5%应变对应偏应力求取强度参数,成桩体较碎石料强度减小超过20%,桩周土混入对桩体抗剪强度及应力变形特性均有较显著的劣化影响。结合初步研究结论,提出了复合地基强度设计参数的修正建议。Abstract: The vibroflotation pile method is used to replace and compact the soft foundation with permeable materials such as gravel to form composite foundation, and in the past, the attention to shear strength of this kind composite foundation is little. Due to the compaction and replacement of gravel and soil around piles, difference is shown between gravel materials and actual vibroflotation gravel piles, regardless of particle composition or stress-strain characteristics. Based on field detection, sampling for vibroflotation gravel piles and gravel materials and indoor geotechnical tests to analyze the difference of physical and mechanical properties between vibroflotation gravel piles and gravel materials, the researches show: (1) The content of fine particles in actual piles is significantly higher than that of gravel materials, which mainly comes from the mixing (squeezing) of soft soil around piles during vibroflotation process; (2) The maximum dry density of piles is higher than that of gravel materials, which is directly related to the poor grading of gravel materials and mixing of fine particles; (3) Refer to actual compaction degree in the field, large-scale triaxial tests are carried out on gravel materials and two kinds of piles with different pile spacings, the strength of piles is about 10% lower than that of gravel materials, considering the difference in stress-strain characteristics, if the strength parameter is taken according to 5% axial strain, the strength of piles is more than 20% lower than that of gravel materials, and mixing of soil around piles has a significant deterioration effect on the shear strength and also the stress-strain characteristics of the piles. Based on the above initial conclusions, the modified suggestions for strength design parameters of composite foundation are proposed.
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图 5 桩间距1.5 m成桩体的应力应变关系曲线
Figure 5. Stress-strain curves of vibroflotation gravel pile with 1.5 m spacing
表 1 研究试样颗粒级配统计
Table 1 Statistics of particle size distribution for research samples
(%) 材料 粒径区间/mm 100~150 60~100 40~60 20~40 10~20 5~10 2~5 1~2 0.5~1 0.25~0.5 0.1~0.25 <0.1 料场碎石 31.44 40.75 17.72 6.31 0.98 0.43 0.24 0.22 0.16 0.54 0.90 0.29 桩间距2.0 m桩体 27.90 28.92 9.36 10.67 7.83 6.03 2.27 1.26 0.78 2.23 1.85 0.91 桩间距1.5 m桩体 30.54 30.15 10.09 10.60 5.54 3.82 1.59 1.32 0.61 1.92 2.99 0.84 
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表 2 成桩材料及碎石料密度测试统计表
Table 2 Densities of vibroflotation gravel piles and gravel materials
材料 现场干密度/(g·cm-3) 最大干密度/(g·cm-3) 密实度 2.0 m间距桩体 2.026 2.249 0.901 1.5 m间距桩体 2.031 2.264 0.897 料场碎石 — 2.130 —
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表 3 大型三轴压缩试验成果表
Table 3 Test results of large-scale triaxial compression tests
试样名称 干密度/(g·cm-3) 抗剪强度指标 c/ kPa φ/(°) 桩间距2.0 m桩体 2.026 36 32.8 桩间距1.5 m桩体 2.031 52 31.7 料场碎石 1.92 42 36.2
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表 4 复合地基抗剪强度计算值对比
Table 4 Comparison of shear strength for composite foundation
材料 1.5 m间距复合地基 2.0 m间距复合地基 φ/(°) c/ kPa φ/(°) c/ kPa 成桩体 26.49 7.76 19.42 10.05 碎石原料 31.17 7.76 22.66 10.05
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