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摘要: 为揭示水泥粉煤灰后压浆对湿陷性黄土桩网结构路基的加固机理，开展后压浆水泥粉煤灰碎石桩室内静载试验，分析了后压浆对桩周土样湿陷系数的影响，研究了竖向静载作用下后压浆桩网结构路基沿深度方向附加应力、桩侧摩阻力及桩端阻力的变化规律；基于Boltzmann数学模型和荷载传递函数，分析了桩侧摩阻力和桩端阻力增强机理，给出后压浆桩侧摩阻力和桩端阻力计算式；利用数值模拟方法，探讨了桩体弹性模量、后压浆深度、桩网置换率和褥垫层厚度对桩网结构路基承载力的影响机制。研究结果表明：在相同荷载作用下，经水泥粉煤灰后压浆处理后的桩周土体的湿陷系数小于自然土样的湿陷系数，且小于0.015；压浆后，静载作用下桩网结构路基中桩顶的竖向附加应力逐渐减小，桩间土的竖向附加应力先减小后增大，桩侧摩阻力较未压浆桩增大了约1.54倍；随着注浆深度的增加，桩身深度方向上的应力最大值呈先增大后减小趋势，且在等桩长深度处取得应力最大值；当桩网置换率提高1倍时，沿深度方向的应力和沉降均减小，其中应力峰值降低24%，沉降量减小26%；桩网结构路基中随着褥垫层厚度的增大，路基深度方向上应力逐渐增大。可见，水泥粉煤灰处理湿陷性黄土路基能减弱路基土体湿陷性，提高承载力，在施工过程中需要考虑桩体弹性模量、后压浆深度、桩网置换率和褥垫层厚度对路基承载力的影响。Abstract: To investigate the reinforcement mechanism of cement-fly ash post-grouting on pile-net composite subgrade in collapsible loess areas, the static load tests in laboratory were carried out on the grouted cement-fly ash gravel (CFG) piles, the influence of post-grouting on the collapsibility coefficient of the soil samples around the piles was analyzed, and the changing rules of additional stress, pile side friction resistance and pile tip resistance in the depth direction of post-grouting pile-net composite subgrade under vertical static load were studied. Based on the Boltzmann mathematical model and load transfer function, the reinforcement mechanism of pile side friction resistance and pile tip resistance was investigated, and their calculation formulas after grouting were given. The influence mechanisms of elastic modulus of pile, post-grouting depth, pile-net replacement rate, and cushion layer thickness on the bearing capacity of pile-net composite subgrade were discussed by the numerical simulation method. Research results indicate that under the same static load, the collapsibility coefficient of the cement-fly ash post-grouting soil around the pile is less than that of the natural soil sample and less than 0.015. After post-grouting, the vertical additional stress of the pile top in the pile-net composite subgrade gradually decreases under the static load, the vertical additional stress of the soil between the piles decreases first and then increases, and the pile side friction resistance increases by about 1.54 times compared with the un-grouting pile. With the increase in post-grouting depth, the maximum stress in the depth direction of pile body increases first and then decreases, and the maximum stress is obtained at the depth equal to pile length. When the pile-net replacement rate is doubled, the stress and settlement decrease in the depth direction, among which the peak stress decreases by 24% and settlement decreases by 26%. With the increase in cushion layer thickness in the pile-net composite subgrade, the stress in the depth direction of the subgrade gradually increases. Therefore, the cement-fly ash treatment of collapsible loess subgrade can weaken the collapsibility of subgrade soil and improve the bearing capacity. In the construction process, the effects of elastic modulus of pile, post-grouting depth, pile-net replacement rate and cushion thickness on the bearing capacity of the subgrade should be considered.
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图 9 桩侧摩阻力-桩土相对位移曲线
Figure 9. Curves of pile side friction resistance and pile-soil relative displacement
图 10 桩端阻力-桩土相对位移曲线
Figure 10. Curves of pile tip resistance and pile-soil relative displacement
图 13 注浆深度不同时桩网结构路基深度-应力曲线
Figure 13. Depth-stress curves of pile-net composite subgrades with different grouting depths
图 14 置换率不同时桩网结构路基深度-应力曲线
Figure 14. Depth-settlement curves of pile-net composite subgrades with different replacement rates
图 15 置换率不同时桩网结构路基深度-沉降曲线
Figure 15. Depth-settlement curves of pile-net composite subgrades with different replacement rates
图 16 褥垫层厚度不同时桩网结构路基深度-应力曲线
Figure 16. Depth-stress curves of pile-net composite subgrades with different cushion layer
表 1 参数相似关系
Table 1. Similarity relations of parameters
物理量 相似系数 长度/m 10 质量/kg 100 应力/Pa 1 位移/m 10 弹性模量/MPa 1 泊松比 1 
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表 2 黄土物理力学参数
Table 2. Physical and mechanical parameters of loess
土的分类 密度/(g·cm-3) 黏聚力/kPa 摩擦角/(°) 压缩模量/MPa 泊松比 粉质黏土 1.78 44.27 29.64 5.8 0.25
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表 3 粉煤灰化学成分
Table 3. Chemical compositions of fly ash
成分 SiO2 Al2O3 Fe2O3 CaO MgO SO3 其他 百分比/% 62.76 23.73 4.02 3.56 2.62 2.79 0.52
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表 4 粉煤灰物理性质
Table 4. Physical properties of fly ash
项目 国家标准 检验结果 Ⅰ级 Ⅱ级 Ⅲ级 细度(45 μm筛余/%) ≤12 ≤20 ≤45 19.8 需水量比/% ≤95 ≤105 ≤115 105 烧失量/% ≤5 ≤8 ≤15 2.0 含水率/% ≤1 ≤1 - 0.9 SO3含量/% ≤3 ≤3 ≤3 2.7
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表 5 水泥的主要物理力学指标
Table 5. Main physical and mechanical indexes of cement
比表面积/(m2·kg-1) 安定性/mm 凝结时间/min 强度/MPa 初凝 终凝 3 d 28 d 357 3.6 65 470 18.1 43.6
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表 6 模型桩与湿陷性黄土物理力学参数
Table 6. Physical and mechanical parameters of model pile and collapsible loess
材料类别 弹性模量/MPa 泊松比 重度/(kN·m-3) 黏聚力/kPa 内摩擦角/(°) 深度/m 湿陷性黄土(修正前) 12 0.25 15.9 27 13.3 0~6 湿陷性黄土(修正后) 6 0.25 17.9 27 13.3 0~6 粉质黏土 12 0.25 17.9 30 26.0 6~20 注浆水泥粉煤灰加固桩 12 000 0.20 21.3 6
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表 7 桩网结构路基不同深度处沉降
Table 7. Settlements of different depths of pile-net composite subgrade
深度/m 沉降/mm 深度/m 沉降/mm E=1.2 GPa E=2.4 GPa E=1.2 GPa E=2.4 GPa 0.0 0.85 6.43 3.0 4.85 8.41 0.5 1.19 6.63 3.5 6.00 9.00 1.0 1.66 6.89 4.0 7.45 9.67 1.5 2.21 7.16 4.5 8.86 10.46 2.0 2.91 7.51 5.0 10.29 11.21 2.5 3.83 7.96 5.5 11.66 11.93
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