Model tests on deep soft ground improvement of existing sand-filled subgrade with squeezed branch piles
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摘要: 高速公路改扩建中既有填砂路基深层软土加固时存在路面结构层和施工场地空间限制的问题。首先，提出使用单盘挤扩支盘桩方案进行地基处理，通过挤扩法成盘于桩顶，以期实现不破除路面结构层的情况下达到设置桩顶承台或托板的效果。其次，利用群桩地基物理模型试验模拟一个桩土单元体研究加固方案的承载过程，通过分析桩体、盘体和土体的力学及变形指标研究支盘桩加固既有填砂路堤的承载表现和土体变形特征，最后，考虑土体参数和桩体尺寸推导了既有填砂路基深层软土支盘桩加固方法的二维简化模型。研究结果表明，软土在恒载下桩体轴力和土压力随时间变化调整，原因在于软土的压缩性和砂土的散粒体属性；桩顶轴力总和与恒载大小近似线性正相关，同时支盘直径越大桩顶轴力总和越大、桩承荷载占比越高，试验发现支盘桩加固法的桩承荷载占比约70%。另外，利用所建二维简化模型进行参数分析发现填砂路基中的最小盘间净距与上覆荷载、填砂内摩擦角、深层软土强度指标和支盘桩几何参数相关。Abstract: There are limitations from the pavement structure layer and construction site space during the deep soft soil reinforcement of the existing sand-filled subgrade in expansion of expressways. Firstly, a new method using the squeezed branch pile (SBP) with one single plate for treatment of the existing soft foundation is proposed. In this method, a plate is set on the top of the piler to achieve the effects similar to a pile cap or support plate without breaking the road structure layer. Secondly, a physical model test on the pile group foundation is performed, and a pile-soil unit is simulated to study the bearing behavior of the SBP. By analyzing the mechanical and deformation indices of the pile, plate and soil, the bearing performance and soil-deformation characteristics of the new reinforcement method are studied. Finally, a simplified two-dimensional model for the reinforcement method with SBPs for the existing sand-filled embankment is derived by considering the soil parameters and pile sizes. The results indicate that the axial force of piles and soil pressure vary with time under constant loads due to the compressibility of soft soil and the properties of loose sand. The total axial force at the top of the pile is approximately positively correlated with the magnitude of the constant loads linearly. At the same time, the larger the diameter of the support plate, the greater the total axial force at the top of the pile and the higher the proportion of pile-bearing loads. It is found that the pile-bearing ratio of the reinforcement method with SBPs is about 70%. In addition, the proposed two-dimensional simplified model for parameter analysis shows that the minimum pile spacing in the sand-filled subgrade is related to the overlying load, internal friction angle of sand-filled soil, strength index of deep soft soil and geometric parameters of support piles.
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Keywords:
- expressway /
- expansion /
- existing sand-filled subgrade /
- squeezed branch pile /
- model test
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图 6 试验#2荷载、盘顶压力、盘面压力时间过程
Figure 6. Variation of parameters with loading time in test No. 2
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图 8 试验#2荷载、盘面压力/盘顶压力的时间过程
Figure 8. Time histories of loads and pressures on bulb surface and top in test No. 2
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图 9 模型#2加载过程中的桩间土压力与孔隙水压力
Figure 9. Soil pressures and pore water pressures between piles during loading of test No. 2
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图 10 模型#2加载过程中的桩端轴力与桩间土压力
Figure 10. Axial forces at pile end and soil pressures between piles during loading of test No. 2
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图 11 模型#2加载过程中的桩端轴力与桩间土压力
Figure 11. Axial forces at pile end and soil pressures between piles during loading of test No. 2
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图 12 本级荷载试验过程时段的土颗粒位移矢量云图
Figure 12. Nephogram of displacement of soil particles during loading at various levels
表 1 模型试验用土性状指标
Table 1 Parameters of soils
土类 w/% γ/(kN·m-3) Gs wL/% Ip Cc CV/(10-4cm2·s-1) Cq/kPa φq/(°) 粒组含量/% < 0.075 < 0.5 天然软土 53.6 16.5 2.65 44 20 1.12 2.30 10 8 54 94 干散砂 — 15.5 2.62 — — — — 0 20 0 99.1 
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表 2 传感器参数
Table 2 Parameters of sensors
测量指标 传感器类型 量程 数量 量测频率 桩体轴力 轴力计 0~5 kN 4 1次/min 盘顶压力 土压力计 0~500 kPa 4 1次/min 盘面压力 土压力计 0~200 kPa 4 1次/min 桩间土压力 土压力计 0~200 kPa 5 1次/min 软土孔隙水压力 孔压计 0~100 kPa 1 1次/min 模型顶板下沉量 百分表 0~15 mm 1 1次/min 软土层压缩量 位移杆 0~10 cm 4 1次/h 位移片 0~10 cm 12 1次/h 模型剖面土层
各点位移矢量PIV设备 — 1 1次/h
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表 3 试验模型参数及控制条件
Table 3 Parameters of model tests
砂层
厚度/
mm淤泥
厚度/
mm模型桩
直径/
mm桩间
距/
mm荷载
级数分级
荷载/
kPa变形稳
定标准/
(mm·h-1)350 260 32 150 12 5 < 0.01
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表 4 模型试验实施情况指标
Table 4 Implementation indices of model tests
编号 试验
时间/
h支盘
直径/
mm盘净距/盘径 盘面积
占比/
%盘净
距/
mm淤泥含水率/% 试验前 试验后 变化量 #1 229.8 75 1.00 19.6 75 59.94 54.81 5.13 #2 220.1 85 0.76 25.2 65 56.56 51.64 4.92 #3 192.3 95 0.58 31.5 55 55.54 52.60 2.94
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表 5 桩体轴力比较
Table 5 Comparison of pile axial force
试验
编号试验
时间/h支盘直
径/mm盘缘距
/mm最大荷载
/kPa桩端轴力
总和/kN桩承荷载
占比/%#1 229.8 75 75 60 7.63 63.6 #2 220.1 85 65 60 8.33 69.4 #3 192.3 95 55 60 8.98 74.8
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表 6 模型#2盘顶、盘面压力
Table 6 Pressures on bulb top and surface of model No. 2
桩位
序号荷载/
kPa盘顶
压力/
kPa盘面法向压
力/kPa盘面竖向压力/
kPa盘顶压力/荷载 盘面法
向压力/
荷载盘面法向压力/盘顶压力 a桩 60 202.99 109.74 137.48 3.38 1.83 0.54 b桩 60 218.10 132.16 165.57 3.64 2.20 0.61 c桩 60 180.01 117.95 147.77 3.00 1.97 0.66 d桩 60 169.09 83.79 104.97 2.82 1.40 0.50
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表 7 试验前后淤泥参数
Table 7 Parameters of sludge before and after tests
试验编号 试验
时间/
h支盘
直径/
mm最大
荷载/
kPa淤泥含水率初值/% 淤泥含
水率终
值/%淤泥孔
隙比
初值总压
缩量/
mm淤泥层
压缩量/
mm#1 229.8 75 60 59.94 54.81 1.618 30.1 15.4 #2 220.1 85 60 55.56 51.64 1.527 27.6 15.0 #3 192.3 95 60 55.54 52.60 1.500 24.2 8.3
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表 8 工况参数与盘间净距
Table 8 Working parameters and disc clearances
材料属性参数 盘几何参数 条件参数 盘间距 γ/
(kN·m-3)φ/
(°)φ′/
(°)c/
kPaL/
mα/
(°)h/
mΔH/m D/m min max max min 17 25 8 15 0.57 25 2.0 7.0 8.25 3.99 2.92 3.0 8.0 9.50 5.89 4.33 4.0 9.0 10.75 7.76 5.72
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