Stress Characteristics and Failure Mechanisms of Plain Concrete Piles of Composite Foundation under Embankment
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摘要: 为了合理分析软土地区路堤下素混凝土桩复合地基的稳定性，基于离心模型试验和仿真分析，研究了不同桩间距条件下路堤下素混凝土桩复合地基桩体的受力特征，引入桩体破坏逐一退出机制，分析了桩体破坏模式. 结果表明：在路堤自重和列车荷载作用下，路堤下复合地基素混凝土桩受力特征和破坏模式具有显著的桩间距效应，当桩间距分别增大至4倍和6倍桩径时，最靠近坡脚的第1列与第1、2列素混凝土桩分别产生了断桩破坏；在桩间距不变的条件下，随着上部荷载的增大，素混凝土桩最大弯矩和剪力均逐渐增大；施加列车荷载后，桩体最大弯矩和剪力往路基坡脚方向呈逐渐增大的规律，桩间距由3倍增大至6倍桩径时，靠近坡脚的桩体最大弯矩由172.9 kN•m增大至601.0 kN•m，大于桩体标定极限弯矩值，剪力由89.4 kN增大至249.1 kN，小于桩体标定极限剪力值，表明离心模型试验中素混凝土桩产生弯曲破坏而不是剪切、受压和受拉破坏，最靠近坡脚的桩最先发生弯曲破坏，随后往路基中心方向呈逐一弯曲破坏模式.Abstract: In order to reasonably analyze the stability of embankment supported on composite foundation with plain concrete piles in soft clay area, centrifugal model tests and simulation analyses were conducted to study the stress characteristics of the piles with different spacings under embankment, and a one-by-one exiting method of piles due to rupture breaking was introduced to investigate the failure modes of the piles. The results show that the stress characteristics and failure modes of the piles under embankment self-weight and train load have significant pile spacing effect. While the ratios of the pile spacing to the pile diameter are 4 and 6 respectively, rupture occurs to the first column and the first and second columns of the plain concrete piles close to the embankment toe. With a constant pile spacing, the maximum bending moment and shearing force of the piles increase with an increase in the upper load. When train loads are applied on the embankment surface, piles bear a larger maximum bending moment and shearing force with a closer distance to the embankment toe. When the pile spacing increases from 3 to 6 times the pile diameter, the maximum bending moment of piles increase from 172.9 to 601.0 kN•m, much larger than the ultimate bending moment value calibrated by test; meanwhile, the maximumshearing force of piles increase from 89.4 to 249.1 kN, much less than the ultimate shearing force value. This indicates that the plain concrete piles of composite foundation under embankment in the centrifugal model are governed by bending failure other than shear failure, tension failure or compression failure. Therefore, the piles closest to the embankment toe fail first; then, failure developing towards the embankment center, piles fail one by one in bending failure mode.
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Key words:
- composite foundation /
- plain concrete piles /
- stress characteristics /
- failure mechanism
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图 3 桩间距为3倍桩径的数值模型（单位：cm）
Figure 3. Numerical model with a pile spacing of 3 times the pile diameter （unit：cm）
图 4 桩间距为4倍和6倍桩径的数值模型平面（单位：cm）
Figure 4. Plan graphs of numerical models with pile spacings of 4 and 6 times the pile diameter （unit：cm）
图 7 第1列桩断开条件下各桩弯矩和剪力
Figure 7. Bending moments and shear forces of piles when the first column piles are disconnected
图 8 第1列和2列桩断开条件下各桩弯矩和剪力
Figure 8. Bending moments and shear forces of piles when the first and second column piles are disconnected
表 1 模型地基土主要力学参数
Table 1. Main mechanical parameters of soils used in the model
名称 含水率/% 密度/（kg•m–3） 粘聚力/kPa 内摩擦角/（°） 粉质黏土 31.98 1 940 30.4 25.1 粉砂土 30.31 1 930 5.7 28.0 
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表 2 离心机试验方案加载历程
Table 2. Loading regime of all centrifuge tests
模拟工况 加速度/（×g） 加载时间/min 持续时间/min 路堤高 2.0 m 16.67 2.04 103.68 路堤高 4.0 m 33.33 1.95 25.92 路堤高 6.0 m 50.00 1.90 11.52 服役 1 a（施加车-轨荷载） 50.00 7.94 210.00
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表 3 数值模型主要计算参数
Table 3. Main calculation parameters of numerical models
部件名称 密度/（g•cm–3） 弹性模量/MPa 泊松比 摩擦角/（°） 粘聚力/kPa 路堤填土 1.95 51.0 0.28 38.0 35.0 加筋垫层 2.10 108.8 0.26 45.0 40.0 桩 2.40 15.0 0.20 粉质黏土（2-1） 1.94 13.8 0.34 25.1 30.4 粉质黏土（2-2） 1.94 13.8 0.34 25.1 30.4 砂性土 1.93 30.6 0.30 28.0 5.7 强风化泥岩 2.20 680.0 0.24 40.0 160.0
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表 4 各数值模型桩身最大轴力
Table 4. Maximum axial forces of main piles of models
模型序号 桩间距/桩径 最大轴力/kN 桩号 C-1 3 349.2 1# 756.8 3# 959.9 5# 443.1 1# C-2 4 811.4 2# 1 205.0 4# 425.1 1# C-3 6 1 160.0 2# 1 436.0 3#
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