Model tests on earth pressure at rest of light weight soil behind rigid retaining walls
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摘要: 为了研究轻量土对挡土结构物的减压机理，通过开展大比尺刚性挡土墙模型试验，采用重物堆载的方式在填土表面施加均布荷载，分析重塑黄土和轻量土作为墙后填土时的静止土压力分布规律。结果表明：轻量土在养护阶段的静止土压力随着时间增长而逐渐增大，但增幅逐渐降低。重塑黄土和轻量土的静止土压力随着填土深度增加近似线性增大，并且静止土压力随着加载而逐渐增大，其中重塑黄土的增幅明显偏大。对比重塑黄土和轻量土的静止土压力，发现轻量土具有明显的减压作用，且填土表面均布荷载越大，轻量土的减压效果越好。重塑黄土和轻量土的静止土压力系数均不是常数，其中重塑黄土静止土压力系数为0.34~0.78，轻量土静止土压力系数为0.22~0.55。当填土表面作用均布荷载时，传统理论在计算重塑黄土静止土压力方面适用性较高，但轻量土计算误差较大。结合模型试验以及传统理论，提出了适用于轻量土的静止土压力修正公式，对比发现其相对误差总体上介于1.01%~23.13%。通过模型试验和理论计算揭示了轻量土的静止土压力特性，对于完善轻量土土压力理论具有重要意义。Abstract: To study the reduction mechanism of earth pressure on retaining wall for light weight soil, the model tests on a large-scale rigid retaining wall are conducted. The uniform loads are applied on the filling surface by heavy stacking. When the remolded loess and light weight soil are used as backfilling behind the wall, the distribution laws of earth pressure at rest are analyzed respectively. The results show that the earth pressure at rest of the light weight soil gradually increases with the increase of curing period, but the increase range gradually decreases. The earth pressures at rest of the remolded loess and the light weight soil increase approximately linearly with the increase of filling depth. Moreover, they gradually increase with the increase of the upper loads, and the increase range of earth pressure at rest for the remolded loess is obviously larger than that of the light weight soil. By comparing their earth pressures at rest, it is found that the light weight soil has an obvious pressure reduction effect, and the greater the uniform loads applied on the filling surface, the better the pressure reduction effects of the light weight soil. The coefficient of earth pressures at rest of the remolded loess and light weight soil is not constant. The range of coefficient of earth pressure at rest for the remolded loess is 0.34~0.78, and that of the light weight soil is 0.22~0.55. When the uniform loads are applied on the surface of the filling, the traditional earth pressure theory has a high applicability to calculate the earth pressure at rest of the remolded loess, but the calculation error of the light weight soil is larger. Based on the model tests and traditional earth pressure theory, a modified formula for the earth pressure at rest for the light weight soil is proposed. It is found that the relative error is mainly 1.01%~23.13%. The characteristics of earth pressure at rest of the light weight soil are revealed through the model tests and theoretical calculation, which is of great significance to improving the earth pressure theory of the light weight soil.
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0. 引言
随着中国交通行业的不断发展,中国桥梁建设水平得到大幅提升,对桥梁跨越能力的要求也不断增长,悬索桥作为所有桥型中跨越能力最大的桥型,越来越成为跨越大江、大河的主要解决方案。但是随着悬索桥跨度的不断增加,锚碇规模急剧扩大,造成锚碇建设成本过高。因此研究锚碇沉井基础的受力变形特性对于悬索桥的锚碇优化设计显得尤为重要。
Alampalli[1]在1994年研究了沉井在承受竖向和水平向荷载时的结构响应；李永盛[2]和李家平等[3]分别在1995年和2005年通过模型试验探讨了沉井基础的变形机制和破坏失稳形式；穆保岗等[4]在2017年通过模型试验研究了水平荷载长期作用下沉井变位的特性；Liu等[5]在2019年通过模型试验结合数值模拟分析研究了重力式锚碇的稳定性。
本文首先进行了在分级水平荷载下的沉井在砂箱中的模型试验,然后基于PLAXIS 3D软件建立了有限元模型,并分析了沉井的位移及沉井前侧和沉井底部的土压力,研究了水平荷载条件下沉井的受力变形规律。
1. 依托工程及模型试验
本文依托南京仙新路大桥北锚碇沉井工程,沉井长度为70 m,宽度为50 m,高度为49.5 m。该工程地基土以粉砂和中砂为主。
本试验采用的模型槽平面尺寸为4.0 m×2.0 m,高1.0 m。地基土采用中砂,其相对密度为2.68,最大孔隙比0.881,最小孔隙比0.463,不均匀系数3.89,曲率系数0.92。模型试验分层填筑地基土,控制每层填土的厚度为0.1 m,最终得到地基土的干密度为1.55 g/cm3,含水率0.63%,相对密实度为59.61%,内摩擦角为34.5°（快剪）。
沉井模型平面尺寸为0.7 m×0.5 m,高0.495 m,由厚度为22 mm的钢板焊接而成,试验过程将沉井看成刚体,不考虑沉井自身的变形,为模拟沉井与土体相互作用的界面,通过在沉井表面黏2～3 mm的砂粒实现[6],如图1所示。
模型试验中设计荷载为62kg,本文中水平荷载分级施加,每级荷载为设计荷载的～0.5倍,试验过程中每级荷载施加持续15 min直至土体破坏（土体破坏表现为沉井盖板处的位移急剧增大）,沉井加载示意图如图2所示。
2. 有限元分析
本研究建立的有限元模型完全基于模型试验,土体及沉井的单元形状均为四面体十节点实体单元,数值模型的网格如图3所示。
砂土的本构模型采用土体硬化（HS）模型,土层参数[7]取值见表1。
表 1 土层参数Table 1. Soil parameters土层 γ /(kN·m-3)e Es /MPaErefoed /MPaEref50 /MPaErefur /MPac′ φ′ /(°)ψ /(°)m 砂土 15.6 0.723 10.2 10.2 10.2 30.6 0 34.5 0 0.5 注： γ 为砂土的重度；e 为砂土的孔隙比；Es 为砂土的压缩模量；Erefoed 为砂土的主固结加载切线刚度；Eref50 为砂土的标准三轴排水试验割线刚度；Erefur 为砂土的卸载重加载刚度；c′ 为砂土的有效黏聚力；φ′ 为砂土的有效摩擦角；ψ 为砂土的膨胀角；m 为砂土的刚度应力水平相关幂值。3. 试验结果与有限元结果对比
3.1 沉井顶部位移
在沉井盖板顶部设置3个位移测量点A、B、C。在位移测量点上放置位移靶标,采用TH-ISM-ST机器视觉测量仪对靶标位移进行测量,分辨率为0.01 mm,靶标布置如图4。
模型试验和数值模拟的位移对比如图5所示,由图易知,模型试验和数值模拟的靶标位移较为一致,本文中取水平位移随设计荷载增加而不断增加的线弹性阶段为水平承载力极限值[4],即安全系数取值为4。
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图 5 模试验和数值模拟的位移对比Figure 5. Comparison of displacements between model tests and numerical simulations3.2 沉井前侧壁土压力随施加荷载变化
在沉井前侧设置8个土压力盒,布置如图6所示,由于土压力盒对称分布,且沉井左右侧完全对称,因此取沉井左右两侧土压力盒平均值作为最终结果,结果如图7所示,其中模型试验中3号及3＇号土压力盒数据较差,本文中已舍弃,余下的沉井前侧土压力盒数据和数值模拟结果较吻合。
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图 7 模试验和数值模拟的沉井前侧土压力对比Figure 7. Comparison of soil pressures on front side of caisson between model tests and numerical simulations3.3 沉井前侧壁土压力随施加荷载变化
在沉井底部设置12个土压力盒,布置如图8所示,同理,取沉井左右两侧土压力盒平均值作为最终结果,结果如图9所示,其中8号及8＇号土压力盒数据较差,本文中已舍弃,余下的沉井底部土压力盒数据和数值模拟结果对比,发现当施加荷载/设计荷载的值小于等于4时较一致,当其值大于4之后,二者的结果相差较大。
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图 9 模型试验和数值模拟的沉井前侧土压力对比Figure 9. Comparison of soil pressures on bottom of the caisson between model tests and numerical simulations4. 结论
本文在已有的研究基础上,通过开展模型试验和数值模拟计算,得到水平荷载下沉井的受力变位特性,主要得出以下结论：
（1）对锚碇沉井基础在砂土中的受力变位特性进行了试验研究和有限元分析,结果显示,水平荷载下锚碇沉井基础在砂土中的破坏模式为倾覆破坏,且安全系数远大于2,说明现阶段规范[8]中锚碇设计较为保守,有进一步的优化空间。
（2）通过PLAXIS 3D软件建立了锚碇沉井基础的有限元模型,采用应变硬化的本构模型,结果表明模型试验的结果和有限元模型计算的结果较为一致,说明数值建模过程中的土体本构模型及参数取值可靠,表明PLAXIS 3D软件能够较好的模拟锚碇沉井在砂土中的受力变形行为。
上述模型试验和有限元分析,只是针对水平荷载条件下锚碇沉井基础在砂土中的受力特性开展的研究,只考虑了单层干砂的地基土层,尚需更近一步探索。
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图 4 不同上部荷载时侧向土压力随填土深度变化规律
Figure 4. Change laws of lateral earth pressure with filling depth under different upper loads
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图 5 轻量土养护期间侧向土压力随时间变化规律
Figure 5. Change laws of lateral earth pressure with curing period of light weight soil
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图 6 不同上部荷载时侧向土压力系数随填土深度变化规律
Figure 6. Change laws of lateral earth pressure coefficient with filling depth under different upper loads
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图 8 重塑黄土静止土压力理论值与试验值对比
Figure 8. Comparison between theoretical and test values for earth pressure at rest of remolded loess
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图 9 轻量土静止土压力理论值与试验值对比
Figure 9. Comparison of theoretical and test values for earth pressure at rest of light weight soil
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图 10 轻量土静止土压力相对误差随上部荷载变化规律
Figure 10. Change laws of relative error for earth pressure at rest of light weight soil with upper load
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图 11 轻量土静止土压力修正公式计算值与试验值对比
Figure 11. Comparison of test values and calculated values by modified formula for earth pressure at rest of light weight soil
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图 12 轻量土静止土压力修正公式计算值与试验值对比（根据侯天顺等数据）
Figure 12. Comparison between test values and calculated values by modified formula for earth pressure at rest of light weight soil (according to data of Hou and Yang)
表 1 陕西杨凌地区黄土物理性质
Table 1 Physical parameters of soil
天然密度ρ/(g·cm-3) 相对质量密度Gs 天然含水率w/% 塑限wP/% 液限wL/% 塑性指数IP 液性指数IL 孔隙比e 1.75 2.72 19.83 20.8 33.9 13.1 -0.07 0.86 
下载: 导出CSV
表 2 模型试验方案
Table 2 Schemes for model tests
试验项目 水泥掺入比ac/% EPS颗粒掺入比ae/% EPS颗粒体积比be/% 含水率w/% 龄期T/d 压实度/ % 轻量土 10 0.41 30 31.0 28 90 重塑黄土 0 0 0 21.6 0 90
下载: 导出CSV
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