Stress-dependent properties of physicochemical interaction of unsaturated saline clay
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摘要: 为了揭示物理化学作用对含盐溶液非饱和黏土压缩行为的影响及其与应力水平的依赖特性，对孔隙含蒸馏水、氯化钠和硫酸钠溶液及控制基质吸力条件下的试样进行了一维压缩试验。然后，根据试验结果测定了不同条件下黏土的压缩指数、次固结系数和屈服应力，并标定了它们随基质吸力和渗透吸力的变化规律。此外，对不同基质吸力和渗透吸力下非饱和盐渍黏土主、次固结行为展开了深入分析，明确了非饱和盐渍黏土物理化学作用的应力依赖特征，并对非饱和盐渍黏土的LC屈服行为进行了探索。结果表明：不同物理化学力下非饱和盐渍黏土的次固结系数与压缩指数的比值Ca/Cc以及屈服应力可以使用基质吸力和渗透吸力来统一描述；由塑性加载区压缩曲线斜率以及特征参量、Ca/Cc与竖向应力的相关性指出非饱和盐渍黏土物理化学作用与应力水平息息相关；此外，化学-水力-力学耦合作用下非饱和盐渍黏土的LC屈服曲线是由MLC屈服曲线和OLC屈服曲线组成的一条光滑曲线。Abstract: To reveal the impact of physicochemical effects on the compressive behaviors of unsaturated clay containing salt solution and its dependence properties on stress level, one-dimensional compression tests are performed on the specimens with pores containing distilled water, sodium chloride solution, sodium sulfate solution and controlled matric suction conditions. Then, the compression index, secondary compression coefficient and yield stress of clay under different conditions are measured according to the test results, and their variation laws with the matric suction and osmotic suction are calibrated. Furthermore, the stress-dependent characteristics of physicochemical action are clarified through an in-depth analysis of the primary and secondary consolidation behaviors of unsaturated saline clay under different matric suctions and osmotic suctions, and the LC yielding behaviors of unsaturated saline clay are explored. The results show that the ratio of the secondary compression coefficient to the compression index Ca/Cc and the yield stress of unsaturated saline clay at different physicochemical forces can be described uniformly using the osmotic suction and matric suction. From the slope of compression curve in the plastic loading zone and the correlation between characteristic parameter, Ca/Cc and the vertical stress, it is noted that the physicochemical action of unsaturated saline clay is closely related to the stress level. Moreover, the LC yield curve of unsaturated saline clay is a smooth curve composed of MLC yield curve and OLC yield curve under chemo- hydro-mechanical coupling.
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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 1 非饱和盐渍黏土的一维压缩试验结果
Figure 1. One-dimensional compression test results of unsaturated saline clay
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图 2 非饱和盐渍黏土在压缩过程中体积应变的变化情况
Figure 2. Change in volume strain of unsaturated saline clay during compression process
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图 3 不同含水和含盐条件下试样的吸力
Figure 3. Suctions of samples under different water and salt contents
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图 4 非饱和盐渍黏土物理化学作用应力依赖性的示意图
Figure 4. Schematic of stress dependence of physicochemical interaction of unsaturated saline clay
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图 5 压缩指数随基质吸力和渗透吸力的变化规律
Figure 5. Variation of compression index with matric suction and osmotic suction
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图 6 非饱和盐渍黏土物理化学作用的应力依赖性分析
Figure 6. Analysis of stress dependence of physicochemical interaction of unsaturated saline clay
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图 7 不同应力水平下Ca/Cc与渗透吸力和基质吸力的关系
Figure 7. Relationship among Ca/Cc, osmotic suction and matric suction at different stress levels
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图 9 屈服应力随基质吸力和渗透吸力的变化规律
Figure 9. Variation of yield stress with matric suction and osmotic suction
表 1 试验用土中易溶盐的离子种类和含量
Table 1 Ion species and contents of soluble salts in experimental soil
阳离子/% 阴离子/% 总含盐量/% pH值 K+ Na+ Mg2+ Ga2+ CO2−3 HCO−3 SO2−4 Cl− 0.0113 0.4470 0.0481 0.0546 — 0.0235 0.3222 0.0912 1.0 7.47 
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表 2 不同基质吸力和渗透吸力下的次固结系数
Table 2 Secondary consolidation coefficients under different conditions of matric suction and osmotic suction
s = 0 kPa s = 50 kPa s = 100 kPa s = 200 kPa π = 457 kPa π = 457 kPa π = 5630 kPa π = 5902 kPa π = 457 kPa π = 457 kPa π = 5630 kPa π = 5902 kPa σ/kPa Ca×102 σ/kPa Ca×102 σ/kPa Ca×102 σ/kPa Ca×102 σ/kPa Ca×102 σ/kPa Ca×102 σ/kPa Ca×102 σ/kPa Ca×102 100 0.1165 100 0.1165 100 0.0764 97 0.0872 101 0.1156 97 0.1161 99 0.0792 97 0.1126 200 0.1685 200 0.1222 200 0.1650 201 0.1676 194 0.1160 194 0.1141 200 0.1451 201 0.1540 300 0.2186 300 0.1318 300 0.1555 291 0.1657 291 0.1220 291 0.1140 300 0.1669 291 0.1673 400 0.2184 401 0.1755 400 0.2141 388 0.2339 387 0.1625 388 0.1159 400 0.1256 387 0.1675 600 0.2672 601 0.1959 600 0.2644 581 0.2626 581 0.1739 581 0.1645 599 0.2098 581 0.2201 800 0.2911 802 0.2645 800 0.3076 775 0.3529 775 0.2040 775 0.1744 800 0.2511 774 0.2722 1000 0.3623 1003 0.3075 1001 0.3952 969 0.4083 969 0.2915 969 0.2510 1000 0.3028 968 0.3217
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