Development of moisture content distribution of triaxial samples during drying process
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摘要: 三轴试验过程中试样含水率分布实时测试对吸力平衡判断、剪切速率确定和湿化变形等研究具有重要意义。通过扩展范德堡电阻率测试理论（vdP），提出了柱状土体含水率测试理论；在此基础上，研发一种厚度仅为0.1 mm的柔性印刷电极及配套的范德堡法集成测试装置。通过设计围压与温度敏感性因素试验，验证装置兼容性和可靠性。最后开展了恒压状态下的三轴干湿循环试验，并对试验过程中试样分层电阻率进行监测。试验结果表明，本套试验装置需要至少120 kPa静围压预压，以保证电极与试样完全贴合；同时试验全程静围压不低于20 kPa以保持电极紧密贴合。温度对电阻率测试精度具有一定的影响，Campbell模型可以有效修正温度带来的测试误差。浸水过程中，试样电阻率从下至上依次降低；排水过程中，试样电阻率由顶到底逐渐降低；最后提出了脱湿过程的试样分层含水率计算方法并且分析了含水率分布演化规律。Abstract: The real-time measurement of moisture content distribution in the triaxial tests is significant to suction balance, shear rate and wetting deformation. The principle of water content tests on triaxial samples is proposed by extending van der Pauw (vdP) resistivity theory. Moreover, the flexible printing electrode (FPE) with 0.1 mm in thickness and the vdP method device are developed. The compatibility and reliability of the device are verified through the sensitivity factor tests such as net confining pressure and temperature. Finally, the triaxial drying-wetting cycle tests under constant pressure are carried out. At the same time, the layered resistivity is monitored. The test results show that the test device needs the preloading of net confining pressure of at least 120 kPa to ensure the complete cover between the FPE and the samples, and the net confining pressure shall be more than 20 kPa to ensure the cover status in the tests. The temperature has a certain influence on the results of resistivity tests, and the Campbell model can effectively correct the temperature-introduced errors. In the process of moisture wetting, the resistivity decreasing from bottom to top can be observed dramatically. In the process of drying, the resistivity decreases gradually from top to bottom. Finally, the method for calculating the layered moisture content during drying process is proposed, and the evolution law of moisture content distribution is analyzed.
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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 9 体变、净围压与电阻率随时间变化曲线
Figure 9. Curves of resistivity, volumetric change and net confining pressure with time
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图 12 电阻率与修正后温度拟合
Figure 12. Fitting results between resistivity and correction temperature
表 1 土的基本物理指标
Table 1 Basic physical properties of soil
土颗粒相对质量密度 液限/% 塑限/% 最优含水率/% 最大干密度/(g⋅cm - 3) 2.71 24.3 15.8 12.4 1.78 
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[1] 叶云雪, 邹维列, 韩仲, 等. 非饱和土孔隙比与基质吸力关系的通用模型[J]. 岩土工程学报, 2019, 41(5): 927-933. https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201905020.htm YE Yun-xue, ZOU Wei-lie, HAN Zhong, et al. General model for relationship between void ratio and matric suction in unsaturated soils[J]. Chinese Journal of Geotechnical Engineering, 2019, 41(5): 927-933. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201905020.htm
[2] 刘文化, 杨庆, 唐小微, 等. 干湿循环条件下粉质黏土在循环荷载作用下的动力特性试验研究[J]. 水利学报, 2015, 46(4): 425-432. https://www.cnki.com.cn/Article/CJFDTOTAL-SLXB201504007.htm LIU Wen-hua, YANG Qing, TANG Xiao-wei, et al. Experimental study on the dynamic characteristics of silt clay subjected to drying-wetting cycles under cyclic loading[J]. Journal of Hydraulic Engineering, 2015, 46(4): 425-432. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-SLXB201504007.htm
[3] CAO Z G, ZHANG Q, CAI Y Q, et al. The effects of suction history on the cyclic behavior of unsaturated road base filling materials[J]. Engineering Geology, 2020, 276: 105775. doi: 10.1016/j.enggeo.2020.105775
[4] 张俊然, 孙德安, 姜彤. 吸力历史对非饱和弱膨胀土力学性质的影响[J]. 岩土力学, 2015, 36(11): 3094-3100, 3120. https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX201511008.htm ZHANG Jun-ran, SUN De-an, JIANG Tong. Effect of suction history on mechanical behavior of unsaturated weakly-expansive soils[J]. Rock and Soil Mechanics, 2015, 36(11): 3094-3100, 3120. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX201511008.htm
[5] 张俊然, 孙德安, 姜彤, 等. 宽广吸力范围内弱膨胀土的抗剪强度及其预测[J]. 岩土工程学报, 2016, 38(6): 1064-1070. https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201606013.htm ZHANG Jun-ran, SUN De-an, JIANG Tong, et al. Shear strength of weakly expansive soils and its prediction in a wide range of suction[J]. Chinese Journal of Geotechnical Engineering, 2016, 38(6): 1064-1070. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201606013.htm
[6] 郭楠, 陈正汉, 高登辉, 等. 加卸载条件下吸力对黄土变形特性影响的试验研究[J]. 岩土工程学报, 2017, 39(4): 735-742. https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201704025.htm GUO Nan, CHEN Zheng-han, GAO Deng-hui, et al. Experimental research on influences of suction on deformation characteristics of loess under unloading-reloading conditions[J]. Chinese Journal of Geotechnical Engineering, 2017, 39(4): 735-742. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201704025.htm
[7] 高登辉, 陈正汉, 郭楠, 等. 干密度和基质吸力对重塑非饱和黄土变形与强度特性的影响[J]. 岩石力学与工程学报, 2017, 36(3): 736-744. https://www.cnki.com.cn/Article/CJFDTOTAL-YSLX201703023.htm GAO Deng-hui, CHEN Zheng-han, GUO Nan, et al. The influence of dry density and matric suction on the deformation and the strength characteristics of the remolded unsaturated loess soils[J]. Chinese Journal of Rock Mechanics and Engineering, 2017, 36(3): 736-744. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YSLX201703023.htm
[8] 王欢, 陈群, 王红鑫, 等. 不同压实度和基质吸力的粉煤灰三轴试验研究[J]. 岩土力学, 2019, 40(增刊1): 224-230. https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX2019S1034.htm WANG Huan, CHEN Qun, WANG Hong-xin, et al. Triaxial tests on fly ash with different compaction and matric suction[J]. Rock and Soil Mechanics, 2019, 40(S1): 224-230. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX2019S1034.htm
[9] 高帅, 骆亚生, 胡海军, 等. 非饱和原状黄土增湿条件下力学特性试验研究[J]. 岩土工程学报, 2015, 37(7): 1313-1318. https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201507021.htm GAO Shuai, LUO Ya-sheng, HU Hai-jun, et al. Triaxial tests on water immersion of unsaturated and undisturbed loess[J]. Chinese Journal of Geotechnical Engineering, 2015, 37(7): 1313-1318. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201507021.htm
[10] 常建梅, 尹荣玉, 王志鹏, 等. 考虑湿化的重载铁路粉黏土基床土体骨干曲线研究[J]. 铁道学报, 2020, 42(7): 141-147. https://www.cnki.com.cn/Article/CJFDTOTAL-TDXB202007020.htm CHANG Jian-mei, YIN Rong-yu, WANG Zhi-peng, et al. Study on backbone curve of silty clay in heavy-haul railway subgrade considering wetting[J]. Journal of the China Railway Society, 2020, 42(7): 141-147. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-TDXB202007020.htm
[11] 冯怀平, 宋慧来, 常建梅, 等. 浸水作用下重载铁路基床动回弹模量衰减规律[J]. 中国铁道科学, 2020, 41(3): 21-30. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGTK202003003.htm FENG Huai-ping, SONG Hui-lai, CHANG Jian-mei, et al. Attenuation law of dynamic resilient modulus of subgrade bed for heavy haul railway under water immersion[J]. China Railway Science, 2020, 41(3): 21-30. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGTK202003003.htm
[12] 郭楠, 陈正汉, 杨校辉, 等. 重塑黄土的湿化变形规律及细观结构演化特性[J]. 西南交通大学学报, 2019, 54(1): 73-81, 90. https://www.cnki.com.cn/Article/CJFDTOTAL-XNJT201901010.htm GUO Nan, CHEN Zheng-han, YANG Xiao-hui, et al. Research on wetting-deformation regularity and microstructure evolution characteristics of remoulded loess in triaxial soaking tests[J]. Journal of Southwest Jiaotong University, 2019, 54(1): 73-81, 90. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-XNJT201901010.htm
[13] 刘奉银, 谢定义, 俞茂宏. 一种新型非饱和土γ射线土工三轴仪[J]. 岩土工程学报, 2003, 25(5): 548-551. doi: 10.3321/j.issn:1000-4548.2003.05.006 LIU Feng-yin, XIE Ding-yi, YU Mao-hong. A new type of triaxial apparatus with γ rays for unsaturated soils test[J]. Chinese Journal of Geotechnical Engineering, 2003, 25(5): 548-551. (in Chinese) doi: 10.3321/j.issn:1000-4548.2003.05.006
[14] SU W, CUI Y J, QIN P J, et al. Application of instantaneous profile method to determine the hydraulic conductivity of unsaturated natural stiff clay[J]. Engineering Geology, 2018, 243: 111-117. doi: 10.1016/j.enggeo.2018.06.012
[15] MUÑOZ-CASTELBLANCO J A, DELAGE P, PEREIRA J M, et al. On-sample water content measurement for a complete local monitoring in triaxial testing of unsaturated soils[J]. Géotechnique, 2012, 62(7): 595-604. doi: 10.1680/geot.10.P.129
[16] CHEN Y L, WEI Z A, IRFAN M, et al. Laboratory investigation of the relationship between electrical resistivity and geotechnical properties of phosphate tailings[J]. Measurement, 2018, 126: 289-298. doi: 10.1016/j.measurement.2018.05.095
[17] 方祥位, 申春妮, 陈正汉, 等. 原状Q2黄土CT-三轴浸水试验研究[J]. 土木工程学报, 2011, 44(10): 98-106. https://www.cnki.com.cn/Article/CJFDTOTAL-TMGC201110017.htm FANG Xiang-wei, SHEN Chun-ni, CHEN Zheng-han, et al. Triaxial wetting tests of intact Q2 loess by computed tomography[J]. China Civil Engineering Journal, 2011, 44(10): 98-106. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-TMGC201110017.htm
[18] 冯怀平, 马德良, 王志鹏, 等. 基于范德堡法的非饱和土电阻率测试方法[J]. 岩土工程学报, 2017, 39(4): 690-696. https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201704017.htm FENG Huai-ping, MA De-liang, WANG Zhi-peng, et al. Measurement of resistivity of unsaturated soils using van der Pauw method[J]. Chinese Journal of Geotechnical Engineering, 2017, 39(4): 690-696. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTGC201704017.htm
[19] 杨志浩, 岳祖润, 冯怀平. 非饱和粉土路基内水分迁移规律试验研究[J]. 岩土力学, 2020, 41(7): 2241-2251. https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX202007011.htm YANG Zhi-hao, YUE Zu-run, FENG Huai-ping. Experimental study on moisture migration properties in unsaturated silty subgrade[J]. Rock and Soil Mechanics, 2020, 41(7): 2241-2251. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX202007011.htm
[20] 王志鹏. 土体电阻率测试及其在动力湿化试验中的应用[D]. 石家庄: 石家庄铁道大学, 2016. WANG Zhi-peng. Soil Resistivity Testing and Its Application in Dynamic Slaking Test[D]. Shijiazhuang: Shijiazhuang Tiedao University, 2016. (in Chinese)
[21] 吴忠学. 考虑厚度效应的vdP法土体电阻率测试研究[D]. 石家庄: 石家庄铁道大学, 2019. WU Zhong-xue. A Study on the Measurement of Soil Resistivity by vdP Method Considering Effect of Sample Thickness[D]. Shijiazhuang: Shijiazhuang Tiedao University, 2019.
[22] CHU Y, LIU S Y, CAI G J, et al. Artificial neural network prediction models of heavy metal polluted soil resistivity[J]. European Journal of Environmental and Civil Engineering, 2021, 25(9): 1570-1590.
[23] ZHANG D W, CAO Z G, FAN L B, et al. Evaluation of the influence of salt concentration on cement stabilized clay by electrical resistivity measurement method[J]. Engineering Geology, 2014, 170: 80-88. https://www.sciencedirect.com/science/article/pii/S0013795213003578
[24] 韩立华, 刘松玉, 杜延军. 温度对污染土电阻率影响的试验研究[J]. 岩土力学, 2007, 28(6): 1151-1155. https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX200706015.htm HAN Li-hua, LIU Song-yu, DU Yan-jun. Experiment study on effect of temperature on electrical resistivity of contaminated soils[J]. Rock and Soil Mechanics, 2007, 28(6): 1151-1155. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-YTLX200706015.htm
[25] 周蜜, 王建国, 范璇, 等. 珠三角地区的土壤电阻率温度修正模型[J]. 高电压技术, 2012, 38(3): 623-631. https://www.cnki.com.cn/Article/CJFDTOTAL-GDYJ201203017.htm ZHOU Mi, WANG Jian-guo, FAN Xuan, et al. Temperature correction models for soil electrical resistivity in Pearl River Delta[J]. High Voltage Engineering, 2012, 38(3): 623-631. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-GDYJ201203017.htm
[26] CAMPBELL R B, BOWER C A, RICHARDS L A. Change of electrical conductivity with temperature and the relation of osmotic pressure to electrical conductivity and ion concentration for soil extracts[J]. Soil Science Society of America Journal, 1949, 13(C): 66-69.
[27] 张世斌, 朱才辉, 袁继国. 降雨条件下重塑黄土中水分迁移模型试验研究[J]. 水利学报, 2019, 50(5): 621-630. https://www.cnki.com.cn/Article/CJFDTOTAL-SLXB201905010.htm ZHANG Shi-bin, ZHU Cai-hui, YUAN Ji-guo. Laboratory model tests on moisture migration in remolded loess under rainfall conditions[J]. Journal of Hydraulic Engineering, 2019, 50(5): 621-630. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-SLXB201905010.htm
[28] DE MELO L B B, SILVA B M, PEIXOTO D S, et al. Effect of compaction on the relationship between electrical resistivity and soil water content in Oxisol[J]. Soil and Tillage Research, 2021, 208: 104876.
[29] LI H, LI T L, JIANG R J, et al. A new method to simultaneously measure the soil-water characteristic curve and hydraulic conductivity function using filter paper[J]. Geotechnical Testing Journal, 2020, 43(6): 20190162.
[30] LEUNG A K, COO J L, NG C W W, et al. New transient method for determining soil hydraulic conductivity function[J]. Canadian Geotechnical Journal, 2016, 53(8): 1332-1345.
[31] 许健, 任畅, 高靖寓, 等. 干湿循环效应下Na2SO4盐渍原状黄土渗透特性与细观机制[J]. 中南大学学报(自然科学版), 2021, 52(5): 1644-1654. https://www.cnki.com.cn/Article/CJFDTOTAL-ZNGD202105025.htm XU Jian, REN Chang, GAO Jing-yu, et al. Influence of dry-wet cycles on hydraulic conductivity and microstructure of saline undisturbed loess with sodium sulfate[J]. Journal of Central South University (Science and Technology), 2021, 52(5): 1644-1654. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZNGD202105025.htm
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