Experimental study on erosion resistance and ecological slope protection of guar gum-treated fiber-reinforcement loess
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摘要: 为了改善纤维黄土的水力特征，维持黄土边坡坡面的长期稳定，促进植物护坡潜能的充分发挥，提出采用瓜尔豆胶对纤维黄土进行固化处理。通过开展直剪试验、崩解试验、渗透试验、土水特征试验以及生态护坡试验，对比分析不同方式处理下黄土的抗侵蚀特性和土水保持能力。试验结果表明：瓜尔豆胶的加入可以有效提升纤维黄土的抗侵蚀特性和水土保持能力，随着瓜尔豆胶掺量增加或养护龄期增长，抗剪强度先增加后减少或稳定，而崩解率和饱和渗透系数均先减小后稳定，最优掺量和最佳龄期分别为1.0%和14 d；最优处理的瓜尔豆胶固化纤维黄土具有较好的持水能力，同时在自然大气降雨条件下表现出优良的生态护坡效果；瓜尔豆胶的固化处理既能促进纤维加筋作用的发挥，又能填充孔隙，黏结黄土颗粒，综合提升纤维黄土的力学及水力性能，可以在黄土高原的工程安全建设和水土保持过程中发挥积极作用。Abstract: To improve the hydraulic characteristics of the fiber-reinforced loess, maintain the long-term stability of the loess slope, and promote the full play of the vegetation protection, the guar gum is used to cure the fiber-reinforced loess. The erosion resistance and the soil-water conservation capability of loess under different treatment methods are compared and analyzed by carrying out the direct shear tests, disintegration tests, penetration tests, soil-water characteristic tests and ecological slope protection tests. The results show that the addition of guar gum can significantly improve the erosion resistance and the soil-water conservation capability of fiber-reinforced loess. With the increase of guar gum content or curing age, the shear strength increases first and then decreases or stabilizes, and both the disintegration rate and the saturation permeability coefficient decrease first and then stabilize. The optimal guar gum content and curing age are 1.0% and 14 d. The optimally processed guar gum-treated fiber-reinforcement loess has better water retention capability, showing the excellent ecological slope protection effect under natural atmospheric rainfall conditions. The curing treatment of guar gum can promote fiber reinforcement, fill pores, bond loess particles, comprehensively improve the mechanical and hydraulic properties of the fiber-reinforced loess, and play a positive role in the process of engineering safety construction and soil-water conservation on the Loess Plateau.
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0. 引言
桩承式路堤能有效控制沉降，在软土地区路基工程中得到广泛应用。当路堤变形时，桩和土间的差异沉降导致路堤应力重分布从而发生土拱效应[1]。土拱效应发生过程中，桩间土承担的土压力朝桩传递，因而有必要针对桩土应力传递规律开展研究。
模型试验及数值模拟被用于桩承式路堤土拱效应及桩土分担变化规律的研究。芮瑞等[2]通过模拟桩间土下降导致的土拱效应，提出适用于砂填料桩承式路堤三角拱计算模型。基于大量试验研究，Van Eekelen等[3]指出桩间软土固结导致桩土应力比和土拱效应增加，且粗填料内摩擦角越大土拱效应越显著。庄妍等[4]通过数值模拟研究了桩承式路堤中的土拱效应，并获得了完整的土拱。吕玺琳等[5]建立了桩承式高铁路基数值分析模型，计算结果与试验结果[6]符合较好，并获得了桩长对路基沉降、桩土应力比的影响规律。马一跃等[7]对土拱效应进行离散元模拟，获得了土拱形成过程荷载传递规律。
为分析桩土荷载分担与桩土差异沉降的关系，通过自主设计的物理模型试验装置，采用电机控制活动板位移模拟桩土差异沉降，研究了桩土荷载分担变化规律及填土高度的影响特性。
1. 桩承式路基土拱效应物理模型试验
1.1 试验材料
试验材料取自宁杭高铁德清站的路基填料。考虑模型试验缩尺效应，剔除直径大于6 cm碎石后进行筛分，得到填料级配曲线如图 1所示，位于高速铁路设计规范（2014）上下限之间。填料平均粒径d50=7.1 mm，d30=2.7 mm，有效粒径d10=0.35 mm，不均匀系数Cu=13.77，曲率系数Cc=1.35。
1.2 模型试验装置
设计的物理模型试验装置如图 2所示。模型箱尺寸为100 cm×100 cm×140 cm（长×宽×高），上部由钢制框架和透明玻璃板组成，底部为模型箱底板和加载控制电机。桩帽板由四块三角形钢板和一块正方形钢板组成，底部固定不动；桩间土支承板由四块活动连接板和四块三角形钢板组成，底部与动力螺纹杆和电机连接，三角形钢板和活动连接板通过角钢连接。试验过程中，控制桩间土支承板以恒定速率下降，从而模拟桩土差异沉降。为减小边界摩擦，在制作试样模型时，边界上涂抹润滑材料。
1.3 测点布置和量测
在桩间土和桩帽不同位置布置薄膜压力传感器，具体如图 3所示。传感器P1~P5距模型边缘为24，36，48，60，72 cm。P1~P3用于测量不同水平位置处桩间土的竖向土压力，P4和P5用于测量不同水平位置桩顶竖向土压力。通过测得的桩顶和桩间土的竖向土压力值，可获得桩土应力比值。
1.4 试验步骤
为研究不同路堤高度条件下桩和桩间土竖向应力随桩土差异沉降的变化规律，设置桩净间距为0.35 m，开展路堤填土高度为0.6，0.8，1.0，1.2 m的4组模型试验。具体试验步骤如下：
（1）对模型箱进行清理和组装，通过电机控制桩间土板上升至与桩帽板齐平，避免在试验前桩间土上方路堤与桩帽上方路堤出现差异沉降，按图 3在桩帽板和桩间土板对应位置粘贴薄膜压力传感器。
（2）为避免大颗粒与薄膜压力传感器接触面不同造成的测试误差，在模型箱底部均匀铺设一层薄的细粒土，保证传感器与路堤土充分接触。每次将等量填料填入模型箱后，采用电机振动密实，直至填料厚度压缩至10 cm，保证每组试验填料密实度基本相同。
（3）试验过程中采用分级施加位移方式进行位移控制加载，控制电机使桩间土板以一定速率匀速下降，桩间土板下降位移即为桩土差异沉降，每级位移后稳定读取薄膜压力传感器数值。
（4）为分析路堤填土高度对桩土荷载分担的影响，重复上述步骤进行4组物理模型试验。
2. 试验结果分析
2.1 竖向土压力分布及变化规律
通过测量试验过程中不同位置处桩间土板和桩帽板的竖向土压力，得到桩顶和桩间土竖向土压力随桩土差异沉降变化如图 4所示。图中传感器P1、P3测定靠近桩间土边缘处的桩间竖向土压力，P2测定桩间土中心处的桩间竖向土压力，P4、P5测定靠近桩帽边缘处和桩帽中心处桩顶的竖向土压力。
从图 4可知，随着桩间土活动沉降板下降，桩土差异沉降产生，不同填土高度下桩顶、桩间土竖向土压力变化曲线均表现为两个阶段。在第一阶段，桩顶竖向土压力随桩土差异沉降增大先急剧增大，当桩土差异沉降达到临界值后，桩顶竖向土压力达到最大值σmax。此阶段由于桩土差异沉降逐渐增大，从而在路堤内形成土拱，导致桩顶竖向土压力逐渐增大到最大值。而桩间竖向土压力随着桩土差异沉降增大而急剧减小，当达到极限状态时，桩间土竖向压力减小到最小值σmin，此阶段与桩顶竖向土压力变化的第一阶段同步产生。第二阶段，随着差异沉降继续增大，桩顶竖向土压力减小然后趋于一个稳定值。这是由于路堤内已形成完整土拱，土拱高度不再随桩土差异沉降增大而增大，故桩顶竖向土压力趋于稳定。随着桩土差异沉降继续增大，桩间土竖向土压力达到最小值后趋于稳定，此阶段与桩顶竖向土压力变化的第二阶段同步产生。
当桩顶和桩间土顶部的土压力达到稳定状态后，得出路堤底面不同水平位置处的竖向土压力稳定值的变化规律，结果如图 5所示。
2.2 桩土应力比
桩顶竖向土压力与桩间竖向土压力的比值（n=σp/σs）是衡量土拱效应发挥程度的重要指标。桩土应力比越大，反映出路堤中土拱效应发挥程度越高，反之发挥程度越低。桩土应力比随桩土差异沉降的变化如图 6所示，桩土压力最大、最小值与不同填土高度的关系如图 7所示。从图 6可知，桩土应力比随桩土差异沉降变化可分为两个阶段。第一阶段，桩土应力比随着差异沉降迅速增大，两者近似呈线性变化，当土拱效应达到极限时，桩竖向土应力比达到最大值。第二阶段，随着桩土差异沉降继续增大，桩土应力比先减小后稳定。桩顶和桩间土顶部竖向土压力极值随路堤填土高度变化如图 7所示。桩顶竖向土压力最大值随路堤高度呈二次抛物线增大，而桩间土顶部竖向土压力最小值则随路堤填土高度近似呈线性增大。基于试验结果可知，可通过增加路堤高度使桩承担更多的上部荷载且桩间土承担较少上部荷载，从而有效减小路堤沉降。
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图 7 竖向土压力极值随路堤填土高度变化Figure 7. Variation of maximum or minimum stress with embankment height3. 结论
通过开展桩承式路堤土拱效应物理模型试验，研究了桩承式路堤土拱的形成过程，并分析了路堤填土高度的影响，得出主要结论如下：
（1）桩土间的差异沉降导致桩承式路堤桩间土上方路堤自重传递到桩顶，使得桩顶竖向土压力增大，桩间竖向土压力减小，从而使得刚度更大的桩承担了更多上部荷载。
（2）桩土应力随桩土差异沉降的变化可分为两个阶段，第一阶段表现为桩顶竖向土压力迅速增大，桩间竖向土压力迅速减小；第二阶段桩顶和桩间土竖向压力基本趋于稳定值。随着桩土差异沉降增大，桩土应力比先迅速增大，之后达到峰值然后减小，最终趋于稳定值。
（3）路堤填土高度越大，桩土应力比的稳定值越大，即土拱效应发挥程度越高。桩顶竖向土压力和桩间土竖向土压力在水平位置上呈不均匀分布，靠近桩帽中心和桩间土中心处的竖向土压力较大，而靠近桩帽边缘和桩间土边缘处竖向土压力较小。
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图 5 纤维黄土与胶-筋固化黄土的崩解过程
Figure 5. Disintegration process of fiber-reinforced loess and guar gum-treated fiber-reinforcement loess
表 1 黄土的基本物理参数
Table 1 Basic physical parameters of loess
天然含水率/ % 密度/(g·cm-3) 液限/% 塑限/% 最佳含水率/% 最大干密度/(g·cm-3) 12.0 1.38 25.2 17.5 14.0 1.60 
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表 2 聚丙烯纤维的基本物理力学参数
Table 2 Basic physical and mechanical parameters of polypropylene fiber
密度/(g·cm-3) 直径/mm 抗拉强度/MPa 弹性模量/MPa 拉伸极限/% 熔点/℃ 燃点/℃ 0.91 0.048 > 358 > 3500 17 > 165 > 590
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表 4 不同试样的VG模型参数
Table 4 VG model parameters of different samples
试样 VG模型参数 R2 θs/% θr/% α/kPa-1 n S1 38.38 12.57 0.013 2.4141 0.9837 S3 40.52 13.60 0.008 2.5671 0.9974
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