Using traditional materials to improve the permeability of silty soils can cause irreversible damage to the environment. Therefore, it is necessary to develop environmentally friendly biopolymers, such as xanthan gum (XG), to replace traditional materials to improve resistance to water erosion by reducing the permeability coefficient. In this study, a series of permeability tests and scanning electron microscope (SEM) tests were conducted on xanthan gum-improved silty soil (XGS). The variations in the permeability coefficient of XG-improved silty soil and the effects of initial dry density, XG-soil ratio, and curing age on the permeability were investigated. Test results show that the permeability coefficient of XGS decreased with the increase of initial dry density, XG-soil ratio, and curing age. With increasing the initial dry density, soil particles compressed against each other, decreasing the actual water flow crossing area, which leads to a decrease in the permeability coefficient. With the increase of the XG-soil ratio, the fill-blocking effect of xanthan gum with hydrogel connections becomes more and more obvious, which leads to a reduction in the permeability coefficient. Xanthan gum hydration takes time, and a lot of crystals are produced in XGS as the curing age increases; these crystals fill larger pores, resulting in the permeability coefficient decrease. At last, a model was developed to predict the permeability coefficient of XG-improved silty soil by using the initial dry density, XG-soil ratio, and curing age. The model can be used to rapidly predict the permeability coefficient of the improved silty soil under different conditions. This research can provide a scientific basis for the safe and scientific application of xanthan gum in seepage damage control and prevention projects. © The Author(s) 2024.

The silt deposited in the Yellow River floodplains in China is weak and widely dispersed. In engineering practice, chemically modified agents such as lime and fly ash are frequently employed to enhance the hydro-mechanical behaviour of soils, which however generates significant amounts of CO2. As an environmentally friendly soil improvement material, Xanthan gum has broad application prospects and is therefore being considered as a solidified material to improve silty soil, which meets the national strategy requirements of ecological protection and high-quality development in the Yellow River Basin. In this study, scanning electron microscope, WP4C apparatus, and an automatic direct shear test system were used to perform several microscopic, water-retention behaviour, and direct shear tests on Xanthan gum-treated silt in drying and wetting paths. According to test results, the differences in shear strength behavior were observed in drying and wetting paths. The average skeleton stress of untreated silt in the drying path is greater than that in the wetting path due to the hysteresis effect, and thus the shear strength in the drying path is higher than that in the wetting path. Xanthan gum-treated silt specimens in the wetting path first were cured at a relative humidity of 47% and temperature of 24°C for seven days, and then were wetted to different water contents. The microstructure of the Xanthan gum-treated silt in the wetting path became firmer, and thus the shear strength was higher than that in the drying path. There is also the hysteretic effect of Xanthan gum-treated silt. However, the curing age is the most important factor affecting this phenomenon. Our findings can serve as a scientific foundation for designing, constructing, and maintaining relevant geotechnical structures in the Yellow River flooded area. © 2024 Elsevier Ltd

Soil erosion in the Yellow River basin in China has seriously affected the stability of the foundations of civil engineering structures. Traditional treated materials used for preventing the soil erosion are often harmful to the environment. Biopolymers have been widely used for soil improvement because of their low cost, environmental friendliness, and feasible application. However, most studies on the water-retention behaviour of treated soils conducted to date have focused on the low or high suction range, while few have examined the behaviour over a wide suction range. Converting the saturated state of treated soils into unsaturated state until it is completely dry will lead to changes in the water retention behaviour of biopolymer-treated soils. Therefore, this study investigated variations in the water-retention behaviour and micromechanism of biopolymer-treated sandy soils over a wide range of suction. Specifically, the water-retention behaviour of treated soil was tested using a pressure plate apparatus in the low suction range and a WP4C apparatus in the high suction range. Scanning electron microscopy was also used to evaluate water-retention mechanism of soils treated with different ratios of biopolymer. Biopolymers were found to absorb water to form hydrogels and biofilms, which reduced the pore spaces between soil particles and improved the water storage ability of sandy soils. The water-retention capacity of both Xanthan gum and Gellan gum-treated sandy soils was higher than that of untreated sandy soils, and increased as the biopolymer ratio increased. The water-retention behaviour of the Gellan gum-treated sandy soil were better than that of the Xanthan gum-treated sandy soil. Finally, a prediction model of the soil–water retention curve that considered the biopolymer ratio was proposed based on the Fredlund-Xing model. This study provides a scientific basis for the application of biopolymers to control soil erosion in the Yellow River basin in China. © 2023 Elsevier Ltd

Adding different materials to soil can improve its engineering properties, but traditional materials such as cement, lime, fly ash, etc., have caused pollution to the environment. Recently, biopolymers have shown many advantages, such as economy and environmental protection, which make them applicable to geotechnical engineering. This study summarizes the effects of biopolymers on soil’s engineering properties and the main directions of current research. Firstly, the advantages and disadvantages of a variety of widely used biopolymer materials and their effects on the specific engineering characteristics of soil (i.e., water retention characteristics, strength characteristics, permeability characteristics, microstructure) are introduced, as well as the source, viscosity, pH, and cost of these biopolymers. Then, based on the theory of unsaturated soil, the current research progress on the water retention characteristics of improved soil is summarized. The key factors affecting the strength of biopolymer-treated soil are introduced. Due to the actual environmental conditions, such as rainfall, the permeability and durability of biopolymer-treated soil are also worthy of attention. In summary, it is necessary to study the variation laws of the engineering properties of biopolymer-treated soil in the full suction range, and to predict such laws reasonably. The relevant results are conducive to the safer and more scientific application of biopolymers in geotechnical engineering practice. © 2023 by the authors.

联合压力板仪和蒸汽平衡法在全吸力范围内对原状与重塑黄土进行持水特性试验，使用非饱和三轴仪对高吸力下的原状与重塑黄土进行剪切试验，同时采用扫描电镜仪和压汞仪进行微观试验，探讨了非饱和黄土结构性差异对其水力力学特性的影响。试验结果表明：随着吸力的增大，原状和重塑土的饱和度、含水率均减小，孔隙比稍有减小。由于原状及重塑黄土的初始孔隙比基本一致，因此两者压汞试验总进汞量接近。由扫描电镜和压汞试验得到的原状及重塑黄土的孔隙结构形态及优势孔径范围不一样，结构性有所差异，导致土-水特征曲线在不同吸力范围内呈现不一样规律。原状和重塑土的应力-应变关系多呈软化现象，吸力为3.29 MPa的重塑土呈硬化现象。且随着吸力的增大，原状和重塑土的黏聚力和峰值强度均明显增加，体变由剪缩趋向剪胀现象。由于原状土具备一定结构性，胶结作用较强，其黏聚力的增幅会大于重塑土，峰值强度也高于重塑土，而两者的内摩擦角基本一致。

Expansive soil is prone to shrinkage and cracking during the drying process, leading to strength and permeability problems that exist widely in water conservancy projects and geotechnical engineering, including foundation pits and cracks at the bottom of channels and slopes. Such problems are closely related to the tensile strength of the soil. In this study, Nanyang expansive soil is taken as the research object and radial splitting tests were performed using a particle image velocimetry (PIV) test system on both undisturbed and remolded expansive soil during the drying process. The results indicated that the load-displacement curve of the undisturbed and remolded expansive soil specimens showed a strain-softening phenomenon and that the peak load increased with decreasing water content. Under the same other conditions, the peak load of the remolded expansive soil specimen was higher than that of the undisturbed soil specimen, with the undisturbed soil specimen having distinctive structural and fractural features. The load-displacement relation curve, displacement vector field, and fracture characteristics had an obvious one-to-one correspondence in the stage division. The compression deformation stage, crack development stage after the peak value, crack maturity stage, and failure stage could be observed via the PIV technique. Moreover, the fracture characteristics of the remolded specimens were more regular than those of the undisturbed specimens. The above research results provide a scientific basis for the design and construction of geotechnical engineering related to expansive soil.

地震和车辆振动等荷载易对路基粉土的强度造成破坏，通常使用水泥和石灰等传统材料对粉土进行改良，但对环境造成了不可逆的损害，研究环境友好型生物聚合物-黄原胶改良粉土动强度特性及机理显得非常必要。通过改变黄原胶的掺量，采用扫描电镜和动三轴仪对黄原胶改良粉土进行一系列的微观分析和动强度试验。试验结果表明：动强度曲线符合幂函数关系，且随着黄原胶掺量的增加而上移。动黏聚力随着黄原胶掺量的增加而增大，强化效果在掺量较大时趋于平稳；而动内摩擦角受黄原胶掺量的影响较小。由微观分析可知，随着黄原胶掺量的增加，黄原胶有效填充了土颗粒间的孔隙并附着在土颗粒表面形成纤维状和网状结构，从而增强了土颗粒间的联结能力并导致了改良土的动强度和动黏聚力的提高。上述研究成果可为相关岩土工程的设计和施工提供科学依据，同时满足了黄河流域生态保护和高质量发展战略的需求。

用于高放废物深地质处置库的压实膨润土砌块因绑扎起吊，会受到拉应力的作用产生拉张裂隙进而影响其使用效果，因此系统研究膨润土的抗拉强度具有重要工程实际意义。基于粒子图像测速技术对压实膨润土进行了一系列的劈裂试验和直接拉伸试验，探求快速且准确获得膨润土抗拉强度的试验方法，同时基于PIV劈裂试验对膨润土抗拉强度公式进行了修正。试验结果表明：压实膨润土的抗拉强度随着含水率的增加呈先增加后减小的趋势，当含水率为13%时其抗拉强度最大。应力-位移关系曲线、位移矢量场和裂隙特征在阶段性划分中存在明显的一一对应关系。当压实膨润土的含水率和初始干密度均相同时，劈裂试验获得的抗拉强度大于直接拉伸强度。采用Frydman提出的修正系数2g和PIV劈裂试验获取的变形参数对劈裂抗拉强度进行修正，修正效果较好。同时在经典巴西劈裂公式基础上引入一个简化修正系数K,结合PIV劈裂试验结果也可快速、准确地获得压实膨润土的直接拉伸强度。

为研究不同初始干密度粉土的抗拉强度及变形特性,基于离子测速技术（Particle Image Velocimetry,PIV）,运用自行设计的单轴劈裂试验装置,对具有不同初始干密度条件下的粉土进行了一系列劈裂试验和裂隙发育演化规律观测试验。同时结合粉土的抗剪强度和扫描电镜所测的微观特征进行了深入的机理分析。结果表明:1)试样在第一次峰值前后仅发生压缩变形,峰值过后出现微裂缝,波谷前后随着裂缝的持续扩张,横向位移和竖向位移均显著增加,在第二次峰值前后,因位移过大,矢量场出现空白区域。2)粉土的黏聚力和抗拉强度均随着干密度的增加而增大,两者基本成线性关系;3)由SEM微观图片易得随着试样干密度的增加,土颗粒间距离减小,相互作用增强,宏观表现为土体强度增加。试验数据为解决豫东粉土路基开裂问题及工程安全生产提供依据。

膨润土作为缓冲或回填材料,在高放废物处置库的使用过程中需要具有高膨胀性、低渗透性、热稳定性、核素吸附性和长期稳定性等性质特征。膨润土具有湿胀干缩的典型特性,即膨润土在非饱和状态和饱和状态之间转变时会发生较大的体积胀缩变形,在此过程中会产生裂隙或裂缝,工程屏障受到破坏进而导致放射性核素的泄露。因此,本文主要研究了膨润土的胀缩性、持水特性和吸力控制条件下的裂隙演化及微观结构特征,具体研究内容如下:（1）利用固结仪对不同初始含水率和不同初始干密度（1.21g/cm3、1.36gcm3、1.43 g/cm3）条件下的膨润土进行—系列膨胀变形试验,试验结果表明膨润土的膨胀率和时间呈现出较好的对数关系,对数曲线均可分为两段,即迅速膨胀（或收缩）段和稳态段。膨润土的干密度相同时,其膨胀力随含水率的增大而增大;干密度与e-p曲线中的参数a、参数b均成线性关系。（2）分别运用滤纸法和饱和盐溶液蒸汽平衡法对膨润土进行了持水特性试验研究,得到不同吸力范围内膨润土的土-水特征曲线。滤纸法和饱和盐溶液蒸汽平衡法测得的土-水特征曲线均随吸力的增大而减小。根据滤纸法量测的试验数据,运用origin软件获得Fredlund and Xing（1994）提出的模型参数,通过建立模型参数与干密度之间的关系,给出了预测膨润土的土-水特征曲线公式。（3）采用不同干化方式对初始含水率为（35%,40%,45%）和初始干密度为（1.35g/cm3、1.25g/cm3、1.15g/cm3）条件下的膨润土进行了一系列的干化过程土体表面裂隙的观测试验,其中干化方式分别为烘箱烘干（50℃C）、烘箱烘干（100℃C）和饱和盐溶液蒸汽平衡法。运用Matlab数字图像处理技术对干化过程裂隙开展的规律进行定量的描述,试验结果表明:膨润土在不同干化条件下裂隙特征的总体特征是:膨润土的裂隙指标均随时间的具有先逐渐增大最终趋于稳定的规律。（4）联合用蒸汽平衡法和扫描电镜（SEM）对特定吸力点（367.54MPa,149.51MPa,71.12MPa,38.00MPa）的膨润土试样进行扫描电镜（Scanning Electron Microscope,SEM）试验研究。本文根据膨润土不同放大倍数条件下的SEM图像的最大孔径,将膨润土的孔径划分为5个阶段:分别为≥50μm、10～5μm、0.5～10μm、0.1～0.5μm和≤0.1μm;与之相对应的SEM图像的放大倍数分别为100x、1000x、5000x、10000x和20000x。将某种放大倍数条件下的表面孔隙率与膨润土的实际表面孔隙率的比值定义为权重Ri,利用Image Pro-Plus（IPP）软件对以上放大倍数的SEM图像进行处理得到孔隙参数,通过调整权重R1（0.20～0.41）、R2（0.22～0.45）、R3（0.31～0.32）得到膨润土在不同吸力时的SEM的表面孔隙率。试验结果表明不同吸力条件下膨润土的宏观孔隙率与SEM表面孔隙率呈定量关系。