To develop a new modification method with environmentally friendly and seismically resistant characteristics for saturated loess foundations, lignin extracted from paper industrial wastes was selected for mixing into the loess. Mixtures with different lignin contents were prepared under static pressure. A series of dynamic triaxial tests were performed by applying sine loads after the lignin-modified loess specimens were saturated and consolidated on the dynamic triaxial apparatus. The microstructural images and the mineral components of the modified loess with different lignin contents were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Based on the test results, the variation regularity of the dynamic residual deformation and the dynamic pore water pressure of the lignin-modified loess was analyzed. The microstructural characteristic parameters of the lignin-modified loess were extracted, and the relationship between these parameters and the lignin contents was obtained. Based on the dynamic triaxial test, the SEM, and the XRD results, the mechanical mechanism of the liquefaction resistance of the lignin-modified loess is discussed. The results show that lignin can significantly improve the resistance of cyclic shear deformation of the saturated loess and can effectively control the increase of the pore water pressure. When the lignin content was 4%, the liquefaction resistance of the lignin-modified loess was the most significant. Compared with the unsaturated compacted loess, the number of small and micropores in the lignin-modified loess increased significantly, and the apparent pore ratio decreased. Additionally, the arrangement sequence of pores deteriorated, the pore structure became more complex, and the pores were more compactly arranged. However, when the lignin content was higher than 4%, the microstructural characteristic parameters reflected a reduction in compactness of the lignin-modified loess. No other new minerals were formed in the lignin-modified loess except for a small amount of albite (${\mathrm{NaAlSi}}_3{\mathrm{O}}_8$ ), which was generated by the ion exchange between the lignin and loess. Furthermore, the liquefaction resistance mechanism of the lignin-modified saturated loess mainly included the filling and cementation of the lignin, thinning of the electric double layer in the modified loess, reinforcement of the lignin fiber, adsorption of fine particles and water by the fiber, and ion exchange between the lignin and loess.

The dynamic stress–strain relationship of a saturated loess modified using different fly ash contents is studied based on dynamic triaxial tests. The relationship between the dynamic behaviour and fly ash content is determined. Combined with SEM test results, the seismic-resistant mechanism of the fly ash-modified loess is analysed. The results show that the dynamic constitutive relationship of saturated fly ash-modified loess obeys the hyperbolic model. The model parameter a is minimum when the fly ash content is 20%. However, with the increase in the fly ash content, the model parameter b decreases, and the dynamic modulus of the modified loess is increased, whereas the damping ratio is reduced. When the weight proportion of fly ash is approximately 25%, the increase in the dynamic modulus and the decrease in the damping ratio with the increase in the dynamic strain tend to be slow. With the increase in the fly ash content, the flocculent cement content and the numbers of fine and micro pores in the modified loess are obviously increased. The improved liquefaction resistance of the loess is largely attributed to the increase in the inter-particle strength and the significant decrease in the number of large pores.

In the seasonal permafrost region with loess distribution, the influence of freeze-thaw cycles on the engineering performance of reinforced loess must be paid attention to. Many studies have shown that the use of fiber materials can improve the engineering performance of soil and its ability to resist freeze-thaw cycles. At the same time, as eco-environmental protection has become the focus, which has been paid more and more attention to, it has become a trend to find new environmentally friendly improved materials that can replace traditional chemical additives. The purpose of this paper uses new environmental-friendly improved materials to reinforce the engineering performance of loess, improve the ability of loess to resist freeze-thaw cycles, and reduce the negative impact on the ecological environment. To reinforce the engineering performance of loess and improve its ability to resist freeze-thaw cycles, lignin fiber is used as a reinforcing material. Through a series of laboratory tests, the unconfined compressive strength (UCS) of lignin fiber-reinforced loess under different freeze-thaw cycles was studied. The effects of lignin fiber content and freeze-thaw cycles on the strength and deformation modulus of loess were analyzed. Combined with the microstructure features, the change mechanism of lignin fiber-reinforced loess strength under freeze-thaw cycles was discussed. The results show that lignin fiber can improve the UCS of loess under freeze-thaw cycles, but the strengthening effect no longer increases with the increase of fiber content. When the fiber content is less than 1%, the UCS growth rate of loess is the fastest under freeze-thaw cycles. And the UCS of loess with 1% fiber content is the most stable under freeze-thaw cycles. The freeze-thaw cycles increase the deformation modulus of loess with 1% fiber content, and its ability to resist deformation is obviously better than loess with 1.5%, 2% and 3% fiber content. The fiber content over 1% will weaken the strengthening effect of lignin fiber-reinforced loess, and the optimum fiber content of lignin fiber-reinforced loess under freeze-thaw cycles is 1%.

In China, engineers have worked to create additional usable land for building construction by flattening the ridges of hills and filling in the adjacent valleys. China’s Loess Plateau comprises a type of soil (loess) with a large pore structure that can collapse and become unstable when exposed to groundwater. Conventional valley fill materials include remolded loess or remolded loess treated with cement, lime, gypsum, or other stabilizing additives. These stabilizers are often detrimental to the surrounding environment. Moreover, loess treated with conventional stabilizers exhibits excessive brittleness, which is not suitable for building foundations. Adequate stability of the building foundations in the filled valleys is required to ensure public safety. In this study, we tested 50 remolded loess samples treated with a lignin polymer compound to determine its potential as a valley fill material. Triaxial tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to study the mechanical characteristics of each sample, determine the effects of the lignin treatment on the loess, and identify the microscopic mechanism affecting shear stress in the lignin-treated loess. The corresponding development of excess pore pressure and volumetric responses under monotonic triaxial testing were also considered. Based on this study’s results, the optimum lignin content in the treated loess samples was 4%; lignin contents exceeding 4% decreased axial stress and increased dilation after saturation. The shear strength and strain-hardening phenomenon of the lignin-treated loess samples increased as the lignin content increased, while the excess pore water pressure decreased. Microscopically, the addition of lignin increased cohesion in the loess samples, while slightly contributing to the internal friction angle. The use of lignin as a stabilizing additive for valley fill material shows potential for controlling building foundation deformation by increasing soil strength and minimizing environmental impacts by maintaining the soil pH and limiting pollutant production.

Freeze-thaw cycles caused by climate change can change the structure and strength of the soil. In seasonally frozen soil areas, the use of improved loess as a filling material must consider the effects of freeze-thaw cycles. With the increasingly severe global environmental problems, the search for suitable new environmental protection improvement materials has become one of the hotspots in soil performance improvement research. The purpose of this paper is to use lignin fiber to improve the engineering performance and freeze-thaw resistance of loess and to reduce the negative impact of engineering construction on the environment of the loess area. Based on a series of triaxial shear tests, the effects of freeze-thaw cycles on the stress-strain relationship, shear strength, and Mohr-Coulomb’s strength parameters of loess reinforced with lignin fiber were analyzed. Combined with the volume change and microstructure characteristics of fiber-reinforced loess before and after the freeze-thaw cycles, the reasons for the effects of the freeze-thaw cycles on the shear strength characteristics of fiber-reinforced loess are discussed. The research results showed that after 15 freeze-thaw cycles, the shear strength of loess reinforced with 1% fiber increased by 0.15%, 2.05%, and 1.35% at 80, 140, and 200 kPa, respectively. The shear strength of the reinforced loess with other fiber contents decreases to different degrees, and the maximum reduction ratio can reach 9.54%. Freeze-thaw cycles changed the variation of shear strength and strength parameters with fiber content. When the fiber content is less than 1%, the shear strength, cohesion, and friction angle of fiber-reinforced loess increase the fastest after freeze-thaw cycles. When the fiber content is 1%, the overall destruction effect of freeze-thaw cycles on fiber-reinforced loess is inhibited, and the soil has the best freeze-thaw resistance.