Desiccation cracking is a common problem of red clay under wet and dry conditions. The presence of cracks could reduce soil strength properties, which is the major reason for shallow failure of red clay slopes. This study aims to propose some additives to improve the crack resistance as well as the strength of red clay. To this end, three natural fibers and three natural gels were primarily selected to modify red clay. Desiccation crack tests were conducted to identify the best fiber and gel in terms of crack resistance. Then, direct shear tests and uniaxial tensile tests were performed on the soil specimens modified by the best fiber and gel to determine the optimal dosages regarding strength improvements. The results demonstrated that natural gels reduced the crack resistance of red clay, while natural fibers could enhance the crack resistance. Lignocellulose was the best additive in improving the crack resistance of red clay, and the modified effect was positively correlated with the fiber dosage. Adding 1.5% lignocellulose into red clay could reduce the crack rate from 2.19% to 0.30%, with a reduction of 0.86 times. The tensile strength, shear strength and its parameters of lignocellulose-modified red clay showed a trend of increasing first and then decreasing with increasing lignocellulose dosage. At the optimal lignocellulose dosage of 0.75%, the shear strength and tensile strength were increased by 44.63% and 18.21%, respectively. The strength of red clay modified by both lignocellulose and guar gum was positively correlated with the dosage of guar gum, and the desirable dosage of guar gum was 1.0%. In engineering practice, the wet red clay mixed with 0.75% lignocellulose and grass seeds can be employed as planting soil and is sprayed uniformly onto the slope surface after excavation using a spraying planting machine. To further improve the strength of the planting soil, 1.0% guar gum solution can be sprayed on the soil surface. The findings could provide useful guidance for the protection of red clay slopes.

Geopolymer-based grouting materials often have a higher early strength, better durability, and lower environmental impact than those of traditional cement-based grouts. However, existing geopolymer grouts face common challenges such as rapid setting and low compatibility with treated substrates. This study develops a new grouting material using industrial byproducts to overcome these limitations while optimizing performance for reinforcing silty mudstone slopes. The base materials used were ground granulated blast furnace slag (GGBFS) and zeolite powder, with calcium lignosulphonate (CL) serving as the retarding agent and NaOH as the alkali activator. The investigation focused on the effects of the mix ratio and water–binder ratio on the setting time, flowability, bleeding rate, concretion rate, and compressive strength of the new grouting material. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were employed to examine the action mechanism of the material components in the slurry. The one-factor standard deviation method and Grey Relational Analysis (GRA) were used to assess the influence of each material component on the slurry performance indices and the correlation between each performance index and its optimal mix ratio. Subsequently, the optimal mix ratio of the new grouting material was ascertained. The results indicate that the setting time is positively correlated with the zeolite powder and CL dosages and the water–binder ratio, while it is inversely related to the NaOH dosage. The flowability is significantly enhanced with increasing zeolite powder and NaOH dosages, but decreases at a higher CL dosage and water–binder ratio. This insight is crucial for optimizing the workability of the grouting material under various conditions. The optimal ratio of the grout is zeolite powder:GGBFS:CL:NaOH = 30:70:5:7, with a water–binder ratio of 0.6. Compared to existing commercial grouting materials, the compressive strength of this new grout is comparable to that of silty mudstone. This significantly reduces the problem of stress concentration at the grout–rock interface due to strength differences, thus effectively reducing the risk of secondary cracking at the interface. These findings provide a new material solution for grouting and repairing fractured silty mudstone slopes.

Silty mudstone is highly susceptible to the influence of hot and humid environments, leading to the deterioration of its mechanical properties presenting a geohazard and even resulting in geological disasters. Accurately characterizing the effects of bedding and water–rock interaction on the mechanical behaviour of silty mudstone is a crucial prerequisite for the protection and reinforcement of silty mudstone slopes. To this end, uniaxial compression tests and Brazilian splitting tests were conducted to examine the mechanical properties of bedded silty mudstone. Based on the test results, the effects of bedding angle and soaking time on mechanical properties such as tensile strength, uniaxial compressive strength and elastic modulus of bedded silty mudstone were revealed. The test results showed that the tensile strength increased exponentially with increasing bedding angle. The observed split failure patterns of bedded silty mudstone encompassed splitting–pulling damage, shear damage and splitting damage. The uniaxial compressive strength of silty mudstone exhibited a U-shaped variation with an increase in bedding angle. The specimen with a bedding angle of 45° had the lowest uniaxial compressive strength at 7.64 MPa. Furthermore, the elastic modulus of silty mudstone was positively correlated with the bedding angle. The failure patterns of bedded silty mudstone under uniaxial compression included splitting-tension damage, shear-slip damage and splitting-shear damage. The saturated tensile strength, uniaxial compressive strength and elastic modulus of bedded silty mudstone exhibited an exponential decrease with increasing soaking time. The test data confirmed the applicability of the Jaeger equation to the uniaxial compressive strength of bedded silty mudstone. Subsequently, a modified Hoek–Brown failure criterion was derived by combining fracture mechanics theory and test results and introducing a softening factor. The parameters A , D and m in the above failure criterion decreased exponentially with increasing soaking time and an empirical Hoek–Brown failure criterion considering the soaking time was established. Compared with previous theoretical models, this model is more adaptable to the actual engineering situation, which results in more accurate calculations.

In order to investigate the influence of the coexistence of clay and silt on the compression characteristics of sand, one-dimensional compression consolidation tests were carried out on reconstituted saturated sand–silt–clay mixtures with a constant initial void ratio, and the effects of fines content (FC) and clay–silt ratio (CS) on the compression characteristics of mixed soils were studied. The mechanism of the experimental results was additionally explained from a microscopic perspective. The test results show that: the compressibility of mixed soil increased with the increase in FC; the compressibility change rule of mixed soils with different CS is consistent under the same FC; the influence of CS on the e–lgp (the void ratio (e) versus logarithm of the pressure (p)) curve of mixed soil is inconsistent when FC is different: when FC = 3%, the compressibility of mixed soil decreased with the increase in CS; when FC = 7% and 10%, the compressibility of mixed soil gradually increased with the increase in CS; when FC = 5%, the compressibility of mixed soil did not show an obvious changing law with the increase in CS, and the compressibility of the specimen with FC = 5%–CS = 1 (FC = 5%, CS = 1) was the largest; when CS was same, the difference between e–lgp curves of mixed soil with different FC increased with the increase in CS. The compression model of sand–silt–clay mixtures was established, which can consider the effects of FC and CS. The reliability and applicability of the proposed model were verified by combining the experimental results of this paper and the test data of sand–clay mixture and sand–silt mixture in other literature.