Background and aims The changes in soil physical properties caused by root exudates depend largely on the chemical composition of root exudates. Our aim was to explore the effects of non-specific root exudates on the physical properties of soil change. Methods Five sugar compounds, five amino acid compounds, and five organic acid compounds were selected and added to loess as three single addition treatments (amino acids, organic acids, and sugars) and four combined addition treatments (amino acids + organic acids, amino acids + sugars, organic acids + sugars, and amino acids + organic acids + sugars). Soil water repellency, aggregate stability, and shear resistance tests were performed on the loess. Results The treatments sugars, amino acids, and amino acids + sugars significantly increased soil water repellency. In addition, organic acids + sugars maximised mean weight diameter (MWD), geometric mean diameter (GMD) and the content of > 0.25 mm water-stable aggregates (R0.25), and minimised the percentage of aggregates destroyed (PAD) in the addition treatments. All treatments except for amino acids significantly increased soil shear strength and cohesion of the loess. Amino acids, amino acids + sugars, and amino acids + organic acids + sugars significantly increased the internal friction angle. Conclusion The single addition treatments had a higher effect on soil hydraulic properties, while the combined addition treatments had a higher effect on soil mechanical properties. Sugars and amino acids substantially increased soil hydraulic stability. Sugars combined with other compounds, especially with organic acids, significantly improved soil mechanical stability.

The Qinghai-Xizang Plateau of China faces challenges like thaw slumping, threatening slope stability and infrastructure. Understanding the mechanical properties of the roots of the dominant herbaceous plant species in the alpine meadow layer of the permafrost regions on the Qinghai-Xizang Plateau is essential for evaluating their role in enhancing soil shear strength and mitigating slope deformation in these fragile environments. In this study, the roots of four dominant herbaceous plant species-Kobresia pygmaea, Kobresia humilis, Carex moorcroftii, and Leontopodium pusillum-that are widely distributed in the permafrost regions of the Qinghai-Xizang Plateau were explored to determine their mechanical properties and effects in enhancing soil shear strength. Through indoor single root tensile and root group tensile tests, we determined the root diameter, tensile force, tensile strength, tensile ratio, and strength frequency distributions. We also evaluated their contributions to inhibiting slope deformation and failure during the formation and development of thermal thaw slumps in the alpine meadow. The results showed that the distribution of the root diameter of the dominant plant species is mostly normal, while the tensile strength tends to be logarithmically normally distributed. The relationship between the root diameter and root tensile strength conforms to a power function. The theoretical tensile strength of the root group was calculated using the Wu-Waldron Model (WWM) and the Fiber Bundle Model (FBM) under the assumption that the cumulative single tensile strength of the root bundle is identical to the tensile strength of the root group in the WWM. The FBM considers three fracture modes: FBM-D (the tensile force on each single root is proportional to its diameter relative to the total sum of all the root diameters), FBM-S (the cross-sectional stress in the root bundle is uniform), and FBM-N (each tensile strength test of individual roots experiences an equal load). It was found that the model-calculated tensile strength of the root group was 162.60% higher than the test value. The model-derived tensile force of the root group from the FBM-D, FBM-S, and FBM-N was 73.10%, 28.91%, and 13.47% higher than the test values, respectively. The additional cohesion of the soil provided by the roots was calculated to be 25.90-45.06 kPa using the modified WWM, 67.05-38.15 kPa using the FBM-S, and 57.24-32.74 kPa using the FBM-N. These results not only provide a theoretical basis for further quantitative evaluation of the mechanical effects of the root systems of herbaceous plant species in reinforcing the surface soil but also have practical significance for the effective prevention and control of thermal thaw slumping disasters in the permafrost regions containing native alpine meadows on the Qinghai-Xizang Plateau using flexible plant protection measures.

Recently, the study of soft soil foundation reinforcement using vacuum preloading technology has received widespread attention from scholars. Along with the emergence of numerous joint vacuum preloading treatment methods, the studies on the monitoring of the treatment process are relatively lacking. Therefore, this study adopts the electromechanical impedance (EMI) technique, with piezoelectric smart aggregates affixed to prefabricated vertical drains, to monitor and research the soft soil vacuum preloading treatment process through four sets of model barrel tests. During the tests, the piezoelectric coupling admittance of the structure is measured, and changes in the soil pore water pressure, shear strength, and moisture content are recorded. The analysis demonstrates that as the soil hardened, the resonant frequency of the admittance shifted toward an increasing frequency, and the peak admittance at the resonant frequency decreased. In addition, the degree of shift differs from layer to layer; the more pore water pressure dissipates, the greater the degree of shift. In addition, we calculate the root mean square deviation values from the admittance characteristic curves and fit them with the shear strength and moisture content to obtain function expressions, further confirming the correlation between the vacuum preloading process and admittance characteristics. The experimental results demonstrate that the EMI technique can effectively monitor the vacuum-preloading process.

Soil detachment capacity (Dc) is an important parameter used to determine erosion intensity in physical-processbased erosion models. Freeze-thaw affects soil detachment processes by altering the mechanical properties of soil; however, due to the compound action of freeze-thaw and runoff on D-c, quantifying the impact of seasonal freeze-thaw on D-c remains challenging. A series of experiments with six freeze-thaw cycles (FTC), six initial soil moisture contents (SMC), three slope gradients, and five flow discharges were conducted to investigate the effect of freeze-thaw and hydrodynamic characteristics on D-c. The results showed that soil shear strength (tau(m)), cohesion (Coh), and internal friction angle (phi) gradually tended to become stable with increasing FTC, indicating that repeated FTC had a cumulative impact on soil mechanical properties, and there was a critical FTC between 5 and 7. When FTC rose from 1 to 15, the reduction in tau m, Coh, and phi was 0.03-23.96%, 2.63-75.21%, and - 5.70-19.24%, respectively, which increased with an increasing SMC, suggesting that the deterioration effect of FTC on soil mechanical properties was promoted by increasing SMC. During alternating FTC, the relative range and variation coefficient of D-c were 2.21-2.43 and 67.87-75.72%, respectively, indicating that D-c was highly sensitive to FTC. Furthermore, D-c increased by 2.37-71.22% after 15 FTC. Alternating freeze-thaw weakened the soil resistance to detachment. Moreover, the promoting effect of FTC on D-c intensified with an increasing SMC, indicating that the variation in D-c was strongly affected by SMC during FTC. A prediction model (R-2=0.955, RRMSE=14.99%) was established to quantify the influence of freeze-thaw and hydrodynamic characteristics on D-c. The explanation rate of variables in the D-c prediction equation was quantitated: the explanation rate of stream power (64.3%) was higher than that of FTC (10.02%) and SMC (3.92%), suggesting that the impact of freeze-thaw on D-c was covered by hydrodynamic characteristics. Further validation is required for the prediction equations when applied beyond the range of construction conditions.

Context Plant roots can increase soil shear strength and reinforce soil. However, wetting and drying alternation (WD) could lead to soil structure destruction, soil erosion and slope instability.Aims This study tried to explore the effects of wetting and drying alternation on shear mechanical properties of loess reinforced with root system.Methods Direct shear testing was conducted on alfalfa (Medicago sativa L.) root system-loess composites with three soil bulk densities (1.2 gcm-3, 1.3 gcm-3 and 1.4 gcm-3) under 0, 1, 2 and 3 cycles of wetting and drying alternation (WD0, WD1, WD2 and WD3).Key results The morphological integrity of the root-loess composites was obviously better than the non-rooted loess after WD. Under the three soil bulk densities, negative power-law relationships were observed between the shear strength, cohesion and internal friction angle and the cycles of WD. WD deteriorated the soil shear strength. The most obvious decrease in soil shear strength occurred under WD1, which was 13.00-22.86% for the non-rooted loess and 17.33-25.09% for the root-loess composites. The cohesion was decreased more than the internal friction angle by WD.Conclusions The most obvious damage to the soil was under WD1. The roots inhibited the deterioration effect of WD on the shear property of loess, and the inhibition by the roots decreased with the cycles of WD.Implications The results could provide new insights into the mechanical relationship between plant roots and loess under WD, and provide a scientific basis for the ecological construction in the loess areas. Wetting and drying alternation (WD) on the mechanical properties of root-soil composites is not clear at present, or if roots can inhibit the deterioration of soil under WD. This paper investigated the effect of WD on the shear strength of root-loess composites. WD was found to deteriorate soil shear strength and cohesion, while roots inhibited the deterioration of WD on the shear property of loess. The results provide a scientific basis for ecological construction in loess areas.