Study on the time effect of shear strength of alfalfa-loess composite

The alfalfa-loess composite, obtained by artificial planting in PVC tubes, was selected as the research object. Through triaxial compression tests, the time effect of shear strength change of alfalfa-loess composite was analyzed, and the effects of confining stress and water content on the shear strength of alfalfa-loess composite  were compared. The results shown that the shear strength of alfalfa-loess complex was significantly enhanced compared to the plain loess. The cohesion of the alfalfa-loess composite significantly increased with increasing growth period of alfalfa (60 days, 90 days, 120 days and 150 days). Under a lower confining stress, the cohesion of alfalfa-loess composite with different growth periods in unconsolidated and undrained (UU) test increased by 7.02%, 18.81%, 26.09% and 36.41%, respectively, compared with that of plain loess, while that in the unconsolidated and undrained (CU) test increased by 27.62%, 50.87%, 77.62% and 158.11%, respectively. Under a higher confining stress, the cohesion of alfalfa-loess composite in unconsolidated and undrained (UU) test was significantly higher than that of plain loess, and increased with the growth period, but there was no obvious pattern in consolidated and undrained (CU) test. In addition, the case of herbaceous plants with shallow root growth was more practical to conduct triaxial test in a lower confining stress. The shear strength index of alfalfa-loess composite  in the saturated water content was significantly lower than that in the natural water content in the same growth period. As the growth period of alfalfa increased, the decrease in cohesion in the saturated water content was more obvious than that in natural water content. Artificial planting of alfalfa-loess composite using PVC tubes was more conductive to modeling the interaction between roots and soil.