Experimental study on root-soil friction mechanical mechanism of Caragana korshinskii Kom. in loess area

To systemically research the shrub roots pull-out friction mechanical mechanism, a shrub Caragana korshinskii (C. korshinskii) Kom. with a growth period of 2 years, which was planted in the self-established testing area in the loess area of the Xining Basin, was selected as the research object. Eighteen C. korshinskii roots samples were selected in the pull-out friction test, and four of them were tested under the condition without lateral roots, which was aimed to evaluate the effect of the lateral roots in pull-out process. The relationships between the maximum root-soil friction and 5 morphology indices of roots (total root surface area, total root volume, total root length, total root dry weight, and lateral root number) were analyzed via regression analysis. Meanwhile, the static friction coefficient between taproot of C. korshinskii and soil was calculated through pull-out friction test under the condition without lateral roots. The test results were as follows: The pull-out process of roots of C. korshinski could be divided into the stage of static friction and the stage of dynamic friction, which were reflected in the relationship curve of root-soil friction and displacement, and when the lateral roots were not cut, a nonlinear increase phase of root-soil friction existed in the relationship curve of root-soil friction and displacement; the major effect of taproot was to provide static friction between soil and roots, and the effect of the lateral roots was to enhance the maximum root-soil static friction, the maximum root-soil friction, and the root displacement corresponding to the maximum root-soil friction to a greater extent. Under the condition without lateral roots, the maximum root-soil static friction of 4 roots samples (1#, 2#, 3# and 4#) decreased by 16.7%, 33.3%, 16.7% and 20.0% respectively, the maximum root-soil friction decreased by 44.0%, 50.0%, 37.5% and 42.9%, respectively, and the root displacement corresponding to the maximum root-soil friction reduced by 88.9%, 88.2%, 85.3% and 84.6%, respectively. The mechanism of lateral roots to improve the capability of C. korshinskii roots to resist uprooting could be attributed to the shear type friction and debonded friction produced by lateral roots. The phenomenon that the lateral roots were gathered around the taproot at the end of the pull-out friction test showed that the above analysis was reasonable to some extent; The maximum root-soil friction tended to increase with the increasing of total root surface area, total root volume, total root length, total root dry weight and lateral root number, and a power function relationship was established between these 5 root morphology indices and the maximum root-soil friction. The correlation analysis showed that total root surface area was the morphology index which had the most significant degree of correlation with the maximum root-soil friction (the multiple correlation coefficient was 0.9562); the static friction coefficient between taproot of C. korshinskii and soil was 0.7389±0.04, and it was significantly greater than that the corresponding static coefficient of soil without roots, 0.5040±0.03, which suggested the friction value of C. korshinskii root-soil interface and its ability to resist deformation were greater than the soil without roots. The research is useful to further investigate pull-out friction mechanical mechanism for shrub roots, and meanwhile this conclusion has a theoretical significance and practical value in preventing soil erosion, shallow landslide and other geological hazards in testing area.