Abstract: To investigate the impact of herb root type and distribution on the unconfined compressive strength of root-soil composites, the root distribution characteristics of cluster-root Viola philippica, tiller-root Ixeris polycephala, and rhizome-root Triglochin Palustre within different soil depths were determined using an image pixel conversion algorithm. Stress-strain curves of the three herb-root-soil composites were obtained through unconfined compressive tests. The relationship between root parameters and cohesion increment of the failure surface of the root-soil composites was examined by quantifying the compressive strength. Results indicated that within the 0-8 cm soil depth range, the root length density (RLD), root surface area density (RASD), and root area ratio (RAR) of Viola philippica and Triglochin Palustre gradually decreased with increasing soil depth, while Ixeris polycephala displayed an increasing trend followed by a subsequent decrease. Quantitative root distribution parameters of Viola philippica ranks first, followed by Triglochin palustre and Ixeris polycephala in sequence. Rhizome-root Triglochin palustre exhibited the highest ability to improve soil compressive strength, with the cohesion increment of its root-soil composite being 1.42 and 2.6 times those of Viola philippica and Ixeris polycephala, respectively. Triglochin palustre has the largest RLD, RASD and RAR of the failure surface, followed by Viola philippica and Ixeris polycephala in sequence. No correlation was observed between root parameters and cohesion increment of the failure surface of Ixeris polycephala. For cluster-root and rhizome-root herbs, RAR has larger correlation with cohesion increment than RLD and RASD do. Therefore, RAR can serve as an important predictor of soil compressive strength improvement for cluster and rhizome root types.

Key words: root type, unconfined compressive strength, root-soil composite, root length density, root surface area density, root area ratio