Shear strength at the soil-root interface under different saturation states

Abstract

Understanding the mechanical interaction at the soil-root interface is essential for predicting how vegetation stabilises slopes, especially under changing vegetation-atmosphere conditions. One of the main challenges is the interplay between mechanical and hydraulic effects, which are mediated by soil matric suction and the degree of saturation. This study presents results from direct shear tests on representative soil-root interfaces conducted at different soil degrees of saturation and matric suctions. It considers two types of soil —silty sand and silty clay with coarse sand particles— and uses root interfaces with different surface roughness, specifically natural roots and treated wood with rough and smooth surfaces, which were characterised experimentally. The tests were performed under constant and low normal stresses (7 to 25 kPa). The ultimate shear strength reached its maximum at intermediate degrees of saturation (typically between 0.4 and 0.5), with decreases observed at both very dry (saturation 0.1) and fully saturated conditions. Natural roots consistently exhibited higher shear strength than treated wood, emphasising the importance of surface roughness.

The authors are grateful for the financial support provided by MCIN/AEI/10.13039/501100011033 and Union Europea Next Generation EU/PRTR.

Peer Reviewed

Postprint (published version)