The Pullout Mechanical Properties of Shrub Root Systems in a Typical Karst Area, Southwest China

Roots play a major role in reinforcing and stabilizing soil. The pullout mechanical characteristics of soil reinforcement and slope protection of the root systems of dominant shrub species (Pyracantha and Geranium) were estimated by in-situ pullout tests in a karst area, in which roots were pulled out from soil to reliably test the pulling force. The goals of this study were to discover the pullout mechanical properties of roots in karst areas and to try to analyse the impact of the root system on landslide control. The F–s curves were multipeak curves with a noticeable main peak and main double peaks. The curves showed a linear increasing trend at the initial stage of drawing and decreased rapidly after reaching the peak. The F–s curves of root systems inserted into rock cracks showed secondary fluctuations in the later stage of drawing, and rock cracks stimulated the tensile efficiency of the root system more effectively. Field in situ pullout results indicate that tree roots fail progressively rather than simultaneously. The maximum pulling force had a linear relationship with the increase in soil thickness and a disproportionate increasing trend with the increasing number of broken roots. The displacement of the maximum peak was different between the two tree species and was concentrated at 5–15 cm and 5–25 cm for Pyracantha and Geranium, respectively. The maximum pulling force of Geranium was 1.29 times that of Pyracantha, and the root system of Geranium had strong pullout resistance. We concluded that the peak distribution of the F–s curves was affected by broken roots and rock cracks, while soil thickness and the number of broken roots had positive effects on the maximum pulling force, all of which is helpful in understanding the effect of root pullout mechanical properties on landslides in karst areas.