Root Tensile And Pullout Mechanism Of Alfalfa And Seabuckthorn In Loess Region

Root tensile and pullout mechanism is a basis for understanding the mechanism of soil reinforcement by plant roots.In this study,root morphology investigation,root tensile test and root pullout test were conducted on roots of 3-4 months Alfalfa(Medicagosativa L.)and 7-8 months sea buckthorn to study the root tensile and pullout mechanism in the loess region.The main research results are as follows:(1)Alfalfa was a straight-rooted plant.The taproot was obvious and thick and long,and the lateral roots were thin and grown in large numbers.Sea buckthorn had obvious taprootand third-order lateral roots.The fractal dimension was 1.39 ± 0.05.The root diameter was mainly in the range of 0-1.50 mm,and the number of lateral roots was large in all directions.The lateral roots with bifurcation angle was greater than 60 °accounted for 62.84% of the total roots.Sea buckthorn was likely to perform a strong traction effect on the soil around the root system.(2)During the tensile process,the root systems of alfalfa and sea buckthorn behaved as elastoplastic materials,and the stress-strain curve was a single peak curve with no obvious yield section.The maximum tensile strength of the root system increased with the root diameter in power functions.The tensile strength and elastic modulus decreased with increase of the root diameter in power functions.(3)The tested roots in soil showed two failure modes: slippage failure and breakage failure,whose L-S curves(between pullout force and displacement)had significantly different variation tendency after elastic deformation due to the difference of root maximum tensile force and ultimate elongation.The L-S curves were divided into three stages under slippage failure: the elastic deformation stage,the root-soil interfacial debonding stage and the post-debonding friction stage.The L-S curves of root breakage failure included two different types.The first type was determined by the maximum tensile force.The second type was determined by the ultimate tensile deformation.(4)Roots in smaller diameters tended to break and roots in larger diameters tended to be pulled out.The critical diameter between slippage failure and breakage failure was 0.30-0.40 mm of Alfalfa and was 0.86 mm of sea buckthorn when root length was 160 mm.(5)The root diameter had a positive power relationship with the peak pullout force,a negative power relationship with the maximum pullout strength,and a positive correlation with the root-soil friction and pullout energy.The pullout energy of pullout failure was greater than that of breakage failure.(6)With the increase of the root depth,the peak pullout force increased linearly and the pullout energy increased exponentially.The peak pullout force(the maximum under θ=60?),root-soil friction coefficient(the maximum under θ=60?)and pullout energy(the maximum under θ=45?)increased and then decreased with increasing embedding angle.(7)The peak pullout force and pullout energy increased and then decreased with increasing soil water content,and they reached the maximum under soil water content ω=9.89%(alfalfa)and ω=8.03%(sea buckthorn).Positive power functions were observed between peak pullout force and soil density,root-soil friction coefficient and soil density.The breakage rate and pullout energy increased with increasing soil density.(8)The root-soil friction coefficient,maximum tensile strength,tensile strength,and elastic modulus of the alfalfa root system were all greater than those of the sea buckthorn root system,so the alfalfa would have greater ability of soil reinforcement in this study.(9)Root morphological characteristics and soil factors were the dominant factors affecting the root pullout mechanical properties.The root tensile and pullout mechanical properties had a greater impact on the properties of soil reinforcement by roots than the root morphology.