Preparation and characteristic testing of transparent soil for vegetation growth

Plant root systems can enhance the shear strength of soil to prevent shallow landslides. Non-invasive visual observation of plant roots can be expected to rapidly develop the transparent soil. Engineered transparent soil with fused silica sand and n-dodecane as the main components have similar transparency and strength as the natural soil. However, it is difficult for the growth of vegetation under the root-soil coupling. A matrix material (Nafion) of transparent soil has also been proposed in the field of ecology. It is still lacking in the survival of inter-root microorganisms and specific mechanical indicators. Transparent soil is then limited to the root-soil composites. This study attempts to modify the preparation of transparent soil for the growth of vegetation. A transparent soil scheme was also proposed to simulate the soil's mechanical properties. Taking platycladus orientalis seedling roots as examples, the planting experiments were conducted to regularly add the nutrient solutions into the root. Then the roots reinforced the transparent soil, thus forming a type of root-soil composite. The porous liquid was finally added to saturate the root-soil composites. The transparency was achieved to observe the root morphology. Direct shear tests were conducted to explore the effects of liquid content (0, 10%, 15%, 20%, and 25.8%) and root content (0, 0.2%, 0.4%, and 0.6%) on the shear strength of root-soil composites. Transparent soil was used as the control. The mechanical properties of the prepared transparent soil were similar to those of natural soil. The Particle Image Velocimetry (PIV) technology was employed to determine the macroscopic and microscopic mechanical properties of the transparent soil and root-soil composite slope under a top load in the saturated slope. COMSOL platform was utilized to verify the behavior under saturated conditions. The results showed that: 1) Plants grew well in the present transparent soil for more than 20 days. Meanwhile, there were the tough branches, the green and healthy leaves. Small fibrous roots were also used to evaluate the mechanical properties of the root-soil composite. When the content of liquid was larger than 20%, the artificial soil shared enough transparency to visually observe the plant roots. 2) The cohesion of transparent soil decreased, as the content of liquid increased. In the transparent bare soil, when the content of liquid increased from 0% to 25.8%, the cohesion of transparent soil decreased from 25.25 to 8.35 kPa, with a decrease of 64.09%. The root systems were added to increase the cohesion of transparent soil. Once the content of root increased from 0% to 0.6% in the root-soil composites under the saturated condition, the cohesion increased from 8.35 to 19.00 kPa, with an increase of 127.50%. 3) According to the improved scheme of transparent soil, the deformation behavior of transparent vegetative soil and root-soil composite slopes can be directly observed by the PIV technique, thus reducing the test cost. The root systems can dramatically reduce the micro deformation of slopes under the action of slope-top loads, in order to effectively prevent potential landslides and natural disasters. The findings can also provide a new material and approach for the root reinforcement.