Hydraulic Properties of Unsaturated Engineering Soils Vegetated with Vetiver Grass in Green Infrastructures

Aiming at green infrastructures which are widely established in countries and regions within the Belt and Road Initiative, both in-situ monitoring and indoor experiments were conducted in the present study. Relationships of hydraulic properties (e.g., soil water characteristic curve and permeability) with root characteristics (e.g., distribution, morphology as well as biomass) and meteorological conditions were investigated. It is found that coarse roots are mainly observed in shallow soils, while fine roots are widely distributed in deep layers. Preferential flow induced by coarse roots and pore clogging due to fine roots are proved in dye-tracer test. The field monitoring is divided into three stages, i.e., T1 (sunny days), T2 (rainy days) and T3 (sunny days after T2). There exist distinct diurnal characteristics of changes of soil suction and water content during the T1 and T3 stages. Moreover, soil suction is negatively relevant with air relative humidity, under absence of rainfalls. Vegetation increases amplitudes of diurnal changes of soil suction and water content. During the T2 stage, diurnal variations of VWC of both vegetated and bare soils are negligible due to rainfalls. Soil air-entry value (AEV) is positively relevant with root biomass. In addition, vegetation improves soil water-holding capability at most vegetation-available suction ranges. As compared to indoor-measured SWCC, field-measured SWCC is more similar to that of ideal coarse-grained soils. Roots improve soil permeability through preferential flows and enhanced water-retention capacity. Results obtained in the present study helps to obtain desired outcomes, i.e., improving feasibility, performance and durability of green infrastructures.