Plant roots can change the soil water retention curve (SWRC) and saturated permeability (k_sat) of vegetated soils. However, there is no model that could capture both the effects of root growth and root decay on these soil hydraulic properties simultaneously. This note proposes a new void ratio function that can model the decrease and increase in soil void ratio due to root occupancy (upon growth) and root shrinkage (upon decay), respectively, in an unsaturated vegetated coarse-grained soil. The function requires two root parameters; namely, root volume ratio and root decay ratio, both of which can be readily measured through root excavation and image-based analysis. The new function is incorporated into a void ratio–dependent SWRC model for predicting the SWRC of vegetated soils. Similarly, the same function can be combined with the Kozeny–Carman equation for predicting k_sat. The model prediction is then compared with a set of new field test data and an existing laboratory dataset for a silty sand vegetated with plant species under the family Schefflera. Good agreements are obtained between the measurements and predictions.

Effects of plant growth on soil hydrological changes need to be considered for long-term vegetation management of geotechnical infrastructure. Most existing studies focused on one particular plant age. This study quantifies the effects of plant growth on the evolution of soil hydraulic properties and matric suction over time, through field monitoring and numerical soil–plant–atmosphere interaction modelling. A full-scale flat landfill cover in China was monitored for more than a year. Four 6 m × 6 m grass plots made of compacted silty sand were formed, three of which were vegetated with shrubs having different plant spacing (0·5, 0·8 and 1·1 m), while the fourth one was left grassed. In addition to monitoring matric suction changes, multiple soil cores were sampled to determine the root length density (RLD) and saturated hydraulic conductivity (k_s) after 2, 4, 7 and 13 months of transplantation. Regardless of plant spacing, k_s always reduced during the first 2–3 months of growth. When a threshold RLD of 2 cm/cm³ was reached, k_s increased substantially. This was especially the case for closely spaced shrubs, whose roots were decayed. Suction preserved upon rainfall depended on the plant growth-induced changes in k_s. Although closely spaced (0·5 m) shrubs preserved the most suction during the first 6 months of growth, the beneficial hydrological effects vanished after growing for 9 months because of the shrub growth-induced increase in k_s. Instead, the widely spaced (1·1 m) shrubs, which showed the least increase in k_s, preserved the most suction in later stages. Nonetheless, the spacing of shrubs did not affect the annual cumulative percolation of the landfill cover very significantly, which narrowly ranged between 3·1 and 4·7% of the annual precipitation during the monitoring period.

Biochar (BC) has been proposed as an alternative soil amendment in landfill cover. Its effects on soil hydraulic properties (e.g. soil water retention) have been explored to some extent, while its influence on landfill gas flow characteristics is not clear yet. The objective of this study is to investigate the water retention and gas permeability (k_g) of BC-amended soil and its relationship with soil water content in unsaturated state. In total, three BC contents (0, 5 and 10%) were considered in clay–sand mixtures. Soil suction, water content and gas pressure were measured continuously using a new in-house developed apparatus. Test results show that the presence of BC increased soil water content and the increased amount was almost the same over a wide range of suction values (up to 3 MPa). k_gincreased linearly with soil suction in semi-log scale, regardless of the BC content. The addition of 5 and 10% of BC could decrease k_gby up to 50 and 65%, respectively, compared with that of the bare soil. k_gdecreased with water content for both soil and BC -amended soils. Moreover, the decrease rate of k_gwith water content became smaller when the BC percentage was higher.