Interplay between ecology and unsaturated soil mechanics for bioengineered landfill covers and slopes

The negative impact of climate change calls for additional sustainable and environmentally friendly techniques to be developed for the improvement of the engineering performance of civil infrastructure, such as landfill covers and slopes. Bioengineering using vegetation can be considered and promoted as a low-cost, aesthetically pleasant solution for greening landfill covers and improving shallow slope stabilisation. The mechanical effects of vegetation as soil reinforcement have been extensively studied, but the hydrological effects of vegetation on soil shear strength and water permeability are unclear. This study therefore presents an interdisciplinary research programme consisting of laboratory and field tests and centrifuge modelling. The programme explores the hydrological effects of plants on the performance of final landfill covers and slope stabilisation. Results show that suction induced by plants under a novel vegetated three-layer landfill cover is preserved better than that under a bare cover even after an extreme rainfall event with a return period of greater than 1000 years in Hong Kong. The laboratory tests and field trials demonstrate that the vegetated three-layer landfill cover system using recycled concrete can effectively minimise percolation at humid climate even without a geomembrane. Novel artificial root systems are developed for the centrifuge model tests. Heart-shaped roots have stronger pull-out resistance and higher preserved suction (hence higher soil shear strength) compared with tap- or plate-shaped roots. The heart-shaped root architecture is thus the most effective type in producing stabilisation effects on slopes.