Vegetation such as grasses and shrubs can improve slope stability, reducing the risk of shallow failures once roots have permeated soil to enhance cohesion. However, establishment of vegetation is hindered by poor soil fertility, frequently a characteristic of disturbed soils used in engineering projects. We evaluated whether compost could improve the rate of vegetation establishment and hence soil mechanical reinforcement by plant roots and therefore protect against shallow failures. Over 1200t of material was formed into a slope 40m long×15m wide, with an experimental soil slope angle of 20°. Washings from recycled mineral fill were used for the surface soil. Five amendment treatments were replicated three times in strips of 1.5m by 8m in a randomised block design on this slope; treatments were a no compost control, standard compost addition at a rate of 35tha-1 and a high level compost amendment at 300tha-1, applied either to the surface or incorporated into the topsoil to 10cm.Thirteen weeks after planting an amenity grass mix, vegetation cover increased up to 6 times compared to the control for 35tha-1 surface applied compost and similarly 20 times for 300tha-1 compost that had been surface applied. Root length density was about 3kmm-3 with no added compost and about 30kmm-3 for 300tha-1 added compost. At 35tha-1 compost, peak shear stress of the vegetated soil at 5cm depth was not affected, but it almost doubled with 300tha-1 compost compared to no amendment. Cohesion from plant roots was 8.1kPa for 300tha-1, in comparison to 2.1kPa for no amendment and 2.3kPa for 35tha-1 compost. Whereas surface application resulted in better vegetation cover, there were no differences in peak shear stress between plots with surface application or incorporation of compost. This study provided experimental evidence in the field that compost improvement to soil fertility has a positive impact on soil stabilisation by plant roots.
- Root reinforcement
- Soil physics