UKCRIC institutes the University of Birmingham and Heriot-Watt University have collaborated with HS2, in a project to demonstrate that excavated London Clay can be used as a growing medium for landscaping and remediation.
The paused construction of HS2 at Euston Station involves the excavation of one million cubic metres of material, the majority of which is London Clay. These clays are beneficially reused for flood protection and landfill capping. Recent developments within the industry have led to greater consideration of how waste materials from construction can be best employed to maximise environmental benefit and aid transition to a circular economy. HS2, in collaboration with Mace Dragados, commissioned a project to investigate the potential for the excavated London Clay to be ameliorated and used as a growing medium capable of supporting plant growth for use in local landscaping and remediation projects.
The aim of the work was to determine whether the addition of compost and sand to the clay can support plant growth either with or without the use of added fertiliser. The desired outcome is to reuse excavated material locally, to minimise excavation waste and maximise resource efficiency. Greenhouse trials were carried out at the University of Birmingham and outdoor growth experiments were also undertaken at Heriot-Watt University.
Physical and chemical properties of the growth media were determined before being planted with three grass species (Lolium perenne, Panicum virgatum and Festuca arundinacea). Plant growth was measured, together with chemical and microscopic analysis of the aboveground biomass, to understand the limitations on plant health and vigour in the ameliorated clay growth media. Chemical analysis of the growth media indicated that the ameliorated clays had high pH and were low in available nitrogen and phosphorus, but relatively sufficient in most micronutrients.
Germination and growth of grass species after the first month was limited without additional fertilisation, primarily because of low availability of nitrogen and phosphorus. Fertilisation alleviated limitation to growth in the greenhouse experiment, indicating that the ameliorated clays are suitable for repurposing as a growth media with appropriate ongoing fertiliser management. Both the greenhouse trials and the outdoor experiments indicate that the London Clays can be sufficiently ameliorated to support plant growth and would be suitable for repurposing in landscaping projects, subject to appropriate amelioration and fertilisation. Key considerations for amelioration are a reduction in compaction and an increase in coarse particle content to improve drainage and permit root growth. This can be achieved with the addition of compost alone, as in the case of Heriot-Watt University’s outdoor experiment, or a combination of compost and sand as shown by the greenhouse experiment at the University of Birmingham.
The successful growth of three species of grass under both greenhouse and outdoor experimental conditions indicates that ameliorated clay from under Euston Station is suitable for repurposing in landscaping projects. Successful planting will depend on the addition of compost and possibly sand to alleviate drainage and compaction and to provide some of the essential plant nutrients. The addition of compost alone or compost and sand may not be enough to sustain good vegetative growth in the initial stages of plant establishment and any landscaping project should also include fertilisation with a management plan for these ameliorated and repurposed clays. Full-scale trials should be undertaken in preparation for the resumption of work at Euston Station, to achieve cash savings and environmental benefit.