In some situations where burial is the preferred disposal method but it is not possible to bury material to the required depth of 5m below ground, infested material can be placed in an underground cell which is fully encapsulated in a root barrier membrane. These cells can be placed under buildings and carparks, incorporated into foundation and substructure design or placed in locations on the site which will not be subject to future disturbance. In the case of Japanese Knotweed, there can be a substantial quantity of material involved, so careful co-ordination is required to protect against the risk to buildings or surfaces from future soil settlement. The cell location should be accurately mapped, recorded and incorporated into the property's documents.
Typical cell construction starts with excavation to the required levels, based on the calculation of the volume of material to be accommodated. If the material is to be drawn from another location, a protected temporary haul route, protected by a proprietary root barrier, should be provided. The cell root barrier membrane is then laid in position using the largest possible sheet sizes, which minimises the extent of on-site jointing required. The infested material should then be laid within the cell, in compacted layers, to the required depth and the top of the cell sealed off with the root barrier membrane. Finally, a capping layer of inert fill or soil is placed over the cell. When the cell is not located beneath buildings or hard surfaces, this layer should be at least 2m deep, which will protect the cell membrane from burrowing animals and future ground disturbance.
It is important to note that protective layers must be installed to both faces of the root barrier membrane, both around the entire cell surface and in other places where the membrane is being used, to safeguard it from any risk of puncture. The entire cell construction process should be closely monitored and supervised, and strict biosecurity measures should be deployed for the entire duration of the works programme.