In the proposed scheme, the tunnel arisings would be used on land to the east of Parsonage Down National Nature Reserve to:
a) blend the new highway embankments into the existing topography and so reduce the landscape impacts of the new alignment;
b) create new chalk grassland and other habitats and extend the existing Parsonage Down habitats. This has been successfully achieved at other sites where chalk tunnelling excavations have been used for habitat creation, such as at Samphire Hoe near Dover using spoil from the channel tunnel.
Construction of the proposed scheme would generate a large volume of excavated material from the tunnel, and this material would generally be unsuitable for use as an engineering fill material in highway embankments The excavated material arising from tunnelling is likely to be in the form of a finely ground slurry or paste and would require processing to reduce the water content sufficiently to make it suitable for handling and re-use. The excavated materials processing plant is likely to be located within the main production area.
Once suitable for use the material would be used for essential landscaping mitigation and new habitat creation to increase biodiversity. The principal area of use would be the land to the east of Parsonage Down National Nature Reserve.
In addition to the need for essential landscape mitigation in this area, the material would be used to create areas of new chalk grassland and other habitats and create an extension of the existing Parsonage Down habitats.
Use of excavated material from the tunnel in this way would minimise the need to transport this material on the highway network to landfill sites. This would minimise the environmental impacts associated with the construction of the proposed scheme, particularly in relation to the air quality and noise impacts of construction traffic on people and communities living along potential off-site excavated materials disposal routes. This strategy would also help reduce greenhouse gas emissions during the construction phase.
In addition to the tunnel arisings, excavation would also be required in places to form cuttings for the highway and this material would then be used to form embankments. The design aims to balance these requirements as far as practicable.
Phosphatic Chalk is known to be present within the Stonehenge Bottom area of the study area and these deposits are likely to be encountered during tunnel boring. (This material is expected to have a higher phosphorus content compared to normal Chalk). The engineering and hydrogeological characteristics and spatial extents of this material will be investigated further as the design develops and will be reported in full in the ES.
The key environmental effect is considered to be the potential adverse effect of the degradation in water quality of the River Till or Avon through eutrophication as a result of nutrient loading, should the material be placed nearby and leaching then occur. To mitigate this, additional investigations and risk assessment, will be undertaken prior to any storage and re-use of excavated materials. These assessments will ensure that the storage and reuse of Chalk arisings at any particular location would not result in an impact on water quality.
Additional investigations are being undertaken to establish the leachate potential of phosphorus in the Phosphatic Chalk at Stonehenge Bottom and the possible risks posed to controlled waters should the material be excavated and re-used within the study area.
There are also uranium-bearing minerals within the Phosphatic Chalk which could give rise to increased radon emissions. However, since radon is a noble gas, it does not absorb to air particulates and in an outside environment, it is dispersed to such an extent that it represents no significant risk. The impact and any possible effect during construction within confined environments, like the tunnel, will be mitigated.
From A303 Stonehenge Preliminary environmental information report (PEIR)