Mersey Gateway – Phase 8

In October 2011, Soil Engineering were awarded Phase 8 of the Mersey Gateway project which comprises the land based contaminated land and geotechnical ground investigations. The Mersey Gateway is a major scheme to build a new 6 lane toll bridge over the Mersey between the towns of Runcorn and Widnes which will relieve the congestion on and ageing Silver Jubilee Bridge. The site comprised a brownfield site on the northern bank of the River Mersey. Previous site usage has left a legacy of contamination including potential radiological contamination.

Soil Engineering Solution

Site works comprised the formation of boreholes using cable percussive rigs, rotary and window sampling techniques. Boreholes were formed to obtain samples for contamination and geotechnical testing as well as to facilitate the installation of combined gas and groundwater monitoring wells. A multiple casing string system and with bentonite plugs was used to prevent cross-contamination between strata during formation of the boreholes. In some cases up to four casing strings were used comprising 150mm, 200mm, 250mm and 300mm diameters.

Works were undertaken by five cable percussive rigs, a single rotary rig and a Terrier sampling unit. Cone Penetration Test (CPT) holes were formed using a CPT rig to provide information on soil conditions. A MIP (Membrane Interface Probe) was used to measure the presence of volatile organic compounds in unsaturated and saturated deposits from the Made Ground and underlying Alluvium.

Soil Engineering Value

During formation of the boreholes samples were screened for radiological contamination. Monitoring of both groundwater and soil gas/vapour was also undertaken. Where solvent odours are  noted in the soil arisings during the site operations a visual examination for free phase product (DNAPL) was carried out. Where visual identification was inconclusive a dye shake test was undertaken on samples using a Sudan IV test kit. If DNAPL or petroleum was present in the sample a red meniscus (or red spots on the side of the jar) appeared within 30-60 seconds providing instant confirmation of contamination.

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