Site workers are dealing with difficukt groundwater on a project exploring a possible new river crossing near the norfolk coastline.
Alexandra Wynne heads to East Anglia to take a look.
Early investigations carried out by the council aimed to establish whether a bridge or tunnel would be the ideal solution. But it instructed contractor May Gurney to carry out a more detailed investigation to determine the nature of underlying soils and measure groundwater levels.
Results will enable the council to decide which of the two options is the most viable. The crossing will be situated along one of seven possible routes between an existing bridge and the sea.
While trying to find out what engineering problems lie ahead, site workers are finding the soils a big challenge. May Gurney operations manager (site investigations) Ben Armstrong says: "It's probably one of the worst ground conditions to work with in the UK. The sub-artesian sand means there is a lot of water in the ground and the pressure of it is pushing the soil up as we work."
The contractor is part of a framework agreement – the Norfolk Strategic Partnership – along with the council and consultant Mott MacDonald, which determined where drilling work was needed along a 2km stretch of land split either side of the riverbank.
Rig operators used A-frame cable percussive rigs – up to four have been used on the project – and drilled the 18 holes needed across the site to explore the possible routes proposed by Mott.
Armstrong says the typical strata on site is Braden formation comprising soft silts and clays down to 12m before the sands appear. Nearer to the river wall there is up to 4m of overlying made ground.
About 1.2m of the top layer of ground had to be excavated using hand tools and a cable avoidance tool because much of the site is industrial land.Site workers were then able to carry out standard penetration tests (SPTs) at regular depths down to a maximum of 40m.
A lead casing is suspended from the rig's wire rope and is tapped into the soft soils to reach the depth required. The upper sections of casing have a 250mm diameter, decreasing to 200mm for the middle sections and 150mm at the lowest point. Because the soils are granular, the casing is used down to the full testing depth to prevent collapse.
Rig operators drill between 1m and 5m long sections of SPT rods with a skinny bailer at the end into the ground. This tool gets through the unwanted sodden material near to the surface. It is repeatedly raised and dropped into the ground, collecting spoil inside its hollow stem and keeping it in place by a sprung trap door (or clack) at the base.
A 450mm long split spoon sampler with a cutting shoe is attached to the SPT rods to collect the soil for testing at the laboratory. Samples are taken at regular intervals down to full depth, however, Armstrong says the first 150mm of each sample has to be ignored because of the soil disturbance when the tool is driven into the ground.
A combination of rigs has been used for the job – a Pilcon Wayfarer, Dando 2000 and Dando 3000. A 63.5kg hammer is used to drive in the tools (applying up to 300kg of pressure).
Although the 2000 and Pilcon models are able to complete the work to depths of 30m, Armstrong says the 3000 model was needed to cope with about one-third of the testing work that went down to a maximum 40m depth.
To cope with groundwater at a depth of between 5m and 9m site workers had to add a head of water above the tool and inside the drilling casing.
Armstrong says it is vital to equalise the water pressure to keep the sand at the bottom of the hole, maintaining the sample and allowing better accuracy. Otherwise the groundwater runs in and up through the drilled well, bringing the sands with it and disturbing the sample.
In addition to the ground conditions, the history of the site held a further challenge to the project. Great Yarmouth suffered extensive wartime raids and particular care had to be taken because of the health and safety risk posed by unexploded ordnance (UXO).
UXO contractor MACC International carried out a survey to check for ordnance prior to May Gurney starting on site. It probed every couple of metres – 3m below the initially proposed 6m – and the presence of UXO meant hole locations were altered in places.
The overall cost of the work for the council is about £175,000 with £120,000 for drilling work and £30,000 for lab testing being paid to May Gurney. The on site work was carried out by the contractor, with the help of local firm James and Milton, between early August and the end of September.
As well as easing existing congestion problems, the council hopes the proposed crossing will provide the infrastructure to cope with the development of Great Yarmouth's Outer Harbour terminal. The terminal is being upgraded to cope with vessels up to 210m long to increase European trading opportunities and create 1,000 new jobs.