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Errand of Mersey

With plans for the new Mersey Gateway bridge pushing ahead, detailed marine ground investigation for the bridge foundations got underway earlier this year. Stephen Mackereth explains the challenges this posed.

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Plans for a new Mersey road bridge to relieve traffic congestion in the Runcorn area were first mooted more than 10 years ago and the £600M cablestayed Mersey Gateway scheme finally got the go-ahead earlier this year.

Nine ground investigations had already been carried out in the process of finalising the route and outline design, but before final foundation design could be undertaken, more information was needed about the ground conditions in the estuary.

With the route narrowed down to a crossing of the River Mersey 1km upstream from the existing Silver Jubilee Bridge in Runcorn, the focus of the previous investigations had narrowed the location for new bridge’s foundations to specific areas in the estuary.

Project promoter Halton Borough Council and its consultant Ramboll appointed Structural Soils to undertake the tenth ground investigation for the scheme that will deliver the essential information to take the project to the next stage.

Structural Soils’ investigation focused on delivering detailed information on the three areas identified as the locations for the foundation for the bridge tower - T1, T2 and T3 - which are labelled from north-west to south-east across the estuary.

Access challenge

Even at the tender enquiry stage, it was clear that this was no ordinary ground investigation. The intertidal nature of the site was not unusual in itself but when linked with the intertidal areas being water locked at all states of the tide, it made access a real challenge.

Add to this the distances involved to the nearest point of access for drilling platforms and support boats and the absence of any significant slipways near the site, and it made the site very remote, despite being physically very close to large centres of conurbation.

Area T1 was positioned straddling a channel of deeper water in the north of the estuary and was flooded every day and at most states of the tide, whereas areas T2 and T3 were positioned on a large 600m wide sandbank in the middle of the estuary.

Tidal graphs provided with the tender documents overlain with ground profiles for exploratory hole locations identified that every day, the exploratory holes on the sandbank were exposed for the majority of the tidal cycle and that for periods of several days to up to two weeks over neap tide cycles, exploratory hole locations would not remain dry.

Structural Soils proposed a dual approach - using its own jack-up platform, the Zee-Jack Inspector, for the deeper water boreholes in Area T1 and a spudded barge with ramps to provide a means of access to drill boreholes directly on the sandbank in Areas T2 and T3.

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During the tender period a kayak-supported visit to inspect the sandbank was carried out. Before anyone could commit to working on the sandbank, assurance was needed that drilling plant could work and move around on the surface of the sandbank.

Confident that the plan would work, Structural Soils proposed to carry out the work on the sandbank using a tracked drilling rig, a cable percussion rig fitted with flotation tyres and two low bearing pressure tracked dump trucks, including one fitted with a hiab.

Temporary roadways were formed using 100 track matts and 300m of heavy gauge reinforcement mesh and geotextile.

The platforms for the overwater work were launched in Ellesmere Docks from road transport, then moved into the River Mersey at Eastham Docks before being towed upriver by a local tug operator.

Getting the platforms onto the water was not the end of the challenge - the 24km tow was on the uncharted and largely undredged upper estuary of the Mersey.

To ensure the best chance of success, low draft towing vessels were selected for the work and Structural Soils employed a local boatyard manager who had personal knowledge of the waters as a pilot.

The tows were timed to coincide with big spring tides, which occur every four weeks, to give the greatest clearance over the uncharted and mobile sandbanks which cross the route.

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Investigation scope

The investigation involved 29 boreholes drilled to depths of up to 55m, and in total 950m of boreholes were drilled.

Each borehole was started using cable percussion techniques to drill the alluvial and locally glacial superficial deposits, as well as the underlying bedrock that was completely weathered.

The boreholes were extended to their full depth by rotary coring into the competent bedrock. The cable percussion component of the drilling went well, but the rotary work presented some challenges.

In some areas horizons of weathered, uncemented sand beneath the rockhead led to core loss and collapse of boreholes, which were overcome by a combination of increasing core diameter, introducing temporary casing and use of environmentally friendly polymer flush additives.

Eight of the borehole locations were preselected at the start of the project, but the remaining positions were determined through the course of the investigation with input from the three shortlisted consortia bidding to construct the bridge.

The investigation involved 29 boreholes drilled to depths of up to 55m, and in total 950m of boreholes were drilled.

Halton Borough Council was keen to provide as much specific information as possible to allow the consortia to fine tune their ideas, so in addition to the borehole locations, the bidders also helped design the insitu and laboratory testing specification.

Establishing the geological profile beneath the three tower locations was only part of the scope of this investigation.

A range of insitu and laboratory tests were also undertaken to determine the geotechnical properties of the geological horizons on which the foundation design would be based. Insitu testing included SPTs, variable and constant head permeability, packer permeability, high pressure rock dilatometer testing and geologging.

Many of these tests were performed in-house and the remainder using a selection of specialist contractors. Overcoming the geotechnical challenges was not a problem but the physical issues of working on this site were the most demanding.

Getting the drilling equipment onto and off site was expected to be the biggest problem, however simply getting the drilling crews and supervisory staff to work locations on a daily basis proved to be difficult.

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Initially a small low draft boat with an outboard motor was used to transport staff to worksites on the sandbank on the main fluvial channel of the Mersey.

This worked fine for the first week of the contract but when the river channel silted up, an alternative approach was needed otherwise staff could only be transported at high water, which was too restrictive. The solution was using a hovercraft but on windy days this proved too dangerous, so a helicopter was brought in to take staff to the rigs.

From the outset this project was daunting and challenging in the extreme, but detailed planning - often hour by hour, a flexible approach to problem solving, an understanding client and the commitment of staff and specialist sub-contractors helped to make this investigation a success.

With ground condition information gathered, Halton Borough Council is now waiting on a decision from the government regarding the funding package for the project, but it hopes to be able to award the contract for the bridge construction during 2013.

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