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Ground investigation: Creating the crossing case

Upper Orwell Crossings - view from the bridge

Work is underway to establish the ground conditions on a tricky tight urban site in the centre of Ipswich. 

Drive around Ipswich and you can see why the new Upper Orwell Crossings are needed. The congested town sitting around the mouth of the tidal River Orwell is struggling to cope with the daily traffic influx and the existing two road crossings in the town are both bottlenecks.

Factor in a marina, working port, multiple businesses and landowners and local residents – all key to the success of the scheme – and you can see that it’s a pretty complicated project in terms of delivery. This is before the ground conditions are factored into the equation too and work is currently underway to give more detail on this part of the challenge and scoring a UK first in the process.

Back in 2015 WSP, which is working with Suffolk County Council on the Upper Orwell Crossings project, undertook preliminary ground investigation work to assess if there was scope to build two new crossings in the town. Fast forward three years and Fugro’s work on the latest phase of ground investigation is just about completed, with a view to take the scheme proposals to the development consent order stage later this year.

Currently the scheme consists of two new crossings, known as A and B, and one crossing – C – that will be renewed and linked up to the existing footways and cycle paths.

 

resized image847477 ipswich 138 bridge bridge c small

 

Crossing A  the largest and most complex of the scheme – will be road bridge for cars, cyclists and pedestrians – and will connect to the south of the Wet Dock Island, which would connect the east and west banks.

Crossing B  a new road crossing the New Cut, which would connect the west bank to the Wet Dock Island. This crossing would be for all road users, including cyclists and pedestrians.

Crossing C  an improved crossing over the Prince Philip Lock, which would connect the east bank to the Wet Dock Island. This crossing would be for cyclists and pedestrians only.

The £100M project, with £77M funded by central government, has been identified as a National Significant Infrastructure Project, due to the relief it will give the A14 trunk road, over the existing Orwell Bridge, when completed.

“The transport benefits are one part of the scheme, but the connectivity it will provide for non-motorised users is another big benefit,” explains Suffolk County Council assistant project manager Sam Cliff. “At the moment, the Orwell really does divide the town centre.”

The scheme is key to a bigger ambition to regenerate the southern section of Ipswich, which consists of a busy working port, marina and multiple businesses. The project will kickstart the redevelopment of the Wet Dock Island and ensure the continued success of the waterfront, marina and port.

Crossing A is proving trickiest as it will cross the main thoroughfare into the marina and the working port. “It’s been quite challenging,” explains WSP’s programme director Andy Indoe. “It’s a challenging alignment. We are constrained by almost the length of it and the rate of which we need to climb to try and maximise the headroom in the navigational channel.”

Upper Orwell Crossings-  view from above looking south

How the crossings will look. View looking south

WSP has maximised the height of the bridge to allow as many vessels as possible to travel unhindered, without having to lift the bridge too often, and disrupt the traffic across the bridge.

“It’s a tight site,” adds Indoe. “There is a railway line to consider, and we need to allow enough headroom for that, as well the deliveries for the port by road.”

Fugro is undertaking the ground investigation work, which started in January, and will be around on the ground until the end of March. Its findings will inform the next stage of the scheme when the scheme goes out to tender at the end of 2018.

The unexpected soft sediments located, at arguably one of the most important parts of the scheme, underneath the abutment is going to be one of the most complicated parts of the project

In a combination of land and marine-based investigation, Fugro will have drilled 28 boreholes when it has completed its investigations, using a combination of cable percussion and rotary drilling, as well as undertaking further cone penetration testing and mechanically excavated trial pits.

When WSP designed the investigation it had three main concerns – that the area is a Chalk environment with associated solution features, there was an historical deep buried channel that needed locating, and there was an element of ambiguity of the ground conditions due to the nature of the site and work that had been done previously in the port area.

Fortunately WSP was able to take advantage of the ground investigation findings for the Orwell flood barrier which is currently under construction. “We have used their ground investigation to get a feel for what we might need to scope up this ground investigation. It’s been convenient and we have used that extensively,” says Indoe.

The 28 boreholes will be bored to depths between 20m and 80m, both onshore and offshore, in addition there are 30 cone penetration tests and 12 trial pits to identify any contaminated land, as there was once a landfill site on the west side. The two jack-up barges, one supplied by Red7, which is coincidentally based in the port, have between 10 and 14 people working at any one time, including two support boats.

“We have found some unexpected ground conditions,” explains WSP project manager Georgia Kainourgiaki. “We found some softer deposits than anticipated and we added extra scope to the ground investigation.”

Upper Orwell Crossings-   plan of 3 crossings

Aerial view of the three crossings

The project is completing both cross hole seismic imaging and down hole geophysics using Fugro’s state-of-the-art nuclear magnetic resonance (NMR) technique, which not been used in this type of application in the UK before (see box).

Initially the scheme was going to use downhole CCTV but that involves 24 hours standing time. “Fugro suggested NMR as it would add value to the project and give you what you are after, rather than the bog standard testing,” explains Indoe.

The technique was chosen for the project as it can provide greater surety of data capture across the target strata than would have been possible using traditional methods such as neutron porosity that uses a radioactive source.

Due to the tidal nature of the river, which has a 4.5m tidal range, WSP is also concerned about the tidal loading of the opening span, and the piers, and have undertaken cyclic triaxial tests.

In addition, unexploded ordnance surveys have also been completed as the area was heavily bombed in the war, and archaeologists are visiting the site regularly to test the soft sediment.

Fugro suggested NMR as it would add value to the project and give you what you are after, rather than the bog standard testing

“We are also clearing the unexploded ordnance risk for the boreholes, and for the project too,” explains Kainourgiaki. “It is complex ground conditions and there is a lot of testing going on in the background. Not only in the labs, but also in situ here on the site.”

The main concern encountered so far is the ground conditions on the east side where the abutment of crossing A will be.

“The unexpected soft sediments located, at arguably one of the most important parts of the scheme, underneath the abutment is going to be one of the most complicated parts of the project,” explains WSP site manager Guy Swains.

But the logistics have also proved complicated. “The logistics of managing a complex number of offshore rig movements in and around the navigation channels and managing the requirement for the active harbour to be maintained has been challenging.

“Especially the moving of the rigs into locations where these pieces of plant shouldn’t be, in very tight spaces and in urban areas, right in the middle of the town centre, has been tricky,” adds Swain.

The rigs are located close to a highway, footpath, offices and a pub, with many people coming out to see them. Close consultation with the harbour master over 20 times a day to log, record and notify every manoeuvre in advance, during and after is time consuming and needs detailed planning.

The logistics of managing a complex number of offshore rigs in and around the navigation channels and managing the requirement for the active harbour be maintained has been challenging

WSP is keen to emphasise that early contractor involvement has benefitted this project, with Fugro brought in early enough to work with WSP to refine the design of the investigation. “We wanted continuous wireline CPT to 80m and Fugro was able to offer that,” explains Indoe. “With the NMR, it has really added value to the project.”

“Fugro has been working with us for 18 months and the collaboration early on has been a definitely benefit to the scheme,” explains Swain, “especially with the complexities of the logistics involved.

“We have been able to redesign the scope ‘on the ball’ due to the unforeseen ground conditions with the contractor.”

The project is also engaging with the local community, and civil engineering students from the local college have been to visit the site to see the ground investigation in progress. “It’s always a good thing to have, but something that never happens on a ground investigation. It rarely happens at a GI stage,” explains Swain

“It’s such a good opportunity to see the jack-up barges in action,” adds Cliff.

“The scheme will be building a full ground model from the results of the investigation and a full 3D BIM model. The BIM model will have the whole structure, the foundations, utilities and everything we have picked, and will be handed on to the contractor. So that gives continuity to the scheme.”

Since GE’s visit, the alignments for the bridges have been announced by Suffolk County Council.

Nuclear Magnetic Resonance Technique

With its origins in the medical sector (where it is commonly known as MRI scanning) the nuclear magnetic resonance (NMR) method has been adapted for a range of geoscience applications. The technique is well-established in the oil and gas and mining sectors but its application in engineering and land geotechnical projects is quite innovative.

Fugro has been using the technology because it provides valuable hydrogeological insight for near-surface site characterisation projects. Despite the term ‘nuclear’ in the title the method uses no radioactive source material. NMR logs both porosity and permeability and can distinguish between mobile, capillary-bound and clay-bound water.

Crucially, the NMR technique can be used in uncased or plastic cased holes even when grouted – eliminating the risk of borehole collapse which often prevents the effective collection of useful data with radioactive source methods. It is effective in air, water or drilling mud and is less sensitive to variations in borehole conditions than other techniques.

It is particularly well-suited to soft ground including carbonates, sandstone and soft sediments and provides unique information in certain geologies (particularly in soft ground) where parameters derived from other downhole techniques would be flawed. Measurements are independent of lithology and information delivered can include: total water content; water porosity (which would otherwise require a combination of two different methods); pore size distribution water fractions (clay-bound, capillary-bound, mobile); and hydraulic permeability.

The down-hole sensor operates at four frequencies to investigate four concentric shells around the hole; this allows discrete evaluation of the ground outside the zone disturbed by drilling.

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