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Ground investigation: Linked investment

Ground investigation work has bought the first flurry of activity to the site of the planned Lower Thames Crossing.

Plans for a new road tunnel crossing of the Thames between Kent and Essex may not yet have a start date but the site is already seeing the results of tunnelling activity. Tunnelling waste from the Northern Line Extension and Thames Tideway is being landfilled at the Lower Thames Crossing site adding some challenges to the recently completed first phase of ground investigation.

The new tunnel is part of a link road planned by Highways England which will reduce congestion on the existing Dartford Crossing and support economic growth in both Essex and Kent. The preferred route was announced in April last year and the initial ground investigation will provide essential detail for submission of the Development Consent Order.

The focus of the work has been mostly on ground characterisation and classification with strength testing to provide information for preliminary design work.

“The aim has been to get the best possible data to work out what’s feasible,” says Lower Thames Crossing (LTC) site assurance lead Simon Harlow.

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CJ Associates carried out the rotary coring as a subcontractor for Structural Soils

Structural Soils, with supervision from Aecom, started the work on the south side of the river in mid-September and then moved to the northern site with work completed in late January. The boreholes to the south extend 2.5km along the planned route, while in the north the boreholes are spread over 2km.

In total 38 boreholes have been undertaken along the route both north and south of the Thames in the current phase. Both cable and rotary techniques have been used, as well as continuous penetration tests. Insitu testing has included high pressure dilatometer, packer testing and surface and down hole geophysics.

Boreholes have extended to up to 65m in some locations but the average depth has been around 45m.

Structural Soils carried out the cable percussive boreholes itself but called in CJ Associates, which is now owned by Structural Soil’s parent company RSK, to carry out the rotary boreholes while the CPTs were carried out by Insitu Site Investigation.

On the northern side, the expected ground conditions were alluvium and River Terrace Gravels, over a thinning layer of the Thanet Sands and Chalk. The thicknesses and depths to the deposits, along with the quality of the Chalk has been different to that anticipated and led to some of the borehole depths being extended.

Structural Soils site agent Mark Honess says that on the south side, establishing where the Chalk is encountered and thickness of the alluvium were the main aims.

“The quality of the Chalk was lower than expected so the depths of the boreholes needed to extended further in order to get some quality samples,” he says. “One of the tests being carried out is flint hardness testing which is important for the design of the tunnel boring machines.”

According to Honess, Structural Soils also carried out some experimentation with different bits, flush and polymers in order to get the best possible samples.

The work itself is straightforward but the logistics and access have been challenging.

On the north side, there is the added challenge of historic landfills – some dating back to the Victorian era – but there is also an active landfilling process underway with tunnel waste from the Northern Line Extension and Tideway raising the ground levels at many of the borehole locations.

Harlow says: “While it is mostly London Clay, the agents added to help the material flow through the tunnel boring machine mean that it is not in a natural state and it has led to some challenging access issues.”

Honess adds: “We have had to use tracked dumpers to tow equipment into position on the tunnel waste but even that has been challenging when taking the water bowsers to locations for the packer testing. We are using up to 4m3 of water per test and up to six tests per hole, so we have had to use pump lines to take the water to the boreholes where. We used 250,000 litres of water on the south side.”

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The ground investigation team has worked in agricultural fields on highways, in industrial areas and through residential access, as well as the Metropolitan Police’s shooting range near Gravesend.

Access in general has been a challenge with lots of stakeholders to consult, the ground investigation team has worked in agricultural fields on highways, in industrial areas and through residential access, as well as the Metropolitan Police’s shooting range near Gravesend.

“Originally we were only allowed to have access to that site in August but we were able to negotiate weekend access to allow those boreholes to be undertaken in this phase of work,” says Aecom site manager John Andrews.

This access did not make the work straightforward though. “We have to carry out the boreholes over two weekends and bury the borehole to reinstate the area between weekends,” says Honess. “Completing the boreholes in that time period was challenging, let alone with the logistics and deadlines.”

Health and safety has been a key focus of the work. “Highways England has been keen to improve health and safety on this scheme,” says Andrews. “There has been a larger scale approach to the ground investigation which has included development of a compound with welfare facilities and use of bunds and fencing around each borehole location to clearly define the work area.”

While the drilling rigs have now moved off site, laboratory testing is still underway and long term gas and water monitoring will be starting on the site soon.

Harlow said that the next phase could start as early as March and focus on the overwater investigation within the Thames channel. “The aim is to have all the ground investigation completed by the end of this year,” he adds.

route map website version

The preferred route was announced in April 2017.

Economic boost

When announcing the preferred route, Transport secretary Chris Grayling said that the Lower Thames Crossing would “create more than 6,000 jobs and boost the economy by £8bn”.

The new 21km link from the M25 near North Ockendon will cross the A13 at Orsett before crossing under the Thames east of Tilbury and Gravesend and link to the A2 close to where it becomes the M2.

The estimated cost of the new tunnel is £4.4bn to £6.2bn and an additional £10M of upgrades are planned to improve traffic flows at Dartford Crossing and to cut “rat runs” in Dartford and Thurrock.

The tunnel will be twin bore with three lanes in either direction and traffic estimates suggest that the new tunnel could carry 4.5M heavy vehicles in the first year.

Highways England has said that the new crossing is essential to prepare for 38% more journeys across the Thames east of London by 2025.

 

 

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