Articles & Case Studies


Posted: Tuesday 23rd June 2009

Darren Watson Ė Technical Sales Engineer - from pump hire specialist, Sykes Pumps, outlines the challenges of keeping theTyneTunnel projectís dry dock pre-fabrication site dry.

Plans to build a road tunnel under the river between Howdon in North Tyneside and Jarrow in South Tyneside were first put forward by Durham and Northumberland County councils back in 1937. Delays caused by the second world war and subsequent funding issues meant that the vehicle tunnel did not finally open until 1967 Ė and no-one could have predicted then the volume of traffic that uses the crossing on a daily basis now.

To tackle the need for greater capacity and ensure that the 40-year-old tunnel continues to provide a vital road link between the two banks of the Tyne, a £260 million scheme is now underway to refurbish the existing tunnel and create a further tunnel just downstream (east) of it. The scheme is being undertaken by TT2 Limited, a consortium appointed under a PPP (Private Public Partnership) agreement, which will not only part finance, design and construct the tunnel, but also take responsibility for operation and maintenance of all the tunnels, including the pedestrian and cycle tunnels, until they are handed back to the Tyne & Wear Passenger Transport Authority in 30 years time. Once constructed, the new vehicle tunnel will complete the dualling of the A19 between Northumberland and North Yorkshire and will carry traffic from north to south, with the original tunnel being used to carry traffic from south to north.

Itís a massive undertaking with the new dual tunnel crossing not due to open until winter 2011. To minimise disruption to road users, design & build contractor, Bouygues Travaux Publics, will build the new tunnel first and this will be opened to two-way traffic initially (scheduled for Autumn 2010) while refurbishment of the original Tyne Tunnel is completed. A significant proportion of the work will be completed on dry land and prefabrication of the concrete tunnel sections that will make up the 1.5km second tunnel is a challenging task in itself, not least because the Ďdryí land being used as the construction site is actually a redundant dry dock, conveniently located 3km upstream but prone to water ingress that could potentially jeopardise the schedule.

Local Heritage

The A&P dry dock being used as the construction site for pre-fabrication of the concrete tunnel sections was once part of the Wallsend Ship Yard, which closed in 2004. Bouygues considered dry docks at a number of sites nationally and abroad but the companyís aim was to find a local solution and Wallsend dock offered the space and facilities needed to construct the tunnel units.

One of the local benefits of selecting Wallsend as the site has been the appointment of Gateshead-based Volker Stevin as the sub contractor tasked with preparing the dry dock for use and constructing the tunnel elements. The sub contractor began preparation work at Wallsend in May 2008 and the first concrete pour took place in the autumn, with construction of the prefabricated sections expected to be completed by autumn 2009. Each of the four tunnel elements will be 90 metres long, 15 metres wide and 8.5 metres high and they have been designed to fit both the adjacent unit and the riverbed. Once construction of all the tunnel sections has been completed the dry dock will be flooded so that they can be floated out towards the tunnel site.

The need to float these substantial concrete structures out into the river following pre-fabrication is the main reason why identifying a suitable dry dock site for this element of the project was so essential. The dry dock at Wallsend was originally designed to allow ships to be floated onto the river, but, just like the tunnel elements, the ships needed to be built on dry land so careful management of water ingress has always been required at the site. Without suitable pumping equipment to stop it, water can penetrate the site through the dry dock gates and through the ground potentially submerging the site, so part of preparing the dry dock for use before the first concrete pour involved removing the ground water at the site and putting in place a fail-safe pumping system to keep it dry throughout the year-long work schedule.

It was, therefore, vital that the site be fitted with a pumping solution that was both effective and efficient as the construction site could not remain operational if water levels were allowed to rise. Whatís more, management of those levels has to be completely reliable, even throughout the night when there is no-one on site.

Fortunately, we are very familiar indeed with the risks and requirements of preventing water ingress at the dry dock as we provided the pumping solutions for the site when it formed part of Wallsend Ship Yard. This experience simplified the specification of the pumping equipment and gave Bouygues Travaux Publics additional confidence that the solution would, indeed, be sufficient for the projectís needs.

Bespoke Specification

We specified a Sykes Pump 22 kW, PX12 N electric submersible pump to remove all water from the dry dock, the same model often used by the ship yard for the purpose when it was operational. The brief from the contractor was that the pump would need to provide a 12m maximum head, (i.e: be able to pump the water at least 12m clear of ground level), however, no maximum flow rate was stipulated as the volume of water ingress can fluctuate with environmental factors such as rainfall and the tides. The PX12 N provides a maximum head of 30m, more than sufficient for the application and, as a maximum flow rate it can pump up to 100 litres of water per second. However, at a head of 12m, the pump can operate at a maximum flow rate of around 60 litres per second and, thanks to the teamís prior knowledge of the site, we knew that this would be sufficient for the projectís needs.

While specification of the pump for the site was little more than a formality, however, designing the complete pumping solution for the particular needs of the tunnel elements construction project was a more complex undertaking. The pump is located in the dry dockís sump hole, a wide pit around 3m deep which was originally designed into the dry dock as a form of overflow for sea water coming in off the Tyne. This has always been used as the location for pumping equipment at the dry dock but the fact that the dock has been disused for several years created an additional complication. While the sump hole was cleaned regularly when the dry dock was in use by the ship yard, the intervening years have seen it clog up with silt so it was necessary to design a holding cage for the pump to ensure that it was protected from the silt and could, therefore, operate efficiently.

From the pumpís location in the sump hole, the water pumped out of the dry dock has to be removed into the adjacent dry dock, several metres away and on the other side of the 12m high wall. In order to achieve this, the pump engineers advised on appropriate pipework with the PX12 N pumping the water through sections of 150mm six metre rigid pipes and three metre flexible hoses running along the dry dock floor then up over the wall into the adjacent dock. As the efficiency of the water management programme depends both on the reliability of the pump and the integrity of the pipework system both these elements are checked as part of a fortnightly routine maintenance regime.

Ensuring Efficiency

Underwriting the efficiency of the pumping solution does not end there, however. Two PX12 N pumps have been supplied and installed at the site, with the second pump fully commissioned and ready to operate as a standby should the need arise. Both pumps are located in the sump hole and are powered by their own 60 kVA generator and connected to their own individual pipework, which runs in parallel. The site team can switch manually between pumping systems if a fault is detected or routine maintenance is required on either pump or generator, ensuring that there is an operational pump available at all times. However, each pump is controlled by its own starter panel and should water levels begin to rise, float switches located in the sump hole will automatically trigger the system to switch to the standby pump and isolate the duty pump. This means that water levels can be controlled again before they reach hazardous levels and the duty pump will be protected from damage until an investigation into the reasons for the failure can be carried out.

There are clear advantages to an automatic default like this, particularly when combined with a manual override like the one being used at the Tyne Tunnel site. However, just to ensure that there was absolutely no risk of an undetected fault the whole system has also been linked to a GSM telemetry device which alerts both Sykes Pumpsí engineers and the Bouygues project team if there is a malfunction.

This remote system automatically dials out to specified Sykes Pumps and Bouygues team members if there should be a power failure or a pump malfunction Ė even though the standby system will have automatically started to address the immediate need to keep water out of the site.

Critical Connections

To date, construction of the four tunnel sections is running to schedule. Once they have been completed they will be floated to Howdon Dock before being lowered into place sequentially, immersed and fitted with an internal ballast tank before being put in place on the dredged riverbed and pulled towards the transition structures on the river banks so that the connection between the two can be made.

Pre-fabrication of the tunnel elements may be just part of a complex programme of works but keeping this work on schedule by ensuring the site remains dry at all times is a critical part of delivering a successful scheme.

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