Fitting A sqaure PEg IN a Round Hole - Newhaven Floating caisson
Newhaven Energy Recovery Facility is designed to export around 16.5 megawatts of electricity to the national grid from waste, which is equivalent to powering more than 25,000 homes a year using 210,000 tonnes of waste.
There were many complex challenges to face in its construction, as the design needed to be sensitive to the landscape. Prior to building, the Environment Agency imposed tight conditions on construction water discharges because the ground had been contaminated by previous industrial activity. Local planning conditions also imposed height restrictions, so it was built at a much lower depth than that of a conventional facility of similar scale.
In order to meet the environmental conditions, the refuse bunker and incinerators needed to be set in a 17 metre deep reinforced concrete pit. To enable the construction of this concrete pit, whilst avoiding excessive water discharge, Hochtief (The main contractor), used an innovative construction method. Firstly, the excavation of this pit was carried out underwater inside a coffer dam, which itself was formed by a 53metre diameter, 28metre deep, reinforced concrete diaphragm wall. The caisson structure (a sealed underwater chamber) was built next to the pit, inside a sheet piled dry dock. Once the excavation of the pit was complete, the dock was flooded, so that the caisson structure could be floated over to its permanent position within the diaphragm wall. This totally eliminated the need for long term ground water pumping, which was usually required by conventional methods, but was not an option in this case because it would breach the conditions of the Environment Agency. This is where Euroflo was asked to carry out several functions:
CONTROL & BALANCE
Euroflo provided a pumping system to control the levels of water compromising 8 ballast tanks, with two placed at each side of the caisson , forming part of its structure. This was to control the weight/volume of each tank, so as to ensure that the caisson remained balanced during the floatation and the moving of the caisson into position. This was achieved by positioning Grindex submersible pumps both inside and outside each tank in order to empty and fill them through a series of 100mm pipework, with special assemblies to aid the construction process. The 16 Grindex pumps, control panels and the ballast tank level monitoring equipment were all wired back to a centralised PC, where the levels could be monitored and controlled as was necessary, so as to keep the caisson structure level throughout its transportation across the water.
The job of the pumping system was to both fill and empty the main caisson structure in order to keep it suitably grounded, while the water filled the temporary dry dock structure. Too much movement during this stage could cause major problems on a project like this. Once the 17metre deep Cofferdam was full, the water level was reduced in order to float the structure into place.
This process was conducted using 2 Grindex Matador pumps from the nearby river. Once completed a number of 4” Grindex Major pumps were used to empty the structure along with some high performance 110v pumps with puddle sucker attachments to empty the chambers within the Caisson to low level of 3mm. This measurement has to be precise in order to maintain the balance.
PRODUCING A MANMADE LAKE
The filling of the Cofferdam effectively produced a manmade lake, which enabled the caisson to be floated across the 53metre area into its intended position. After this task was completed, the main Cofferdam was emptied back into the river through 8 metre long pipework while ensuring that all of the environmental regulations were met. These processes were performed using 6 Grindex Matador pumps; 3 for the filling and 3 for the emptying. While emptying the Cofferdam, it was necessary to carefully balance the caisson structure until it was able to reach the foundation base, which was some 20 metres below the water level of this man made body of water.
Once the structure was lowered into its final resting place, the groundwater level still had to be maintained until the mass of the structure could be supported by the normal surrounding ground water levels. Grindex Major pumps were connected to 450mm diameter pipes, which were together controlled through a series of control panels with probes and alarms. This was to ensure that the groundwater around the structure was maintained at a level so that backfilling around the caisson structure could take place with minimal disruption. The construction continued until a sufficient weight/mass was achieved, so that the structure would be entirely stable in the 17m deep pit in which it would permanently rest.