The North London Heat and Power Project (NLHPP) will replace the existing ‘Energy from Waste’ plant at Edmonton with the ‘greenest’ such facility in the country. However, two foul water sewers were located where foundations for the new facility were proposed.
Negotiating a Foul Water Sewer Bottleneck
North London Waste Authority (NLWA) and, OTB’s client, Barhale Ltd, needed to negotiate the bottleneck caused by these sewers, keeping them fully operational while new shafts were constructed. Until the old sewers could be removed, it would be impossible for other key works to progress.
That is where OTB Engineering’s expertise came into play.
OTB identified the opportunity to design a pioneering tunnel drive shaft solution by launching two pipe jacks simultaneously from a single shaft. Technical challenges have historically discouraged attempts at such an engineering feat but OTB helped identify an approach which ultimately enabled Barhale Ltd to deliver time, cost and environmental benefits to their customer.
The core challenge was to construct the main shaft with a live sewer running through its middle, severely constraining working space. Additionally, the temporary works needed to be designed to minimise the thrust force of the pipe-jacking on the shaft and on the live sewer.
Two 15 tonne Iseki 1200 Tunnel Boring Machines (TBMs) were used. The proximity of the live sewer, and the risks associated with causing damage to it, meant the TBMs required constant monitoring. Because the new shafts had to be installed below the existing sewers, they had to be suspended above them for part of the project. Nonetheless, the pipe jack was completed successfully in just ten days – achieving several financial and environmental benefits for NLWA.
Financial and Environmental Added-Value
Manpower requirements were reduced as the same team could manage tasks across the two pipe-jacking operations and items such as slurry and lubrication management were automated.
There were numerous carbon savings. Idle time for the shared equipment such as separation plant, crane, excavators, generators and pumps were reduced considerably. Diesel consumption was halved.
Modifications to the tunnelling equipment optimised equipment efficiency, reduced power demand and lowered carbon generation.
The amount of water used during the slurry recycling process was estimated to be between 50% and 60% less than if a conventional solution of consecutive pipe jacks had been used.
The programme bottleneck caused by the two foul water sewers was cleared ahead of schedule. The need to delay building whilst waiting for time-consuming sewer diversion work was avoided. In fact, NLWA was able to start subsequent works early on a programme which promises to deliver a climate-friendly, state-of-the-art, ultra-green waste energy and recycling plant for North London.
Innovation in Tunnel Construction
OTB delivered simplicity of design, value engineering, sequencing and certainty which resulted in time and cost savings for the customer and their contractor. That’s part of what we do. By demonstrating that a simultaneous pipe jack solution could be successfully implemented, OTB has also provided an innovative breakthrough for future application.
Technical Excellence and Winning Delivery
OTB Engineering is a dynamic and progressive civil engineering consultancy practice that specialises in tunnel, construction and geotechnical engineering and rail assurance. It is known for its consultative, collaborative, creative and agile approach. The team builds client relationships that last, based on technical excellence and winning delivery. They work on civil engineering’s biggest challenges: building core infrastructure and contributing to economic development nationally and internationally.
