Thames Water Ring Main AMP4 London Resilience Programme

At present the TWRM forms a complete ring around the major water supply zones within the Greater London area. The TWRM was constructed in several phases in the 1980s and 1990s. It comprises of 80km of tunnel and 11 No. pump-out shafts. The existing TWRM is supplied by 4 No. input shafts, each linking to a Water Treatment Works in the Thames Valley.

As part of the Asset Management Period (AMP) 4 Consultancy Framework Agreement between Thames Water Utilities (TW) and Mott MacDonald, we are providing specialist engineering design and planning advice for the Thames Water Ring Main (TWRM) resilience programme involving northern and southern tunnel extensions and new pump-out shaft projects.

The TWRM AMP4 resilience programme comprises the following;

• Southern Tunnel Extension from Brixton to Honor Oak
• Northern Tunnel Extension from New River Head to Stoke Newington
• Coppermills Water Treatment Works shaft connections
• Hampton Pump Out shaft

The two tunnel extension projects are both approximately 4.9km long and 2.5m internal diameter. The southern extension will run from the existing pump out shaft at the TWRM site in Brixton to TW’s Honor Oak pumping station site. This will give Thames Water greater operational flexibility for supplying the South East area of London. The northern extension runs from the TWRM New River Head site (near King’s Cross) to Stoke Newington and thereby linking the existing ring main to Coppermills Water Treatment Works in the Lee Valley. The new pump-out shaft at Hampton is to replicate the function of the existing High Lifting Pumping Station (HLPS) at the Advanced Water Treatment Works. The new transfer pump shaft at Coppermills Water Treatment Works will transfer water to and from the TWRM during resilience events. All four projects aim to enhance the operational capabilities and robustness of the London clean water network.

Our co – located design team have been working with Thames Water Engineers in the client project office at Maple Lodge, Rickmansworth, and our head office staff has provided a supporting role to Thames Water. Together we have developed the design through the feasibility optioneering stage up to outline design and preparation of tender documents. The southern extension project has been presented an interim CEEQUAL› award for environmental excellence in civil engineering design. The northern extension project is expected to achieve the same.

The geology to be encountered by the tunnels for these projects is highly variable. The Brixton to Honor Oak tunnel will encounter the Woolwich and Reading Beds, Upnor Formation, Thanet Sands and Chalk. It will also cross the major Wimbledon Fault. The extension will be lined with a 2.9m ID single pass segmental lining. Each shaft is between 55.0m – 60.m deep and will be sunk by dry caisson jacking techniques. The shafts at Brixton and Honor Oak will both be 11.0m internal diameter to house pumps, high integrity valves and delivery pipework connecting to the clean water network distribution.

The two tunnel extension contracts have been awarded under a Design & Build type contract. Construction works for the tunnel extensions has commenced and with support from Mott MacDonald engineers on site supervising the construction works. The contract to construct the Hampton pump out shaft has been tendered and design is currently on-going.

The projects are expected to be fully commissioned by the early 2010.