The West Ham Flood Alleviation Scheme (FAS) serves a densely populated area of East London, approximately one mile (1.6km) south of the 2012 Olympic Park, and one mile (1.6km) north of West Ham’s football ground at Upton Park. In the 19th century, a combined stormwater and sewage system was constructed through this area, gravity draining to Abbey Mills Pumping station where it was pumped into the Northern Outfall Sewer, running to Beckton treatment works to the east. In the intervening time, a rapid expansion of housing followed by a steady growth in the construction of extensions and hard-standing has over- stretched the system, and since the 1950s occasional flooding of streets or basements has occurred.

Under its commitment to The UK Water Regulator (Ofwat) Thames Water was required to take more than 5,500 properties out of flood risk by 31 March 2010, with more than 650 of these from the West Ham scheme. A number of options were considered including underground storage tanks to be used during overflow events, and tunnelled outfalls in a number of different directions to connect with adjacent systems. The selected option was to provide a new interceptor tunnel running lower than the existing system, pumping from the terminal shaft to the existing pumping station at Abbey Mills.

AECOM provided a reference design in preparation for a design and build tender under NEC3. Detailed studies and modelling of the existing system revealed that 22 interceptions onto the existing sewers were required, which ultimately resulted in the layout as shown in Figure 1. This included the construction of 3.3km of 2.87mID tunnel, 2.5km of 1.2mID microtunnels and 2km of open-cut.

The tunnels were to be driven in the Thanet Sands and the clays, sands and gravels of the Upnor formation at the base of the Lambeth Group, as shown in Figure 2. The route was to pass over the Olympic Development Authority (ODA) cable tunnels and the Channel Tunnel Rail Link (CTRL) tunnels, and beneath a number of London Underground (LU), Docklands Light Railway (DLR) and Network Rail lines, a tower block and a medieval church.

To enable the scheme, some 98 utility diversions were required, with 90 per cent of these let prior to award of the main contract. At T5 alone, the bill for the combined utility diversions exceeded GBP 500,000 (USD 757,000).

In March 2008, following a competitive tender process, Costain was appointed design and build contractor, with Halcrow as its designer. The team’s objectives were safety, managing third parties and delivering a product that would last for 150 years by the 31 March 2010 deadline. Many of the sites would be within roads and adjacent to housing and businesses, inevitably causing significant local disruption. A substantial community relations programme was launched. More than 28,000 letters were put out in 20 languages, staffed drop-in centres were positioned at the sites, local functions were supported, schools were visited and a competition was held for local schoolchildren to decorate sections of site hoardings.

Drive Shaft, T1
An initial site investigation borehole within the grounds of the Abbey Mills pumping station revealed the ground around T1 to be contaminated with benzenes and other aromatic hydrocarbons. Further investigation showed this to be limited to the Harwich and Channel Sands immediately below the London Clay. The source could not be identified but is thought to be from a former gasworks to the north. It was vital that the contaminants were prevented from reaching the lower aquifer, which is a source of drinking water nearby.

The shaft sinking methodology was therefore adapted:
• To cut off the contaminants, a secant pile cofferdam was driven to a level below the contamination and yet above the lower aquifer
• Dewatering wells were drilled into the Thanet Sands to relieve lower aquifer pressure. The discharge from these was sampled, demonstrating that no contaminants reached the lower aquifer either before or during construction
• A 15mID shaft was sunk to near the base of the secants by underpinning with the contaminated material within the shaft removed, requiring continuous monitoring, protective suits, forced ventilation respiratory protection and a strict occupational health regime
• A 12.5mID shaft was then constructed as a caisson from within the 15mID shaft.

The methodology worked well, though a number of secant piles were insufficiently vertical, causing some leakage into the cofferdam and requiring remedial grouting.

Main Tunnel Drive
The 2.87mID tunnel was driven with a new Lovat M132 EPB with a hydraulic drive and, unusually for a TBM of this size, a cable reeler. The large diameter screw conveyor can be changed for a belt, though the West Ham tunnels were all driven in EPB mode.

With the launch from T1 directly into contaminated ground, Costain proposed an elongated screw (from 14m to 20m) to take the discharge further away from the build area, thus allowing the creation of a well- ventilated zone for most of the workforce for most of the time. Continuous monitoring, the provision of personal and respiratory equipment and occupational health monitoring were continued for the first section of the drive.

The intermediate shafts were part-filled with foamed concrete to allow the TBM to mine straight through, though the TBM stuck while mining through T4 on a 200m radius curve and had to be partially dug out to relieve some thrust pressure.

The contract specification required settlement of the surface or structures to be less than 10mm, but the final settlement was generally no more than 4mm, demonstrating an effective face loss of one per cent. Only minimal movement occurred to the CTRL tunnels and there was no reported damage to utilities. The most severe property damage was only in the ‘very minor’ damage category.

Microtunnelling
The scheme included 18 microtunnel drives of varying lengths totalling 2.5km of 1.2mID pipes. These were intended to be driven by two TBMs, but at one point eight were employed. Two major problems occurred:

• The pipe following the TBM driving from MT10 to MT9 became stuck when the TBM was just 30m short of the reception shaft. Interjacks showed that the TBM would still move, but the pipe could not be advanced without major structural damage, indicating something had settled onto the tunnel.
• The TBM driving from MT4 to MT5 veered off course to such a degree that it could not be corrected. An additional shaft was sunk to recover the machine, and an additional machine was used to drive the remaining tunnel the other way.

It was not possible to sink an additional shaft to recover the MT10 to MT9 machine, and so a hand shield was used to drive a recovery tunnel the 30m from MT9. In preparation for this, permeation grouting was carried out above the tunnel, and ejector wells were installed beneath. Controls were introduced to minimise the vibration effect on the miners from using pneumatic tools, and thin 1.63mID steel liner plates were used to line the tunnel. These performed well, as long as they were built with good circularity. The 1.5m OD TBM was pulled through the temporary tunnel, which was then lined with 1.2mID concrete pipes.

Programme
Following early delays due to the contamination at Shaft T1, the team was constantly focused on the required “flows on” completion date of 31 March 2010. On the 2.87mID TBM, which broke through into T5 in mid December 2009, 24-hours a day, 7-days a week working was introduced. At peak, the TBM constructed 260m of tunnel in one week.

The recovery operations on the two microtunnelling machines were carried out just in time, with the last pipes installed in late March 2010.

The open-cut sections were completed without incident, the various connections made to the existing system and the shaft finishings installed. Mechanical and electrical installations included the pumps and pipework at T1.


Figure 1 – To update the West Ham FAS, Thames Water chose an interceptor tunnel, running lower than the existing system Figure 2 – Tunnel alignment and geology. The new tunnel route passes over and under a number of rail tunnels operating in and around London Building the inner caisson in T1 Contamination at Shaft T1 required shaft sink to be adapted Shaft T1, nearly complete