Sheet piling and hydraulic shoring is allowing a sewer system to be re-engineered. David Taylor
Most people know about Portsmouth’s history as an important naval port but few realise that it is also Britain’s only island city, most of it lying on Portsea Island where the Solent meets the English Channel.
Foul sewage and rainwater flow under gravity to the Eastney pumping station where it is pumped several kilometres to the Budd’s Farm sewage treatment works in Havant. After treatment, the clean effluent is pumped back to Eastney and then released into the Solent via a 5.7 km outfall.
There are very few natural water courses on Portsmouth’s Portsea Island and so when it rains, most of the run-off ends up in the sewage system and heavy flows are contained in storm tanks at Fort Cumberland. “Within 20 minutes of a heavy downpour we can have up to forty times the normal ‘dry’ flow entering Eastney” says senior project engineer with utility company Southern Water Ben Green. “It’s like a wall of water and it can fill up those storm tanks pretty quickly”.
Now, in line with the recommendations of the Pitt Report published after the disastrous floods of June 2007, Southern Water is adding extra pumps to help cope with the increasing likelihood of such extreme events. Four new submersible electric pumps, capable of pumping up to 9,000 litres per second, will provide much-needed support to the six existing diesel pumps at Eastney.
The work involves tapping into one of the two main interceptor sewers bringing effluent into Eastney and diverting part of their flow via the new pumps and a new 2.2 m diameter pipeline to the rising mains transferring flow up to the storm tanks at Fort Cumberland. The new pumping station, like most of the plant at Eastney, is underground; “Eastney’s like an iceberg – what you see above ground is only a tiny fraction of it” comments Mr Green.
The new pumping station is housed in a circular concrete structure 21 m in diameter, created by a series of 1.2 m diameter secant piles driven 20 m into the earth. On the side of this huge concrete cylinder, main contractor 4 Delivery (a consortium comprising United Utilities, Costain and MWH) has also built a 5.5 m by 6.5 m x 15 m deep square concrete box to receive the inflow from the interceptor sewers.
From the circular pumping station the 2.2 m pipeline snakes for 120 m across the site to join up with two 1.8 m diameter steel pipes which carry the treated effluent from the six existing diesel pumps to the outfall at Fort Cumberland.
Excavating the trench for the 2.2 m pipeline meant negotiating the numerous underground obstacle and hidden services (including high-voltage electricity cables, and several deep shafts) at Eastney, all within a very restricted site.
“Working space was very restricted, with residential properties to one side and an existing pumping station five metres to the other” explains Martin Tresidder, site manager for 4 Delivery. “Minimising noise and vibration was essential when selecting equipment for the installation of the sheet piles. We worked to ensure a silent piling system was used and all sheet piles were installed with no disturbance to properties neighbouring the site.”
Ground conditions were also highly variable, ranging from made ground at the surface to, in places, a very soft clay alluvium layer below and firm and stiff to very stiff clay in other areas. To install the pipeline, 4 Delivery employed Milton Keynes-based piling specialist Dawson Construction Plant to drive a series of 9 m long L605 and 7m long L604 interlocking steel sheet piles to support the side walls during excavation and installation of the pipeline.
The extreme depth of the excavations combined with the poor ground conditions imposes high horizontal loads on the steel sheet piles supporting the sides of the 8.1 m deep trench. Lateral stability for the excavations was provided by specialist hire company Groundforce Shorco, which supplied its 1500 Series high capacity hydraulic bracing system and MP125 modular struts to prop the sheet piles.
“These are deep excavations which demand a flexible support system which can be adapted for the prevailing ground conditions” comments Keith Brown, technical sales representative for Groundforce. “In one excavation we’ve installed four levels of hydraulic bracing frame to withstand the horizontal loads”.
The 1500 Series is a high capacity hydraulic bracing system comprising modular steel members in lengths from 3 m to nearly 17 m. Various lengths can be combined to give the necessary span, and the integral hydraulic rams used for final adjustment and pre-loading. At Eastney, the frames are being used as waling beams along the steel sheet piles lining the 4.6 m wide trench.
Lateral support is provided by the MP125 bracing struts which span the 3.7 m gap between the waling beams. These struts comprise a hydraulic ram within a tubular extension piece fitted with swivel connectors at either end. These swivels allow the struts to be fitted as knee-braces across the corners of an excavation if required. Like the 1500 Series frame, the hydraulic ram provides final adjustment and is fitted with an integral lock-off valve for security.
The pipeline is being installed in about 6 m lengths with each section backfilled as soon as practicable to allow the next section to be excavated and installed. “The key factors influencing this decision are firstly economics” explains Mr Tressider. “It would be very costly to hire the support equipment for a total of 120 m of open trench. Another reason is the programme: we want to complete the work in as short a time as possible. If we wait until we excavate the total length of trench before starting to install pipes, the work will take much longer to complete.”
A third reason is safety. “Protection of small sections of trench is more easily managed than a 120 m long run” says Mr Tressider. The maximum length of trench likely to be open at any one time will be 40 m, he adds.
The use of a modular hydraulic shoring system allow the support structure to be quickly removed on completion of each pipe section to allow backfilling, ready for excavation of the next section of trench. To embark upon a project of this kind using the traditional method of fabricating bespoke steelwork to support the trench sheets would be a slow process.
“The design of the trench and supports for the installation of this 2.2 m diameter pipeline took into account the many challenges faced when carrying out the works” continues Mr Tressider. “The trench had to be wide enough to accept the new pipe, but also to allow passage of our excavating plant within the trench corridor.
“We needed to develop a design to suit our site needs and the sheets and propping system had to take into account the varied ground conditions. The propping spacings needed to be set up so that 6 m long pipes could be lowered into the trench for welding, and waling positions located to allow welders free access around the pipes while they are still in position” explains Mr Tressider.
Work started on the project in June 2008 with the construction of the new pumping station shaft in Bransbury Park, a public amenity area adjacent to the Eastney site. When completed, the pumping station will be buried and grassed over so the area can be returned to public use again.
Installation of the pipeline is now well underway and progressing according to schedule. The whole project should be completed and commissioned by mid-summer 2010.
Preventing a flood waiting to happen
With its scope for physical expansion thus limited, Portsmouth today is one of the most densely-populated places in Europe; the only place more crowded in the British Isles is Central London. And like any crowded urban community, Portsmouth’s population imposes a heavy burden on the city’s network of drains and sewers.
“Here, all sewers lead to Eastney” says Ben Green, senior project engineer with utility company Southern Water. Eastney is where Southern Water has its main pumping station which drains the whole city of Portsmouth. “The city’s like a tea tray – very flat”, says Mr Green, “and everything drains to Eastney, in the south-east corner. It’s a key site for the whole city”.
In fact, Eastney is so critical to the flood protection of Portsmouth that it is listed as one of only three designated nationally important ‘critical infrastructure’ locations for Southern Water. But despite several upgrades within recent years, it is still below optimum capacity. This was demonstrated dramatically in 2000 when a ‘one-in-108-year storm event’ inundated the Eastney pumping station, submerging its six key diesel pumps.