Removing a disruptive element
Murray Howitt, marketing manager with Stanton Bonna Concrete addresses the benefits of trenchless technology, highlighting projects at Welwyn Garden City and Heathrow Airport.If the UK construction industry has a fault, it is a reluctance to adopt - and to adapt to - new technology and techniques. As a result, many traditional and inefficient methods hold sway. One area that has been of concern is the lack of uptake in the UK of the comprehensive range of widely available, non-disruptive trenchless methods for pipe and cable installation and repair. It is estimated utility operations on highways result in a largely avoidable extra £2Bn each year in community and environmental costs as well as unnecessary disruption and damage.
Fortunately, a growing number of clients and contractors are taking advantage of the technique, using products such as those manufactured by Stanton Bonna. Essentially, trenchless construction consists of the creation of a number of excavated shafts from which a remote-controlled, or hand-dug, tunnelling shield is launched and behind which a succession of straight-walled concrete pipes are jacked. When the shield reaches the destination or reception shaft, it is either re-launched in a different direction or removed to another location and the process repeated. The excavated drive and reception shafts are usually converted to finished manholes once the pipeline installation is complete.
Particularly high levels of installation accuracy can be achieved with microtunnelling systems since they use sophisticated steering and guidance methods based on laser technology, combined with automatic computer or manual control.
'Pipejacking' is a term used for essentially the same process as microtunnelling but is generally applied to person-access sizes of over DN 900. The concrete pipes are manufactured to the requirements of BS 5911: Part 120: 1989, Precast concrete pipes, fittings and ancillary products. Specification for reinforced jacking pipes with flexible joints. This standard covers concrete jacking pipes from DN 900-3000. Apart from some of the smaller microtunnelling pipes, most jacking pipes are reinforced and the concrete cover to the steel cage reinforcement is generally more than the minimum 12mm specified in the British Standard. Additional cover can be provided if required.
There are two basic types of jointing system for jacking pipes - in-wall and steel-banded joints. For in-wall joints, the jacking thrust is taken on two faces with a fixed or rolling elastomeric sealing ring between them. Joint packers are provided on the concrete faces and these may typically be medium-density fibreboard, about 18mm thick and 650kg/m3 density. This type of joint is often used for shorter hand-dug drives and the pipe wall thickness tends to be slightly higher than for steel-banded joints.
The second type of joint involves a cast-in steel collar to form a socket end of the pipe and a spigot end with an elastomeric sealing ring, which may be self-lubricating. Packers are again used to prevent concrete-to-concrete contact. Microtunnelling and jacking pipes with stainless steel collars can also be supplied although they are not normally necessary as the low oxygen levels, at the typical depths at which these pipes are installed, prevents corrosion of the steel collars. For both types of joint, the design is for a horizontal thrust evenly distributed around the internal jacking face. If any joint is deflected so there is a gap between the end face and the packer, the permitted end load will be reduced.The kindest cut
The advantages of using a trenchless method such as microtunnelling can be substantial. Any attempt to dig long stretches of trenches often results in severe disruption to the community, delays and diversions to traffic. In addition, pollution takes place through excessive dirt, dust, noise, use of fuel for transporting granular surround/backfill and taking away excavated trench spoil. Further environmental impact occurs through quarrying for imported materials and landfill sites for surplus spoil. There can also be loss of profit for local businesses, damage to properties or other buried pipes and cables. Many of these items are usually referred to as social costs and are nearly always absorbed by the community rather than paid as direct engineering costs.
For many building and engineering projects, there can be a considerable advantage in using a minimum disruption method such as microtunnelling for installing drains and sewers. Obstacles such as roads, railways, rivers and canals can be negotiated far more easily and other buried services including pipes and cables can be avoided. Also, where necessary, new drains and sewers can be installed directly underneath existing buildings with a minimum of impact and/or damage.
One current project is the Welwyn Garden City Southern Outfall contract for Thames Water, which will relieve the Beehive, Commons and Penshanger pumping stations and connect into the Mimram Valley Trunk Sewer. The three pumping stations overload and flood during storm conditions. The contractor is Network North Alliance - a combination of Barhale and Subterra, with Barhale installing the jacking pipes and Subterra the open-cut pipes. Begun last September, the project is due to finish in October 2004, the jacking period being January 2003 to June 2004. In this case, as the sewer had to be installed at depths up to 26m, jacking was the only practicable approach.
In total, around 3,600m of DN 1200 Stanton Bonna jacking pipes are being jacked in 17 drives from 140-320m long, including approximately 25 interjack stations. A further 240m of DN 600 microtunnel pipes from another supplier plus 1,000m of DN 450 and DN 525 Stanton Bonna circular pipes will be installed in open cut. To date, Stanton Bonna is the first and only UK manufacturers to successfully produce machine-made, vertically-cast jacking pipes. Jacking work is being carried out at depths of 5-26m, 60% of which is through hilly open countryside and a subsequent 40% within Welwyn Garden City.Rapid progress
Barhale's project manager, Paul Salmon, and key support manager, Arthur Gugan, are both delighted with the rapid progress made to date. The company originally programmed to jack four pipes per day (10m) but is regularly achieving seven pipes per day (18m) in a nine-hour shift. This excellent progress is largely attributable to low jacking pressures and good ground i.e. chalk. Later, Barhale expects to jack through ballast seams and clay. Any delays are normally experienced as the closed-head Iseki TBM (tunnel boring machine) passes through changing soil types, as this necessitates switching separation tanks. As the project lies under a flight path, a gantry crane has been used to lower pipes down the shaft to the thrust pit, instead of the traditional and much taller, overhead crane.
To date approximately 1km of the 3.6km of jacking pipe has been successfully installed without incident and Barhale is currently running eight weeks ahead of programme. Deliveries from Stanton Bonna are running smoothly with the contractor keeping four days of stock on the ground at any one time. Barhale has also used an innovative system that enables electronic selection of where to pump lubrication for the pipes being jacked. The system is unique to Barhale and has proved highly efficient.
Archaeological interest in the site is high. The Museum of London Archaeological Society (MOLAS) has representatives on site watching all open-cut work and the sinking of the initial 3m of each shaft. Chalk spoil removed as slurry from the tunnelling face is being recycled by the landowners (mostly Lafarge tenant farmers and other private landowners) to reduce and rebalance the acidity in the soil. The Environment Agency has issued a licence of approval for this activity. Another project making use of Stanton Bonna products is at Terminal 5, Heathrow Airport, the client in this case being BAA (British Airports Authority). Here, Morgan Tunnelling is in joint venture with Vinci Construction to construct the tunnels. The tunnels included are:
- Airside Road Tunnel,
- Heathrow Express Extension,
- Piccadilly Line Extension,
- Stormwater Outfall Tunnel,
- A3044 Service Tunnel.
Of these, the A3044 Service Crossing is being constructed to take 33kV cables and water pipes underneath the A3044 to the airport. Contractor Morgan Vinci Joint Venture is installing the first DN 2500 jacking pipes, supplied by Stanton Bonna, in a three-year contract begun in April 2002. The crossing comprises two 126m-long tunnels, 2.5m internal diameter, constructed by pipejack. The pipejacks are being launched from an 11-24m diaphragm wall box and driven into two reception shafts, each 6m internal diameter.
The project is the first to be supplied with Stanton Bonna's new DN 2500 jacking pipe and comprises 117 pipes, each 2.325m long, weighing 12.5 tonnes each plus two interjack stations. To date, one tunnel has successfully been installed with the second tunnel close to completion.
The contract has progressed extremely well from the outset, with Morgan Vinci working round the clock installing up to eight pipes per day. Project Manager Ken Henderson is delighted with the rapid progress made.
In spite of the slow uptake, concrete microtunnelling and jacking pipes are playing a significant role in reducing the social and environmental costs associated with pipeline engineering works. They have a number of benefits compared with other jacking pipe materials, such as a durability measured in centuries - which is particularly applicable for machine-made jacking pipes due to their low water/cement ratio and low concrete porosity. Most concrete pipes of this type are Kitemarked under the BSI Quality Assessment Scheme, providing assurance that pipes meet the specified requirements. For the larger pipe sizes concrete provides the only material choice.