Top trenchless techniques

Nick Sheehan of Perco reports on low-impact trenchless technology

Over the past 20 years, trenchless technology has established its place in pipeline installation and rehabilitation. Many examples have passed into the realm of tried-and-tested techniques favoured by utilities and major contractors, and others are coming through to widen the scope of available technology. Even so, the trenchless approach, as a whole, cannot be said to rank as first choice in all situations.

We are still delaying or diverting traffic and sending countless tonnes of spoil to landfill, with the excessive fuel consumption, noise and dirt that entails. As ever, the main issue is one of cost. Much of the real cost of open-cut working is met not by the utility company but by the locality, in terms of disruption and environmental impact totalling an estimated £2B/pa.

As long as these unseen costs are unaccounted for in project estimating, there will always be a case to be made for open-cut, where trenchless techniques are judged not to be essential. However, there are strong signs this may change, as a matter of government policy in the UK. Announced in the Queen's speech last year, a new Traffic Management Bill will focus greater government attention on the disruptive effects of streetworks.

A probable consequence of this interest will be greater promotion of technologies that counter disruption to traffic and pedestrians. Not every trenchless solution is more expensive than the open-cut alternative by any means and legislation favouring trenchless methods may not prove to be an undue burden for utilities. Already, four UK water utilities have commissioned a portfolio collaborative research project by WRc, with findings that so far appear to support this view. The Real Cost of Streetworks to the Utilities and Society research examines planned rehabilitation projects - such projects can generate serious disruption to the community adjacent to the streetworks. As Doctor Andy Russell of WRc Utilities stated: "For these situations the indirect costs (for example, those the utility are not legally responsible for and which are met by the community adjacent to, and commuters passing through, the streetworks) can be many times (four-16 times) greater than the direct costs the utility pays."

The project developed cost models for the major indirect cost elements and the indirect benefits to serviceability. Russell added: "These have been enhanced with the WRc waterfowl whole-life-costing software, resulting in a versatile model that estimates the direct and indirect costs associated with specific water mains rehabilitation projects, which enabled the overall benefits of various rehabilitation techniques to be quantified. The initial findings indicated that, as the degree of urbanisation increases, the benefits of using trenchless or minimum excavation techniques becomes greater." Now the trenchless technology industry has come of age in the UK, with comparatively few of its techniques still tied-in to individual contractors, choice and availability have multiplied and achieving maximum cost benefit is a more complex matter than ever before.

The question 'open-cut or trenchless?' is becoming superseded by 'which trenchless technique?' Specialist contractors are now offering the answers by assembling broad trenchless technology portfolios. This new diversity promotes impartial advice, through which best practise solutions can be reached for any project. Moreover, contractors are introducing least impact techniques, with less associated environmental disruption than the established trenchless systems, enabling utilities to drive down effective costs further.

One trenchless technique of this kind was used to avoid disrupting a site of special scientific interest (SSSI) this year, as part of Severn Trent Water's extensive flood alleviation measures in Kenilworth, Warwickshire. Perco was called in by North Midland Construction to burst an existing nine-inch sewer and install 120m of 450mm MDPE pipe.

The job was completed on time and budget, using a 225t rod puller. Stuart Proud, operations manager for North Midland Civil's engineering division explained: "As sections of the sewer ran under trees and a wall, Perco's choice of non-percussive bursting was critical. The SSSI is close to Kenilworth Castle and the work area was given a precautionary survey by English Heritage archaeologists before work started. With a river adjacent there was also a potential risk to the local newt habitat and this was minimised by erecting a newt barrier, 12 weeks in advance."

Inevitably, trenchless installation and rehabilitation processes carry their own environmental cost - in some cases releasing chemical pollutants or impacting on landfill sites. Others, requiring minimal excavation, are still used when even less disruptive alternatives now exist. For example it is no longer necessary to excavate launch and recovery pits for pipe-bursting in cases where compact equipment is available to work from one manhole to another.

An instance in the field of sewer renovation is the recent introduction to the UK of EcoCIPPTM. This German development in trenchless renewal allows a liner to be installed without the pollution problem of most cured-in-place-pipe (CIPP) systems. Pollution occurs with conventional cured-in-place felt liners when they are heated, to activate curing, by flushing with hot water. This causes styrene, a volatile, potential carcinogen, to wash out of the liner into the water, which is then lost to drain. The new system avoids the pollution drawback by using ultra-violet (UV) light, instead of hot water, to activate curing. It also uses a fraction of the energy consumed by conventional CIPP, as water heating is not required.

The process is faster and there is no need to bring a boiler truck on-site. Provided the sewer is not badly distorted, an EcoCIPPTM liner can be deployed straight into the old sewer pipe, following cleaning, thorough inspection and removal of any obstructions, such as projecting service lines or grown-in tree roots. The liner is winched through the pipe between manholes and then closed at both ends using a special sealing system. At this stage the liner is very flexible. It is inflated with compressed air, which presses the liner against the bore of the existing pipe to produce a tight fit. The final step is to draw the system's UV curing lamp/camera unit along the length of the liner, hardening the resin and forming a high-strength pipe-in-pipe.

One of the additional benefits of the system is the superior strength of the liners, due to their high glass fibre content and seamless construction. At a given wall thickness, the carrying capacity of the liner is 100% higher than that of a typical, laminated felt liner, allowing thinner wall sections to be employed. A prominent trenchless technique, applied to pipe installation, has gained popularity due to the predominance of PE in both water and gas distribution. Horizontal directional drilling (HDD) can install pipes under roads, rivers and railways with all the attendant advantages over open-cut work. While HDD avoids the potential need to import backfill materials to site, conventional HDD entails an amount of 'spoil away' due to the use of drilling muds.

Muds based on Bentonite clay can vary in consistency almost to the heavy solid and the high degree of structure in the material is difficult to break down in landfill. Alth-ough this binding property is of benefit to farmers to improve soil structure, the facility for farmland disposal has been withdrawn since 2001. With a limited number of licensed disposal sites available and disposal costs at stake, unauthorised dumping of drilling mud has become a problem. Thankfully, the solution is now coming from contractors and mud tankering companies, working together to ensure this aspect of drilling is handled responsibly.

As a result, contractors to the utilities should now be using mud mixing and recycling plant, on-site, so deliveries and waste removals are minimised. Even biodegradable muds are now available, enabling drilling contractors to reduce the long-term impact of disposal. This approach was followed in September, with a series of four road crossings completed by HDD near Hemel Hempstead.

The contractor, Fabricon, suggested HDD in order to complete a 40m section of a fire hydrant system, which it is installing for a BPA fuel depot. This method offered the fastest way to install the pipelines without open-cut works across the road. Whenever a cutting head crosses a soil interface, close control is essential to keep it on track and in this case Perco had to negotiate layers of sand, gravel and flint soils. Total precision was vital, as the drill ran underneath a gas main and four fuel lines and the latest tracking technology was employed. The project installed HDPE pipes, running 40m in parallel, with a 1m separation and drilling muds were mixed and recycled at the site. Trenchless technology has earned the reputation for minimising disruption due to engineering works, particularly in pipeline installation and rehabilitation.

Often the environment benefits because the landscape is left intact, there is less noise and the extra exhaust emissions from queuing traffic are avoided. Water utilities are increasingly considering the wider effects of streetworks and other planned maintenance or renewal activities, and as the trend gains momentum, trenchless technology is ready to reduce the indirect costs.


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