Trenchless technologies such as microtunnelling, directional drilling and moling were originally developed for installing small diameter utility cables and services with minimum disruption.
But recently these techniques have been finding novel applications within geotechnical process and foundation engineering.
Directional drilling has been used on a number of reclamation projects at contaminated sites. Several petrol forecourts in US and mainland Europe, for example, have used directional drilling to introduce insitu clean- up agents deep into the soil. Accessing contaminant hotspots using pipes installed horizontally requires less drilling and is less disruptive than drilling vertically from the surface.
A similar concept has seen directional drilling used below unlined landfill sites for the installation of drainage pipes to prevent unwanted leachate migrating from the landfill.
Microtunnelling has been used to support ground in advance of large scale tunnelling. On the Jubilee Line Extension project at Westminster in London, a particularly sensitive section of tunnel was excavated below an arch formed by closely spaced microtunnels. This structural canopy within the ground meant tunnelling could take place with greatly increased protection for the historic buildings nearby.
In a smaller scale but nevertheless innovative application of a trenchless technolgy, impact moles operated vertically have been used to form driven displacement piles for house foundations.
Yet trenchless technologies have not been embraced by the UK geotechnical community with the enthusiasm of its mainland European counterparts. European geotechnical contractors which got involved in trenchless contracting at an early stage have a headstart as new applications arise within the geotechnical arena (in the UK contractors tend to be trenchless technology specialists).
Significantly, not one company in GE's most recent Geotechnical Services File listed trenchless technology as an area of specialisation.
Nevertheless a number of geotechnical consultants are developing small trenchless technology groups which may help to integrate the method within the arsenal of geotechnical techniques.
Evidence that changes are afoot recently came from consultant WS Atkins which announced its involvement in a consortium that had secured EC funding for research into the development of jet grouting using navigational drilling techniques.
One of the main objectives of the three-year Navijet project will be the development of a compact navigational drilling system tailored to jet grouting devices. Ultimately it is intended that the project will facilitate the construction of foundations in tight working spaces to depths exceeding 50m.
Improvements in trenchless systems should also increase the potential to apply the equipment to new areas. For example, Kent-based Powermole International has successfully developed a small diameter directional drilling system that can drill efficiently through rock (see box).