DUMPED DOWN BUT NOT OUT

In these environmentally conscious days, when a new disposal site is being planned some form of ground protection is always laid first.

Lying beneath the waste, this protection, usually an impermeable membrane, ensures that contamination does not penetrate into the groundwater. However, this has not always been the case. Many European countries are facing a growing ecological problem: how to clean up old disposal sites, where various hazardous wastes are percolating down and threatening to contaminate the groundwater.

A lead has been taken by Denmark in addressing this problem. The local council in Copenhagen takes part of its water supply from the groundwater near Ejby, 20km north-west of the city. In this area, the water was threatened by chemicals leaching from a nearby disused disposal site.

As part of an overall clean-up scheme, the council initiated a pilot remedial project for the old dump, involving two local contractors: Kemp & Lauritzen, which did the preparatory surveys and project specification; and Rasmussen & Schiotz, which implemented the project using No-Dig technology. As a pilot project it would be carefully monitored and if successful, be repeated at other sites in Denmark. It would also point the way to a solution for other countries facing this dilemma.

The dump had been in use since 1935 for the disposal of household, industrial and construction waste, but was abandoned in 1972. From desk research, Kemp & Lauritzen found little information on what hazardous chemicals, toxic substances or physical obstacles the dump might contain.

A site survey showed the presence of oil and tar pollution, high ammonium levels and various organic acids. More seriously, it also found the dump was active, with a risk of explosion if the surface was penetrated. By now the dump site was in a park with buildings nearby and to open the dump would have posed serious health and environmental risks.

Pump tests for hydraulic conductivity showed that remediation could not be performed by pumping from the secondary reservoir. So, to minimise pumping from the primary reservoir, Kemp & Lauritzen decided to collect the contaminated water leaching through the soil in run-off drain pipes shot under the dump, above the water table.

'Standard' pipe installation methods using excavators or trenchers would cause too much disruption to the site and there were too many physical obstacles. Instead, the project specification given to Rasmussen & Schiotz called for an innovative technique borrowed from other underground applications - using an underground guided boring system. These systems have a steerable drill head that can be manoeuvred past unseen underground obstructions and, as a trenchless technology, digging down through the dump site is not needed.

However, it was discovered that very few guided systems on the market could cope with the particular demands of this project. In some places the waste depth was in excess of 15m, beyond the limits for locating the drill head of most systems. There were also extensive variations in the local geology within the site which, when combined with the sheer size of the site (about 100,000 m2) and the hazards outlined before, made it a large and challenging undertaking.

At this point, Rasmussen & Schiotz turned to Nordisk Vermeer, the Scandinavian dealer for the entire range of trenching and trenchless technology equipment from Vermeer Manufacturing. Vermeer makes a complete range of guided underground boring systems called Navigators and although Rasmussen & Schiotz already owned a smaller unit, a D7, it was obviously not heavy duty enough for this project. The company approached Nordisk Vermeer about using a D24, currently the largest machine in the Navigator range which can produce over 8,000kg of thrust at the drill head and a pullback of nearly 11,000kg.

The project specifications called for six bores, with lengths between 120m and nearly 300m, using pipes specifically made for the purpose - 140 mm perforated HDPE 160 PN10. Obviously, to drain off the water the pipes would have to be installed at a constant slope down to the lowest point, in this case into catch wells about 6m below ground level, where pumps would be installed. So unusually, the Navigator was set up below ground level, in the pit already dug for the main pump, and positioned to bore upwards under the dump hill.

Working below ground level, in a pit dug into an old dump site created its own difficulties, and not just because of the restricted space. Methane, CO2 and other gases were a constant danger, so a pump was installed to remove the gases continuously; gas detectors were placed all around the pit to give an early warning in case of gas build-up.

To protect the operators from contact with hazardous chemicals, complete protective suits were worn during boring, with over-pressure helmets to keep them from inhaling gases. Obviously no welding was allowed in the pit and metal-to-metal contact was kept to a minimum. Finally, samples were taken regularly from the outflow, while the pipes were being installed, to be analysed for hazardous chemicals as part of a continuous safety-checking process.

The D24 had the power to handle the boring but the other 'unknowns' made the entire project very much new territory to Rasmussen & Schiotz. To help out, Vermeer provided Navigator specialists from its international office in Goes, the Netherlands, to be on site to train Rasmussen & Schiotz and provide general assistance. Digital Control Inc., which makes the Digitrak drill head locator, flew over an experienced engineer from the US to help.

Starting below ground level and shooting up resulted in a required location depth of over 15m in some places and ground conditions were very difficult to navigate because of the largely unknown materials that had been dumped. With help from the Digitrak specialist, Rasmussen & Schiotz found that the Digitrak Mark II deep locator could pick up pitch-and-roll updates down to 14m, with the depth measurement continuous and constant down to 15.5m.

Information from the locator head is passed to the remote control display, set up alongside the Navigator itself, providing the operator with the accurate gradient and depth information. Problems were anticipated, due to the depth of the starting pit and the severity of the terrain between the receiver and the remote display, but in fact it picked up the signal with no problems throughout the length of the bore.

Rasmussen & Schiotz had another problem. This stemmed from the use of perforated pipe - its tensile strength is lower and friction in the ground is higher on a perforated pipe than normal, limiting the pulling force that could be used. Therefore it was doubly important to make the pull through as easy as possible by making sure the hole stayed open.

Bentonite is usually used for this, but it is an effective water barrier and would have blocked the pipe. Also, local anti-pollution laws now prohibit leaving such chemicals in the ground and even in a site already polluted, it was important not to add to the problem.

This looked as if it would cause a major headache but the Vermeer team found a unique solution. It studied well developed techniques and the use of a biodegradable polymer gel, which perhaps could be used as a bentonite replacement. Initially, used 'off-the-shelf' it seemed ineffective but the team persisted and after a lot of experimentation and practical trials, came up with a complete 'recipe' and exactly the right operating conditions. Using this, the team found it could keep the bore walls open long enough to pull back the pipe, after which the mixture used would decompose.

Both Kim Brandt and Tom Kjar, in charge of the Rasmussen & Schiotz team at the disposal site, were very satisfied with the way the project progressed and with the performance of the D24 Navigator.

Initial work on the drilling phase of the project started in September 1994, with only 10 actual drilling days in a three week period from mid-November into early December. By mid-January, all but one of the pipes were in. Kjar attributes the speed of the project firstly to good equipment, but also to the support and assistance the Rasmussen & Schiotz team received from Vermeer and Digitrak. 'It really has been a co-operative effort to solve a particularly difficult problem,' he said.

There is much about this pilot project that is unique. It is the first of its type in Denmark and possibly anywhere in the world, and in many ways it is a completely new type of pollution prevention for this sort of landfill site. Bores like this have never been tried successfully on landfills before, and the depths and lengths required were pushing the limits of the technology. Using specially made pipes in conjunction with Vermeers' biodegradable polymer recipe is certainly a first.

Due to its success, Brandt and Kjar foresee that guided boring systems will replace traditional solutions, as they cause so little inconvenience and offer cost-savings. Already, more projects of its kind are being planned in Denmark and it is expected that this project provided a welcome solution for other countries.