Many older landfill sites were constructed with little consideration of the need for leachate control or the possibility of groundwater contamination. Traditional methods of leachate control at these sites generally involve retro-fitted vertical wells, but their impact is often limited by a number of factors including the reduction in wetted area (through which flow occurs) at large drawdowns.
Horizontal wells might offer a more effective solution, partly because they do not suffer from this problem and partly because their greater length makes their overall performance less sensitive to installation through zones of low hydraulic conductivity within the waste mass.
Recent advances in directional drilling techniques have seen horizontal wells used for a range of environmental applications. For example, they were installed beneath the runways at New York's JFK airport to intercept spilt aviation fuel and in association with the Cardiff Bay Barrage Scheme to control groundwater levels. A number of horizontal wells have been installed beneath landfill sites, but in late 1998 it appeared that few, if any, had been installed within waste.
A research project involving a highly instrumented field trial was initiated at the University of Southampton to investigate the feasibility of using horizontal wells for leachate control in old landfills. The project is funded by Cleanaway Ltd under the Landfill Tax Credit Scheme whose objectives include research into the minimisation of pollution and harm from waste.
Site investigation and initial trial drilling
Rainham landfill in Essex was considered to be an ideal site for the field work, mainly because it has elevated leachate heads with no pre-engineered drainage layer;
it is substantially a landraise, which would allow for a simple drilling profile (Figure 1); and previous operation of vertical wells met with only limited success.
Site investigation boreholes were drilled and samples taken to establish the composition of the waste. The team was expecting directional drilling to be impeded by large solid items such as concrete blocks and car engines.
However, the investigation indicated the main problem was likely to be the difficulty of steering the drilling rods through the highly compressible waste.
In March 1999 a 12 tonne pullback capacity drilling rig was brought to Rainham to install an initial trial well using the backreaming technique. This involves:
drilling a guided pilot borehole from entry point to exit point;
enlarging the borehole by pulling a succession of reaming tools back through it, removing any cuttings with drilling fluid; and pulling the well screen into the borehole.
A pilot borehole with a maximum depth of 10m was successfully drilled to an exit point 100m away. The guidance system lost accuracy at depths greater than 8m because of interference from metallic objects in the waste.
And, as anticipated, the low compressive strength of the waste made the drill rods difficult to steer.
Despite these problems, drilling progressed rapidly and the pilot bore was complete in four hours. Three backreaming passes were made using progressively larger reamers each time, with a final reamer diameter of 250mm.
No cuttings or drilling fluid returned to the surface at any time. Finally, the 150mm diameter wellscreen was pulled 25m into the borehole before the pullback capacity of the rig was reached. No further progress could be made and the trial was abandoned.
The team believe the reason the pullback capacity of the rig was reached after only 25m of screen had been installed was that the backreaming tool tended to compress the waste as it passed, rather than remove it to form a stable bore. Friction builds up much more rapidly when there is an all-round contact stress between the waste and the wellscreen than when there is a clearance gap between the wellscreen and a stable open bore.
Recent experience in the US Following the trial bore at Rainham, we found that horizontal wells had recently been installed at two landfill sites in the US. In Wisconsin, a 360m long well had been installed using the backreaming technique. Three backreaming passes were made, and the 100mm diameter HDPE wellscreen was pulled in through a 250mm diameter carrier casing which was subsequently removed.
It is thought the pilot rods were installed along the surface of the basal clay layer and that subsequent backreaming cut deeper into the clay. This would have improved borehole stability during carrier casing installation, but probably had an adverse impact on the effectiveness of the completed well.
At a landfill in Illinois, a number of pilot bores and backreaming passes were made to form a borehole large enough for installation of 150mm diameter HDPE wellscreen. This wellscreen broke on installation and was replaced by a 100mm diameter steel screen. The open area of the steel screen was very small (less than 1%), which might explain the low leachate yield achieved. Despite this, two further horizontal wells were being installed at the time of a site visit in October 1999.
Problems due to borehole collapse during backreaming were encountered with these further wells and, as at Rainham, development of excessive friction between the wellscreeen and the surrounding waste eventually prevented complete installation.
Second Rainham trial
After discussions with German drilling company DrillTec, a different installation technique known as overwashing was adopted for a second trial.
The principal advantages of overwashing are that the borehole is held open by an overwash casing during installation of the wellscreen, and the borehole need not exit the landfill. The four stages of overwashing are drilling a guided pilot borehole to the desired profile; installing the overwash casing over the pilot rods, which act as a guide (Figure 2); pulling the pilot rods out and inserting the wellscreen into the overwash casing; and pulling out the overwash casing, leaving the wellscreen exposed to the formation.
During installation of the overwash casing, cuttings can be removed by means of drilling fluid pumped in through the casing and flowing back along its exterior. The wells were installed entirely in the waste, within 1-2m of the base of the site to maximise the potential impact on leachate levels.A schematic drilling profile is shown in Figure 1.
In March 2000, DrillTec's nine-man crew arrived at Rainham with a 100 tonne pullback capacity rig and began work on three 250m long horizontal wells.
Drilling and installation of the first well took six days.
Initially, drilling of the pilot bore was attempted using a blunt jetting device which failed after 30m. Following its replacement by a tri-cone reamer, drilling proceeded without serious difficulty to the full 250m length.
Biodegradable drilling fluids were used, but as in the initial trial, no cuttings or fluid returned. Bentonite-based fluids were avoided to prevent blocking the waste with clay particles. A wireline guidance system replaced the walkover system used in the initial trial. By transmitting information along a wire running through the pilot rods, interference problems were substantially overcome. However, this system required a surface-induced magnetic field, which itself suffered some interference.
A 150mm diameter overwash casing with a cutting shoe at the leading end was used to enlarge the borehole.
The teeth on the cutting shoe broke off, and were replaced by a rough tungsten carbide face which proved more successful. Even so, progression beyond 200m was slow and it is unlikely that a well longer than 250m could have been installed using this drilling rig. As before, no cuttings or fluids were returned from the bore.
The pilot rods were removed, and the 100mm diameter wellscreen was assembled and inserted into the overwash casing without difficulty over a period of seven hours.
Finally, the overwash casing was removed leaving the wellscreen exposed to the waste. The first few metres of casing were difficult to pull out and required the full capacity of the rig, but the remaining length followed relatively easily. With the benefit of experience from installing the first 250m well, two further trial wells were completed in three days each.
The shape, size and spacing of the apertures in the wellscreen, and the material from which the wellscreen is made, may all have a crucial impact on the performance of the well. At Rainham, stainless steel wire-wrapped wellscreen was chosen for its strength and corrosion resistance. The slot width of the screen used for the first 250m well was 1mm, with an open area of 28.6%. In the second and third trial wells, slot widths of 2mm and 5mm were used (Table 1).
Monitoring The performance of each well is being monitored using arrays of instrumentation that include:
gas and leachate flowmeters for each well;
a total of 46 piezometers to monitor continuously the leachate levels at various distances from each well and depths within the waste;
four magnetic extensometer arrays to measure settlements resulting from the dewatering process;
three neutron probe access tubes to monitor changes in moisture content; and three geophysical investigation lines to measure changes in the electrical resistivity of the waste in an attempt to relate them to changes to moisture content (this geophysical investigation forms a partner project being undertaken by Joanne Moore and Dr Ron Barker at the University of Birmingham, funded jointly by the Engineering and Physical Sciences Research Council, EPSRC, and the Environmental Services Association Research Trust, ESART).
With the monitoring network and pumping systems now in place, pumping tests have commenced. Table 2 summarises some of the results so far.
The response of the piezometers to dewatering has been twofold.
Porewater pressures in the lower waste horizon in which the horizontal wells are located have fallen quite dramatically, by up to 7m, with reduction of pressure observed up to 100m away from the well.
Porewater pressures in the upper waste horizon have not responded to dewatering. Evidence suggests that there is a confining layer 6m above the level of the horizontal well.
The perched leachate above the layer has not been dewatered.
Findings to date
The difficulties of installing horizontal wells in heterogeneous landfilled wastes using directional drilling techniques have been substantially overcome.
Initial results, in terms of both leachate yield and reduction in leachate heads, are encouraging. Gas yields are also high. The gas is typically 60% methane and 34% carbon dioxide, and could be used for energy generation.
The extensive instrumentation is providing valuable data on the wider geotechnical properties of waste, in addition to horizontal well performance.The impact on well efficiency of important details such as aperture size is currently under investigation.
None of the wells has yet been developed properly: development will be carried out in due course, and has the potential to increase well yields significantly.
The project is being carried out by the Department of Civil and Environmental Engineering at the University of Southampton, funded by Cleanaway through SUnRISE, the University of Southampton Environmental Body.
The related geophysical project is being carried out by the University of Birmingham, with funding from EPSRC and ESART. Those involved with the project are Darren Cole and Mike Dyer (Cleanaway); Ron Barker and Joanne Moore (University of Birmingham); and Richard Beaven, Stephen Cox and William Powrie (University of Southampton).
For further details, contact Dr Anne Stringfellow, SUnRISE Centre manager, Department of Civil and Environmental Engineering, University of Southampton, Highfield, Southampton SO17 1BJ. Tel 023 80 593988; fax 023 80 677519;
e-mail firstname.lastname@example.org; www.soton.ac.uk/~sunrise