Over 350 boreholes, sunk to depths of nearly 60m, have been needed in Copenhagen as part of one of the largest geotechnical and hydrogeological surveys ever undertaken in the Danish capital in preparation for a new DKK15bn (£1.6bn) underground metro circle line.
The mass of data collected from the extensive geophysical logging and thousands of soil and water samples will be used by engineers from the project's joint venture (JV) consultants, Cowi, Arup and Systra, to design the underground stations, deep access and ventilation shafts and main twin-bore running tunnels.
The new 15km Cityringen will, like the existing metro, be a driverless system controlled by computers and have 17 underground stations and a depot. Five of the stations will connect the Cityringen with existing main S train lines and the metro lines that run from Vanlose in the west to Copenhagen Airport at Kastrup and Vestamager in the south east.
When the Cityringen is completed, an estimated 85% of all homes, workplaces and institutions of higher education in the densely populated areas of the capital will be located within 600m of a metro or S train station.
This will, in most cases, mean a walk of less than 10 minutes to the nearest station, dramatically improving the city's connection to public transport.
Owner and operator Metroselskabet anticipates that the new twin-track circle line will be open in 2018. Passengers will be able to ride in a driverless train from Copenhagen Grand Central Station to Rådhuspladsen, Kongens Nytorv, Østerport, Norrebro and Frederiksberg, before returning to Grand Central Station 24 minutes later.
Metroselskabet let the about £5.34M, 10-month ground investigation contract to a pair of Danish contractors. Onshore and offshore geotechnical consultant and drilling contractor GEO pooled its expertise with civil engineering contractor Per Aarsleff in the JV called GEO-Aarsleff.
Broadly, Aarsleff focused on preparing the individual borehole sites, drilling works, pumping tests and permanent borehole inspection, and access covers. GEO also focused on drilling, as well as performing all the complex geophysical logging, laboratory testing and analysis of samples, and writing the final report for Metroselskabet.
The JV used a combination of up to 11 Rotamec, Fraste, Nordmeyer and Knebel hydraulic rotary lorry and crawler track-mounted drilling rigs to install the 350 boreholes in a variety of locations. One of the most difficult was through a rail track next to a platform, right in the centre of Grand Central Station. Per Aarsleff had to modify one of its lorry-mounted rigs by removing the normal road wheels and replacing them with steel wheels to run on the track.
GEO-Aarsleff was given a short programme for track occupation and completed the borehole in just four days.
The GEO-Aarsleff team of rigs drilled boreholes of 100mm to 300mm in diameter using a combination of plain auger drilling, core drilling, down the hole hammer drilling (DTH) and DTH combined with the Symmetrix system of drilling through overburden with casing, to suit the sampling required.
The boreholes range from 5m to 57m deep and are installed through Moraine Clay and Sand overburden to penetrate about 30m into the underlying Upper and Intermediate Copenhagen Limestone bedrock, which typically starts 10m to 30m below normal ground level.
Depending on the type of borehole, disturbed and undisturbed soil samples were taken during drilling to full penetration. Geophysical and hydrogeological properties of the soil and inflowing groundwater were also recorded using different probes to measure a combination of borehole diameter, radioactivity, electrical resistivity, density, porosity and temperature, and conductivity of the borehole fluid.
The flow of incoming groundwater was continually recorded as the probe was slowly retracted from the borehole to determine the hydraulic values of the limestone bedrock. Site workers then lowered slotted PVC screening tubes to full depth and filled the surrounding annulus with a sand filter, prior to rechecking all the recorded parameters.
Included in the total are about 90 environmental boreholes needed to obtain details of any possible soil and groundwater contamination in the proposed station locations.
After sampling, all boreholes were temporarily plugged before being later used for observation – both during and after construction of the Cityringen.
GEO-Aarsleff drilled about 11km of boreholes and expected to take about 2km of undisturbed soil samples and about 8000 disturbed samples.
"This is a vast geophysical and hydrogeological survey and by far the most comprehensive I have been involved with," says GEO-Aarsleff project manager Jesper Furdal. "A typical ground investigation for a new office building may only require a single-cored borehole and maybe five to 10 shorter geotechnical boreholes.
"But for this Cityringen project we had a team of about 35 installing 350 boreholes, doing about 400 geophysical surveys and taking and recording thousands of soil and water samples. This is quite a technical and logistically challenging project where we needed to provide very accurate information for our client in a very tight programme and make a profit."
As well as making regular visits to the individual borehole sites, Furdal was able to keep a close eye on the progress of each borehole from his desktop computer. He imported the exact location of each one on to a Google Earth map, which was shared among the GEO-Aarsleff team.
The JV started on site in the middle of August last year and completed all the fieldwork by the middle of June. GEO-Aarsleff was expected to take a further month to complete all the testing and write and submit its report to Metroselskabet. The JV consulting team of Cowi, Arup and Systra will use the data to design the stations and tunnels, and produce the tender documents.
The design of the new Cityringen is expected to be similar to the existing metro lines. The 5.5m outside diameter twin-bore running tunnels will be driven through the stable Upper and Intermediate layers of Copenhagen Limestone, and run between 30m to 35m below ground level at their deepest point.
The Cityringen tunnels are expected to dip between stations, as the gradients and difference in levels of the track before and after the stations will reduce energy consumption during acceleration. This may help reduce construction costs as the stations could be built closer to the surface. Many of the stations are expected to be built in dewatered open box cofferdams.
Metroselskabet is aiming to complete the Cityringen design and issue tender documents by the middle of next year. Construction is expected to start in 2010 and be complete and ready for opening in 2018.