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Ground rules

SITE INVESTIGATION

Ground investigations for CTRL involved nearly 6,000 holes and 50km of drilling in six phases spread over seven years and involved challenging logistics and public relations work.

'Site investigation drilling contractors were the vanguard of the CTRL project, ' says Rail Link Engineering (RLE) lead hydrogeologist David Whitaker.

As the first visible sign of the project, site investigation had to be handled with sensitivity to local residents and businesses.

Drillers were some of the first to find themselves under the strict health and safety rules laid down by the project. 'Health and safety is at a level well in advance of the construction industry at the moment, ' Whitaker maintains.

'CTRL did give the site investigation industry a bit of a leg up, ' he adds.

After five years of consultation, site investigation began in January 1994. 'A phased approach was taken, ' explains Whitaker. There have been six.

'Phases one, two and three were carried out by Union Railways in the pre-award phase, ' he says. As this was before CTRL had received final government approval, land had yet to be acquired which generated a number of difficulties.

'During the first three phases, construction of CTRL was some way off, so much of the site investigation had to be done on thirdparty land, ' Whitaker explains.

This work was carried out under a legal agreement with landowners who were paid for the privilege. 'We had no powers during this stage, ' he adds. 'If there were any gaps, they had to be filled in during later phases.' 'Each phase is a step up in detail, ' says RLE ground investigation co-ordinator Chris Parks.

'Phase four covered the entire route, but phases five and six are specific to Sections 1 and 2 respectively.' 'But in terms of the level of infill, they are the same, ' Whitaker adds.

The change of approach came at the end of 1997. That spring, Whitaker explains, in the middle of the phase three site investigation and just as the first major tunnelling contracts were about to be awarded, the project was closed, following the collapse of funding. The project restarted under the control of Railtrack and Union Railways (South), although only on Section 1.

'Section 2 was consent-related work, ' says Whitaker.

Phase five site investigation began soon after, but because detailed design for Section 2 did not start until the beginning of 2000, it was not until September that year that phase six began.

This involved 'all the difficult bits' says Parks, including positions with difficult access and investigations for design changes.

More than 5,900 investigation 'holes' were put down using various methods, including rotary coring, cable percussion drilling, static cone testing, trial pitting and window sampling. Many of the boreholes were deep and involved high frequency of sampling and insitu testing, with piezometers installed in most. A variety of geophysical methods, including seismic refraction, ground penetrating radar and resistivity, were also used.

'The clever thing was to work out what was appropriate for design and any modifications, ' says Parks.

Several groundwater monitoring holes were also put down at specific locations along the route, for example, next to public water supplies, to obtain information on existing groundwater levels and chemistry, and to confirm that construction would have a minimal impact on the surrounding hydrological regime. Gas monitoring was also carried out in areas where the route crosses contaminated land.

The CTRL investigation work was supplemented by information from more than 2,700 'historic' holes acquired from various sources including consultants, government agencies, local authorities and utility companies.

Site investigation work was carried out by various contractors including Soil Mechanics, Foundation and Exploration Services and Bactec. TES Bretby carried out soil testing and the vast amount of site investigation data generated was handled by a database set up using MZ Associates' SID package (see data management box).

Work was carried out under the ICE 6th contract, as opposed to the main civils work being carried out under NEC target cost contracts.

The reason behind this is historical, Whitaker explains. 'RLE decided to use the NEC contracts, but site investigation contracts had already been awarded under the ICE 6th and these simply continued. Because this form of contract does not lend itself to change and because the contracts organisation was geared up for NEC, it meant that we had to work to interface the two, ' Whitaker says.

'There is a very striking contrast between the two contract types, ' he adds. 'The ICE 6th can be adversarial and NEC is more open, but ICE 6th is tried and tested.' However, he admits that working with the NEC contract 'made us think that it might be appropriate to site investigation.' Site investigation has been more strictly monitored on the CTRL than on other projects, Whitaker says. 'For example, we needed specific Environment Agency consent for any sampling point below the water table. We did not get blanket approval - it was almost borehole by borehole.' Specific requirements were laid down for drilling into aquifers, for example. Here, triple tube drilling was used to ensure that there was no chance of contamination, with large casing drilled to just above the aquifer and grouted in. After cleaning the borehole, smaller diameter casing was used to drill through the grout plug and into the aquifer.

'This meant some of the boreholes started in 300mm and 250mm diameter casing, ' says Parks.

Phase four included pumping tests at Ashford, one in the Weald Clay and one in the Atherfield Clay. 'A detailed investigation with very closely spaced boreholes was carried out at two locations, ' says Whitaker. A number of cored boreholes with piezometers at various levels as well as a central pumping well, were installed. Results were used to investigate material variations within the clay and help with the design of the Ashford cut and cover tunnel. 'It was an unusual investigation for an impermeable material, ' says Whitaker.

Four large-scale construction dewatering tests, 30m into Chalk, were also carried out for the London Tunnels, with pumps shifting water at between 30 litres/s and 42 litres/s for three months.

Other highlights include extensive Dutch cone testing, to pick up an underground chalk cliff just south of the Thames Tunnel's south portal, and x-ray diffraction to identify clay minerals in the Weald Clay and Atherfield Clay. The materials are often difficult to distinguish, so work aimed to see if the boundary between them could be fixed, Whitaker explains.

Another big challenge arose during the latter part of the investigation. 'For the last phases, we started to have main contractors on site, ' says Whitaker. Because the sites were in contractors' hands, site investigation work involved them directly. 'In practical terms, the site investigation contractors had to deal with the main contractors and they had to work around the site investigation, ' says Parks.

'On Railtrack land, it became very complicated when the main contractor, site investigation contractor and Railtrack were on site - it was a logistical nightmare, ' he adds. Despite the long lead-in times for planning and gaining approvals so that all parties knew what they were doing, it was still 'very stressful'. The problem was eased somewhat by using a formalised system for working on Railtrack land, Parks says.

'However, this was not needed on phase six, because we arranged that boreholes would be positioned where possessions were not needed. Even if they were, we moved the borehole. While this was not ideal for design purposes, it was a pay-off between getting the best information and trying to reduce time and cost, ' he says.

'A few boreholes and other positions were moved but not to the detriment of the project, ' Whitaker confirms. Sampling methods were also changed to ease the process - from conventional boreholes to window sampling, for example.

While the main site investigation contracts have finished, some work is still ongoing, but is being carried out by the main contractors for their own purposes, says Whitaker.

'The 5,900 boreholes put down for CTRL and the 2,700 historical thirdparty boreholes used represent about 50km of drilling, costing £16M and equivalent to about 0.8% of the civil construction costs, which is a good benchmark figure for projects of this size and complexitiy, ' he says.

CTRL ground investigation

Phase Period Boreholes Trial pits Other

Totals 1Jan-Jul 94 193 230 184 607 2

Jul-Nov 94 424 488 335 1247 3

Oct 95-Nov 96 450 390 291 1131 4

Apr 97-Jul 98 492 598 93 1183 5

Aug 99-Jul 99 279 281 301 861 6

Aug 00-Dec 01 343 225 333 901 5930

'Historic' 2739

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