Over the summer Ritchies is doing extensive grouting tests on site following in-house tests at its Kilsyth centre, where it looked at 10 different materials.
'We have examined some 80 batches of grouts with different additives which has meant thousands of test results, ' says Ritchies' onsite project manager, Iain Robertson.
He says grouts need to be 'fairly clever' because the requirements are quite stringent. Most will be microfine cement based materials but the behaviour of these can be quite different depending on the supplier, their raw material sources and the rock they are used in.
For the fast-setting 'blocker' grouts a 30 or 40 micron grind will probably be used, with about a half hour gel time. But for the high penetration work the cement could have a 12 to 16 micron fineness - 'which is at the limit of what you can get'. Gel times should be about two hours.
Robertson says all these cements are milled to order and vary between makers. A key problem is the 'dust' of particles from the grinding, especially those between one to six microns. Sieves to remove them do not work at this fineness and a completely uniform grind is impossible to achieve.
'These small particles can behave rather oddly - no one is sure why yet but it may be electrostatic binding effects or ultra-rapid chemical reaction times.
Anyway, they cause blocking.' A second problem with the superfine grouts is their watery thinness and a tendency for the particles to separate during penetration. 'You make them thicker like cream and then add superplasticisers and other additives, ' Robertson says. Even laboratory tests do not show the full behaviour, and specific tests in site rock are important.
Ritchies has devised a new testing system for this using down-the-hole pH detectors in three borings around a test bore at a distance of several metres.
These pick up the alkalinity of the cement at up to one part in two million sensitivity.
'Detectors will sit in a part of the bore sealed off with inflatable packers and the test injection will be made at the same level, ' Robertson says. Holes will be examined beforehand with optical and acoustic TV as well as a gamma detector which picks up (natural) radioactivity in the rock layers.
Despite the greater cost, Ritchies is using HQ wireline core drills for the work so rock is brought up intact, meaning it can be examined if necessary.
Warren Jones, senior project manager for the UKAEA's major projects and engineering division says it will also aid disposal on the final job. 'Only the rock which has any measurable contamination need be treated and the rest is normal spoil, ' he says. A rotary grinding drill or similar would mix all the crushed spoil together and it would all therefore need treatment.
Jones says the drills are remote operated Boart rigs, allowing the operators to stand away from them during drilling, a useful safety precaution, though it is not expected that much radioactivity will be found in the boreholes.
Ritchies will follow up the penetration bore trial with a full-scale dewatering test around a trial shaft comprising a 350mm diameter bore 60m deep. It will make a grout curtain around this to see its impact.