Case Study

Where conventional drilling and grouting contracts fill up or consolidate any spaces left underground from the extraction of minerals (i.e. mineworkings), on this contract GDC ltd were employed to undertake a unique drill and grout exercise whereby an old landfill site required grouting unlike typical grouting contracts the purpose was to raise the ground levels, which were monitored by on-site surveying throughout the grouting contract.

Approval was given for a 3-metre grid spacing across the footprint of all the planned highways and infrastructure on the site. GDC mobilised Casagrande C6 drilling rigs, and for height restricted areas a Masenza MI3 to undertake the rotary drilling works and a large 8-tonne grout mixing plant, supplemented by pumps and water tanks, plus associated hoses to undertake the grouting operation.

To ensure the full depth of the landfill was grouted a special type of steel threaded tremmie, with elongated slots was drilled into the ground on each of the borehole positions. The slotted casing was then left in-situ, as the drilling rig continued onto the next location, shortly followed by the grouting team who connected grout lines onto the casing with a t-piece and pressure gauge. Grout was then injected through the drill string in ascending stages until the desired pressure was achieved, whereby the casing was lifted 1m and repeated. The slotted drill pipe ensured that grout would fill the full depth of the landfill wherever it was required. Once pressure is achieved the drill pipe is extracted from the ground.

Grouting constituted of a Cement:PFA mixture in the ratio of 1:9 respectively and water to be no more than 40% of the overall weight. Standard flow, bleed (The bleed capacity shall be limited to 5% maximum) and cubes testing strengths were required (The minimum strength at 28 days should be 0.7N/mm²).

In summary an average of 8.77 tonnes was injected into each treatment hole.