The concrete floor of the garage for 220 tonne haul trucks was beginning to crack.
At this Hunter coal mine the workshop building serves to garage these trucks in eight large bays each of about 1,000 square metres.
The slab sub-base was hard crushed rock but pumping concrete under the 350mm floor slab had not proved successful in stopping the cracking.
The reason why that did not stop the cracking problem was not immediately clear, so the mine engineers called in Tom Bailey, senior mines project manager of Uretek Ground Engineering.
When he considered the whole building from a distance, Bailey realised that it would have been built on a cut-and-fill site. The relatively soft fill excavated from the upper end would have been used as the base for the lower end.
Bailey's speculation turned out to be right: bore logs confirmed that there were weak strata deep down under the sub base. There appeared to be quite soft fill at about 800mm depth, consistent with cut-and-fill practice.
Deep Injection of Uretek structural expanding resins was used to compact the sub-base and re-support all the garage areas.
As each area was treated Bailey positioned a haul truck right there to increase the weight on the concrete floor.
Asked why, Bailey explained: "The heavier the load on the element that we are supporting, the more compaction pressure we can put into the foundation ground. In this case we added the weight of a truck itself while we worked.
Enormous ground compaction pressure
Bailey continued: "We can put in enormous pressure when we have to. More than 1000 tonnes per square metre, if necessary.
"In this case the 220 tonne weight of the trucks enabled great compaction pressure to be brought to bear as the expanding resins were injected into the weak strata.
"Full support was restored with minimal interference to the normal workshop activities. The mine engineers were so pleased that they called in Uretek again later to re-support the extensive apron areas outside the garages.
"Uretek's engineered resin methods are often used these days for exremely heavy jobs at mine sites. For instance re-levelling stacker-reclaimer rails and large mine conveyer belt systems.
"Those are sometimes built on ground that is not compacted enough for the big loads they support. We can usually fix these issues quickly with our injections, saving a lot of down-time."
More information: www.uretek.com.au or Bailey Bailey 0418 288 277.