A DRILLING system designed to cope with difficult granular ground has been used on a metro extension in Chile.
Sandvik Rock Tools' Tubex system was used to install anchors for the new Bellas Artes station on the Line 5 extension on the Santiago Metro.
Anchors were being installed to support the piled retaining walls of the station box but local geology of alluvial sand, gravel and boulders meant that conventional drilling systems would have struggled.
Instead, Chilean foundation contractor Pilotes Terratest decided to use the Tubex system. Working like an under-reamer, as the pilot bit turns, a reamer swings out, cutting a hole slightly bigger than the casing which is tapped in as the hole progresses. When the hole is finished, reversing the drill causes the reamer to swing back and allows the drill to be removed, leaving the casing in place.
Pilotes started work on the station, one of two on the new line, in January 1998. Limited space between buildings either side of the site meant that it was impossible to install vertical walls, so the firm had to sink 130, 1.2m diameter inclined piles between 15m and 21m depth.
Main contractor Neut Latour then excavated the 16m wide, 140m long and 15m deep station box in stages, allowing rows of anchors to be installed at depths between 3m and 7m. The 120mm diameter cased anchors were placed at 2.4m centres at up to 30degrees and were 13m to 20m long, installed either through or between the piles and grouted for half their length.
Once the excavation was deep enough, the station's roof was installed and excavation continued below. The station is due to open next year.