Within a five-week construction period, a total of 552 micropiles with a diameter of 270mm were installed using the continuous-flight-auger (CFA) method.
"The CFA method is a rotary drilling technique that uses an auger as drilling tool. Once the tool reaches the final depth, the hollow stem auger concretes from the bottom up," explained Michael Doll, project manager at Bauer Spezialtiefbau.
The drilling depth at the construction project in Amerang was 11m. Additionally, GEWI threaded rods were fitted into the drilled and concreted CFA piles and set as an uplift prevention. A projection of 0.65m was calculated for the GEWI head structure.
Due to the various and unfavourable ground conditions, tensile tests were carried out on 16 test piles in the run-up to the actual foundation works. "Based on the results, the length and number of the envisaged micropiles could be adapted to the requirements of the subsoil," according to Doll.
The soil conditions at the construction site were gravelly to soft or mushy. Unbreakable boulders found in various depths also demanded a quick response.
"The rocks needed to be secured or the pile grid adjusted as necessary," said Doll. The tight construction grid of 3.2m meant that the production sequence needed to be precisely planned and coordinated so that the drilling work executed with a Bauer BG 20 H drilling rig would not be impeded due to already installed GEWIs. The works carried out by Bauer with a total of 6,072m drillings were successfully completed at the end of March.
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