Removing the sheet piles

The sheet piles that were inserted into the ground were only ever intended to be a temporary measure to prevent the side walls of the basement excavation from collapsing. Consequently once the concrete shell of the basement was complete the sheet piles could be removed. The sheet piles were never intended to be an integral part of the basement. Moreover the steel Larsen piles used in this case are expensive items and on removal and return to the piling contractor result in a substantial discount to the price of the piling. There was therefore a significant financial incentive to ensure that as many as possible of the sheet piles were removed and sent back.

The method for extracting the sheet piles was to use a remotely operated lifting device which incorporated extremely strong hydraulic jaws that clamped tightly on to the pile to be lifted at which point vertical hydraulic rams lifted the pile out of the ground. At the same time a crane with its lifting tackle attached to the top of the pile was used to secure the pile from toppling over as it was extracted. The vertical reach of the pile extractor hydraulic lifting rams was limited to less than one metre therefore to extract the full length of the sheet piles, which in most cases were 13 metres long, a repetitive process was employed that lifted the sheet piles incrementally as follows: clamping on to the pile, extending the vertical hydraulic rams to lift the pile, releasing the clamp jaws, retracting the lifting rams back to their starting position and starting over again. As the pile was extracted the crane took up the slack on its lifting tackle at each successive partial extraction. The hydraulic pile lifting device was electrically powered from a nearby mobile generator and it was controlled by an operator using a hand held remote control unit connected to the device by a cable.

Surprisingly the sheet piles were quite difficult to extract despite the 160 tons of vertical force the pile lifting device could exert. This meant that the process was quite slow since most of the sheet piles had to be cajoled out with successive pushes and pulls until they became loose enough to be pulled out. Even then for seven of the sheet piles which just could not be extracted they had to be excavated as deeply as possible and then cut off cut off with an oxy-acetylene blow torch, leaving the remainder behind in the ground.

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