 The ground on which the wheel is built was previously used as an open cast fire clay mine , a coal mine , and a tar works , resulting in contamination of the canal with tar and mercury . 20 metres ( 66 ft ) of loosely packed backfill from the mining operations containing large sandstone boulders was not considered adequately solid foundation for the size of the structure , so deep foundations with thirty 22 metres ( 72 ft ) concrete piles socketed onto the bedrock were used . 
