After 18 months, with £26,000 spent, Hardwick resigned. Work continued and by February 1816 the first prisoners were admitted, but the building creaked and several windows spontaneously shattered. Smirke and the engineer John Rennie the Elder were called in, and they recommended demolition of three of the towers and the underpinning of the entire building with concrete foundations: the first known use of this material for foundations in Britain since the Roman Empire.[22] The work cost £70,000, bringing the total cost of the building to £458,000.