The iron curb was was needed to sink the shaft on the northern shore at Wappping. It had a diameter of fifty-five feet, and was delivered by John Rennie in September. The Rotherhithe and Wapping shafts were both seventy feet deep and fifty feet in diameter. Because of the problem posed by the watery surface gravel, Brunel's method was to build about half the depth of the shaft in the form of a brick tower above ground, and then excavate inside it, so that, as digging progressed, it would gradually sink under its own weight through the gravel into the firm clay beneath.