A tunnel extension to the Thames Water Ring Main has recently been completed in SE London. Two new deep shafts were built at either end, using jacked caisson construction methods. One of the shafts was excavated through much of the London Basin Palaeogene geological sequence and into the Chalk. It is the world's deepest large-diameter jacked caisson. Close geological supervision throughout the construction process validated the pre-construction ground model. This in turn has facilitated predictions of and contingency measures for adverse ground ahead of excavation. Observation and monitoring of the engineering behaviour of each of the stratigraphic units during construction has demonstrated how field data can be gained subsequent to the pre-construction ground investigation. In doing so, construction data have been used to augment and verify the ground investigation borehole and laboratory test data on which shaft design was based, in addition to providing assurances to the on-site construction team on the absence of unforeseen ground conditions.