The Thames Water Ring Main has been driven through the various Palaeogene deposits of London and the underlying Cretaceous Chalk. Much of the tunnel alignment was designed to keep within the London Clay Formation over the western half of the project, although discovery of a number of Quaternary scour hollow features, during the ground investigation, led to revisions in tunnel design. In addition, a scour feature was also intersected during tunnelling, necessitating changes to the tunnel lining and use of compressed air. Over much of the eastern half of the project, the tunnel alignment was unable to avoid the lower Palaeogene deposits of the Lambeth Group, and Thanet Sand Formation, as well as the underlying Chalk. These deposits were found to contain hazardous features, such as sand-filled channels, sub-artesian groundwater and hard ground, complicated further by geological faults along the route. Intersection with these features could have had serious implications for the health and safety and efficiency of tunnelling operations. These adverse geological conditions were avoided by changes to the alignment during design or by ground dewatering prior to construction. When intersection occurred, the open-faced machines used compressed air and lining changes to mitigate the risk. From 1990, the project used Earth Pressure Balance Machines (EPBMs) for the first time in the UK. These allowed better control of problematic ground and allowed the project to set an all-time productivity record.