In the design of major construction works, the better the ground conditions are known, the more control there is on the assessment of risks for construction, contract and personnel, and ultimately on final costs. Understanding of the ground conditions is usually expressed as a conceptual ground model that is informed by the results of desk study and of dedicated ground investigation. Using the GSI3D software, a 3D geological model (a model composed of attributed solid volumes, rather than of surfaces) can be constructed that exactly honours geologists’ interpretations of the data. The data are used in their true 3D position. The 3D model of faulted Lambeth Group (Palaeogene) strata in the area of the proposed new Crossrail Farringdon underground station, in central London, has several types of benefit. These include allowing optimum use of available ground investigation data, including third party data, with confidence. The model provides an understanding of the local geological structure that had not been possible using other commonly used methods: in particular, it shows the likely distribution of numerous water-bearing coarse deposits and their faulted offsets, which has potentially significant effects on groundwater control. The model can help to focus ground investigation, constrain design and control risk.