An investigation was carried out into the magnitude of undrained volume loss due to tunnelling by open-face shield in London Clay as part of PhD research concerning ground and building movement caused by tunnelling on the Jubilee Line Extension (Dimmock, 2003). The investigation was based on the volume loss observed at greenfield sections across St. James’s Park (Nyren, 1998; Harris, 2001). The objective was to link particular phases of tunnel construction to resulting volume loss. One section was monitored comprehensively for both surface and sub-surface ground movement. Other sections across St. James’s Park were just monitored for surface settlement. Most significantly, a modification to the Load Factor Approach (Mair et al., 1981; Macklin, 1999) to estimating undrained volume loss for open-face shield tunnelling in London Clay is proposed.