When designing geotechnical structures engineers need to assign soil parameters. Soil design parameters are often inferred through correlations with basic site investigation data. The objective of this work is to determine the shape of the undrained stress-strain curve of a heavily overconsolidated Eocene clay in such a way that it may conveniently be used in simplified deformation mechanisms to predict ground movements due to construction. A database of London clay triaxial test data is presented. Use of a power model to predict strength mobilization is demonstrated for 17 previously published triaxial tests on high quality cores of London clay. A novel method of normalising these mobilization curves is demonstrated (using a reference strain at 50% mobilization of shear strength) different relations are shown to apply to different magnitudes of strain. The parameters that influence the variation of the reference strain are studied.