A firm whose judgement has been questioned on a previous occasion has been entrusted with the strength testing of rock in a site investigation project. During their first uniaxial compression test, the equipment failed to measure the peak axial load, but the technician did record that the specimen failed by the formation of a single fracture inclined at 20 ° to the loading axis. In a subsequent triaxial test, as the confining pressure was being increased before application of the axial stress, the specimen failed prematurely when the confining pressure in the Hoek cell was 85 MPa. On the basis of these results, propose a failure criterion of the form σ1=aσ3 + b for the rock.