This paper discusses the derivation of soil strength from the vane shear test, examining factors such as rotation rate and waiting time that may influence the observed result. In addition, several different vane geometries (differing height/diameter ratios) have been tested to examine the effect of aspect ratio on the observed vane shear strength. Results from vane shear tests are compared with those obtained from other in situ tests, in particular T-bar penetration tests. A new form of "helical" vane test Is discussed, In which the standard vane apparatus is continuously rotated during penetration into the soil sample, resulting in a complete profile of soil strength. In the discussion, profiles of soil strength determined with varying penetration and rotation rate are compared with strength measurements obtained from more traditional in situ test methods.
Accurate determination of soil strength is a fundamental aspect of foundation design for offshore structures. At present, in situ assessment of soil strength is comprised primarily of cone penetrometer and vane shear testing, combined with core sampling to retrieve material for laboratory testing. More recently, the T-bar penetrometer has been used to estimate in situ soil strength. As part of a centrifuge modelling study of the performance of skirted foundations for offshore structures (Watson, 1999), a number of in situ strength measurement techniques were examined. Tests were conducted in a range of material types, with varied stress histories ranging from normally consolidated to heavily overconsolidated. Testing included different penetrometers including the cone (CPT) and T-bar, as well as a range of rotational devices including the vane shear test (VST). To complement the in situ tests, a suite of laboratory tests (including simple shear, triaxial compression and triaxial extension tests) was also conducted.