A series of field vane shear test at different rates of vane rotation was conducted in order to study the effect on vane shear strength for Japanese clay. Vane was rotated either at 6 deg/min or at a constant rate to achieve the time to failure of 2 min in reference to the procedure which is under deliberation in ISO. Results indicate that the strength under undisturbed condition is not affected by rotation rate however the strength under remolded condition decreases with increasing of rotation rate. In this paper, relationship between rotation rate and sensitivity, plasticity index and liquid limits are discussed.
Field vane shear test (FVT) is one of the popular methods in the world to determine in-situ undrained shear strength of soft clay. Various factors of influence on the measured undrained shear strength obtained from FVT are pointed out as follows:
reduction of shear strength due to the disturbance and change of pore water caused by insertion of vane;
gain of shear strength during waiting time following the insertion;
rate of vane rotation;
anisotropy of soil;
The first three points are considered as main factors having an influence on measurement of undrained shear strength of clays.
Vane insertion leads to disturbance of soil structure and displacement of soil particles in the ground. La Rochelle et al. (1973) reported undrained shear strength decreased with increasing thickness of vane blade so that sensitivity is a factor in controlling the effect of vane insertion disturbance on the measured undrained strength.
Torstensson (1977) shows that significant increment in strength was not observed when waiting time is less than 10 min while 20% of increment was obtained in case of more than 24 hours of waiting time. Subsequently, 5 min of waiting time was recommended for the testing.
In order to study the effect of rate of vane rotation, FVTs with a wide range of different rotation rates were performed. Studies reported by Cadling and Odenstad (1950) for Swedish clays with rotation rates ranging from 6 to 60 deg/min, and Wiesel (1973) and Torstensson (1977) for plastic Swedish clays with rotation rates from 0.0002 to 200 deg/min indicate that the obtained undrained shear strength increases with increasing rotation rate. Meanwhile, Roy and Leblanc (1988) reported that the shear strength decrease with increasing rotation rate up to 12 deg/min whereas only very small increase of the shear strength above 12 deg/min based on results of the test for plastic Canadian clays with rotation rates from 0.6 to 120 deg/min. In contrast, Blight (1968) reported that undrained shear strength of silts increases while rotation rate decreases. Therefore, the effect of rotation rate of the vane on the measured undrained shear strength is not clear yet. The rate effect is important to determine the shear strength for design from both in-situ tests and laboratory tests, since it is obvious that the actual strain rate at failure in the field is different from those tests. Accordingly, a series of FVTs with different rate of vane rotation was conducted in an attempt to confirm the effect of the rotation rate in the range of the reasonable values for a practical procedure.