This is Nick again from Vietnam. I need some advice on Vane Shear tests. I got some results - a peak torque of 134 Kn-m and according to the formula for a rectangular vane where D = diameter of vane = 0.1m
6 x Peak Torque/7 x pi x D(cubed) results in 151 Kn/m2
This to me is rather high, since the SPT N= 1 both immediately above and below the testing interval. The clay is saturated, extremely soft, almost liquid. When the test for remolded strength was performed, the residual strength torque readings were twice as high as the initial readings for most of the observations, stabilizing to 160 Kn-m. Is this possible? I remember somewhere in the back of my memory, that quick clays can have a remolded strength greater than original.
The guys performing the test couldn't tell me what depth they were at. they reported 26.9, then changed it after the test to 26.5 I was concerned during the test that the vanes were in the cuttings, and not 4 diameters below the borehole.
The bottom line is, should I believe these readings?
Hi, The vane test may be perfomed in a thin sandy sublayer existing inside the very soft layer which has not been traced by the driller.
The other point: It is useful to check the test setup. The torque registered may be due to the friction of the vane rod or the drilling rod with the accessories inside the borehole. Hence, I usually order some calibrations (i.e. registering the torque needed to rotate all the inserted equipment into the borehole except for the vane itself). Some times due to the poor setup, the torque for the calibration is too high. However, the final torque value is the trque of the test minus the calibration torque. Another check: Have you find this problem with all the readings or just at one unique depth? If just one, then it is better to neglect that.