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?
I have since ordered such calibration tests - the rods without the vane to get rod friction, and it is now subtracted from the total torque. Although, because these tests are in a borehole, the length of rods in the soil is minimal (40 cm)
As for the instrumentation itself, it is a gear driven setup made in China, and the manual is in Chinese, so I am not clear on the torque readings.
You idea about the thin sand layer is very possible, and I am going to look into this. The work is being carried out by a local Vietnamese contractor, and is quite sloppy and careless. The foundation engineers will use the data for total stress analysis and will consequently overdesign in any case.