CPT Negative Readings from Barge
CPT Negative Readings from Barge
(OP)
During different CPTu projects over the last few years I have been observing negative readings of sleeve friction in very soft silt (85% silt size particles). The cone is calibrated and performs fine in firmer material. The negative skin friction values measured are about 1 to 3% of the full scale output of the cell. I am pretty sure that they are just noise. However, last week I was pushing from a barge in a soft organic silt within 2 to 3 metres of the river bed (WOH to 5 blows per foot) and the sleeve and tip resistance both went negative and stayed negative over the next 10 to 15 feet of the push. The negative values were more around 10 to 15 % of the full scale output. Just wondering if anyone has had any similar experiences. Thanks.





RE: CPT Negative Readings from Barge
RE: CPT Negative Readings from Barge
Although lake/sea bottom silt is very soft most electronic cones should show some increase in the tip and friction sleeve values with depth. The fact that you observed neg values over a 15 meter interval might be cause for concern. Possibly the cone was not properly calibrated at the low range? Were you using a cone with a 10 MPa cone tip capacity or was the cone equipped with a 200 MPa tip?
Although I have never seen it happen I guess it might be possible for a 200 MPa cone to show some small neg values at the very bottom end of the scale.
Did the neg readings come from an electronic cone or a singing cone?
Best of luck
Coneboy
RE: CPT Negative Readings from Barge
RE: CPT Negative Readings from Barge
When you put the cone 1.5 ft below ground level for half an hour, it was only compensated for the temperature at that depth. The temperature might be different as it was advanced deeper.