CPT Negative Readings from Barge

[eccs]

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.

 

[Robert168]

Though it is not deep below grade, the changed temperature may be the contributing factor leading to negative reading. A precondition is that the soil must be clayey and very soft.

 

[coneboy]

I agree with Robert168's comment. The problem could be that you are pushing a warm cone into very soft, cool soils.

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

 

[eccs]

Thanks for the feedback. Initially i thought it might be either temperature effects or a bad cone. However, I am pretty sure it is not a temperature effect as the negative readings have appeared on first pushes of the day and even after several successive pushes. I have even placed the cone in the ground (approx 1.5 ft) for about a half an hour prior to pushing and still read negative readings. My company has a 2 ton cone and a 20 ton cone and I have observed similar results with both cones in the same soils. The cones have been calibrated within the last couple of months and I usually do a very dirty check in the field by placing a known weight on the cone. My last pushes were from a barge. I am wondering if anyone has noticed any negative readings when pushing from a barge. Thanks.

 

[Robert168]

The sleeve and the pressure gage are so installed(as least so with those I used)that no negative reading would be possible even though the sleeve is sucked downward, which is very unlikely to happen in a soft soil.

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.