CPet-IT : SBTn number = 0

[kauri]

Hi,

I'm using CPeT-IT, dealing with some very fine organic soils. The soils I'm interested in are determined to be SBTn = 0, which I understand to mean undefined. As a result, I cannot obtain a number of key parameters.

I've figured the reason for this: my soils have very low fs (uncorrected), and Fr < 0.1% (in some cases negative), thus falling off outside of the Modified Robertson SBTn chart. I imagine it would look something like the below:

As some examples of the resultant issue:

I do not have u3 and cannot correct the fs. How can I calculate other parameters (such as OCR, sensitivity) without manually doing so?

Cheers,
Kauri

 

[GeoEnvGuy]

Cpet-it provides its correlations in the valid range for each equation. This is typically based on index behaviour, if the material is outside the range of IB for the equation the program does not plot the data. Unless you manually edit the data to increase the minimum sleeve friction the program will not compute it. I do not endorse altering or corrections to the data if it is accurate.

The equation rules around IB are clearly shown when you review the peak friction angle and undrained shear strength correlations side by side.

 

[kauri]

Thanks for your response. Could you please provide a reference for the valid ranges? I haven't been able to find such a reference, and don't follow the peak friction angle/undrained shear strength correlation.

 

[GeoEnvGuy]

Cpet-it user manual has the ranges for behaviour number for each correlation. The pdf report the program generates also includes the correlation and behaviour type rules applicable. The program is also largely based on the work of Peter Robertson, you can review his CPT guide on the Gregg drilling website.

An example of the behaviour rules is how behaviour numbers 1 to 4 and 9 plot undrained shear strength and not peak effective friction angle. Conversely behaviour numbers 5 to 8 plot peak effective friction angle but not undrained shear strength.

 

[EireChch]

SBTn is based on pore pressure. Show us your pore pressure profile. I bet you have negative pore pressure, this can occur in over consolidated clays and very dense sands

 

[kauri]

Attaching this FYI. It doesn't seem to be related to pore pressure; the SBTn is 0 where the sleeve friction is very low or negative. Following discussion with a Geologismiki representative, this can occur in clays that are consolidating at a significant rate (as we have). Also, given that fs is the most uncertain result and the impact the cone can have on this, I suspect the engineers who did the testing may have used an unsuitable cone - later CPTs performed by us had no such problem.

Edit: I realise I attached a different CPT to the original post. Theres 5 CPTs all with the same issue.

 

[GeoEnvGuy]

Took a quick glance and it looks as though the input is in the wrong units. The negative pore pressure response from 2.5 to 7.5 meters is odd as there is only an initial spike in penetration resistance followed by no penetration resistance.

I would confirm the units in the raw data and import again converting to the standard metric units. The inputs are probably in imperial units but that is just a guess.

 

[kauri]

I could not confirm the units are not in imperial, as the raw data is just a bunch of numbers. However in New Zealand it would probably be negligent, if not criminal to provide imperial data without stating units. The data is also similar to our own CPTs, with lower sleeve frictions.

 

[KKasin]

Hi kauri
There seems to be several problems with the CPT data that you have posted.

The first thing that struck me is the lack of pore pressure responce after the layer at about 2,5 m. I would interpret that as lack of saturation due to cavitation through a dilatant layer at 2,5 m. This happens quite often when using water as a saturation fluid. Also you should note that this will influence the tip value, as one need to correct qc for the influence of the porepressure. Also there seems to be a lack of response in the friction, perhaps due to a calibration error, zero reading error, or a lack of sensitivity in the cones sensor. But anyway I would be confident in putting the data that is giving SBT=0 into SBT= Sensitive fine grained.

I would be very interested in knowing where the data is from, as we have similar datasets here in Norway, in highly sensitive soft clays)(quick clay).

 

[kauri]

Thanks KKasin,

The data is from Whangarei, New Zealand. Similar datasets are not uncommon. For your interest, a new commercial building suffered >60mm differential and unknown amounts of total settlement within 2yr.

One of our interpretations regarding fs was that the cone might be low quality or uncalibrated. A Geologmiski representative stated that he has seen such readings in soils settling at a significant rate, producing "low-quality data".

 

[BigH]

Just a question - did you not do a regular boring to get physical samples and hence determine the soil types that your cones were being pushed through?

 

[kauri]

Hi BigH,

For context, the data was obtained by a defendant in a lawsuit I was assisting. I was not seeking SBT-n to determine the soil type; rather so that CPet-IT would calculate a bunch of other parameters reliant on an SBTn.