Hi, I need to calculate the permea

[aj_k]

Hi,
I need to calculate the permeability, k from cone penetration data.
The formula I have found is by calculating the soil behaviour type index, Ic , which is calculated from the normalised friction ratio (Fr) and normalised cone resistance (Qt).
The equations for calculating Fr = [fs/(qt-σvo)]*100 and Q = [(qt-σvo)/Pa](Pa/σ'vo)ⁿ
For some soil layers, I am getting a negative value for (qt-σvo), since my qt values are smaller than σvo. Does anyone know how to continue with these negative values for calculating soil behaviour type index, Ic.

 

[GeoEnvGuy]

First off I hope your negative values are not at the surface which was drilled out through a road or another cemented material. Second I recommend you review the cpt manuals published by Paul Mayne and Greg Robertson.

 

[aj_k]

Thank you for your reply.
I have searched for a lot. But I could not find any documents regarding a negative value for (qt-σvo).
And my negative values are not on the surface. It's in the soil layer below ground level at a depth of 8metres or 10 metres.
Since I get a negative value for (qt-σvo), the values for Fr and Q are also negative, and I cannot go any further with those calculations.

 

[GeoEnvGuy]

Did you try and perform a pore pressure dissipation test in the interval of interest? The pore pressure dissipation allows for a better calculation of the permeability or will indicate if you are in a low permeable deposit if the test was not run to completion due to time constraints.

From the information you have shared i can only speculate that your total stress calculation may be wrong for a variety of reasons, you may consider incorporating a density calculation to more accurately calculate the total stress. I also want to highlight the permeability based on soil behavior types zones is for naturally deposited materials, not man made fills. It can be true if the material is truly in a state where the effective stress is zero or negative. If your data shows the 8-10 m zone having a large pore pressure response then you can be in a very soft clay, if there is no spike you could be in a free draining organic soil Zone 1, an underground void would be near the origin of the plot, or a liquefied material near the upper boundary of zone 1.

The general trend however is as you get closer to negative values for normalized penetration resistance, the Q is low and the Fr is high, that puts you on the bottom right of the chart below showing Ic contours.

This would put you in zone 2 identified as organic soils - clay according to the table below

If you truly were in a very soft clay you should expect to see a large pore pressure response due to the low permeability.

If you wanted to show the plot of q, fs, fr, and pore pressure I could probably offer a better opinion of the zone of interest.

 

[aj_k]

Thank you for your help.
I think i could get the data from the above mentioned.