DCPT ressults on silty clayey soils

[GeoGrouting]

It would be appreciated if you could help me to obtain the clay/silt properties from a series of DCPT results.

The objective is to calculate the pile skin friction or bearing capacity of soil. The DCPT values varied from 2 to 9 for the first 37' and from 14 to 25 between 37' to 45'. The DCPT on the underlying sand below 45' varied from 12 to 33.

 

[BigH]

GeoGrouting - before anyone can provide some insight, you need to specify the details of the DCPT. Why? - because in different parts of the world it means different things. If you look at the recent Soil Testing Engineering forum, you will find reference to a pentest - a 140 lb hammer dropping 30 inches to drive in a cone; but you will also see Ron posted a DCPT using a 15 lb hammer - then there is also the miniature DCPT used in highway work (developed in the UK). As you can imagine, the choice of the DCPT will affect the "blows - and also, blows are counted differently for different DCPTs.

 

[GeoGrouting]

Here is the specificactiins for my DCPT test.

Soil strength was estimated by DCPT (ASTM D 6951). Dynamic cone penetration testing is done by driving a standard 60º cone attached to a ~ 70 mm drill rod into the soil by blows of a 65 kg hammer falling from a height of 75 cm. The blow count for every 30 cm penetration is made to get a continuous record of the variation of soil consistency with depth.

 

[BigH]

That's the standard Canadian pentest. In the first Canadian Geotechnical Engr Manual, they had correlations for SPT to pentest results - it was something like 1.5 pentest = 1 SPT for the first 15 or 20 ft, then it changed; don't have access to my older files - perhaps someone else can help . . .

 

[geoman110]

I really do not recommend designing a pile based on SPT and/or DCPT in clayey soil. This is just not sensible engineering and also may lead to wrong design which is dangerous. Unless, you try to knowingly be over-conservative in the design.


With these low blow count numbers you can do field vane shear tests. You can also collect Shelby tube samples and conduct tri-axial test. Even if client does not want to pay for tri-axial test, you can obtain lots of information for design by conducting Field Vane Shear Test and the good old consolidation test (plus a few typical Atterburg tests and moisture content offcorse)

 

[GeoGrouting]

Geoman - I really do not recommend designing a pile based on SPT and/or DCPT in clayey soil.

Could SPT/DCPT be related to clay material properties based on litrature.

Unless, you try to knowingly be over-conservative in the design.

Do you think using SPT/DCPT approach would be over-conservative?

you can obtain lots of information for design by conducting Field Vane Shear Test.

What a about laboratory vane tests if a shelby tube sample would be present?

You can also conduct tri-axial test and the good old consolidation test.

Can a UCS test suffice if budget would be tight. The pile tips would be on sand with DCPT values about 18 to 34. What do we miss if we do not do triaxial or oedometer tests? I.e. can we use emperical values for the long-term soil parameters (in fact for skin friction only)

 

[geoman110]

Hi

SPT can be related to shear strength of the clay. There are couple of correlations around in the reference books. However, these correlations are pretty vague. For example, the associate shear strength for SPT values of 4 to 8 is about 25 kPa to 50 kPa and this does not necessary means a linear correlation that 4 corresponds with 25 kPa and 8 with 50 kPa. If the SPT is 6, your shear strength could be anywhere between 25 to 50. I have see soils with SPT of 6 and shear strength of 35 kPa and even personally experienced clayey soil with SPT of 6 and shear strength of 80 kPa.

Therefore, if you only rely on correlation between SPT and shear strength, you have to use the lower limit of shear strength provided for each SPT bracket. This may lead to a conservative design and in contrast, if SPT is the only mean, anything beside it could be risky and dangerous.

I have not seen any direct correlation between DCPT and shear strength. If you are thinking to use the same blow count of DCPT and use the SPT correlations, that would be another problem. The correlation between SPT and DCPT is yet to be proven. I have seen people that insist the blow counts between SPT and DCPT are comparable. I have seen soils that we conducted DCPT to 20’ (6 m) depth and the numbers between SPT and DCPT were a very good match. On the other hand, I have seen plenty of sites that after penetration of about 5’ (1.5m), the DCPT values start increasing and deviating from the SPT values (due to skin friction). I personally could not figure out what are the condition that the DCPT and SPT are comparable and when are not. I believe, looser/softer the soil, more comparable are the numbers. However, until someone shows me strong evidence, I only use DCPT as a qualitative tool, not quantitative (e.g., to find the depth of hard soil underneath of soft soil).

A laboratory vane shear test is fine as long as you have good sample.

An Unconfined Compression test is also good for simple design (much better than SPT correlation for clayey soil).

All of my comments were in regard to obtaining the parameters of clayey soil for calculating skin friction of your pile within clayey layer.

For calculating the tip resistance within sandy soil, you have no choice other than using correlations with SPT. I still do not assess the tip resistance based on DCPT. Conducting tri-axial test on undisturbed samples of the sand may seem appealing, but it is not practical for a real project of regular importance. Even for important project, people do not usually try to do this kind of stuff (tri-axial on sand). For important project, you do a pile load test before finalizing the design.

 

[GeoGrouting]

If I have the choice between laboratory vane test and UCS, which one is going to be more useful for pile skin friction calculation? Thank you.

 

[geoman110]

It is hard for me to answer this as I have not done too many Laboratory Field Vane test.
I prefer the UU tri-axial test to Unconfined Compression Test and lab vane. It is as fast as UCT and even with one sample you can get good results (e.g, multi stage test). However, if you have no choice and have to chose, I personally believe UCT is better than lab vane. However, as noted before, I have not done too many lab vane and my comment could be a bit off base in this regard.
Regardless of what you do (UCT or lab vane), I recommend conducting a consolidation test. There are good correlation between shear strength and Moisture Content, Plasticity Parameters, and Preconsolidation Pressure. You can double check your results of (UCT or lab vane) with these parameters.

 

[GeoGrouting]

thanks