CPT's in peat

[JWB46]

Hi guys,
Has anyone experience of CPT's in peat. Field vane tests are known to be unreliable in peat, are CPT's any better?
Any help would be appreciated.

 

[BPin]

The reference book "CPT in Geotechnical Practice" by T. Lunne, PK Robertson and JJM Powell references a work by Landva in which "Landva concludes that due to the fibrous nature of peat, and the frequent presence of obstructions like stumps and roots, small-scale in situ tests like the cone penetrometer or the vane tests are of little engineering use for the design of road embankments. He recommended using carefully selected laboratory tests or large test fills. However, in organic soil of non-fibrous nature, CPT and vane tests can be very useful tools."

The paper by Landva is: "In site testing of peat". Proceedings of the ASCE Specialty Conference In Situ '86: Use of In Situ Testing in Geotechnical Engineering, Blacksburg, 191-205, American Society of Engineers.

 

[JWB46]

Thanks BPin. That's really very helpful.

 

[coneboy]

To JWB46

I am aware of many soil strength and consolidation correlations for use in silt/clay soils when using CPT data. Unfortunately, none of them can be applied with any great certainity to highly fibrous soils such as peat.

I agree on not using a field vane in peat. The fibrous matter in the peat just raps itself around the vane and you get a reading that is far to high.

Many times in the past we have augered into the peat see where the fibrous peat ends and where the underlying organic silt starts. This procedure can tell you the depth at which your CPT data can be used as the organic silt is more amendable to CPT interpretation.

If you can assume the peaton your site is normally consolidated, then use the moisture content and determine the settlement/consolidation parameters using the Cc/1 + e0 method. I know that there is more than one approach to this problem, but I have had good success with this method.

Best of luck
Coneboy

 

[coneboy]

To JWB46

I thought of few other items that will answer your question better.

Peat has a very low tip resistance and some sleeve friction. This produces a very high friction ratio, which is the typical trade mark of normally consolidated peat. If a low capacity electric cone was used then you should see some high friction ratios through the peat. The less fibrous, organic silt which typically underlies peat will not have a high friction ratio and may be interpreted as a silty clay.
If a high capacity electric cone was used then the peat deposit may not show up clearly, or at all. In my experience, an adjacent auger hole will help clear up any lingering questions.

If the peat has been has been preloaded (no longer normally consolidated) then the interpreted soil behavior type will most likely be "Clay" through the peat zone.

Peat is one of the most difficult soils for the electric cone to accurately identify as it's measured properties are right at the bottom limit of the load cells capacity. If you are really worried about locating the peat layer get the CPT contractor to use a low capacity (5 ton) cone.

Hope this helps
Coneboy

 

[JWB46]

Thanks Coneboy. That's very helpful. I'll let you know how I get on.
Regards,
JWB46

 

[toolpusher]

JWD46,

One other problem with trying to interpret CPT results in peats is that once you get through the fibrous cap, you typically encounter an "amorphous" or decayed organic zone beneath. This often is very weak material and is not soil at all, but material that looks black and definitely was fibrous once.

The problem with interpreting shear strength from the CPT in very weak soils has been covered by coneboy, who drew attention to the equipment-sensitivity problem. So, if you cannot make much use of the cone penetration resistance, you are left with trying to use the penetration pore pressure response to evaluate undrained strength. In most fine-grained soft soils, this is a much more accurate method than qc, however in your peat you may very well encounter an unsaturated pore pressure response - even below the GWT! I have tried CPT's with pore pressure measurement in such conditions and found lots of gas in the section, which partially damps out the pore pressure response to penetration.

So long as you check the response with some dissipation tests at rod breaks and use low-capacity equipment, you stand a chance at getting a decent measure of shear strength, but only within the amorphous zone.