Hand vane shear strength vs hand penetrometer

[Geoinvestigate]

Is there any way of converting the results from a hand penetrometer into Cv<br>
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Thanks in advance<br>
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Regards<br>
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<br>
andy

 

[Ginger]

Andy,<br>
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Not sure if the question relates to the title of your query. The hand penetrometer is a device to measure the cohesive shear stength of a soil Cu. This is done directly and your penetrometer should come with a calibration sheet so you can convert the penetrometer reading directly into a cohesive strength. A hand shear vane will also give an instant Cu reading from the calibrated dial on top of the rotating head. If you are asking for a measurement of Cv (coefficient of volume compressibility) then I'm afraid you need to perform an oedometer test in the lab. There is no way I know of of obtaining Cv from a penetrometer as Cv requires accurate measurement of volume change which a penetrometer clearly cannot provide.<br>
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Best regards<br>
<br>
Ginger

 

[dirtman]

A hand penetrometer is an extremely poor piece of equipment. I personally would never use one, but would instead use a pocket torvane apparaatus for measuring the undrained saturated strength of a clay soil.

 

[bmike]

If by Cv you mean coefficient of consolidation, or as Ginger has suggested coefficient of volume compressibility, the simple answer to your question is no, although you may be able to find some empirical correlations, of dubious accuracy, which will be quite dependent I am sure on your estimate of the over consolidation ratio.

A hand pentrometer purports to measure unconfined compressive strength (referred to as Qu typically). Vane measurements yield undrained shear strengths (referered to as Su or Cu typically).

Unconfined compressive strength is one half of undrained shear strength.

I have used the hand penetrometer frequently in both normally consolidated clays and in heavily over consolidated tills. The vane is the preferred field instrument, however the large amount of stone in the tills makes it necessary to use other means. I have found that the penetrometer shows reasonably close agreement to the results of unconfined compressive testing.

I have not subjected this to any statistical analysis, but think I am accurate in stating that the penetrometer results agree, within 25% to 50%, to the results of Qu testing, when performed on tills.

Having said all this, however, I would not think of the results as anything more than a strength index, and would not stretch the results to the extreme of using them for consolidation or compressibility parameters.

 

[jetman]

I need to convert vane shear strengths of soils from kPa to psi. I have used the standard conversion of 0.14504 psi/kPa but the numbers seem incorrect. Example: 73 kPa = 10.59 psi. This is a pressure conversion from SI units to English units. I am trying to relate these numbers to soil strengths.. Can someone give me a list of soil strengths.

 

[Ginger]

Your conversion appears to be correct. A shear strength of 73 kPa suggests you have a firm clay (assuming t's a saturated clay). The shear strengths of typical generic soils can be lifted from most soil mechanics text books.

Regards Andy Machon

 

[bmike]

Jetman, your conversion is correct. If you simply think of 100 kPa equalling 1 tons per square foot, you will be pretty close (its actually 95.7605180 kPa per ton per square foot). The difference is pretty subtle when one considers the accuracy of a pocket penetrometer, or, unfortunately, most field vane tests done too quickly and measured with an automotive torque wrench with a resoltuion of 5 ft-lb, if you're lucky.

In your case, 73 kPa is about 0.75 tsf, or 1500 psf, or a little more than 10 psi.

 

How friction angle would change if unit weight or density of the soil changed, i.e. how does the value of the friction angle change as a granular soil's density increases? What difference would there be in results if Ottawa Sand were tested in a wet condition? How the is the friction angle used in the design of shallow foundations in granular soils?

 

[Ginger]

The angle of internal friction would increase as the soil density increases. This is a function of the particle interaction in the soil. It will take more force to cause the soil particles to slide past each other when the soil is more dense. When water is added then there is a buoyancy effect on the soil particles with a corresponding reduction in the angle of friction.

The friction angle parameter (phi) is fundamental in ascertaining the bearing capacity of the ground. Whichever method you use (Prandtl, Reissener, Terzaghi etc), they all rely on a function of phi. When phi increases, so does the bearing capacity of your foundation.

Regards



Andy Machon