Pressure Meter Data and Soil/Rock Modulus Interpretation

[garrettk]

I've become involved in a project that has had numerous subsurface investigations over the last 7 years and one piece of the data gathered is Pressure Meter testing.

In the results of the PM testing, there are two modulus values provided, unload/reload modulus and initial modulus. The unload/reload modulus ranges from 3 to 10 times the value of the initial modulus.

The geologic setting consists of a relatively thin soil overburden with severly weathered shale, claystone and thin seams of sandstone underlying. The material is actually called Flysch which is German for flow (I have observed numerous landslides in the area).

The grading plan calls for removing approximately 20 meters of soil/rock from the site prior to construction of foundations which will typically consist of heavily loaded mat foundations - pressures between 400 and 500 kPa.

The question becomes, which modulus should be used when estimating settlement? Most texts reference the use of the initial modulus. However, because we are removing such a large overburden pressure, would it be more appropriate to use the unload/reload modulus provided our foundation loads are less than the overburden pressures?

Obviously this makes a huge difference in the estimated settlements.

Thanks in advance for any advice.

 

[BigHarvey]

If you remove overburden you are entitled to use the reloading modulus for loads up to what you are removing. pressuremeter rules give you coefficient of multiplication in cases where cyclic tests have not been performed.

 

[garrettk]

I'm not sure I understand your last statement regarding a coefficient of mulitiplication. Could you explain this further?

Thanks for your previous response.

 

[BigHarvey]

If you run a atandard pressuremeter test and not a cyclic one , you will be allowed to use a eloading modulus equal to 2 times the virgin modulus for clays ( 3 times for silt and 4 times for sand and gravel).
If you have run cyclic test then you must use the real values obtained

 

[garrettk]

Ok, that makes sense. We have cyclic tests and will use that information directly - with a Factor of Safety applied.

Thanks for you assistance.