Load Rate Effects

[Ed J. Garbin, Jr., E.I.]

I am looking for literature concerning the rate of test load
application and its resulting effects on the measured strength of
soil.

For example, conventional static load testing gives a direct bearing
strength value for the soil, but dynamic testing adds inertia and
acceleration into the equation, and these must be factored out to
obtain a static equivalent.

Have there been any such studies done?

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--
Ed J. Garbin, Jr., E.I.
University of South Florida
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On the WWW : http://www.eng.usf.edu/~garbin
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[Anthony Burke]

I am currently doung some research on the effect of a water table rise
on the settlement of a shallow foundation in granular soil. Terzaghi said
the settlement doubles when the watertable is at the foundation depth, and
there is no effect when the water table is below 2B under the foundation. Is
there any ideas on what happens in between?

Anthony Burke
Anthon...@jcu.edu.au

[Ed J. Garbin, Jr., E.I.]

[Rob Jeffrey]

Ed,

I assume you are well aware of the strong effect and interaction of load
rate with pore pressure (and strength) in soil mechanics.

Rob

[Ed J. Garbin, Jr., E.I.]

Of course. However, my research is focused on testing of shallow
foundations, which are not typically rested on clays but on granular
soils. In my case, the influence zone of the test apparatus is always
well above the water table, and the soils are granular, non-cohesive.
Therefore, pore pressure is not a concern.

It was Thu, 23 Jul 1998 12:38:47 +1000, when Rob Jeffrey
<r...@dpr.csiro.au> wrote:

>Ed,
>
>I assume you are well aware of the strong effect and interaction of load
>rate with pore pressure (and strength) in soil mechanics.
>
>Rob

[Ed J. Garbin, Jr., E.I.]

Thanks for the information. For your knowledge, there is a relatively
new test method for determining the ultimate capacity of a foundation,
be it deep or shallow. It is called Statnamic testing, and you can
find some information on it at:

http://www.berminghammer.com/frstat.htm

Upon analyzing the results of many tests on both shallow and deep
foundations, it has become apparent that inertia plays a significant
role in correlating Statnamic data with Static data.

Ed G.

It was Mon, 27 Jul 1998 10:49:28 +0200, when Lars Andresen <l...@ngi.no>
wrote:

>Hello Ed J

><P>As I know, all field an laboratory tests for measuring shear-strength
>are considered to be quasi static with no inertia effects present (even
>with different loading rates within the range mentioned above).&nbsp; I'm
>not aware of any dynamic testing methods which may be used to reveal soil
>shear-strength (except maybe for SASW - Spectral Analysis of Surface Waves
>which determines maximum shear modulus Gmax which again can be correlated
>to shear-strength).

[Martin Brungard]

Lars' comments bring to light some questions I have had regarding the rotation rate for the vane shear test. I wondered where the maximum rotation rate came from (I seem to remember 0.1 degree per second?) and I take it that the 1958 reference in Lars' post has the answer.

 

Here is what I am hoping to resolve. There are at least 2 methods for turning the vane, a worm drive mechanism or a torque wrench. But, when you think about the methods, they are quite different. One is a controlled strain test and the other is a controlled stress test (sort of!!). I'm in the middle of a project where I'm using a torque wrench to actuate and measure the vane resistance. I doubt that the rotation rate applied with the torque wrench is as slow as recommended. Does anybody have any quantification of the difference in the shear strength when the shearing rate is higher than standard? The materials I'm testing are a marine organic clayey silt and a mining waste that is primarily a silt. Both are saturated.

 

Of course, there is the Bjerrum correction to apply based on the P.I., but is there some other correction published to account for too rapid shearing? I assume the recommended shearing rate for the vane was based on some sort of data, I could use some assistance in determining what that data is.

 

Thanks,

--
Martin Brungard, P.E.
Tallahassee, FL 

Lars Andresen wrote in message <35BC3F18...@ngi.no>...

  Hello Ed J.It is a well known fact that the measured soil shear strength generally increases with increasing loading rate.  This effect is small, in the order of 1-10 %, for drained tests (e.g. triaxial) performed on dry granular specimens (see e.g. "Soil Mechanics" by Lambe and Whitman).  But for a saturated soil tested under undrained conditions, the effect can be quite substantial (see e.g. Bjerrum, Simons and Torblaa, "The effect of Time on the Shear Strength of a Soft Marine Clay", Proc. Brussels Conf. on Earth Pressure Problems, Vol. 1, pp. 148-158, 1958).

The loading rate considered is typically in the range from 10e-3 to 10e+1 %/min axial strain.  This effects are however not connected to inertia as a strain rate of 10e+1 %/min still is considered to maintain quasi static conditions.  The differences in measured shear-strength are believed to be due mainly to the inherent nature of the clay like visco-plasticity or to internal migration of pore water.

As I know, all field an laboratory tests for measuring shear-strength are considered to be quasi static with no inertia effects present (even with different loading rates within the range mentioned above).  I'm not aware of any dynamic testing methods which may be used to reveal soil shear-strength (except maybe for SASW - Spectral Analysis of Surface Waves which determines maximum shear modulus Gmax which again can be correlated to shear-strength).

-
Lars Andresen
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