Elastic Methods for Offset Surcharge 1

[RFreund]

Basically there are 3 equations:

Point load or area laod, Line Load and strip load.

The most commonly used for point load and strip load are derived from the boussinesq equation and were modified by Terzaghi (and others) after experiment. However for strip load I do not see anywhere that this equation has been modified. All three commonly used equations can be found in NAVDAC DM-2 or USS sheet pile manual or Poulus and Davis. I have two questions:

1. Bowels suggests only using boussinesesq's equation with finite are point loads and adjusting values for Poisson's ratio and factors of safety. However when you compare the results of this type of analysis for strip loads you do not find the same results (using a ratio of 0.5 and FS=1.0). Why is this? Is there some modification to the strip load equation?

2. Is there a text that presents the experiments and modifications to elastic analysis surcharge approximation that may shed some light on my above question?

Thanks in advance



EIT

http://www.HowToEngineer.com

 

[FixedEarth]

See attached.

 

[RFreund]

Forgot to say thanks for the attachment.

I still don't understand why the strip load equation gives different results than using discrete point loads and the Boussinesq equation.

EIT

http://www.HowToEngineer.com

 

[aeoliantexan]

Soil Engineering, Fourth Edition, by Spangler and Handy, treats this subject on Pages 564 - 571.
They present the equation for a point load based on Boussinesq plus experiment, and then integrate it for line and area loads. Spangler conducted some of the early experiments.

The pressures given are higher than those given by Bousinnesq because elastic analysis assumes the soil is a half space, so deflections occur in the soil across the plane where the wall will be. If the wall is rigid, the pressure will be higher because the soil can't strain as much.

 

[RFreund]

aeoliantex - Thanks.
I don't think my wife will allow me to by another soils book without finishing the 5 that FixedEarth has recommended to me in various threads. However I think you have answered my question and I may look for the book as this question has been bothering me for sometime.

Basically you are saying that Spangler and Handy start with the Boussinesq equation that is modified by experiment and integrate it. There for the strip load equation that is in the Navdaq and USS sheet pile among other places is not directly based on the Boussinesq equation (point load) but is based on a modified Boussinesq equation. Right?

Thanks.

EIT

http://www.HowToEngineer.com

 

[FixedEarth]

RF- Didn't mean to get you in trouble with books!

To illustrate your question- I had a similar problem on a real life project recently. See attached. The Chimney has a setback that does not require for it to be accounted per Boussinesq stress distribution, but if we use the Spangler results (or Boussinesq modified by experiment), we get higher lateral stress. So what did I do? Spangler won over Boussinesq and we used larger soldier beams. If I had an instrumentation budget, we could have found out!

 

[RFreund]

FE - Thanks for sharing. You going to have to work on that budget, I would like to see the results!

EIT

http://www.HowToEngineer.com