Surcharge Load Approach

[InDepth]

Apparently Civil Tech Shoring Suite 8 uses the following methodology (see below). Key in on the fact that they take the strip loading surcharge and modify it to obtain area and point load surcharge. Is this methodolgy typically used in practice? I usually use the Terzaghi equations listed in the USS Steel Manual for point loads (modified by experiment & listed in almost every shoring manual).

I believe that the strip and area loading is based on elastic methods, while the USS Steel Manual equation for point loads accounts for the inelasticity of soil (modified by experiment).

There appears to be great differences between using the Civil Tech methodology and the USS Steel manual (Terzaghi)methodolgy. Any thoughts would be greatly appreciated.

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We use the following equations:


1. Pstrip = k* Q/pi * (beta-sin(beta)*cos(2alpha))

where k=1 for very flexible wall. k=2 for rigid wall

This equation can be found in P16 of USS manual. It is modified Boussinesq equation and widely used for shoring design. It called Wayne & Teng Equation. Wayne & Teng Equation is widely used in shoring design. We found that using Boussinesq equation to calculate Area Loading and Pint Loading do not match the results from strip loading calculation of Wayne & Teng Equation. We have to modify Wayne & Teng Equation for area loading to keep the results consistent.

Following are our equation:

2. Parea = f * Pstrip
Where f is length factor:
f = 1- 1/(0.25 * L/(X+1) +1)
L - length of area loading; X - Distance to the wall.
When L is infinitive, f = 1, Parea=Pstrip

We plotted curve from Boussinesq equation and use this curve to scale down to fit Wayne & Teng Equation. Then we get the above equation.

3. Ppoint = Parea when Length=1 and Width=1.

 

[PEinc]

Why don't you call James Su at CivilTech and discuss this with him?

http://www.PeirceEngineering.com

 

[fattdad]

There is not one ? in the OP. Is there some direct question related to other's approach to designing a surcharge?

f-d

¡papá gordo ain’t no madre flaca!

 

[InDepth]

I apologize for not providing a clear question. What I am after is an understanding/confirmation of the assumptions behind surcharge load equations. What are the assumptions behind the equations cited in the texts (USS Steel, Caltrans, NAVFAC, etc..)? Is it only the Terzaghi point load /line load surcharge case that has been modified by experiment? Did the strip load case never get modified by experiment and is it still an elastic solution?

Which equations assume (Point, Line, Strip, Area)
1) elastic medium or inelastic medium
2) rigid wall vs flexible wall
3) equations modified by experiment
4) backfill or natural soil
5) Soil/wall friction (some elastic solutions don't account for wall friction)

Based on my research you'll notice that the USS Sheet Pile Manual, Navfac DM, and Poulous and Davis reference are all the same equation for the strip loading surcharge case(for poulous multiply by 2 to get same result). You'll also note that all these strip load equations are based on elastic theory. To obtain this equation, the Boussinesq elastic solution was superimposed. The assumption was an infinite strip load in an elastic medium.

The point load and line load equation on pg 15&16 of USS Sheet Pile manual have been modified by experiment by Terzaghi. Thus, the modifications changed the shape of the Boussinesq to account for soil inelasticity. Unfortunately the strip loading solution provided in most texts was never modified to reflect soil inelasticity.

With this said, it appears that the Civil Tech software has an underlying assumption that the surcharge provided is based on elastic theory (nothing wrong with their approach).

 

[fattdad]

I've never used a software to design a surcharge. I have designed surcharges for different cases:

to minimize secondary compression (force aging).
to arrive at 90 percent primary consolidation quicker.
to force elastic compression prior to large areal loading.

In each case, I've used some version of Bousinesq or Westergard elastic solutions to arrive at the change in vertical effective stress with depth. Not sure one is better than the other, just used engineering judgement for that site condition.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[jdonville]

InDepth,

The elastic solution (Boussinesq, Westergaard, etc) is conservative (assuming that you have your flexible vs rigid assumptions right), therefore safe.

BTW, when using ctShoring, I always use an area load - that way I can be sure that the loading area matches the pile spacing. Based on the proximity of the load to the top of the wall, you may also want to modify the elastic loading so that loads outside of the assumed failure wedge don't penalize you unduly.

J

 

[rowingengineer]

sorry don't have the time to read everything so i may just be posting something stupid, but I believe the problem could be half space, take all at this link.

http://www.ejge.com/iGEM/Articles/FactorOf2/FactorOf2.htm

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rowingengineer]

why is when you want to do things fast they always take a heap of time, PDF for those who want it.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[InDepth]

It's interesting, because EM 1110-2 indicates 1 for yielding walls and 2 for non-yielding walls. See attached.

jdonville, I like your points. I usually select elastic solutions for stiff shoring systems like diaphragm walls or secant walls with tightly spaced tiebacks. I select the Terzaghi (modified by experiment solutions) when dealing with soldier pile and lagging. i.e. Surcharge selection (and even earth pressure selection) should be stiffness based. Are there any papers or reserach on this?

Some engineers/softwares seem to use:
Boussinesq elastic solutions multiplied by 0.5 for flexible wall and 0.75 for semi-flexible walls and 1 for rigid.

 

[fattdad]

If you are using an elastic solution to derive the change in horizontal effective stress and if you are then using these values to design loads acting on a retaining wall, you need to double the values.

My professor (J. M. Duncan) was clear on this and there was no wiggle room for whether it was the active case or whether it was the at-rest case. I'm talking about retaining walls, not temporary shoring systems.

f-d

¡papá gordo ain’t no madre flaca!

 

[InDepth]

2 Questions:

1) Elastic solutions also have a shear component. Would you also double this shear component?

2) If the elastic solution provides stresses (horizontal and shear) that exceed the soils ultimate capacity, what would you do? Would you cap the elastic stress levels at the ultimate soil capacity....but then how would you achieve force equilibrium?

 

[fattdad]

You'd double the horizontal component of the shear force.

Not sure what you mean by "the soils ultimate capacity." Not trying to be dense, but the soil's ultimate capacity is determined by shear strength and if you are designing a shoring system or a retaining wall, you are already dealing with soil strength and shear capacity. Your structure will make up any force deficate and allow for the safety factor.

f-d

¡papá gordo ain’t no madre flaca!