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Cofferdam/trench box lateral surcharge load

Cofferdam/trench box lateral surcharge load

Cofferdam/trench box lateral surcharge load

I apologize in advance if the question is somewhat remedial.  This is not typically my field of expertise.  However something has come up for me that requires some braced cut analysis...

I have been reading the previous posts, and it seems this issue is largely split on how to properly apply the surcharge load.

When designing for a cofferdam style shoring system (or any other temporary shoring system),
One must account for lateral soil pressure(rectangular distribution in fine soil), however lets say for example the contractor wants to use a large crane (200 ton) to work within the shoring system at a distance of X' back from excavation.

Question is do I simply use Q*H*k for this?
Q = vertical pressure
H = excavation height
k = active soil pressure coefficient

and furthermore, should it be multiplied by a factor of 2?

For simplicity:

Say the vertical pressure is 1900 PSF for the crane with a track length of 24' and width of 4' per track and the excavation is 20' deep with soil pressure coefficient = .39.

Would it then be
1900PSF * 20' * .39

Seems like this number is grossly large, especially if multiplied by 2.
Should it then be distributed over the entire wall of excavation to a PSF?

If so, should the smaller side of cofferdam (assuming a rectangular unit) be used to be conservative.

Where does the distance from the excavation come into play?

Is there a surcharge load that would be considered conservative for all cases that could be used in place of specific calculation of crane or any other equipment/spoils pile/etc for that matter?

Thank you in advance for your help!!

RE: Cofferdam/trench box lateral surcharge load

If I remember correctly, for a uniform lateral soil pressure,isn't the additional lateral soil pressure from surcharge pressure be half the value throughout the depth of the trench,anotherword 800 lbs/sqft?
Also consider to place the crane on dunnage to substantially reduce surcharge soil pressure  

RE: Cofferdam/trench box lateral surcharge load

The lateral pressure from a vertical area surcharge is very often taken as p = q x Ka (PSF).  

q = 1,900 psf
Your Ka = 0.39
p (lower case) = 1,900 PSF x 0.39 = 741 PSF lateral load against the wall.
This pressure is assumed to be a uniform rectangular load against the wall.

Your calculation of 1,900 PSF x 20' x 0.39 [P (upper case) = 14,820 PLF = 741 PSF x 20'H] would represent the total lateral surcharge per linear foot of wall due to a vertical area surcharge of 1,900 PSF imediately behind a 20' high wall.

However, from your description above, it sounds like your 1900 psf is the bearing pressure beneath each of the cranes two crawlers.  If so, you should be looking at two strip loads, each 4' wide by 24' long (or 4' long by 2' wide, depending on the orientation of the crawlers).  You can use a Boussinesq analysis for strip loads.

You could also place the crane on properly sized and designed crane mats to distribute the entire load over a rectangular or square area that is greater than the area of the two crawlers.  For example, if the total crane surcharge equals 24' x 4' x 1,900 PSF x 2 ea. = 364,800 pounds and you placed the crane on 30' x 30' timber mats that are stiff enough to spread the load uniformly, the vertical pressure would be (364,800# / 900 SF = 405 PSF.  The lateral surcharge would then be 405 PSF x 0.39 = 158 PSF (uniform, rectangular, full height of wall), assuming that the surcharge is located immediately behind the wall.  158 PSF is much less than 741 PSF.

The lateral surcharge load also depends on the distance of the surcharge from the back of the wall.  Cranes do not usually sit immediately behind a wall.


RE: Cofferdam/trench box lateral surcharge load

Thank you for the reply PEinc.
Very helpful.

In the case of 741PSF you mentioned, I assume you are considering a crane immediately against the edge of wall?

Being that I will be recommending a setback requirement for the crane, I am trying to ascertain the recommended setback requirement which will allow proper access to excavation while keep system secure.

Where does the distance from the wall come into play and when/where does it get applied?
Best method of determining the minimum setback requirement?

RE: Cofferdam/trench box lateral surcharge load

If you do a Boussinesq analysis, you can enter the setback distance for the surcharge.  The greater the setback, the lower the lateral surcharge pressure.  There are different Boussinesq equations for area loads, strip loads, line loads, and point loads.  Pick the equation that most closely represents your crane loading condition.  The resulting lateral pressure districbution will not be either uniform or linear.

The best method for determining the setback is to keep the surcharge located at least beyond the active failure plane.  This would be about half the height of the wall.  This would not be a problem with a larger crane because most crane operators will not position their crane that close to the excavation because the crane boom would need to be too vertical to make a pick that is close to the sheeting wall.


RE: Cofferdam/trench box lateral surcharge load

The 741 PSF pressure would be for a uniform 1,900 PSF vertical surcharge that extends to the back of the wall.  However, be careful.  Crane crawlers usually have variable bearing pressure depending on the load and the pick radius.  Also, while the wall may be designed for the surcharge load, a crawler or outrigger pad might be too close to the wall that it easily damages and fails timber lagging or overstresses one or more components of the wall.  If the crane needs to be close to the wall, use properly designed crane mats or outrigger cribbing.


RE: Cofferdam/trench box lateral surcharge load

Very helpful and much appreciated.

RE: Cofferdam/trench box lateral surcharge load

I am a contractor that routinely runs crawlers on job.  Our bmp is keep the tracks the same distance back as the depth of the trench.  So, ten ft trench means stay ten foot back.  Crane mats and plates can significantly reduce your load as well.  I would also use an unfactored track load, because load is known.  I would however, calc the heabiest pick over the trench ide of the tracks and runs numbers for that scenario.  Long story short, its totally safe and doable when planned out.

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