surcharge estimating 1

[enko]

Hello
I have to perform e sheet pile design for an excavation.
The building to be retained is a 3 storey r/c structure with pad foundations.
Dimensions of the existing building in plan 9x32 m. The sheet pile wall will be along its long dimesion (32 m).

Anyone has any idea for calculating the surcharge loads coming from the pads?

In the previous designs I have had, the existing building foundation (to be retained)had been strip footing, which allowed me to restrict the design in 1 m pile wall.

Is this case different because of the concentrated loads coming from pads?
Thanks in advance

 

[a2mfk]

That seems like some pretty serious surcharge, with some critical consequences if your piling deflects an undesirable amount. I would get the footing reactions from the EOR of the building and get a geotechnical engineer to give you lateral pressures based on the exact soil at your site. But I suppose once you get the reactions you could calculate the surcharge yourself, I would want a licensed geotech providing that in writing myself for a project of this caliber...

 

[enko]

Thanks for replying
Actually our office deals regularly with these projects.
We get the geotechnical report from a licensed geotech but the surcharge evaluating is our issue.
As I mentioned, the cases when the existing foundation is a strip foundation or even a mat foundation is clear because the loads are treated either uniform or surface loading respectively.

Even in this case the concentrated load can be converted into uniform (conservatively) which has been our practice.

I am interested for any more detailed method concerning pad foundations.
I'm focusing in the "Boussinesq" concentrated load pressure

 

[Lion06]

I was going to recommend the boussinesq equations - Bowles is a good reference.

 

[a2mfk]

Gotchya, when doing retaining walls I have had instances where geotechs provide the lateral load pressures once we give them the soil bearing loads and locations...

 

[dcarr82775]

Boussinesq is a fine method, and there are equations for both a strip and point load. It assumes a rigid wall so it gives conservative loadings. the spacing of the pads and how far behind the wall they are will influence whether a strip or point loading is more accurate.

In the past I have done a simple trial wedge analysis and then converted the loads from active to at-rest since that is the soil condition you are working with.

 

[enko]

I agree with you guys.
I personally like the "conservativeness" of the geotechnic designs.
Not to mention that the discuss focuses on the "plane deformation state", or 2d calculation.
Anyway, I will try to make a 3d model of the soil in
SAP 2000 modeling as solid(just for fun).
The question is at defining "E" (deformation module).

 

[FixedEarth]

Whether a sheetpile is an appropriate earth retention structure depends on the lateral distance between the sheetpile and the building, the type of subsoils, location of other nearby structures and the height of the excavation. For example, sheetpile being a flexible retaining structure would not be suitable if you are only 10 feet away from the building. The sheetpile would deflect away from the excavation and this will cuse the soil behind the sheetpile to yield.

Sometimes you will need to underpin the existing footing and choose a rigid earth retention solution like a tiedback slurry wall.

As a quick guide, however, you may use 250 psf uniform surcharge for every 1 story of RC building. Attached are the surcharge charts to estimate your actual lateral stresses.

 

[enko]

thank you Fixed Earth

Anyway, the analysis methods we've been using so far allow the soil yielding.
On the other side,we monitor the max. deflection of the sheet pile not to overpass the allowable value.
I don't think if it's anything wrong with that.

 

[PEinc]

I would not use an average psf surcharge for a building with spread footings. I would try to get the plans for the existing builting and then calculate the loads on the nearest footings. If no plans are available (they seldom are), then I would visit the building and make measurements of the wall height, construction, and thickness to determine the wall's dead load. Then I would check out the slab construction, floor loads, and column spacings to determine the column loads. Then, when I have calculated the total load on the footings (DL & LL), I would check to see if my calculated bearing pressures are reasonable. No mater what system you design, you should visit the building to check its condition amd do a preconstruction survey. During construction, the building should be monitored for movement.

As FixedEarth said, this may be an underpinning job rather than a sheeting job. Don't forget any required lateral support for underpinning.

http://www.PeirceEngineering.com

 

[Ron]

Agree with PEinc....since you won't be changing the size or section of the sheet piling along the wall, design for the actual isolated footing loads closest to the wall.

 

[FixedEarth]

Anytime enko. I agree with PEinc & Ron, for actual shoring design that you may submit, use the exact foundation loadings.

As a curiosity, I did a quick check on lateral stresses of 750 psf uniform loading near a 30 ft excavation and then compared it to a wall line footing of 8k/ft and isolated column load of 100 kips at a distance of 10 and 20 ft respectively. Guess what--the earth thrust values are within 15% of each other.

As you stated, if you limit the deflection then that ofcourse would help. However, if you have to dewater the bottom of the excavation and you have granular soils & other nearby structures, be extra cautious with soil settlement effects.