Grade beam soil pressures

[PAstructural08]

I am design multiple grade beams running between spreadfooters. The spreadfooters are for building columns which are isolated from the mat foundation that I am also design. So the spreadfooters and grade beams are the perimeter of the mat.

The spreadfooters go below the frost depth (2m) and the grade beams are just in place for the mat foundation, obvioulsy I can't make the mat 2m thick for the frost depth.

My question is should I apply the at-rest pressure to both sides of the grade beam, or apply at-rest pressure on one side and active or passive on the other side??

Thank you

 

[dcarr82775]

Can you thicken the mat around the edge? If not, is that the only portion of the mat that will be on fill?

 

[Ron]

First...they are "footings" not "footers". Next, why are you concerned about lateral earth pressure when the structural member is only 1 or 2 meters below grade? The lateral pressure is negligible compared to the other loads.

 

[PEinc]

Also, doesn't a grade beam usually have about equal backfill on each side? Therefore, little or no lateral earth pressure?

http://www.PeirceEngineering.com

 

[paddingtongreen]

There is the construction condition when they often backfill and tamp the inside so they can go ahead with the slab, and follow up on the outside later.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[PAstructural08]

dcarr82775: We are not able to thicken the mat around the edges due to the clients instructions. Also, the entire mat will be on fill.

PEinc & paddingtongreen: There shall be equal back fill on each side of the grade beam but as paddingtongreen has said the inside will be backfilled first during the construction process.

Ron: I have heard the use of both "footings" and "footers". Also, the lateral earth pressure is not negligible compared to other loads. The only other loads acting on the grade beams is the surcharge load. I might not have explained the grade beam properly.

I have a 2m heigh pier with a 0.6m spreadfooting. Say, the top of the pier is @ EL. 300mm, than grade is @ EL. 0mm. The grade beam will extend from grade to the bottom of the spreadfooting. So, it is not between just the spreadfooting itself but between the piers also.

 

[BigH]

I'm sorry - have just never heard anyone worry about this type of problem for the shallow depths involved. Interesting (?) I have prepared quite a few geotechnical reports and have read so many more by a variety of experienced geotechnical engineers and, having a pretty extensive library, have never seen this condition addressed.

 

[PAstructural08]

They might be shallow depths but the spans, I consider to be large. The load on the grade beam is 1.5kip/ft...the grade beam length is 30'.

I've attached a sketch to clarify what I'm doing.

 

[PEinc]

6' of unbalanced fill? Sorry, but that's a retaining wall/foundation wall, not a gradebeam.

http://www.PeirceEngineering.com

 

[PEinc]

Sorry, make that 6'-9" of unbalanced fill. Even worse.

http://www.PeirceEngineering.com

 

[apsix]

My question is should I apply the at-rest pressure to both sides of the grade beam, or apply at-rest pressure on one side and active or passive on the other side??

My answer; apply active pressure on the inside only.
Why would you use lateral loads on both sides when designing for the temporary case with backfill on one side only?

 

[PAstructural08]

Well our client for this project refers to the "retaining wall" as a grade beam....therefore as do we. Also, when soil surrounds the object (inside & outside), I don't consider that a retaining wall. But I agree when the inside is filled with backfill and the outside is not...that is considered a retaining wall

Thank you apsix for answering my question.

I was applying lateral loads on both sides (at-rest pressure on the outside and active pressure on the inside) for a worst-case design.

 

[PEinc]

The worst case would be at-rest pressure on one side and no pressure on the opposite side. I would check it for active pressure on one side and air on the opposite side. I would also make sure that the backfil on the inside of the "grade beam" gets compacted with a smaller, walk-behind tamper. However, I would recommend that both sides of the grade beam be backfilled concurrently and somewhat evenly.

http://www.PeirceEngineering.com

 

[TDAA]

PEinc has the right approach. At rest on one side would be the case I would design for. If you use loads on opposite sides, the forces cancel, and would definitely not be worst case.

Only use active if you expect the grade beam to deflect due to the loading. At rest will result in more force, and designing for it should decrease the potential for wall deflection.

And as Ron said (first thing I thought when I read the OP) is that they are footings. Footers are at the bottom of the page. Just because people use the wrong term does not make it right.

 

[PAstructural08]

Thanks everyone for the comments back, I appreciate it.