Trench cave in impact Foundation forms. 1

[SRO]

Trench cave in impact Foundation forms.



I have a cast in place foundation wall located 4’ away from the edge of excavation which is a vertical cut 10’ deep. OSHA wants the inner forms braced incase the soil gives way, and caves the forms in.

Obviously the best approach is to retain the soil in place. To accomplish this I suggested trench boxes between the forms and the soil then install braces on the inside of the forms. However, the contractor wants to avoid this if possible.

In the event that I do need to account for the dynamic load of the soil impact, can anyone give me some feed back what they’ve done in this situation.

 

[PEinc]

The concrete forms should be the least of your worries. Unless the excavation was made in stable, competent rock; the excavation should have had some type of shoring system installed BEFORE anyone went into the hole. I don't see how bracing the concrete forms will accomplish anything. You would have to brace the outer wall form to the inner wall form at two or more elevations. These braces would penetrate through the concrete pour. Then you would have to likewise brace the inner wall form to the ground or to something else. If dirt falls against the outer wall form, you could have trouble removing the form safely.

I assume that the installed forms prevent installation of any type of cross bracing. I also assume that the excavation is wide and that there is not another excavated face to which you can cross brace. If so, then the most appropriate way to support the excavated in order to PROTECT THE WORKERS is to drive some cantilevered, steel sheet piling against the excavated face and sufficiently below the excavation subgrade. If the sheets can be driven, you can most likely drive the sheets without the need for workers to enter the unshored excavation. You might also be able to drive some cantilevered soldier beams and then drop in some timber lagging panels or steel plates to retain the soil between soldier beams. This won't be easy or cheap; but it can be done.

Your other (cheaper?) choice may be to excavate some more and safely slope back the 10' vertical cut to meet OSHA requirements.

Whether you install sheeting or do more excavation, you need to consider the effect of either method on any adjacent structures or utilities. This situation should have been addressed pre-bid and before any excavation was made. Contractors do stupid (yes, stupid) things like this every day and usually get away with it. Sometimes people get hurt or killed. You were just "unlucky" enough to get caught by OSHA.

 

[ishvaaag]

Part of the problem surely is that for small works the contractor has not by himself the means to do properly the work, and sometimes those that have are not in the mood or time-span in their planning to afford others to use them at some reasonable cost. Major local contractors doing street works (here, since about only 5 years ago) have usually all the necessary means to properly shield the trenches and hence people and standing buildings, but this can't be said from those doing building work, for whom letting weighty hardware seems always to be from unavailable through unaffordable to some economically unwanted or -improperly- uncounted nuisance.

So the matter stands, and so building contractors get imaginative -they have the custom- on how avoid such major works. Fortunately dynamic contractors are overcoming such trend by embracing the safety in works, making a point from the start on using means thant ensure the safety of people and workers, and where in the past they would have accepted, say, a foundation made by shields or batches one besides another may well today ask themselves to do some pile, micropile or bentonite trench wall. They find at such instances also at those instances the incentive of a more expensive foundation where to mitigate any reduction of benefits they had to make in the bid process.

But of course safety anytime is paramount and we must help with our professional activity to it become custom. So much for if that makes the building more expensive; we are being pushed (I would say slowly, we can't, it seems, diggest and negotiate at a quicker pace both the stupid new nuisances from some codes and the true real betterments) toward better quality works. This enters the question on how a house or building is a one time shot, and hence even if made of industrial products rarely enough input is given to it the same quality levels that CAN (but not always is) given to industrial products made in the hundreds of thousands or even millions; and what the relative cost should and can be. But, even when our professions are in engagement with the economy of the works, our true task is in enabling the making of the works, i.e., in our task economy is only ancillary to design and construction, for there are others (owners, commissioners, construction planners, cost controllers, financial parties) that of themselves must care with us of things being their worth. So ahead with safety, and if they can't afford, let the thing on the table to see what happens.

 

[PEinc]

In most situations like this one, the most expedient solution usually is to backfill the excavation and then re-excavate while taking the proper precautions to shore the excavation. However, this situation is a little different in that the wall footing is already constructed and the wall forms are in place. In order to backfill the excavation, you would need to first remove the wall forms, which may be hard to do without putting workers into the unshored excavation.

 

[SRO]

This contractor has no clue. He wants to avoid doing anything between the soil and forms. After trying to convince him that the best bet is to keep the soil in place (i.e. a trench box or other alternative). I told him the magnitude of the load would be blown out of proportion if we had to resist the dynamic force of slope failure. It fell deaf ears.

I need to run some really quick calcs (because I feel this job is, at best, going nowhere) to give him a very preliminary design. Any ideas for a quick and dirty procedure to get the dynamic load? I was thinking F=MxA for a 1' width.

 

[PEinc]

F = MA will be difficult to accurately calculate. After all, what is A? What is the distribution of the force to the wall forms? I don't think it is wise to try designing a structure to fit in less than a 4' wide space in order to resist (absorb?) collapsing soil. Even if you could place a trench box or some similar shield between the excavated face and the outer wall form, you would still need to brace both wall forms. If the trench box or shield gets hit by falling soil, you could damage the wall form and the form ties. The entire wall forming system would shift laterally, if not crush. Repairing the formwork after the soil falls against the forms would be more dangerous than the current situation.

The contractor now knows what needs to be done. He just doesn't want to do it.

There is no cheap, easy way to remedy this shoring problem. The contractor took a chance and lost. Now it's time to do the right thing. Anything less could cause additional problems. This is a contractor safety issue. As the engineer, you should stay out of it.

 

[SRO]

I think you are right... once again.

Thanks for the input from both of you, and additionally I want to thank PEinc for all his contributions to this forum. It seems you are in every other post helping all of us out. I just hope one day I end up w/ half the knowledge as you.

Thanks!

 

[PEinc]

SRO, thanks for the very nice words. I sure have you fooled!