Bottom of Sheeting Bearing on Granular infill

[RFreund]

At the bottom of a braced excavation the contractor has hit refusal in a tough clay. We instructed them to add an additional row of bracing near the bottom. However they would like to pour the concrete slab then backfill with granular material between the sheeting and the concrete slab. Then remove the lowest row of bracing. This requires the granular material to act as a continuous strut between the sheeting and the concrete slab. To put some numbers on this:

Top of slab to bottom of sheet pile = 1.5 feet
Distance between Sheeting and edge of slab = 2.5 feet
Total retained height = 40 feet
Retained soils are mostly very stiff clay
Reaction force at the bottom of the sheeting is about 4.5 kip/ft

I'm curious to hear anyones thoughts on this. My concern is that the the granular material will heave but I'm not sure how to prove this or what the relationship should be as far as depth vs length of granular fill between the slab and the sheeting.

EIT

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[SlideRuleEra]

I would not consider using granular fill for that application. There is too much uncertainty in how will perform under load. IMHO, flowable fill would be a much better choice. Poured unreinforced concrete fill would be best.

An addition way to evaluate the Contractor's proposed granular fill is to determine how much sheeting movement can be allowed before the wall becomes unstable. I would expect the answer is "not much". If true, and you present the movement limit requirement to the Contractor, his confidence in granular fill probably will go away.

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[Ron]

Agree with SRE.

Further, if you turned the "picture" on its side, you would have the equivalent of 4.5 k/ft bearing on sand. That's a bit more than you'd like to see for bearing pressure on sand. Still further, the lateral "bearing capacity" of sand in this case is much lower than a vertical bearing capacity of the same material. There is no top confinement of the sand.

 

[GeoPaveTraffic]

I agree with the others. Either pour the concrete slab all the way to the sheeting or use something like flowable fill between them.

Mike Lambert

 

[PEinc]

I agree. Options: Flow fill, pour the slab against the sheeting, or place blocking between the edge of slab and the sheeting (if the slab is thick enough and continuous enough to accept the load). Also, is the braced sheeting strong enough to support this new design case when the bottom brace is removed?

http://www.PeirceEngineering.com

 

[RFreund]

Thanks for the input!

EIT

http://www.HowToEngineer.com

 

[RFreund]

The contractor now has offered to to put 2'x2'x 4' long precast concrete 'deadman' on top of the granular area (in order to help keep it confined). The price of concrete must be going up! I'm still leery... Would this sway anyone's opinion?

EIT

http://www.HowToEngineer.com

 

[Ron]

No.

 

[PEinc]

If the SSP or soldier beams are too short, it might not be possible to excavated to subgrade, pour the slab, backfill the slab, and then remove the blower bracing level. There could be insufficient toe embedment to allow excavation to subgrade.
The same is true for placing a 2'x2'x4' concrete "mafia" block. The sliding resistance of a block would probably be insignificant compared to the required toe reaction.
The PE who designed the 40' high sheeting wall needs to re-analyze the wall, including any temporary condition, such as before the lowest brace level is installed, before the slab is poured, or before the "mafia" block is set.
I suspect that the solution is to install an additional level of bracing, a couple of feet above the top of slab (brace location chosen to assure sufficient toe embedment before installing the brace level), pour and backfill the slab, and then remove the added brace level.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

Another "No".

Braced excavations and cofferdams differ from most other structures. The calculated loads are large (compared to member capacity), the loads are "real" (not statistical probabilities, like wind, seismic, and even live loads), and the braced structure has to stay in equilibrium so that all structural members share load as expected. Finally, if there is a structural failure many times it is a sudden, dramatic, and total collapse. Casualties can be expected if this happens while work is underway. The bottom line is that you DO NOT take chances with short cuts that "might" or "should" work.

If the Contractor wants to change his plans, take PEinc's advice and reevaluate the complete structure to come up with an intelligently thought-out revised approach.

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[RFreund]

Thanks again. In short, ensure that this: Link doesn't happen

EIT

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