Slurry backfill behind Soldier Pile Lagging 1

[reggeo]

My drilling contractor wants to backfill the cavity behind 4"x12" timber lagging with lean concrete ("slurry"). He says this is common practice.

I am worried that this material will not be permeable enough and excess hydrostatic pressure could develop. Still, waterproofing is required for the concrete face of wall, so maybe lean concrete would "weep" enough(?)

Does 2-sack lean sand/cement concrete (slurry) have adequate transmissivity for this purpose? Also, is low-slump slurry acceptable as backfill over a 4" diameter slotted PVC/gravel/geotextile subdrain?

 

[PEinc]

No

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[reggeo]

I didn't think so. I will require pea gravel to fill the cavity behind the lagging and above the subdrain

Thanx, PEinc

 

[PEinc]

Be careful with placing any type of loose granular material behind lagging. Walls with lagging are most often built from the top down with lagging being installed in about 5' lifts. If you overdig for lagging and then backfill behind with loose granular material, it can all run out when you dig for your next 5' lift. When installing lagging, you should dig just enough to fit in the lagging boards. Do not overdig. If there is a small amount of overdigging, the space should be filled in and packed tight with the same excavation spoils.

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[reggeo]

PEinc

Thanx for your reply

Lagging is already in place. A small slough concavity occurred before lagging could be installed; there is no void space below or behind the lowest lagging but there is a 6"-12" wide ~5' high void behind the upper lagging. It's too narrow to mechanically compact, so I've considered "self-compacting" pea gravel. We could also lower a geotextile across the bottom of the void area to help keep the pea gravel in place.

I've also thought about jetting the native DG into place behind the lagging. The jetted water would seep out between the gaps in the lagging.

 

[PEinc]

Don't put water behind the lagging. If the void is only 5' high and extends to the top of the wall, dump in the gravel full height or dump in some granular soil in lifts and tamp (pack) the lifts with something like a long handle 8# sledge hammer.

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[PEinc]

I meant to say don't put loose clean stone behind the lagging unless, for some reason, the wall is cantilevered and is being built from the bottom up. In that case, the wall should be of relatively low height and you would stack the lagging from the bottom up with stone placed behind the lagging.

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