Compacting behind retaining wall 1

[asixth]

Hi guys,

I have a situation where I need to provide an elevated slab on ground 600mm (2') above natural ground level. The architect doesn't want to tie the wall to the slab because they are using some crib wall system and the builder is worried about subsidence and thinks we should have piers down to the natural soil.

My questions are, can settlement still be a problem if we have adequate compaction behind the wall and eliminate the need for piers?

Is this the best solution? I think I should tie the slab to the wall so it prevents any horizontal movement of the wall away from the slab in the future.

 

[SlideRuleEra]

Suitable compacted fill on top of good existing soil can, and will settle... but for a two foot thickness, with proper compaction the answer would be not much. The real question is how much settlement is acceptable.

Your sketch does not show if the fill extends outside the perimeter wall. If it does, then you may not need the footings. A thickened slab to support the walls may be sufficient.

If you do use the detail shown in your sketch, I would not tie the slab to the wall. Even small differential settlement would tend to "rip" rebar ties out of the slab, possibly causing concrete spalling that would look bad and be difficult to repair.

If the wall with footings in existing soil is used, you may not need the perimeter slab build-downs (shown in your sketch). A slab of constant thickness should be adequate.

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[Ron]

With the sketch as noted, I would be concerned about loss of material from beneath the slab more than compaction settlement.

I assume they will be doing some type of decorative wall such as segmented block. These are not sealed systems and can allow lateral material migration.

I would prefer to see the slab resting on a stemwall and tied to it. You can put any kind of "face" you want on the wall, but this type of construction will allow confinement of the subsurface soils as well as integrity for the slab/wall system.

 

[apsix]

You should be able to get adequate compaction except immediately behind the wall.
If it's a concrete block type wall I would tie it into the slab, see attached for examples.
If tieing in is not an option run the slab edge down to the wall footing. Reinforce and use additional internal beams as per AS2870 recommendations if it is for a building.
If it's an external slab provide adequate falls to cater for some subsidence in the centre (and edge heave if there is reactive clay).

 

[BAretired]

The answer depends on the location. In my area, frost heaving would be a major consideration. I tend to agree with Ron that it is better to use a foundation wall under the slab and that the slab should be tied into the wall. Probably, the slab reinforcement should be reinforcing bars rather than welded wire mesh.

If the distance between walls is not too large, use a structural slab and omit the fill under the slab. In that way, neither swelling nor settlements can affect your foundation.

If the walls are too far apart to use a structural slab, then the situation has to be properly evaluated.

Best regards,

BA

 

[asixth]

Thanks guys for all your input, it is very much appreciated.

I spoke with so senior site guys at work and they also agreed that adequate compaction is achievable except immediatley adjacent to the wall. So I thickened the slab edge thickenings and increased the top steel incase settlement does occur.

BAretired,

I am in a sub-tropical climate so frost action is not a concern.

 

[BigH]

asixth - maybe you are in area of seasonal moisture changes? If so, for some soils this can create a problem.

 

[DRC1]

Consider using crushed stone under the slab