Expert: Robert Cummings, P.E. - 2/19/2008

Question
Hi. I have a 48' x 24' detached two car garage on a slope.
It has a concrete slab and the slab is not cracked.
The location is near Charlotte, NC and the soil is sandy clay.
On the downhill slope and sides(rear and sides of garage) the retaining wall consists of 8"Dx10"Wx16"L hollow concrete block, 80" high.(10 courses)
They rest on a concrete footer that is appx 10" wide and 8" deep. It roughly outlines the poured foundation.
The retaining wall does extend around to the sides of the structure all the way to the front, where it narrows to 2 blocks tall.
On the bottom of the top block there is 1/4" separation all around the structure.
There are stair stepped cracks on the lower sides of the wall where the meet the back side.
This wall appears to have been built around 1985 and has a sill plate(2x4" i believe) on top course where it contacts the 3 walls of the garage.
The back wall bows out(downhill) at the top center of the wall appx 1 1/2". The base of this rear wall appears to be straight.
I would like to take remedial action on this rear wall, including drainage, to shore things up so it does not get worse.
One recommendation is to use 16"x16" piers(CMU), built on a poured footer(16"x16") filled with "grout" and #4 rebar at "appropriate distances across this back wall, then tie each pier into the existing poured slab using 8" bolts and epoxy. These bolt would be tied into the 16x16" pier(I don't know the details).
In addition to this, to place 1" drain holes about 1' off the ground about every 4'  and place pvc pipe with metal grid protection to filter mud/rocks. and of course seal all existing cracks and add 4" drainage around perimeter.
Would something like this method be enough?
Or should the wall be Demo'd and rebuilt on a wider and deeper footer(12"x12") with 12" blocks filled and #4 rebar every 4', with the new wall tied into the excavation using poured concrete "anchoring"? and 4" drainage around perimeter.

Answer
I suspect that this is mostly a drainage-induced bearing capacity or settlement problem but need a little more information.

I understand that the garage slab extends out from the slope -- in other words, the wall retains backfill.  Correct?

Do the garage walls rest on the top of the retaining wall or is there a terrace around the garage perimeter?  If so, how wide is the terrace?

How steep is the slope?  Does the ground slope steeply from the base of the retaining wall or is there a shelf that can catch water?

Are there any springs or seeps nearby?  How is the garage roof drainage handled?  Is there runoff from the surroundings that can enter the ground in the garage area?

Has cold weather presented the possibility of frost heave?

Do you have any idea whether the wall was provided with backwall drains or what the backfill soils were?

As for the remedial design, I'm having trouble picturing how these piers will be constructed in order to lend support to the wall.  How is the native soil beneath their footers to be compacted if it's loose or wet? Are they below or to the side of the wall?  I also am having trouble with the idea of tying external piers horizontally into a poured slab if the slab is not post-tensioned.  Or is the slab to be underpinned by the piers, and if so, what is the spatial relationship between the piers and the wall?

As for the alternative, what do you mean by poured concrete anchoring?  

If it were me, because the wall has not yet failed, I would not jump straight into the pier and underpinning idea without trying drainage first.  Without knowing the site conditions and without having the answers to the above questions, I don't know how appropriate this would be, but you should consider installing horizontal drains by drilling on 3-4 ft centers about 1 ft above the base of the wall at least 3 ft (preferably 4-5 ft) deep, and inserting slotted PVC drain pipe or, alternatively, perforated drain pipe wrapped in a woven geotextile fabric or burlap. If you can, install vertical drains in the embankment fill right behind (within 1 ft of) the back face of the wall by drilling vertical holes, inserting perforated drain pipe and filling the pipe with clean washed -1/2 inch gravel.  Drill the holes as deep as the wall foundation base and at half spacings to the horizontal drains.  Excess moisture in the backfill should migrate readily to vertical and horizontal drains.  This will reduce lateral pressures on the wall and also the pore pressures in the foundation which may be affecting bearing capacity.

When you drill, pay attention to what is coming out.  If there is no backwall drain material or free draining (sandy or rocky) soil then the drains will not be as effective, although in a wall this size they should help.

Monitor the movement.  If the wall does not support a structure it can tolerate a lot of deflection without failing.  If it continues to rotate you may have a bearing failure problem and should consider demolishing the rear wall and rebuilding it with proper backwall drains, a properly compacted foundation, and engineered fill.