foundation wall

[fargofarmer]

Hi,

Can Somebody give me an idea to fix an exisitng 6-foot tall foundation wall? The foundation walls were constructed with 4 inch brick and 4 inch CMU block bonded with ties. The walls are neither reinforced nor grouted. The air gap between two courses is approximately 1-inch. Both brick and CMU block are supporting the superstructre load (roof and load bearing walls. In order to bring the pour garage slab at the planned elevation, the ingterior of the garage was filled with 5 feet of unontrolled fill. The maximum foundation wall height in this area is approximately 7 feet. Because of the imbalanced condition, the contractor placed 3-foot tall soil berm againt the exterior of the garage walls. The problem is the right and rear walls of the garage are in the crawl space. The placed berm obstructing the vents and crawl space door. It also created an unbalanced condition. I am planning to recommned the client to construct a retaining wall approximately 5 feet fromt foundation walls.
As an alternative, I am planning to recommend to attach 6-inch thick solid grouted CMU block to the existing wall on the exterior of the foundation walls. If they attach the new block with Portland cement paste, the final wall product will act as a compiste wall. Per building code an unreinforced and un grouted wall can support 5 feet of backfill. I don not know if my recommendation is technically correct or not? Do you think that there will be any cons with this solution? Can somebody suggest me any other solutions. The construction of the house is in the middle.

 

[BridgeSmith]

The solid grouted CMU solution would seem simpler than a separate retaining wall. I don't know that I would count on the composite action with the existing wall, although it would be best to grout between them to rigidly support the existing wall. A partially reinforced CMU wall (w/ reinforced "columns" at intervals) or steel stiffening ribs would also be worth considering.

 

[fargofarmer]

Thank you for your feed back and suggestion. Actually, I was planning recommending rebar in the solid grouted CMU. Do you mean the partial reinforced CMU wall/steel stiffening ribs should be attached to the existing wall? If we use steel ribs, don't we need use tie-backs?

Thanks
Kumar

 

[cetiger]

Similar to HotRod10, I would think a 6", definitely 8" CMU wall would work. You might consider a cast in place concrete retainign wall poured against the exterior foundation walls, as you'll probably get a thinner wall compared to CMU, if that's an issue. Design the wall to retain the soil and any superimposed loads above. Concrete would save labor cost and time, but there's always a cost for formwork. If you do go CMU, I would recommend providing vertical rebar spaced as req'd per your design, provide an 8" deep bond beam at the top and grout all cells solid, as a minimum.

 

[brickmason]

a 4 inch block is never to be used as a structural unit, the cells are too small to hold rebar and concret. a 6 inch block is still a bit small to hold back any backfill. i strongly suggest an 8 inch block wall as a minimum for retaining walls. i also reccomend grouting the vertical cells every 24 inches on center and a bond beam on the top course. reincorcment wire placed in every course is commonly used in retaining walls

 

[msquared48]

I STRONGLY agree with brickmason here, plus I would add that per code, any CMU foundations that are below grade need to be FULLY grouted below the highest level of soil.

I would also be concerned with water penetration and freeze/thaw action on the CMU isf left ungrouted and unsealed.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[fargofarmer]

Friends,

I really appreciate your suggestions. I attached an hand sketch here for clarity. Finished product may look ugly. Comments please.

 

[fargofarmer]

I am sorry here is file link

 

[concretemasonry]

fargo -

As it I now see (with a sketch) and understand that the slab is a floating slab inside the brick and the brick is not on the exterior as I had earlier supposed.

The first thing to do is determine the loads on the load bearing portion of the wall and recognize the dead loads in an analysis of the wall with an unbalanced soil load. You may find out that the wall carrying the dead loads may be enough for the low effective distance used for the unbalanced lateral soil load.

Is there any type of common tie, such as a solid CMU or bond beam at the top of the masonry wall to make the brick and block to carry the load jointly. If not the wythes will carry the load separately (uncoupled)and be weaker than a coupled or bonded masonry wall. Filling the 1" gout space is not really an option since there is no way to know if the 1" void is really clear or the mortar excess can cut off grout spacing. Placing grout in such a narrow space (not meeting any code requirements for minimum grout dimensions) is very questionable. Even the lowly ACI 530 specifications for "Empirical Design" specify a minimum wall thickness, but other ACI 530 methods allow 6" walls that are very common around the world.

Unfortunately, the option of adding a 6" partially grouted wall (which can be engineered easily), but the wall would end up being on the wrong side (compression) of the surcharge, so the effects of the steel are not very beneficial. Also tying the footings adequately to involve the rest of the structure would be problematic.

It is not necessary to solidly grout all of the cores of CMUs below grade according to any major code I am aware of (there may be local codes and traditions)and never came up when the ACI 530 was written over a 2 year period. Grouting is only necessary in the cores or cells where there is reinforcement, although in the older shapes of block, slop and grout migration into other cells was common, causing problems with other uses later. All too often, full grouting makes people feel good because it seems "stronger".

Since the unbalanced load due to the elevation difference even with slopes and possible slab surcharges, the wall may still work. If the dead load is sufficient to reduce the tension in the wall.

I suspect this is an addition on a previously designed residential structure that was built with a non-typical detail to avoid a brick ledge for the veneer.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[BAretired]

I have some concerns about the detail shown. The dowels between the new footing and old footing may not be adequate to make them act as a unit.

A cantilever retaining wall of that height needs a footing approximately three feet wide.

Another solution, if the slab has not been poured, is to tie the top of the new wall to the slab by means of dowels drilled through the existing wall and spaced at, say four foot centers.

BA

 

[fargofarmer]

Dick and BA,

I don't think there is a bond beam at the top. This is a new construction and half of the work was completed by the time we got the work. It looks like the load is on both existing brick and CMU block. As you said, filling 1 inch void with grout is not possible now.

BA,

Your suggestion is also good. I was thinking about connecting both old and new walls with dowels. I did not think about connecting the new wall with slab. Anyhow, the slab was not poured yet.

thanks
Fargo Farmer

 

[BAretired]

fargo,

concretemasonry says:

As it I now see (with a sketch) and understand that the slab is a floating slab inside the brick and the brick is not on the exterior as I had earlier supposed.

I seem to be interpreting your sketch differently. I believe the left side of your sketch is the exterior. Top of slab adjacent to the 4" CMU is 5' above top of footing. Exterior grade is not shown.

Please clarify.

BA

 

[concretemasonry]

BA -

I agree in general. I took the left side as the exterior since I noted the right side was the "dirt" placed to support the floating slab, which is not unusual in some residential work for a garage slab.

The additional "fix" of adding additional fill on the external side was probably not noted or included on the drawings, but just seemed a logical "fix" to me in an attempt to reduce the effects and unbalanced load on the minimal wall. I could be in error. Unfortunately, an uncontrolled fill sloping away does not give much relief.

The information provided is really not very complete for a definite suggestion to "sign off" on.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[BAretired]

Five feet of uncontrolled fill under the slab does not exactly inspire me with confidence in the outcome.

BA

 

[fargofarmer]

Dick and BA,

Sorry for the confusion. But you are right, the left side of the sketch is the exterior. Approximately 1 foot of fill will be placed on the exterior side. Do you guys think it will not work. unfortunately two more houses were built in similar fashion.

Thanks

 

[BAretired]

I would not be comfortable with the solution shown on your sketch.

Whether or not it will produce satisfactory results in the long term depends on several variables which we cannot assess from afar, namely:
[ul]
[li]Location of project...freeze/thaw conditions[/li]
[li]Quality of uncontrolled fill under slab[/li]
[li]Compaction of fill under slab[/li]
[li]Adequacy of attachment of new footing to existing[/li]
[li]Adequacy of ties between existing brick and block[/li]
[/ul]
A local engineer would be better able to review the work and provide an opinion.

BA