src: Good question. I note that a similar question is posed by allisch (reinforcement in foundations) in this forum. Unfortunately, the game is controlled by the Building Codes of many jurisdictions in North America, which in my opinion need to be seriously reviewed, in relation to residential foundations.
I will stray a bit from the question to provide some reasoning why there is so much confusion and debate on the question asked by src and by allish. It is lengthy and apologize. It will be concerned more about construction on engineered fills. The discussio/comments are pertinent to well behaved namaterials in non seismic areas.
Many of the recommendations regarding building on fills, requirement for reinforcement for foundations and walls for residential houses are followed verbatim. Unfortunately, many of these homes suffer tremendous problems which are costly to remedy.
Of course, such problems have resulted in other construction techniques such as underpining,grouting of wall cracks, installation of additional drainage measures against basement walls and so on which seem to go hand in hand with new construction these days. These have become very lucrative to the other contractors. Who eventually pays - the homeowner. Just like purchasing a new car, problems tend to occur after the major warrnty is over.
In Ontario, Canada, the Provincial Building Code(OBC) does not specifically stipulate reinforcement for footings for residential houses of up to three storey in height, nor reinforcement for concrete basement walls up to about 9 ft high.( Section 9, OBC) However, depending on how one reads and interprets the Code, not untypical of many other Codes, for foundations on engineered fill,the design should be done in accordance with Section 4 which pertains to structural design. This is does not appear to be practised by many.
Consulting engineering companies engaged in home inspection have generally adopted the approach that if footings are to be built on native ground no reinforcement is required. If however, the footings are to bear on fill which exceeds about 1 m above native ground then the 2 N0 4 or 2 N0 5 bars are required. The concept used is similar to that stated by Lcruiser i.e to span soft spots or to cater for non uniformity in compaction levels along the footings. Very often inspectors have asked for the rationale for this application. Since structural Engineers do not as a norm get involved, the geotechnical report often contains this type of recommendation. When asked about the rational, the geotechnical engineer often cannot provide a sound answer.
It is typical for may companies to send an inspector out with a 4 -5 ft T-bar about 0.5 or less in diameter to probe the ground and based on how it feels he/she makes the decision on reinforcement in native ground etc. There is no gauge on this T-bar and the decision is based on how far it is pushed and the opinions of the inspector. In many instances the Builder goes along with the recommendations of the inspector.
Since the one to three storey building loads are considered light, one can undertake a beam analysis (footing spanning an imaginary or concieved void and come up with two or three No 5 bars). Similarly, a nominal reinforcement of one or two horizontal rebars about 12 inches below the top of the wall and no vertical reinforcement is often specified. some have argued that the reinforcement should be at the base of the wall above the footing and that no rebar is required in the footing. The concept of a deep beam has been postulated. No one seems to have a proper answer mainly because we have treated the housing construction of this type not to require serious engineering except for expansive clays and in semic areas. This neglect is in my opinion a mistake.
These nominal reinforcement are often thrown in without any transeveres ties and/or spacer blocks etc. Contractors engaged in residential house buildings do not carry steel benders or rebar installers as the Codes do not generally specify rebar for thse situations.
However, I have noted that building regulations of many cities, and counties in the USA have rebar specified for foundations and walls irrespective of whether on fill or not. It would seem that for fill situations, the concept is based on good practice or some imagination that onec the rebars are in all will go well. Sometimes it does and at other times it does not. When it goes well, we still do not know why it worked out that way.
I would like to suggest that regardless of whether we build on native ground or on engineered fill, there are several aspects of the basement-footing construction that allows problems to surface which reinforcement as specified cannot in my opinion prevent.
We need to examine the drainage situation at the weeping tile level vis a vis the nature of fill material and even native ground especially if we are in glacial tills. In the case of the engineered fills, existing materials from site grading is utilized. Compaction is often diligently checked without a recognition of soil moisture content but only percent compaction. How often do we examine the saturation curve wrt to the compaction curve and the shape of the compaction curve in deciding how to treat the fieldf situation. Not many look at these important characteristics these days. Today's approach is unfortunately making money- just send someone with a nuke get some numbers, doctor them if too much time is spent so that the comapsction can be passed. there is a lot of adjustments made to proctors, perhaps justifiable but more than often not. Homeowners do not know of such.
When water seepas down to the foundation footing level, we expect that all will be taken care of by the drainage pipes. We however forget that water seeps downwards in areas where gradients are very small. With compacted fill that is perhaps dry of optimum but achieves the percent compaction desirable, the fill settles especially if it is silty in nature and of Cl, or SM, SC etc classification which it often is in some areas.
This aspect of drainage is not recognized by the building codes or by many geotechnical engineers.
What we need in those situations is to create a system that does to allow water to percolate downwards. this can be provided by a geotextile impermeable liner sandwich placed below the foorting and extending to the exterior of the basement wall by 2 to 3 ft.
The concept of eliminating granular material against basement walls and replacing with synthetic drains against the wall and backfilling same with engineered fill material that was excavated to construct footings and walls in engineered fill has brought in my opinion a culprit of immense proportions.
The above is often considered to be drainable system and as such the design of the basement walls up to aroud 9 -10 ft may not require reinforcement since the lateral pressures are only due to earth, water being zero. Unfortunately, no one bothers with the characteristic of the engineered fill in relation to the type of drainage board/waffle board/synthetic drain etc that is being used.
The practice is literally to throw in the type of material. Does any one check this. Perhaps, but more than often not. Cracks in the walls if they occur has some safeguard that they may not allow water to infiltrate since there is an impermeable back against wall. However if this ruptures then we are back with the problem we know so well about. We can maodify this backfill design but it woud be a bit more expensive and of course builders have been accoustomed to the cheap approach.
The answer to the questions posed in this thread and the one by allisch lies in the examination of the above comments.
There is an urgent need in my opinion for both structural and geotechnical engineers to be involved with the wall footing system of residential homes not only in areas with problematic soils but in totality since billions are being spent in this essential industry and year after year problems surface that repetitive. It is time we realize that there is a need to engineer these taken for granted structures as it affects all of us whether we are engineers or not.
