Spread Footer on Fill

[src123]

I'm working with a guy who wants to put a garage on a 6-year-old push out made from blasted decomposed granite that came from his house excavation. A few years ago, an engineer who was at the site leaned on a piece of rebar, which sank, and said a garage couldn't be put there.

The fill seems well-graded and compact, and there are no signs of settlement. I plan to have compaction and basic soil testing to determine if the site is suitable. In shopping for geotech services, I talked to one geotech engineer that said a footer should never be placed on fill, and another who saw no problem with it.

Anyway, I'm interested in some other opinions... whether the garage should be put on a spread footer on fill, and additional testing recommendations.

Thanks in advance for your input.

 

[LCruiser]

It is done, but only if the fill is brought up and compacted in lifts. Bridge Abutments are sometimes built that way. It sounds like in your case (just pushed out from excavated materials) that it's a very risky situation. Just because it's not settling doesn't mean it's structurally sound. The bigger the zone of influence the deeper it will be felt, so that loose fill a few feet down would be subject to subsidence. I wouldn't do it...

 

[civilperson]

A engineered fill is great for placing a foundation, as long as the entire foundation is sitting on similar soil. Over excavate the non-fill area by 12-18" and fill back using the same material and compactive effort as the fill portion. Use two longitudinal #5 bars in the wall footing to insure capability to span any soft spots. Place the concrete floors with expansion material against the wall/foundation.

 

[LCruiser]

But you can't just overexcavate and compact 12-18" into a pushed in fill several feet deep.

 

[oldestguy]

It all depends on the owner's degree of fussyness.

As see it most garages, as a separate unit, can be built on a concrete slab, with thickened edges over almost any soil condition that has been "at rest" for several years. This applies where ever you are, if a little distortion of the garage can be accepted.

To minimize distortion, reinforce the slab, make it thicker than normal, say 6 inches instead of 4.

If you are more fussy, dig in some trenches criss crossing the site to give the slab further rigidity and, with the help of a structural engineer, consider that any part of the slab can span over a clear zone space say 8 ft. by 8 ft.

This can be further improved ahead of time by running a loaded dump truck slowly back and forth to put two rear tire tracks at every place.

However, if the owner is real fussy, spend the money on the advice shown in prior posts. For most garages, it is not worth the expense.

 

[civilperson]

LCruiser,
Are you disagreeing with my post or elaborating on a different condition? An engineered fill is placed with level lifts at close to optimumum moisture using compactive effort to achieve the specified % of the dry density of a proctor. If in the fill area under the building, 18" was removed and then the area compacted to specfied density and the fill continued up to subgrade elvation with same methodology, I would classify that fill as a "engineered fill"

 

[LCruiser]

Like oldestguy said, it depends on the owner's degree of fussiness. I would not put 18" of fill over a "pushed in fill" of varying depths and expect, for example, the garage door to open and close with an electric opener without adjustments. The deeper grid idea is an improvement and a garage is certainly not a high psf structure, but I would consider things like changing the infiltration pattern of the lower loose fill, long term behavior under the vibration of a car on the slab over and over, etc. So, yes, civilperson, I guess I am disagreeing with your statement that an 18" engineered fill over who knows what would necessarily be acceptable in the long term.

 

[msucog]

how thick is the fill that you are talking about?

if it's thin (say 5' or less), then you could always do many hand augers with dcp just to try and get a general idea of what the fill looks like. however, the owner still must accept a substantial amount of risk since hand augers can't possibly tell the big picture. also, if you've got blasted rock mixed in the fill, i hope the Hulk has gone into the geotech business since you'll not likely get far with hand augers (plus dcp will likely provide useless data). so, the owner (notice i did not say you) might consider trying test pits to try and identify potential questionable materials. if the owner wishes to do nothing and build, everyone involved should disclaim the fool out of any documentation, thoroughly explain to the owner the risks, and proceed with extreme caution after talking with your company's loss prevention guru. i think the most logical choice would be to further explore what is there so that the owner and team can actually try to understand what sort of risks you might be talking about.

i would also disagree about fixing on the upper 18" if there's several feet of not-so-good fill. the only times i might go along with something like that is if there's say 20-30' (depending on the loads that will be applied, how much settlement the structural engineer can stand, and how much risk the owner is willing to accept) of well compacted materials over a relatively thin layer of not-so-great stuff. since the more significant stresses will propogate to about 2.5 to 4 x B (footing width) for column and continuous footings, i'd prefer to have some buffer in addition to that. also, for rock or rocky fill, there's other ugly "things" that could happen.

as far as fill in general: fill, when placed in thin lifts and well compacted to at least say 95% standard Proctor, is usually fairly better than the residual soils here where i'm at. but, if i don't see it go in, then that's a problem. also based on current building codes, fill materials must be observed (i'll say "to at leat some extent") during placement.

one last point: i notice the phrase "in shopping for geotech services"...unfortunately, i see this happen to often. get a respected geotech that will provide real recommendations/opinions and go with it. just like with contractors, there's some geotechs here that will prostitute themselves just to make a few bucks. this doesn't help the owner, it doesn't help the industry as a whole, and it only opens them up to additinoal liabilities (even though they think they're "safe" since they disclaim like crazy because they're not providing "real" recommendations). so, the moral of the story is that as with anything, you will get what you pay for...except that when you hire cheap, i often see them spit out ridiculous recommendations as to how the owner should fix the problem where as the geotech that is little more expensive provides more effective recommendations with an overall lower cost to the owner. then sometimes, the cheap guy just overlooks the issues and tries to look like the hero by not doing much at all to remediate the problem. either way, it ultimately does not help the owner in the long run.

 

[VAD]

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.

 

[msucog]

here in the south where we're dealing with sandy silts, compressibility drives most recommendations here. likewise, since most of the soils are sandy silts with a upper crust of maybe a clayey silt, we don't get too wound up about what goes behind a cast-in-place wall as long as it's well compacted and there's a drainage system in place against/at the bottom of the wall. 99% of the problems i've seen with cast in place walls is that either the thing was put on top of absolutely horrid soils (in a creek floodplain or something like that) or the backfill materials were backdumped and push up to grade with little/no compaction where no drainage is provided. since the backfill is so loose, it becomes saturated (whereas a well compacted fill soils here are typically not permeable...this is one of the reasons why mse walls here have so much problem...the designer "designs" for a sand but the onsite materials typically found here are sandy silts with 40-70% passing the #200 sieve--but there's no adjustment to the design...they just disclaim the liability on to the owner).

i can't imagine not putting any reinforcement in the foundation..that just seems weird. like i said, fill soils here are quite often better than the natural residual soils in many aspects.

 

[fattdad]

re: Original Post.

I'd do a proofroll of the fill pad and see if there are any obvious week zones - that'd be good for the garage slab.

I'd overexcavate the foundation trenches about 3 or 4 ft and replace the bearing soils with compacted structural fill (i.e., 95 percent Standard Proctor) - either that or use an open graded aggregate (i.e., AASHTO 57).

I'd then build the garage. It's not like you're talking about a 5 story office building or anything. . .

(Sorry if this was covered in the other posts, but I didn't have the time to read everything that was mentioned already.)

f-d

¡papá gordo ain’t no madre flaca!

 

[Mickney]

I agree with fatdad. It is a garage, very lightly loaded structure. Just be careful that you do not undermine any existing foundation in this process.

Question: What is blasted "decomposed" granite?

 

[msucog]

here's some rambling that i tend to do from time to time (in the event i'm able to help someone/anyone out there with my "limited" knowledge and experiences):

i've seen a rather massive "garage" in my short career on more than one occassion...they happened to have 2-3 stories above it, retaining walls on one or more sides of it, and/or was part of a multi-million dollar home. i've also seen your average garage supporting nothing but a bass boat crack and settle all to heck (and the owner-who happened to be my rough and tough old superintendent-was not a happy man).

as far as residential versus commercial type work, the residential home owner is much more likely to see any minor problem and ask for money back because they don't understand the risks (which is not necessarily the engineer's fault...but it is still a "real" issue you might have to deal with) and they generally want to keep their home in as good or better shape than they bought it and are likely still paying for. plus, cracks tend to sometimes cause the overall selling price to be a little lower which hits them where they feel it most--their pocket book.

i'm cautious of any old crappy fill...especially if it's thick with something sitting on top.

 

[fattdad]

the residential home owner is much more likely to see any minor problem and ask for money back because they don't understand the risks

Point well taken. These are the problems with residential work (why I don't do it) and the problems with cyberland - we don't know the loads or the true nature of the proposed construction. That said, if it is non-compressible fill (i.e., from broken rock), been in place for awhile and passes proofroll, it's likely o.k. for slab-on-grade support. We've used engineered subbase on old fill (of similar nature) successfully on lightly-loaded stem wells (i.e., strip footings) when bearing on old fill. This is not just a local (i.e., Central Virginia) case, but true in other cities with reclaimed land (e.g., Seattle).

In light of all these unknowns, I'd consider digging a test pit away from the footprint of the garage and looking at the condition of the fill. There's no substitute for a direct visual evaluation (and a trained eye).

f-d

¡papá gordo ain’t no madre flaca!

 

[Mickney]

I guess I should have provided more insight to my previous post. All recommendations and the potential "risks" associated with them should be discussed with the owner and project team. This should be done in a written report and in-person. In this case and provided the garage is lightly loaded, the risk for settlement can be reduced, but not entirely eliminated, by a conscientious field observation program including proofrolling, shallow test pits, etc... If the owner is not willing to accept some risk, then the fill should be removed and replaced in a controlled and documented manner.

 

[msucog]

i knew what you guys were referring to since i've seen both around enough to see that you take rational approaches to real world "stuff". and yes, in cyberland, it's easy for me (or anyone else) to play the "what if" game from behind the computer screen. i've picked up a lot from these boards so i was hoping to help someone else do the same (or at least pick an idea in the back of their mind so that it kicks in one day when they see it in the real world).