Sloped Footings?

"I'm pretty sure the engineer is going to say it's ok, but at the same time he accepts no responsibility if I have problems in the future."

The individual or firm needs to have indemnity insurance cover so that liabilities that might ensue are covered. If there is none, then change them for someone who is more responsible, otherwise there is no assurance for the next occupier of the building, which may affect its value.

Local building inspection authorities will need to see this loop closed, as they will check (and thereby assure) the calculations.

The foundation contractor is looking to complete his work in the shortest time so as to move onto other things. Get the engineering sorted before work starts.

See Petrobras 36 for an example of doing things contrary to established practice.

The house won't tend to slide down sloped footings unless it drops 2 feet from north to south over a very short distance, and even then it probably wouldn't move without the aid of an earthquake, but you failed to let us know what that distance is.

Footing contractors will ordinarily step the footings if the slope is too great anyway. Block masons won't lay block on a sloped footing that has too much slope, cause it requires too much hogging with brick. If I was doing it, I'd put in stepped footings, and, of course, I'd make sure the steps equal 1 course of block (usually 8").

The distance is 70 feet from end to end. The 2 feet is gradual across this entire span. There are no building inspectors involved, as it's being built in a rural area where there is no building code. That being said, I want the house to be built to code as do not want to have any problems with it in the future. There have been numerous other buildings in the area that have had problems, slabs cracking with one end of the house dropping a few inches, brick walls collapsing and etc. That's why I'm being as cautious as I am. Generally, builders in the area do not even do soil tests, let alone consult with structural engineers on the design, except for in the metro areas where it's required by city codes.

I'm sure there are engineers in the area that would put their seal of approval on it and stand behind it, but this particular one is more less just functioning as a consultant unofficially and offering advice based on his experience with the soil in the area. Prehaps I should just bite the bullet and hire a structural engineer to design the foundation.

Back to the issue at hand, if I step the footings would the house not then tend to want to split/settle where the footings are stepped? I guess what I'm wanting to know is what is the best way to deal with the grade so that I don't have to worry about it ever giving way and cracking the brick or interior drywall? I realize the house is going to settle some and that concrete is going to crack, and that's fine. I just don't want any structural problems with it. If there is not another thing about the house that's right, I want the foundation done right. Because if it's not then no matter what else you do you end up with a 6 figure pile of junk.

Well, in 70 feet, the span of each block (16") rises ~1/2" from one end to the other, so putting a thick layer of mortar under each block is not the most desirable situation for the bottom course. Additionally, it's still a pain to step the blockwork 2 feet in 70.

Slabs cracking are not uncommon on the best dirt. Concrete shrinks, and if the slab is large enough, there's not much you can do to stop it. Concrete has excellent strength in compression, but nearly none in tension, comparatively speaking. Long concrete footings will crack even with steel reinforcing bars.

Foundation footings, on the other hand, may be independent of each other but should be placed where load bearing points are and designed accordingly, such as in a steel building with load-bearing columns. That being said, a continuous, non-stepped, footing around your building will not have much strength without reinforcing bars, and the difference between it and a stepped footing's strength can be measured in the 0.01 percent range, so that's not an issue. What is important is that you dig down to a reasonable depth and put in a wide (not long) enough footing to bear the more or less uniform weight it will carry. In other words, each concrete block will bear more or less 1/50 of the load in a 70 foot long wall. If the portion of footing each block is sitting on will bear that load, then a steel reinforced footing, as a whole, will have more load bearing capacity, regardless of stepping.

I suspect the structural engineer consulting for you wanted you to get below silt and/or top soil on the surface. Once you have reached dirt which has a decent load bearing capacity, and you dig a trench wide enough and deep enough to put in a footing for the load, you shouldn't have any problems. I'd follow his suggestions.

As far as other buildings in the area are concerned, I'd bet that they weren't built according to the instructions that you have been given.

Thanks, Bill, for the discussion. For me, I need to understand the how's and why's of doing something a certain way versus the other, instead of simply relying on others to make those decisions for me, so this helps me tremendously.

As to the reasoning behind going 4 feet deep with the footings. In my talks with the engineer, the intent on doing this is to get down to depth where soil's moisture levels are generally stable more than anything else. With the clay soil I'm dealing with it is variations in the moisture content that destroys foundations, hence it expands and contracts depending on the moisture level. If I had the money, I'd excavate the entire building pad to the depth of good solid soil and backfill it with stable compacted fill. But in this case, we're talking approximately 12 feet to what's referred to as chalk with it being expansive clay all the way to this depth. So that would cost a small fortune to do, and the engineer ensures me that I'd be seriously overreacting since the soil's moisture content is considered stable at 4 feet. He also stated that backfilling the trenches with 2 feet of large crushed stone would be ideal, since then the stone would carry the brunt of the weight exerted by the footing, further reducing the load on the bad soil.

So if I understand correctly, you'd have no reservations at all about using a stepped footing as opposed to leveling the site and pouring a continous level footing.

And I'm sure you're right, about other buildings not consulting an engineer and having problems, because that is in fact what most area builders do. They just use their judgement to build the foundations, which most of the time their judgements are probably driven by their bottom line. :)

Another question, in drawings and diagrams I've been able to locate online, it appears that the pressure on the soil is exerted out triangularly from the bottom of the footings. Meaning that soil beneath and immediately to the side footings also sees pressure from the footings. Albeit, I'm sure that the most pressure is directly under them. What I'm getting at is there justification for enlarging the width of the trench making it wider than the width of the final footing in order to get compacted stone at the footings' sides? Doing so would mean needing to then form the footings with lumber before pouring, when the general way of doing it here is to just let the sides of the trenches be the forms. In other words, is it worth the extra expense involved to get good compacted stone to sides of the footing bottom?

You wrote: " . . . it appears that the pressure on the soil is exerted out triangularly from the bottom of the footings."

That's correct, sorta. The pressures vary with soil types, but they are generally distributed in a truncated triangle, the soil type dictating how much the sides of the triangel slope.

The gravel is as much for economy of not using all concrete as it is for drainage (has greater surface for allowing water to seep into surrounding soil) and providing a good soil below footings. If a 12' excavation depth is required to get to good load-bearing soil, then what you will essentially be doing is floating the footings on the soil you have. In that case, a wider footing (which you haven't told us how wide your consultant hs recommended) is better.

The USGS probably has recommendations for your particular area, as well as other government agencies. Your county extension agent may even tell you sources of information that your consultant may not know about.

In the big scheme of things, a solid foundation will prevent disasters. It's better to spend a little more there now than later.

Good luck.

Bill

Great thanks for the info on other sources of info, I'll check those out. I dropped the blueprints off at the engineers office this afternoon and he'd said he should have something for me next week, so I have to wait and see about the footing sizes. I'm being told by some of the contractors I've talked to that I'm looking at something between a 24" to 36" footing width. We'll see what he says.

We recently installed a large chiller, and the engineer who designed the foundation called for 8 _ 3 foot-diameter, drilled piers to 18 foot depth (or until good soil was encountered) based on test borings. These holes were filled with concrete with lots of rebar. Oddly, the cost was not all that high. The drilling company charged about $190 per foot, the concrete and rebar was another $800 per hole, so in the big scheme of things, the foundation was only about 4% of the total job cost.

Your structure wouldn't require such large piers, so the cost would be much less.

Driving pilings to resistance in poor soil is also an option. Pouring reinforced concrete on top of pilings spaced out around the periphery is a common method of construction.

Good luck.

Expansive soils are difficult to work with. As the chalk layer is 12 feet down, and assuming that it is stable; it would be best to build your structure on cast-in place piers. Depending upon how expansive your soil is, the rather large footprint of your structure may create differential lateral and horizontal pressures on an on-grade slab with footings resulting in cracking.

Consult with the geotech engr and a structural engr. to preclude any problems.

Are there really no building codes in rural areas?

Is this why there are so many damaged houses in tornado alley?

In Darwin, Australia, there are building codes for nearly everything you build on your property. For example a 3m by 3m garden shed needed a council inspector to witness the footings before the cement pour. 10mm thick floor with a 20 cm deep rat wall around the entire outer edge. And embedded Rebar mesh. Once shed was build it needed 34 -10mm* 100mm Chemical Set Anchors around the bottom to hold the shed onto the slab. Then inspected again. Even aluminimum window shades are rated to 340km winds. Get second or third opinions. I know sometimes its cheaper to do the easier way but in the long run especially if your spending a couple of hundred thousand dollars do it the right way so when you sell it in a couple of years a selling point would be your dilligence in doing the correct enginerring. just because other people have done it one way doesn't mean you have to do it.

Good Luck.

Are there really no building codes in rural areas?

The building code still applies. It falls to the county to review the plan rather than the city itself. State building codes will always apply.

Even if they didn't, it would be smart to follow them anyway as they offer a standard set of guidelines for construction. Without them the general contractor can do as he wants and the results may not be favorable.

<Back to the issue at hand, if I step the footings would the house not then tend to want to split/settle where the footings are stepped? I guess what I'm wanting to know is what is the best way to deal with the grade so that I don't have to worry about it ever giving way and cracking the brick or interior drywall? I realize the house is going to settle some and that concrete is going to crack, and that's fine. >

<if I step the footings would the house not then tend to want to split/settle where the footings are stepped?>

Answer:If each footing is non-sinking/moving--the house tied to these columns is standing too.Walls can follow the gentle contour directly on Clay. Wall loads are only downward. Clay is good,pristine,in situ.Clay has much more bearing strength than loams.

<best way to deal with the grade so that I don't have to worry about it ever giving way and >--Please clear topsoil down to 1' and build wall direct on pristine compact clay. 2' in 70' is not "Sloping"!

If you want a very seasoned friendly expert Architect of 38 years experience to look your site/house plans up-- you can visit my brother at St. Louis MO after 25th April07. He is cmahanta@charter.net. His own house( which he designed and built) slopes down steeply to the Missouri river.

If you used a poured wall, you could use the xloped (gentky) footing. TYhe modular block makes stepping in 8: increments cheaper.

RichH

Has your geotech given you the soil boring results? Usually in expansive clays you need to be concerned about lifting and settling as rain and dry spells occur. I am surprised he did not recommend a system such as helical piers to transfer the load past the questionable soil. You can use these same piers as anchors to tie the foundation into the slope. Often this application is done after the house is built and settlement and shifting occur. This system is not inexpensive but could save you many thousands in the future. Where in the world is this structure located?

Under no circumstances should you install footings on a slope. Your concern about spending an additional $3K will diminish to a dot when you are faced with substantially more to repair the damage that will likely result from your proposed contractor's ill-conceived plan. Stepped footings are still continuous as the reinforcing steel must step with them and there is an overlap in the concrete at the step. Stepping concrete footings is a common practice and recognized as appropriate by virtually all building inspection entities I have ever encountered.

Sounds to me like you should find a contractor who actually knows what he is doing. Or you could simply stop someone driving down your street as you may have a better chance of getting someone with sense enough to do what he is instructed.

In regards to "compacted gravel" or crushed stones, I had an engineer insist on us compacting the rock prior to pouring a slab. he actually wanted us to get a portable compactor and "pound" the rock together! YOU CAN"T. When your contractor installs the crushed stone or other medium, make sure it is compacted, but it needs to be done correctly. I can't tell you exactly how to compact larger rock but we used about 1" gravel and used a vibrator to "compact" it. The engineer had absolutely no idea what he was talking about. also, a wise old concrete contractor once told me this. "it's not whether or not concrete is going to crack, it's only a matter of where". As I understand it, lots of steel, a stepped foundation and make it wide to distribute the load. I have just one question. Why the block? why not just pour the foundation all the way up?

You may not be able to "compact" open stone (even pea gravel) to a specific density, but you CAN and BETTER consolidate it to a stable condition. With 24 inches of gravel, a 1% consolidation under load is a 1/4 inch settlement available. You more likrly have more like 3 to 5 % available consolidation. Also, as expansive clays swelll and shrink, they "move" up into the open gravel, resulting in a lowering of the bottom of the stone. /the clay fills the voids. I would recommend either a bottom layer of compacted graded stone under the open stone, or compacted "engineered" fill in 8" lifts. Open stone will also allow water to accumulate under the footing, causing further swelling and later shrinkage than graded stone.

Non-compactable does not mean non-consolidatable, but excavators forget that part. If the stone moves underfoot when you walk on it, it is not consolidated. Look at a haul road with open gravel. After several wet periods of use, the clay from beneath has worked it's way to the top. The rock does distribute some of the load, but each individual rock point has little distribution amd pushes it's way down. During dry seasons, the "dust" works it's way up.

Maybe the old treated timber piles with a grade beam could be the way to go, for durability. 12 ft below grade with 4 ft of excavation means that a standard 8 1/2 rairoad tie would be a suitable pile. If there is a fair size town around, or a large national chain store building around, go see how they handle it. Also, check the International Building Code (the IBC). That is your best friend.

If you are far enough south not to worry about frost, you may have to worry about termites. Open rock leaves them wonderfully easy paths.

Hope this helps.

RichH

The single critical issue on your site is the expansive soil (which explains the cracking and settling foundations in the region). The geotechnical consultant (who is also an engineer) is giving you the best solution to that issue. Don't second guess him. Ignore nonprofessionals who say things like "the engineer doesn't know what he's doing." As one person correctly commented, a drilled pier and grade beam foundation is an option in expansive soils. They are often equivalent in cost to a complex standard foundation. Both systems will be fine so long as they are correctly designed and constructed.

-- CA architect

"don't second guess him" is the worst advice I have ever heard anyone give. PERIOD!

I no longer work in the field. I merely inspect. I recently had a contractor installing footings and walls for a 5 6 story masonry building. One short piece of wall required 2 ft overdig, and the soils eng was happy to pour on freshly dumped 1 1/2' limestone. I wouldn't let them pour without consolidation. About 2 " worth.

The lesson should be that the fill is not compactable to a "measurable standard". It CAN be consolidated to about 90% of its loose as-placed VOLUME.

Pea gravel should require a license to purchase.

RichH

Have LOTS of experience in this type of soil--here in Bellaire Tx, we have 'Beaumont clay' a highly expansive soil with 40% shrink/swell. Two possible foundation types work-

1) is a 'post tensioned' floating slab poured on top of 18" select fill compacted to 90%. Have to remove the top XX" of original soil to get slab elevation to proper height. This is a precisely engineered, installed, and tensioned reinforcing system. Not for unskilled or unfamiliar folks.

2) Design a 'pier and beam' foundation, done by a competent engineer with certification of 'foundation design in expansive soils'. Typically here--10" dia bores down to 'hard yellow clay' which is equivalent to your 'chalk' layer. Then belled out on 30 deg angle to 3' diam. Insert reinforcing cage of Min 3 -5/8" rebar continuous top to bottom, bottom flared out to 2' dia, held 6" off bottom of shaft. These rebars must extend min of 12" above top of pier to tie-in to cast in place concrete beams. Poured, reinforced concrete beams on 10' centers, with a pier at each intersection. Larger piers under fireplace or other heavy loads as engineered. Typical beam dimensions here for 2 story brick wall house is 10"wide x18"deep, 1/2" rebar reinforcing cage, min of 8 rebar/beam , with 4" 'void boxes' under bottom of all beams to prevent 'heaving' from soil expansion. Slab poured integral with 'beams', min 6" thick, steel reinforced with either rebar per engineer (1/2" on 12" centers typical) or #10 welded wire mesh, 6" grid, both types supported by plastic 'tees' 2" off bottom of slab, continuous over top of beams.

Important to 'vibrate' the concrete in beams and all areas to ensure good aggregate coverage and no voids. Seal top of slab with spray sealant or cover with vapor barrier to ensure damp surface for 7 day min for proper cure.Suggest '4000# -30day' (4-sack mix) strength concrete minimum.

Top 18" of in-place soil removed and replaced with 'select fill(non-expansive sand/silt/clay, compacted to at least 90%), covered with 20 mil black polyethylene , including sides and bottom of trenches for 'beams'. Termite treating of soil under the poly sheeting. This is not a cheap foundation, but properly installed, results in a true firm, rigid foundation, with NO slab cracking of any kind. Additives in the concrete will prevent shrink cracking. Critical area is proper design (not difficult) and ensuring the drilled holes do not have loose 'muck' on the bottom immediately before filling (from bottom up with a hose,not dumped from the top) --which will defeat the load bearing design.

Since there are known foundation failures in your immediate area, I STRONGLY suggest you do it right. The extra $5-10K for a REAL good foundation will be repaid many times during the life of the structure and handsomely when it comes time to sell. You have no idea just how costly it is to maintain and repair a brick/wall board -sheetrock structure that moves with the seasons, droughts, wet spells.

Another thought to the problem of laying block on your footings. Why not have the footing trench dug so the bottom is fairly level then use a styrofoam cement form on top of the footing hight/thickness, and bring the foam forms up to the top of your foundation wall. The foam form does not get stripped off after the cement is cured. Level the floor inside as desired or not at all, start the floor of the house on top of the poured foundation walls. Dont forget to bring in your utilities before you pour the foundation walls, or you will have to dig under the footings or punch a hole in the new wall.

Re: Keith E Bowers, PMP

Excellent, Keith. I would just emphasize the importance of the 'void boxes' under the grade beams. The void allows any clay soil to expand without putting upward forces on the concrete beams. This is critical since the concrete grade beams are designed to resist downward loading, not upward loading.

Thanks guys for the continuing discussion, it's very helpful.

As to the question about building codes. As I stated previously, there are absolutely no building codes (state, county, city, or otherwise) in force in the area I'm building. In fact with the exception of a few of the more heavily populated counties and the costal counties (hurricane codes) the entire state is building code free. There are no plan reviews. The plans do NOT have to stamped by an engineer or architect. In fact you don't even technically need any plans at all. I've had several builders tell me that on most of the houses they build all they have to go by is what the owner has either sketched out on a piece of paper or printed/copyed from a book/website on a standard 8.5 x 11 sheet of paper, and it just works out every how it works out. Now you can surely imagine why there are so many foundation problems in the area. That said, almost all the cities within the state have codes that are enforced within their city limits. The state legislature has taken the issue up a time or 3 in the past few years, but have so far decided not to put codes in place. The local county board of supervisors also took the issue up last fall, but have tabled it for now. It will eventually work it's way through though, it's just a matter of time. That's one of the reasons I'm spending the amount of time that I am researching areas of construction that I'm personally not familiar with, like foundations on bad soil. Here it's not necessarily that the contractors are knowingly bad, it's just that they've always been able to do pretty much whatever they wanted without any inspectors looking over their shoulders, and they may not know how to do it correctly.

Anyway, back to the issue at hand. Here's what the soil test report said. They did 2 borings to a depth of 5 feet. Boring 1 sample:
N Value = 21
% Field Moisture = 23
Liquid Limit = 60
Plasticity Index = 29
Ground Water: None Found
Description of Material:
1 foot: Tan Silt
2 foot: Tan Clay
3 foot: Tan Clay
4 foot: Tan & Grey Clay
5 foot: Tan & Grey Clay

Boring sample 2:
N value = 17
% Field Moisture = 29
Liquid Limit = 58
Plasticity Index = 27
Ground Water: None Found
Description of Material:
1 foot: Tan Silt
2 foot: Tan Clay
3 foot: Tan Clay
4 foot: Tan & Grey Clay
5 foot: Tan & Grey Clay

I know the clay stops and chalk begins at about 12 feet because I had a pond dug on the property last summer and had the guy dig down to the chalk with his excavator just behind where the house will be built. Took him about 30 seconds, btw, amazing at the amount of dirt those things can move in a hurry. The soil pretty much cam out of the bucket in large clumps all the way, so I know it's bad even without the engineer's report.

As to his recommended method of foundation construction. Here's exactly what the report says:

It is our opinion that the clay soils found near the surface are not suitable for use as a subgrade material for shallow foundations. Should the house be planned to be founded on a typical slab-on-grade foundation system, the entire house site should be undercut and the expansive clays removed to a minimum depth of 4 feet beneath the surface. The undercut area should extend a minimum of 5 feet outside the building perimeter and be backfilled with a well compacted granular fill material compacted to a minimum of 95% Standard Proctor Density. The fill used should be a sand-clay material with a minimum liquid limit of 35 and a maximum plasticity index of 15.

If a rasied slab foundation is desired, which minimizes the amount of cut and fill over the typical slab-on-grade and allows for an elevated floor level relative to the outside grades, it can be utilized here with good success. The perimeter footing wall should be located a minimum of four feet beneath the surface. Concrete blocks can then be used to bring the structure up to desired floor levels. To minimize block work, the perimeter footings can be dug a minimum of four feet in depth and backfilled with approximately two feet of crushed stone and a footing then constructed on this compacted stone course. Concrete blocks can then be used to raise the floor level to desired grades. The interior void can then be filled with compacted fill and slab placed on this structure. The fill should be a minimum of 2 feet in thickness and should be placed in successive stages of no more than 6 inches at a time so that maximum compaction can be acheived.

Net allowable soil bearing pressures should not exceed 2500 pounds per square foot for spot footings and 2000 pounds per square foot for strip or continuous footings. The typical house will not have loads that will cause excessive footing sizes using these values. Footings and grade beams should be adequately reinforced for maximum stiffness should some slight differential settlement occur in the future.

The report goes on for many pages with drainage considerations and so on, but that is the jest of it. Again, I'm not questioning the engineer's expertise at all. He has over 30 years experience dealing with these soils, so I'm sure he knows what he is doing. I was questioning the "foundation contractor's" suggestion of using a sloped continuous footing and just didn't have anything to contradict it since the geotech's report doesn't get that deep into it.

It was mentioned above about using large stone and that stone possibly allowing water to pool under it and it sinking into the clay under it. I had wandered about this myself. I recall asking which was better, compacted stone or compacted fill, when I first got the report and he said stone but that I could use dirt so long as it was "good" dirt. I am however contiplating doing a little of both. Prehaps take the footings to 5 or 6 feet deep, backfill the first 2 feet with good compacted dirt and the next 2 feet with the stone, or maybe leave the stone out altogether and do 4 feet of compacted dirt under the footings. I intend to run this by the engineer. What are you guys' thoughts on that?

Bill, someone else had mentioned to me that there is a guy within range that does the drilled piers and the prices sounded very reasonable to me. I will investigate that further.

Someone above asked why use the block and not go with concrete all the way. Well I'd prefer to do just that and I do intend to compare the 2 cost wise. I'm not sure how the extra weight involved would effect the design though, so I'll need to check that also. At the end of the day all I want is well built structurally sound house that I can enjoy trouble free for many years to come. At the same time I can not afford to go crazy and overbuilt it when it's unecessary. But I've seen some where the slab has broke completely into to a point where you could get your fist between the concrete, so I'm very cautious about how my foundation is being done. The contractor I end up selecting for the job will probably be sick to death of me by the time he leaves the job site because I will be there every step of the way, because that is the only way I can ensure it's done right. That being the case, I need to know what right is, which is where you guys and the engineer I'm consulting come in. Thing is you and he can not be there every step of the way. :) So again, thanks very much guys for educating me on the issues at hand. You have been awesome.

Crushed graded gravel meeting the state htghway spec should hold anything you need, and if it meets that spec, and is compacted in 6 to 8 " lifts to 95%, holds up the highways of the nation, from Fla to Ak. It is more impervious than all but the best clays.

the local quarry probably has 2 piles of CA6 (Course Aggregate). One meeting state spec, and the rest. Guess which one you want.

Over your subslab stone, place 1 to 2 " rigid foam insulation to fight localized differential swelling bearing points.

The rigid stuff, not the soft stuff. They're now floating highways across peat bogs on the stuff. They use meter cubes of it instead of 1 or 2 ".

Let the bottom of the footing slope, and step the top. Minimum at step of 8", 2X8 header to 16", 2 #4 (#13 metric) bars, lifted (mucked) from the bottom into the pour, should hold most home loads. A little rust coat is fine. Lap 12" with 3 wire ties for continuous strength. 24L24 corner bars tied similarly.

The extra concrete in the steps is cheap material compared to the labor involved in cutting block on an angle. Hook 6X24 L bar #4 (13) at 8' centers midline of your block wall, slush a 3' bar with mortar filled cores, and you should be fine. Hooking the l-bars under the footing bar helps. The brickie won't work over longer L's. Epoxy anchors into the slab at 4' centers should hold against any wind load. Don't let them just nail or shoot anchors in for your TREATED wall plates. Sprinkle heavily Boric acid onto the plates before insulation and drywall to avoid cockroaches and silverfish or firebugs. Insulation batts should be applied from the outside

Good luck,

RichH

Going down 5' is not a boring, it is a test pit. To get accurate boring information you should be going down till you get the best soil to build on. Often that is 20 to 30 feet or more. We have had projects were we had to go over 100' 5' is way too shallow to determine if you will have a problem in future.

The footing slope is not an issue provided the slope does not exceed about 10 percent.