Compacting house pad

[herdfan]

My builder has a dilema, which is giving me a headache!

Before my question, I have to give a little background info.

We are building a home with a walkout basement, and the garage is off of the first floor. There is a creek down the center of our property, which we have contained in a 4' culvert and covered over. The basement floor of the house is above the level of this creek. The edge of the garage runs adjacent to this culvert pipe, but does not go over top of the pipe.

So - basically, we are building the house and covering up the front of the basement with fill to make a house on a hill with an exposed. walk-out basement in the back. The dilema is with the garage on the first level. The garage base needs to be built up with fill as well to raise up the garage footer to the first floor. The second floor of the house is built on top of the garage, so the garage footer is pretty important. Our builder says that he can't compact the fill appropriately because it's too "spongey." The fill he is using is mostly red clay. He is now saying that he may have to dig down under the clay (even though he spent the past 2 days putting this in in 6" lifts) and lay in a footer, lay some block up to the 1st floor level, and then fill this in with compacted gravel.

This sounds a bit overkill to me (not to mention a bit costly), but I want it done right. Would a different type of fill work? Has anyone had this problem come up? I've seen house seats being built up with fill, but haven't ever seen a footer poured and then the entire house built up 8 or 9 feet on block before!

Does it sound like this guy is on the ball or should I start looking for another contractor? /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

 

[dltimmons]

It may not be the type of fill being used, but the underlaying soils that are the problem.
Do yourself, and your contractor a favor, and hire a proffesional, licensed, structural engineer, to do a set of engineered foundation plans. First thing to get is a soils test, this will tell the engineer the bearing capacity of the soil, and allow him/her to design the appropriate foundation.

A foundation should always rest on undisturbed soil, with a sufficient bearing capacity. It is never a good idea to build any foundation over fill. If fill is required, structural fill, compacted in no more than 6 - 10" lifts is normally used, and then a soils test should be performed before pouring any footings.

When the existing soil is either unfit for a normal foundation, or has expansive properties, usually caisons are drilled down to soil with good bearing, and the foundation is poured over the top of these caisons.

The last thing you want is to get your new house done, and a year or two later, have doors that wont close, cracks in the walls, and your garage breaking away from the house.

Find an independant engineer to look at this. It should only cost $500.00 - 1,000.00 and it could save you a whole lot more.

If you have a walk out basement and your garage is on the first floor, the rear wall of the garage must act as a retaining wall for all of the fill inside the garage plus the load of the concrete floor, plus the weight of any autos etc. inside the garage. This wall may or may not be exposed at the rear of the house, but construction of it needs to consider all of the above.

I can't give you any better advice without a lot more detail on your building site and home design.

Good luck,

DT

 

[neilly2]

The fact that this moron put clay material under a structural element (even a slab) during a two year rain, makes me cringe.

I hope this guy is on a lump sum contract. If he's trying to get you to pay for this mistake tell him to get bent. Or give me his number and I'll call him. (i'm not kidding I have unlimited long distance). It drives me crazy when people screw over other people that don't know the particulars of the business.

I recently put a building up back on the East Coast over wet silts and clays (this was the virgin material). If we put more poor soil over it, the soils engineer would have flipped. We dealt with it by putting a layer 3-4' of stone. We started with 4" stone and pounded it into the poor soil then came up with 3/4'' clean and DGA(road mix). What this does is bridge the poor soils. Then we came up to bottom of footing elevation with structural fill.

I can get a lot more detailed with your project if I get a few more particulars.

Some questions-

Who designed the house? The builder should be consulting them.
Not many Non-mansion homes have to be concerned with soil bearing capacaties, so doing a test of the soils would be a waste of time. Just make sure there are no sink holes in the area.

You need to worry about compaction which if you do not have you will get differential settlement as the cause of cracks. Compaction is a function of soil type, moisture content, and density.

If you are concerned get a geotechnical engineer. However, this sounds like a fairly straightforward issue that the Architect or knowledgable builder can handle.

By the way, the moron builder is on the right track with the gravel, but his idea of putting a footing down to virgin soil could actually do more damage even though it may seem stronger.(in fact if I am picturing the issue right he's definitely going down the wrong road) I can give you more detail on this if you wish.

Virigin soil is not necessarily buildable, it can have adequate bearing capacity but if not properly compacted and stable will be useless

If your desinger has your plans on AutoCAD send them to me and I'll take a good look.

Sorry for the disjointed thoughts but I can barely keep my eyes open.

 

[herdfan]

Thanks for the replies, guys.
I don't have the plans on the computer, but the plan is this one online House Plan
The basement is essentially the same as the first floor with the garage area being unexcavated earth on the plan.
I'm thinking the problem may be the soils in the creek be, because the soil for the rest of the basement (away from the creek) has just a little clay in it and it contains quite a bit of shale. The contractor dug down beside the culvert pipe to a solid base (although it was clay) about 5 feet below grade and then compacted in the fill (predominately clay) in 6" lifts with a vibratory roller.
He did say that he was waiting on a call from some guy to let him know what to do (but I think this "guy" was the one who told him to lay down the footer on the clay and then go from there). I'm trying to find out who this "guy" is, exactly.
The bigger issue is, obviously, does this contractor know enough to do this job correctly? I surely don't want a detached garage in a few years! /forums/images/graemlins/blush.gif

 

[DiskDoctr]

Herdfan,

Nice house plan, sharp looking.

I am more from the world of "yeah I shoulda done this ahead of time, but now what can I do to alleviate the problem."

First, let me warn you that I am not an expert in this area. Anything I say should be discussed with your contractor(s) or engineers as "would it help to ..." then go with what they say.

Could you use sonatubes to make a pier type foundation? Basically, dig some deep holes to solid material, place these forms in ground and fill with concrete to make pillars. Then your floor structure could use these.

I've seen people use these for partial support (floor still rests on ground and these pillars act as anchors), and as complete support (beams and support structures don't touch ground anywhere except on the piers).

I've also seen a commercial building put up where it was necessary to dig out all the earth to at least 5 feet deep and replace it with different material (shell, I think). This was a large manufacturing facility with a bunch of CNC machines and such.

If it's any consolation, these builders (very experienced and well respected) didn't know this until they started excavating and weren't satisfied with the results from the compacting efforts.

Whatever you do, don't take chances with your new home. If you skimp now and you have troubles later, you'll never forgive yourself. Also, don't try to rush it and compromise because you are anxious to move in.

Of course, this is just my opinion and some suggestions. Really hope it works out quickly for you.

Let us know what you work out.

-JC

 

[johnday]

Herdfan; look at your PM's and talk to me. /forums/images/graemlins/cool.gif

Also go on line to JLCOnline.com and go to building science, we even let layman ask questions /forums/images/graemlins/smile.gif

 

[EddieWalker]

When I was in California, one of the residential developers I new put in a tract of spec homes on the side of a hill. He had earthquakes to deal with, so the footings had to go down 20 feet or more in places. I don't remember the specs, but the total foundation, per house was in the neighborhood of 140 yards of concrete for 2,000 sq ft of living area. These homes sold for $500K and he said he didn't make 10% on them.

Good luck,
Eddie

 

[Darren]

There's more involved here than just compacting the soil in 6" lifts and doing a compaction test. For one thing soil mositure is critical. If it's not within a certain range, you won't get the compaction needed no matter what. I've seen high clay content soils actually sink under a load and then spring back after the load passes. Depending on the type of clay you could also have shrink and swell problems.

Clay is absolutely the worse material your idiot contractor could have chosen. You're guaranteed to have problems with the basement walls you've backfilled with clay.

The other potential problem is the creek. Given the increase in flash flooding in West Virginia we've seen, you've got a problem sooner or later. You're building in a floodplain. I'm not sure how you got a permit, if the county you're building in enacted an ordinance based on the FEMA requirements to enable residents to buy flood insurance.

WV has a lot of problems with the way the permit process is being conducted in some counties. If you're in one of those, the county isn't doing it's job and they just haven't been nailed by FEMA yet. FWIW, that comes from the state flood insurance coordinator.

BTW, were you required to get an elevation certificate to apply for the permit?

 

[herdfan]

I talked with the builder again today and he is arranging some soils testing.
The creek shouldn't be a problem. It is basically a collection of 2 smaller creeks from the hollars at the back of our property (mostly runoff, but some springs too). We put in 4' poly pipes, which are well above what is needed to handle the volume of water.
As far as the flood plain, we are outside of the 500 year floodplain, although I do understand that this does not mean that we will not get flooded or get a mudslide come in on us. If that does happen, though, then there will be a bunch of other folks in a far worse condition than us since they are about 25 feet lower in elevation than we are!

 

[herdfan]

</font><font color="blue" class="small">( total foundation, per house was in the neighborhood of 140 yards of concrete for 2,000 sq ft of living area. )</font>

I hope I don't need that much concrete! /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif
That would kinda ruin my foundation budget /forums/images/graemlins/crazy.gif

 

[johnday]

Eddie; There are a lot of special engineering on the "left" coast. It's really a beautiful part of the country with the weather and landscape, but you "REALLY" pay to be there full time. I guess the prices are whatever the market will pay. /forums/images/graemlins/confused.gif

 

[Darren]

The soil testing is a moot point. Whoever does the test, if they know what they're doing, will have to put togther a graph based on the soil density obtainable through a standard process for a range of soil moistures. The problem is, it sounds like the material was too wet when it was placed. That means the pore pressure was too high which might account for the sponginess the builder reported. I'm guessing that the backfill material will be contained so over time the second story garage will not be undermined when the clay settles or washes out from the bottom. I've ranted about using clay as backfill in many of the retaining wall posts. Make no mistake about it, It sounds like at least two walls of your walkout basement as currently planned will be retaining walls.

You'd be better off reworking the design to use a precast slab for the garage floor and leave the area underneath open. If you're using block for the walls, there are some other things the builder needs to do. It may cost more now, but if it isn't done right the problems will show up at some point. In some cases I've seen the cracks show up not long after the walls were laid. Over time, as happened to a friend, a basement wall can collapse which is a real PITA if you're using it as a finished family room and you end up with several tons of mud lying on top of your carpet not to mention the new view of the outdoors.

Rather than use block, if that's what you're planning, there are a lot of concrete contractors in MD and maybe closer that can construct a basement of reinforced concrete. In many areas the reinforced concrete is cheaper than block, especially if masons are few and far between. That will eliminate a lot of the potential problems. If you go that route make sure they install a waterstop in the basement floor slab to tie into the walls.

The other point to keep in mind is that the 100 and 500 year floods are mathematical constructs. They are statistical calculations based on the history of flooding in your area. Since statistics works best when you have a large number of samples, it's obvious that a hundred years of records isn't enough. Even in Europe with 300 or 40 years of records they get surprises. In my area consistent recordings go back to 1929. We've had three floods that exceeded the 100 year flood since then. The 85 flood was beyond anything they could calculate. Keep in mind a 500 year flood in most cases isn't much higher than a 100 year flood. The recent flood in NJ east of the Cherry Hill area resulted from 11" of rain. One newspaper account termed it a 1000 year flood. That was BS. I doubt given the circumstances, several dams failed, someone actually did the math.

In your case, I'd head for the local NRCS office which should be located in or near the county seat. they have a free booklet on designing ponds that goes into how to calculate runoff. The local Farm Service Agency which may be co-located with the NRCS office will have aerial photos of your area. They also have a device called a planimeter that they can use to calculate the acreage upstream from you. It wouldn't be hard to run some worse case calculations and see just what might come downstream. The booklet gives you all the equations and examples to follow. I'd plug in at least a 10" rain. With maybe an hours work, you'll have something to think about.

This may seem like a lot of effort, but there're folks in many areas of this state that have recently seen flash floods that have exceeded anything before. BTW, since you're not in a flood zone, you can buy flood insurance at the preferred rate which is around $100 per year. You might want to do that based on the results of your calculations.

Another factor to consider is unless you own all the upstream property you can't guarantee what may come downstream one day. Whether it's brush or childrens' toys. Your culvert might end up being a dam once it's blocked. Where does the water go then? Water can undercut a foundation very quickly that close to the creek.

 

[have_blue]

I agree with what dtsimmons said.

The type of soil beneath the foundation is critical. If it's not a good type, even good "spec dirt" with a lot of red clay sand in it must be compacted by time or by fancy equipment. Just mashing it down with a dozer won't do it.

Don't take chances with the foundation. You wouldn't want to be watching TV on Sunday afternoon and hear and FEEL what sounds like a rifle shot, indicating your slab just cracked. (yea, it happened to me). At least dig deep and wide enough to get to settled soil and spread crushed limestone to distribute the load. Make your slab plenty thick with plenty re bar and steel mesh. Make the footings wide, deep, and the trench bottoms smooth and rounded. Consider using fiber concrete, or a 6 bag mix. Consider the "Cable Lock" system.

You may come out cheaper hiring a structural engineer than to take every precaution known to man. Probably so!

 

[herdfan]

Thanks for the info, Darren

I'll check into the planimeter. It will be interesting to come up with the different runoff scenarios.

We are taking precautions to keep the culvert inlets clear. We own to the ridge-tops pretty much all around us, and the creeks originate from our property, but we have found several dumped items around the property from the previous owners (old refrigerators, ovens, tires, pretty much everything). I have seen a pretty good sized log come on down the hollar once too /forums/images/graemlins/blush.gif

The one positive that may come out of this is the potential gain of the basement area underneath the garage. It's an oversized 3 car garage, so there is a lot of potential square footage that we could put to use.

I think the builder is planning on using concrete-filled, rebar-reinforced Ivany block for this part of the basement. How does that compare to the reinforced concrete wall system you had mentioned (I assume this is the poured concrete form system)?

Hopefully we can get this project back on track this week.

Eric

 

[herdfan]

</font><font color="blue" class="small">( You may come out cheaper hiring a structural engineer than to take every precaution known to man. )</font>

Either way, it's still way cheaper to do it right the first time than fixing a problem after the house is built. I'll bet that was a bad feeling you had that Sunday afternoon!
We recently moved back to WV after being in Mississippi for a few years and I remember that they had lots of problems with foundations down there because of "Yazoo clay." The foundation repair and inspection businesses in Jackson seemed to be in pretty high demand.

 

[Darren]

I'm not familiar with that kind of block. The thing I don't like about block basement walls is the difficulty of leakproofing them. If you use the waterstop with formed concrete, you've eliminated the potential for leaks at the floor to wall joint. With the block or the reinforced concrete someone will need to figure out how the precast garage floor slabs will be tied in if you go that route. Someone will need to figure if the block wall will need anything done to support and anchor the slabs.

The difference between block and formed concrete costs depends on your area. This is not a good time of year to find a contractor who has equipment and forms walls. Like any field if you find a contractor that isn't busy and can do something for you immediately, you better wonder why.

 

[herdfan]

</font><font color="blue" class="small">( Like any field if you find a contractor that isn't busy and can do something for you immediately, you better wonder why )</font>

There is definitely a lot of truth in that statement!

 

[kevinro]

You should, however, take heart that your builder pointed out the problem. A crappy builder would have just plowed right through. So, we should give your guy some credit for realizing there may be a problem.

Contractors, even the really good ones, are not experts in every sub-trade. When you hit a hard problem, its always best to find an expert in that particular field. Its part of the cost of building.

Soils and geology are as much black art as science. Get a pro.

Kevin

 

[orezok]

I live in the desert so building on sand is what I do. Sand is an excellent base to build upon if it is done properly.

Sand has several properties that make it excellent and a couple that can be problematic. If you're trenching for utilities and backfill with sand, all you have to do is hydro-consolidate it and no compactive effort is required. Sand is non expansive and will not "settle" after it is consolidated so that is a good property.

Sand is obviously easy to grade as long as you properly moisture condition it.

Building a retaining wall? Sand is the best backfill as it will not expand/settle and it drains easily preventing hydraulic loading of the wall.

Now the bad part. Sand is totally stable under a building as long as it is constained. Sand on an exposed surface is friable and subject to water and wind erosion.

The key we use out here is to build the pad and then butress the edges with a more cohesive material. The pad will then be very stable.