eq1
Registered Users
Posts:24
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| 11/19/2007 5:04 PM |
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The lot that we bought has following issues:
1. High bedrock, about 2 - 4 feet underground.
2. High seasonal water table, in Spring you can see the water on the ground.
3. The surface is almost flat, with 2-3 ft variations in elevation.
Under the top soil we have sand, until the bedrock. The septic system (conventional) will have to be raised about 5 ft above grade, because of the water table. ICF walls, 2 storey house, total 3600 sq.ft., no basement.
The question is where to put the footings. Me and a couple of builders I spoke with, thought to put the footings on bedrock, for best support. The geotechnical and structural engineer that visited the lot said that I don't have to do that, that is much cheaper to go with a short FPSF foundation, on the sand.
In addition to that, the septic needs gravity, that means I have to raise the slab on grade about 6 ft above ground.
One option I am considering is to bring in lots of fill, to compact it, and to build a short FPSF foundation on it, 2 ft deep, which will allow a 6 ft elevation of the slab on grade for the gravity septic system.
My concerns are:
1. Building on bedrock means lots of costs for ICF, concrete and steps. Not justified since I don't have a basement.
2. If I put the footings on sand I might have fill settlement, "downhill creep", tree root lift, animals digging under the footing, erosion, excessive moisture around the building, earth tremors, etc.
What do you guys think about this? I am looking for a solution both economical and robust. Can you suggest a better solution?
Many thanks! |
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drogers
Registered Users
Posts:68
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| 11/19/2007 7:07 PM |
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Are you sure you want this location? You did not say what area of the country this is in, but if you have heavy frost I can see problems ahead. Is there any natural drainage to this site or does it dry by evaporation? A silly question you might think but if the bed rock traps water on the site you may be setting yourself up for major problems. Mother nature had a lot of fun in past ice ages, at times she scooped out depressions and filled them with soil. Make sure you have a truthful picture of what is beneath the surface before you do anything. |
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eq1
Registered Users
Posts:24
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| 11/19/2007 7:16 PM |
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| Location: Ontario, Canada. There is some drainage on the side of the road, but doesn't help much. The solution proposed was to bring in lots of fill to raise the house above water table and also to create the gravity required for septic system. Insulation under the footings to prevent frost heave. |
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xexpat
Registered Users
Posts:18
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| 11/19/2007 7:43 PM |
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I would personally be nevous about placing footings on compacted fill, especially with footings so close to the frost line unless someone could show me a lot of successful projects more than 10 years old. Footings on SOUND bedtock sound a lot better to me. What is local practice? Post and beam might be interesting.
What about doing a ventilated crawl space rather than bringing in lots of fill.....or do you need to do that anyway for the septic system? Are aerobic systems a viable option?
xexpat
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icfblocks
Registered Users
Posts:265
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| 11/19/2007 7:49 PM |
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Drogers raises some good points. Another location may be the best option given the described situation.
But if you just have to live in that location you could scribe the ICF's to the bead rock, and pin the concrete to the rock and not need footers at all. The ICF 's could be used for stem walls to raise your house the needed height to give you the necessary flow to your septic tank.
Another option would be a Legalett style slab. |
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Thanks,
Tom
www.advbuildingtech.com |
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eq1
Registered Users
Posts:24
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| 11/19/2007 8:00 PM |
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xexpat: I am also not comfortable with footings on fill. I'd like to use a conventional/gravity septic system for lower (maintanance) costs. I heard scarry stories about crawl spaces. An option is to have a low basement, let's say 6ft high, to put there the furnace/water heater/storage.
icfblocks: We like the place, so we will build there. Legallet is not an option (I want to heat with a wood furnace). The idea of ICF walls pinned to the bedrock may not be possible, since the bedrock is not flat, variations of at least 2 feet. |
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ReadyToRetire
Registered Users
Posts:195
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| 11/19/2007 8:09 PM |
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eq1,
I might have misinterpreted him, but I didn't read drogers as meaning surface drainage. If there are depressions or issues *in the bedrock* not visible from the surface, they could create problems.
What will happen to the known water flow when you obstruct it with your construction?
What will happen to your structure during the winter freeze?
Will your elevated field work in winter?
etc. . . .
I'm risk adverse; I'd want to speek to a hydrologist, geologist, and PE that *know* the region. Does the site's advantages justify the known additional costs and risks of problems and future costs?
Good luck,
Larry
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dmaceld
Registered Users
Posts:495
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| 11/19/2007 8:32 PM |
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Posted By eq1 on 11/19/2007 8:00 PM
I'd like to use a conventional/gravity septic system for lower (maintanance) costs. I heard scarry stories about crawl spaces. An option is to have a low basement, let's say 6ft high, to put there the furnace/water heater/storage.
With the high water table you say you have there, are you going to able to use a conventional septic system at all? Have the local authorities granted sewer permits for your land? You need to have good percolation for a septic drain field to work, and high water can prevent that. You would probably be better off to use an anaerobic septic system. It's basically a two compartment septic tank with air being continuously injected into the first chamber I think. They're used extensively in Louisiana. Unfortunately it's difficult to find info about them on the 'net because in LA anyway, they can only be installed by licensed contractors.
What kind of scary stories have you heard about crawl spaces? If you only need to go down 2' to 4' to bedrock, and the house is ~1' above grade, you only have a 3' to 5' high crawl space. That's not excessive at all. Drainage issues, etc., won't be any different for a crawl space than for a basement.
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Building house - what a way to spend retirement! |
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Quantum
Registered Users
Posts:262
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| 11/19/2007 9:06 PM |
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First of all, almost no site is unbuildable. This one doesn't have problems that haven't been solved many times before. Four foot slope in the bedrock? There's a big clue aboot (hehe) where the water's going.
You could have dozer scrape the bedrock to level. (Do they have bulldozers in Canada? {hehe}) You could blast out a basement and septic tank pit. You could run a line from the tank to a remote drainfield. You could trench in the rock around the site to bleed the water elsewhere. You could dig a pond for a place to put the water, maybe which feeds a further depression that could become a stream.
But two things are for sure:
- You need a different engineer. It is dumb to build on compacted soil in this situation. Put it on rock.
- You need more information about the subsurface, like where does the water come from, when and how much, and where does it try to go. Don't fight the water... help it. |
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eq1
Registered Users
Posts:24
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| 11/19/2007 10:07 PM |
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Lots of valuable info. Thanks!
I also feel that I should put the footings on bedrock. I prefer a conventional septic system because much lower maintanance costs. Even if I have to raise the house few feet. I already got the permit for a raised septic bed to about 5 ft. While the slab on grade elevation should be at least 6 ft.
How can I get info about the subsurface? Is there a device for that? I used a metal rod with a hammer to test the bedrock level in various spots.
The lot is still treed, so I can't bring in an excavator, but we think also to this option, to dig in few spots to see what is under.
And another question, please: can I have a very low basement, let's say 6 feet high, where I can place the wood/oil furnace, oil tank and oil boiler? |
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irnivek
Registered Users
Posts:272
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| 11/19/2007 11:13 PM |
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You should talk to your well driller if you are using one, the best ones always know the geology.
Many structures are built right on the Canadian Shield, but in some localities and on faults we have had engineers dictate they want a bondbreaker between rock and footer. A couple inches of sand was spec'd for us in the past...
FYI Compacted structural fill will often be far stronger in structure and support than native, undisturbed soils. In the past we have removed soils and reduced moisture content, installed drainage systems, and replaced the same soil in lifts with proper compaction and density tests to achieve 98% compaction....
We have installed plenty of raised septic beds in Ontario in the past, these used lift pumps, and I've never heard of a problem with them, just thought I'd throw it in although I know you specified you (and I) like gravity better... A lot of the newfangled septic systems turn out to be a maintenance nightmare, more than "worrying" about a pump..Many need certified maintenance once or twice yearly, that seems like a pain and costly long term.
If it were me, I'd do my best to keep the home a slab on grade and put a battery backup pump in the tank with alarm to let you know if there ever is a problem....
Yes, you can have a short basement, you need to condition this space as if it were living space.
Kevin |
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eq1
Registered Users
Posts:24
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| 11/19/2007 11:56 PM |
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| Thanks Kevin, I actually thought of this scenario, where the tank sits on the bedrock, the slab on grade 4 feet above bedrock, and I use gravity only to the tank, and then a pump from the tank to the raised septic bed. Can you recommend such a tank? A precast concrete tank? |
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gregj
Registered Users
Posts:168
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| 11/20/2007 10:37 AM |
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| Just my opinion but if you sometimes have surface water on the lot in it's current condition then you wouldn't want the crawlspace or basement slab below the current grade height or you will always be fighting water. I would want the slab above the highest known water level. |
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drogers
Registered Users
Posts:68
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| 11/20/2007 1:36 PM |
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Posted By eq1 on 11/19/2007 7:16 PM
Location: Ontario, Canada. There is some drainage on the side of the road, but doesn't help much. The solution proposed was to bring in lots of fill to raise the house above water table and also to create the gravity required for septic system. Insulation under the footings to prevent frost heave.
Your approach using a fill that drains well should keep you out of trouble. If you could fill it this fall and let it go through the winter the freeze thaw cycle will help to consolidate the material.
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eq1
Registered Users
Posts:24
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| 11/20/2007 1:47 PM |
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By now sounds that footings on bedrock is a much safer approach, and trouble free in the future. I believe that the bearing requirements for the fill under the slab on grade are not that important, as if I'd put the footings on the fill. Am I right?
The geo engineer that we hired said that there is no difference, even if I go with footings on bedrock I need to bring in exactly the same fill.
I hoped that, if footings are on the bedrock, I can at least save money on the engineered fill under the slab.
Your advice is greatly appreciated. |
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eric monkman
Registered Users
Posts:195
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| 11/22/2007 2:23 PM |
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Beno : The cheapest and most structurally sound solution for your lot is:
1. Footings on bedrock.
2. Frost wall (conventional concrete) to 1 course below ground floor finish elevation. In your case approx 4 ft.
Pour pads & columns on bedrock to support your steel jack posts and eliminate interior foundation load bearing walls.
3. Pour one course of ICF to top of ground floor level,and fill interior with clean sand.
Maintain your ground floor 12 in above existing grade and use sand backfill to ease underfloor utility install.
4. Then ICF upward to Hambro and roof.
5. Pump your shit up to your raised bed and keep the natural lot grading. Buy a good pump. LOL.
6. Find an intelligent excavating contractor and ICF / Concrete contractor.
Good Luck.
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eq1
Registered Users
Posts:24
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| 11/22/2007 2:57 PM |
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Thanks Eric, sounds good!
I think is a good idea not to have ICF in the wet soil underground (also there is no basement).
One comment: Since the Hambro allows enough span for our design (30ft) I guess I don't need anymore this:
"Pour pads & columns on bedrock to support your steel jack posts and eliminate interior foundation load bearing walls."
All the load will be on the exterior ICF walls/footings. Maybe the ICF walls will require more rebar. |
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rpaulpen
Registered Users
Posts:3
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| 06/25/2008 10:36 PM |
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I'm far from a contractor or engineer, but have seen many cottages in my area (Magnetawan/Parry Sound) build on bedrock and will surely outlast many of us. My neighbour has 27 pads that vary 8 feet. He gets water flowing through there every year and actually pumps from the pool under his house in the early spring before the ice is out. His block work and footings are in perfect shape. He basically has a crawl space before the basement floor and vents everywhere to keep it dry. Conversely, my other neighbour built about the same time and used packed fill just above bedrock and his blocks were crumbling after 7 years. He ended up having a new foundation put in on solid rock and it's been fine ever since.
You cannot underestimate the experience of a reputable local contractor who has most likely overcome most of these problems. These guys know their business and have built a reputation on many years of happy customers. I suggest you ask around and get some good names and talk to them. Their experience is invaluable.
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yogia
Registered Users
Posts:78
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| 07/01/2008 12:24 AM |
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Hi eq1:
You have been offered several feasible options. Some other things to consider which you may have already done ...
1) information about other houses built in the area ...
a) what type of foundation was used in those houses
b) is there any history of problems with the foundation, drainage, ... so forth
c) talking to the Engineers, Contractors, Building Officials who know the area and have worked in the area
2) if you do decide to put your foundation on the fill, an option would be to use a mat (or so called raft) foundation
that should minimize concerns about localised / differential settlement
3) if you do decide to go a few feet below grade, you may want to consider use of a split level
4) you may want to have a local Architct/Engineer custom design the house ( the layout, the elevations, ... ) to suit your
specific site conditions and your own needs and preferences
5) ....
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Regards!
Yogi Anand, D.Eng, P.E.
Energy Efficient Building Network LLC
http://www.energyefficientbuild.com |
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