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Building on Bedrock - foundation styles

Posted by Brian-the-Architect (My Page) on
Tue, Sep 4, 12 at 21:56

Hello Everyone :)

This is my first thread here - I've been lurking for a longgg time and my lurking has been more frequent lately so I figured I should sign up.

I am an architect by trade and have been building and designing homes for quite a while. I've been surrounded by construction my whole life, and me and my wife will be building our first home shortly.

I have an interesting scenario that I thought I should run past all the creative people here. I have acquired a property (50+ acres) which we will be building on. However, the property is 90% surface granite bedrock, and anything that is not is only 1-2 feet of topsoil.

In my professional experience I have never dealt with a property this challenging, and I have narrowed my problem down to three scenarios, relating to the foundation type.

1) Full foundation
Pros:
Would allow for a full basement - but wouldn't change the ground size we are planning on building.
Cons:
We would need to bring in fill to surround the house and essentially create an artificial hill for the house to sit on.

2) Shallow Foundation
Pros:
Cheaper that option #1 (less fill and less concrete)
Cons:
No usable basement, just crawlspace.

3) No Foundation - just structural slab on grade
Pros:
Cheapest option - fastest build and least headache.
Cons:
No basement
Mechanical nightmare (attic furnace/difficult plumbing).

We are leaning towards option 3, as we would be having a radiant flooring system either way and this option would take the best advantage of that. The house is also designed for passive solar, both for the heating and ventilation. I've uploaded a photo of the property. There is a large wetland on the property that floods in the spring, so we will be deeping it in the fall so that it will retain water year round and be a nice lake - already got environmental approval.

What do you think would be the best option and why?


Follow-Up Postings:

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RE: Building on Bedrock - foundation styles

As a retired architect, my thought is that any excavation or blasting needed to remove bedrock may be a serious budget issue. You don't say where you are located, but if you are in the northern U.S., there's also the issue of freeze and thaw. You haven't mentioned any subsurface water issues, so I assume there are none.

I personally hate to spend money where it doesn't show and/or give me something functional. Thus, I might consider: 1) slab on grade; or if raised finished floor is desireable, 2) an 18" raised continuous stem wall perimeter foundation, that is filled with tamped sand to receive a reinforced (#4 bars both ways) floor slab.

Hope this helps. I'm working on a retirement house, so I'll ask a return favor soon, I'm sure. Good luck!


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RE: Building on Bedrock - foundation styles

We are located in Canada, so essentially the same as northern US, just a few more heating days.

There is no subsurface water, and excavating/blasting the bedrock is not an option ($$).


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RE: Building on Bedrock - foundation styles

I'd say #3

Trying to put a foundation in bedrock only creates a bathtub effect full of water. Building a foundation on fill is no way stable enough to give me a secure, solid feeling. Crawl spaces are not useful IMO.

You don't necessarily need an attic furnace if that is your issue as you can go with the raised ranch style home ... No need to make it look like one though...where the lower level is 1/2 basement if you will and the upper story is the living level. If you design the house properly, you can make that utility space on the north side and keep the living area on the southern, sunny side.

Don't know how well it works in rock...though theoretically it should...just might raise drilling costs...but I would look into geothermal heating...we are so happy with ours.

We are located in Canada, so essentially the same as northern US, just a few more heating days.

Not true! You guys have health insurance and Canadians are all so polite!
;)


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RE: Building on Bedrock - foundation styles

For yet another architect's opinion:

It is not difficult to avoid a freeze-thaw problem on bedrock.

Some of the options are puzzling.

I suspect the cost of filling against a raised house would be far greater than blasting and would serve no practical purpose other than raising the view at the expense of a steeply sloping yard.

I have designed 2 houses on bedrock. For one we blasted for a full basement and for the other we blasted for a small basement mechanical room. These basements do not have a water problem. It is not difficult to drain the footing to lower ground unless the house is on low ground in which case it should be moved to higher ground for good drainage.

It is not difficult to put a heating system in an attic if it is designed for that condition. The equipment and ducts would be included within the conditioned space of the house which means the insulation would be located in the rafters. I don't think you indicated if the house would have more than one story.

If a slab-on-grade is poured on gravel over the bedrock I can't think of a reason for it to be structural unless the rock elevation varies greatly and it is not practical to level it for uniform bearing. If the elevation of the rock is highly irregular it might be better to remove part or all of it.

This is a design issue that should be discussed in the field with an engineer and excavation/foundation contractor familiar with this condition in your area. If you do that I think you will arrive at a very different list of options.


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RE: Building on Bedrock - foundation styles

Slab on grade will still need some excavating, and you will need to devise a way to insulate it from that bedrock, especially if you are going to use radiant. Mind you, I'm only familiar with slab on grade construction here in the sunny South, where winter temps are "cold" if they drop below 30 Fahrenheit. I can't imagine the amount of R value needed where temps may go to -30! I would imagine that thermal isolation of the slab from the bedrock to be even more important! And perhaps my ignorance of northern building science is showing here, but I can't seem to wrap my mind around how you would both have a foundation secured into bedrock and at the same time, create enough of a thermal break from that bedrock with the R values that you would need in that situation. I hope that a couple of the posters more familiar with northern construction scenarios (worthy, lzerarc) will comment on this, as you can benefit greatly from their perspective and I can fill in some gaps in my own knowledge.

Perhaps more of a "pier and beam" type of foundation into the bedrock would work, with enough crawl space to be able to insulate the living space well would work. It's essentially how many homes on the Gulf coast are done here to be able to raise them above flood level. I guess that would be sort of your #2 solution, but with an entire stem wall instead of the point loads of the piers.


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RE: Building on Bedrock - foundation styles

I too, would love to know more about "pier and beam" type of foundation, and whether this is sometimes referred to as pin type foundation. I vaguely remember reading about pin type foundations used over wetlands or other environmentally sensitive sites -- not sure in what climate. In a cold climate, the main issue then is how to insulate the bottom of the house against the exterior cold? What about the water line into the house? Maybe one excavates a very small area for mechanicals, but otherwise uses a pin foundation?


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RE: Building on Bedrock - foundation styles

A "pin foundation" is basically a block of precast concrete with angled opposing sleeves that is placed in a shallow pit and steel "pins" are mechanically driven through the sleeves into the ground. It is a good system for temporary buildings, decks and in cold climates buildings that can tolerate some movement from below grade frost.

It really isn't that difficult to remove the rock and use a conventional system. A foundation is rarely a good place to save money. Rigid foam immediately below a slab over gravel fill is not installed any differently because there is rock below the gravel. There may be some need to anchor the perimeter foundation/grade beam to the rock to resist exterior wall uplift forces but not for a slab. If the slab contacts the rock only in some places it would need to be reinforced to resist differential settlement forces.

A good material to use in place of compacted gravel fill under a slab is "flowable fill" from a concrete plant. It is like a soup and fills irregular depressions and holes without the need to be compacted but you would still need a perimeter foundation system to support the superstructure..


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RE: Building on Bedrock - foundation styles

Why would you remove a wetland? There are many reasons to keep a wetland...they provide habitat for wild life and increase biodiversity. Digging a lake will alter the ecology and hydrological balance in the area. Not to mention, that cant be cheap...use the money for an excellent foundation ... since you have special needs I'm sure the foundation won't be cheap.

I'm certainly no granola cruncher, but this is just one more problem for our environment. Wetlands are extremely important to your area's health. I'm surprised you could do that in Canada.


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RE: Building on Bedrock - foundation styles

A foundation is rarely a good place to save money.

Absolutely right. An old architect I knew from way back always said, put your money in the ground....it's the one place that you don't want to try to get to again to fix any problems.


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RE: Building on Bedrock - foundation styles

Why would you remove a wetland? There are many reasons to keep a wetland...they provide habitat for wild life and increase biodiversity. Digging a lake will alter the ecology and hydrological balance in the area. Not to mention, that cant be cheap...use the money for an excellent foundation ... since you have special needs I'm sure the foundation won't be cheap.

I'm certainly no granola cruncher, but this is just one more problem for our environment. Wetlands are extremely important to your area's health. I'm surprised you could do that in Canada.

While there are many reasons to keep a wetland, there are as many reasons to remove one (for us). Our area has an abundance of wetlands, which, as a result, creates an area that is not diverse for ecology. We brought in an environmentalist and they believed that the property would better suit the area as a strong duck habitat - we made our case and the ministry of natural resources agreed. Also, other than my time and diesel, it won't cost anything. I've got the excavator and we'll be using the soil removed as topsoil for the lawn around the house, so it should break even.

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To set the record on this thread (as it seems to be going off trail), we will not cheap out on the foundation.

In regards to Renevator8's post, I completely agree with what you've said. The ground in fairly level, so it should be easy to pour a nice level slab. Do you have some more info on the 'flowable fill', costs wise? I've seen it before but never on residential projects, just on roadways and pipelines etc. I imagine it would be the same cost once you trucked in the 3/4 stone and tamped it.

If we went with option 3, this is a sketch of the footing for the slab. We would be using 2" or 3" of XPS foam above the fill.


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RE: Building on Bedrock - foundation styles

Brian, your detail should work fine and is likely the most economical approach.

Two issues to consider and verify: 1) that surface water runoff will be positive away from the footings throughout the perimeter; 2) changes in finish grade may require a deeper stem footing where finish elevation is greater than what is shown.

Neither issue is a biggie if considered and verified during construction. Good luck! Post your plans!


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RE: Building on Bedrock - foundation styles

We are currently in the design phase of our home and have a simliar piece of property with much bedrock both exposed and only a few feet below the soils. We live in the Northesst and have been researching slabs versus basement for a while now. One contractor suggests we save the money we would put in blasting and a basement foundation and put a utility room somewhere off the main floor. My wife and I have several concerns being storage, utilities, radon & resale. One discussion I came across seemed to focus on the hardness of slab floors on your joints? I am not sure I buy into the joint problem issue. Given the topography of our land, I think we could blast three to five feet of surface rock and build enough basement foundation for our utility room and some storage or even a small walk out living area below our great room? We will have ample storage above our attached three car garage, but I am nervous that we will reget not having a basement? Any input would be appreciated.


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RE: Building on Bedrock - foundation styles

You have the cheapest foundation, and you do not even realize it. The Bedrock is the foundation!. There is rock exposed in Northern NJ, where one rock is clearly under three houses, hillside! The houses were built on the rock by just drilling rebar into the rock, and pouring a small wall, as the foundation walls. There is NOTHING below the rock. Two houses were built this way, in the 1950's! They are both some of the most unique TINY homes I've ever seen. The gotcha? One homes basement has a slope that is about 30 degrees. The homeowners lived with it, and built a platform for a hot tub. The other gotcha....in order to put anything underground around the home, you've got to cut into the rock and build a trench. You've got power to outbuildings, you need a trench. You've got an old well, you've got a trench. etc. The other gotcha......can be slick in the winter. But even after years of salting the rock, there is no evidence of damage. two of them have been there 65 years and one has been there since 1930. The weirdest thing I've ever seen, is the oldest home was built in 1930. Some of the floor joists sit on river rock, and this keeps them off "the rock" by less then an inch. (5x5 floor joists sit on rounded pebbles) , and the house does not appear to be anchored at all, but sits on dry stacked stone. they built that home off of a "shelf" on the rock, so, there is a basement under part of the home. No man in the world, would build a home this way today, but it's been there for 84 years.


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