frost heave

[deereman]

I am designing a house in Charlotte, nc where the contractor wants to run a section of the wall footing above ground so that a tree can be saved. The distance above ground will be 26 feet so he doesn't want the beam to span like a grade beam but to simply bear on top of the ground. Obviously if the footing is made heavy enough and reinforcement is put in the top it could resist any frost heave, but what force do I design it for. Has anyone ever done something like this before.

 

[CarlB]

Frost heave uplift can be several hundred psi; it could and has lifted up corners of houses. I assume you are just trying to avoid digging into the root system? Will not putting a building over these roots be just as harmful? (I don't know myself, ask a horticulturist)

 

[VAD]

CarlB has given you an idea of the pressures. Have you determined if you will indeed get frost heaving. Three components are essential for this to occur - a frost suscptible soil, cold temperature and a source of water. The absence of any one of these eliminates frost heaving. You may be well advised to check these through a geotechnical engineer which event may be a cost savings on your part. Just a suggestion.


Regards

 

[cvg]

I would not build your foundation over the tree roots, as the roots can also cause heaving. It sounds as if the tree may be too close to the house. You should evaluate either removing the tree or roots, or moving the house.

Even if your beam bears on the ground, you will have to compact the subgrade, probably for a depth of 6 inches or more, and may still damage the roots.

 

[deereman]

A horticulturalist has evaluated the situation and determined that the house should be placed at least 3 or 4 feet from the tree. He says the tree will survive if the footing is on top of the ground at this location. The home owners are determined to keep the tree, and their lot is very small, so they want to build right up to it. Thanks for all the input.

 

[Imagineer]

While it is absolutely true that "if the footing is made heavy enough and reinforcement is put in the top it could resist any frost heave.." In practically it is not. Very few buildings are heavy enough and strong enough to resist frost heave. (This means NO houses and few building short of sky scrapers and concrete bunkers. And, I'd lay even odds on the frost finding someway to win against the concrete.)

The weight of the structure will not compress the frost. In order to resist the heave, the structure would have to be heavy enough to push the frost heave down and compact or compress the soil underneath. If the structure is heavy enough to do this then it is too heavy to bear on the soil.

I know they don't like the idea but your only real choice is to use a grade beam as a lintel to span over the roots. Quite simply, using the ground surface as the support will eventually crack the structure.

Imagineer

 

[MotorCity]

Building over tree roots is not advisable, especially large trees. If the tree dies during the life of the structure, it will immediately begin to decay and rot, thus creating soft spots or voids below grade. Even if you decide to use a grade beam, it will have to span an adequate distance beyond the drip line of the tree.

As mentioned above, most structures are not capable of resisting frost heave, or at least without extensive insulation.

 

[dirtguy4]

What type of soils are at the site? Gravel, sand, clay, silt? What type of tree is it? "Most plants are capable of applying 1-2 MPa (10-20 atm) of tension to the pore-water prior to reaching their wilting point (Taylor and Ashcroft, 1972). These types of pressures on expansive soils can create shrinking and swelling with seasonal moisture variations, providing buildings with very poor, if not unacceptable, foundation performance. The roots systems are also known to obstruct drain tile performance. If the building is being constructed on a free draining gravel or sand, you may be safe, if not free draining, I would stay at least one if not two mature tree heights away with my foundation.

 

[Slowzuki]

Why not bore some piers through the root structure to support the span? A couple of holes could be placed such that they don't hit major roots and would coexist with the tree.

 

[garrettk]

I personally would never recommend that, nor would I purchase a house designed like that.

An option that disturbs a minimal amount of soil and should be readily available is "pin-piles". Basically small pipe piles that are great for carrying the relatively light loads of a residential structure.

Probably the easiest solution is to move the house if the tree must stay.

 

[DRC1]

Frost heave is not something that is resisted by strength. But 26 feet or so is not a long span for a properly reinforced concrete beam. The house loads should be fairly moderate, so I think it would be reasonable to span it. Even if you use piers or piles, you will have to properly reinforce the beam, so I would first look at the option of using 3 or 4 ksi concrete and some rebar.

 

[unsrtn]

No matter what you decide to do - beam on ground, or beam on mini-piles, you will need to place a void-form material between the underside of the beam and the ground surface. This is a compressible foam material that deforms when the ground heaves upward. The uplift force on the beam is usually taken as the compressive strength of the void form. I believe that Beaver Plastics is one manufacturer of void-form products. Likely DOW is, as well. I'm not sure how deep your frost penetration is in your neck of the woods, but the depth depends on soil type, moisture content, degree-days below freezing, snow depth, heat loss from building... I'd go with a minimum of 4 inches thick void form. Void form is commonly used in areas such as mine (northern Canada) to deal with both frost heave and swelling clay pushing on the underside of grade beams. Keep in mind that the load applied downward by the beam must be sufficiently high to resist the compressive strength of the void form product. A lightly loaded beam would simply lift up with the frost, with no deformation of the void form occurring.