Drilled piers and frost depth 1

[AlmostPE]

If I have a grade beam on drilled piers (residential with fairly tall crawlspace), Do I need to burry bottom of wall to frost depth? The main level of the house is a little high.

 

[civilperson]

No, place void form beneath the wall.

 

[GeoPerson]

Pile cap and grade beams should be embedded below the frost depth. Frost action will tends to heave the ground causing the wall to crack. Unless if your taking that into consideration.

PS: I don’t have a full understanding of your situation.

good lock.

 

[AlmostPE]

Do I need to put the bottom of my crawlspace beam below frost depth? Or can I just put it just slightly below grade? I will have drilled piers supporting the crawlspace wall instead of footing.

 

[CarlB]

What's the frost depth in your area? Is the crawl space heated (or at least the floor is not insulated)? What does local building code/official tell you?

Where I practice the maximum frost depth is 10 feet. Strip footings and perimeter grade beams for HEATED BUILDINGS are required to extend at least 42" below outside finish grade, AND have 2" subgrade insulation against the wall to direct heat below footings to keep frost from penetrating below footing/wall.
Alternate insulation schemes are possible to reduce the 42" cover requiremment but to not less than 2 feet.

 

[AlmostPE]

I just want to know where the frost depth is for a beam that sit on drilled pier. The drilled pier goes down 20 ft which is well below frost depth. Do I need to make the bottom of the beam to frost depth also or not?

 

[GeoPerson]

Usually the top of the drilled piers are set below the frost depth, so that the bottom of the pile cap/grade beam set below the frost depth. Here in NJ, we have the frost at depth ranging from 3 to 3 ½ feet below the finished ground level. Therefore, all piles after driven and/or drilled usually cut at that depth. This is the common practice.

Also, the frost is usually occurring in fine sands and silty soils. So if applicable use a free drainage material similar to ¾ inch clean crushed stone.

I hope some one could confirm this.

Good Luck

 

[civilperson]

The bottom of the grade beam can be above the frost depth if void form is used below the grade beam,(leaving a void or space beneath the grade beam.)

 

[AlmostPE]

civilperson, I do have void form because this foundation is built on expansive soil. So I set the bottom of the wall about 1 foot below finish grade.

 

[emmgjld]

I have heard of some instances where frozen ground has become attached to the tops of drilled piers, lifting the piers due to normal frost heave scenario. This would be similar to leaving the tops of the piers 'mushroomed' and have the surface clays pulling the piers upwards.

I recommend the bottom of the grade beam be at or below the frost depth.

 

[csd72]

civilperson,

Just playing devils advocate, but what if the void gets filled up with water and then freezes?

 

[civilperson]

Frost heave may be prevented with gap graded gravel as a drain layer beneath the slab/footing/void form in combination with insulation. Good grading practices of minimum slopes away from the building and existing water table below the possible capillary rise should also be used.
See threads discussing alternate depths of footings in deep frost locales using granular subbase and insulation.
My opinion: ice has 10% gain in volume over water and if the building requires drilled piers for support then the soil has a potential to deform from the increase in volume when water turns to ice. If the soil deforms, then no damage or detremental effects from frost below grade beams.

 

[JrStructuralEng]

Up here in cold canada. We never place the bottom of grade beam below frost level. Our frost level is typically 6-8ft.

I agree that void form (preferably polystyrene) should be placed beneath the piles. 4" is typical for residential applications, 6" for commercial. As a rule of thumb, we generally place the pile to a minimum depth of 15ft (or twice the length of the frost line).

Hope that helps.