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Anonymous
 
Posted on Friday, August 13, 2004 - 01:28 pm:   Edit PostDelete PostPrint Post

Has anyone has experience with using crushed stone as a base for concrete footing in lieu of taking the footing to frost depth? The project is in Bristol, CT. This is for a building canopy area.

Marc C Chavez
Senior Member
Username: mchavez

Post Number: 41
Registered: 07-2002
Posted on Friday, August 13, 2004 - 01:41 pm:   Edit PostDelete PostPrint Post

On a regular basis my contractors often have to over-x the foundation. When they do so they place fill (that has been specified by the Structural engineer as capable of holding the load) and compact it to 95% or better. When this is done things work well. We have a much thinner frost zone than you do and the code depth for foundations reflect that But the concept should still be valid.

This fill has been everything from soil-sand-gravel mix (percentages per state transportation spec) to crushed rock to crushed building (concrete and brick)

OR did I miss the point of the post.
Richard L Matteo
Senior Member
Username: rlmat

Post Number: 51
Registered: 10-2003
Posted on Friday, August 13, 2004 - 02:12 pm:   Edit PostDelete PostPrint Post

Having grown up and spent most of my career in Connecticut (Hamden/New Haven) I would take the footing all the way to the frost line (42 inches more or less), especially in Bristol - this is a "snow belt" area. However, the final word on this should be your structural and geotechnical engineers. Also, "crushed stone" in CT tends to be trap rock, another consideration. Here in California, we regularly do the same thing Marc mentioned for structural fill, but not concrete or brick.
David Axt, AIA, CCS, CSI
Senior Member
Username: david_axt

Post Number: 334
Registered: 03-2002
Posted on Friday, August 13, 2004 - 02:30 pm:   Edit PostDelete PostPrint Post

I'm not sure if this would solve the problem, but look at GeoPier (www.geopier.com). We have used this system on a few of our projects and saved money over a conventional driven pile or augered pier system.
John Bunzick, CCS, CCCA
Senior Member
Username: bunzick

Post Number: 252
Registered: 03-2002
Posted on Friday, August 13, 2004 - 03:27 pm:   Edit PostDelete PostPrint Post

I have heard of this technique, but could not comment on the specifics of how to do it. Basically, the fill that is used is a capillary-break drainage type. Thus, moisture readily drains through it, and capillary action will not drawn moisture into it. Therefore, the fill will be relatively dry and it won't expand when the ground freezes, which is why footings are placed below the frost line. (And, there is space for some moisture to expand without displacing the fill particles--kind of like air-entrainment in concrete.) Even if the geotech engineer is okay with this (which I'd make sure of) you may still need the code official to agree with this "compliance alternative" to footing depth.
Richard L. Hird P.E. CCS
Senior Member
Username: dick_hird

Post Number: 7
Registered: 02-2004
Posted on Friday, August 13, 2004 - 03:43 pm:   Edit PostDelete PostPrint Post

The concept of using frost free granular material as a foundation is not new, but is has never had great acceptance. Frost heave results from freezing and thawing cycles of material in the microscopic pores of soil material having fine soil structures, clay material typically.

Granular material is considered to be frost free which is one of many reasons it is used for base material below pavements. Although I have never seen where granular material to the frost line was used in lieu of rigid foundation, I was told this approach was used in countries where concrete is much more expensive.

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