I have native silty clay with a liquid limit ranging from 55 to 73, and plastic limits of 26 to 29. The consistency of the clay ranged from firm to very stiff with SPT "N" values ranging from 7 to 16. I am curious if this clay will heave on shallow footings. What soil properties are important when determining whether or not it is susceptible to heaving? Thanks in advance.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

yes, silty clay is susceptible

http://www.eng-tips.com/viewthread.cfm?qid=187204

http://www.pavement.com/Downloads/TS/EB204P/TS204.3P.pdf

http://www.eng-tips.com/search.cfm?pid=256&action=Search&q=%22frost+susceptible+soil%22

Probably all except solid rock

Fractured rock is also susceptable.  It goes boom real big!

Mike McCann
MMC Engineering

 

As shown in cvg's second link, a clay is not as frost-suceptible as a silt. This is because moisture does not migrate through this soil quickly enough to build up significant ice lenses in the soil as the freezing front advances.
However there is still some potential for frost heave, especially of there are variations in the soil. As mentioned in the other thread, would be best to insulate footings having inadequate cover(side of wall below grade and laterally over footing) to maintain soils thawed under footing. In Anchorage Alaska the design frost depth is 10 feet, but the standard foundation design (for heated buildings) is to provided 42" of soil cover, and 2" insulation on the foundation wall.  It works.

If you have fine material in the form of silts... any percentage in excess of about 7% or 8% can lead to frost susceptability... it is sufficiently free draining to permit the flow of water and sufficiently fine to permit water flow by capillary action.

Water has been found in highly plastic clays in a supercooled state at -40degrees... the clay is too tight for the water to change phase to ice.

Dik

should have added the 7% or 8% passing the 200 sieve...

Dik

Highly plastic clay can heave a lot if frozen slowly.  I investigated a frozen-food warehouse floor that had heaved 8 inches over a period of years.  There were thin ice lenses less than an inch apart in the clay.  Most interesting was that when the floor was heated and the frozen soil thawed, the floor settled 16 inches - twice the original heave.  The explanation was that the clay consolidated from negative pore pressure when the water migrated to the lenses.