Does this foundation need to modified into a frost protected shallow foundation with perimeter drainage and damp proofing or is it durable as is? My inlaws have a home near Vancouver BC that is on a shallow footing (estimated 6-8″ below grade, but extends above grade.) The soil is compacted gravel and cobble, some sand and well drained. You need a pick ax to dig and a 5 gallon pail of water poured into a hole disappears in seconds. The water table is several feet below the surface as measured in well. The terrain is flat and there are not puddles. There is a 16″ block wall on top of the footing which is insulated with 2″ XPS on the inside. The footing is not insulated and receives some heat from the crawlspace (no floor insulation). There is no perimeter drainage and no foundation damp proofing. There is a poly layer under a skim coat of concrete on the floor of the crawlspace and it is dry (no noticeable mold, rot, relative humidity is 42%, vented in summer, gets heat through house floor in winter).

Local code calls for footings 18″ down or frost protected method but this was built in an area that had no code and they are trying to sell. Buyer’s house inpsector said that its at risk of structural damage due to frost heave and that you would have to jack up house to fix foundation. He’s also stressing the need for perimeter drainage and damp proofing. Is that all necessary for this site?

The house is 16 years oid and went through a very cold winter (for the coast) in 08-09 without any signs of foundation cracking. What I could find on the net suggests that there is very limited frost penetration on the coast, that the soil and drainage would preclude frost heave here. I imagine that they could just put down 4″ crushed rock, 2 ” XPS wing and some soil or/and cement on top if really necessary.

“The large amount of precipitation on the outer coast ensures that soils there are constantly moist. The small annual range of temperature implies slowly changing conditions within these moist soils. absent in any but extreme surface layers. Heavy snowpacks insulate soils from frost at higher elevations. In the lowlands surrounding the Strait of Georgia, warming and drying of the soil normally results in a mid- to late-summer moisture deficit estimated to range from less than 100 to about 200 mm depending on the site. Again, winter temperatures are moderate with frost penetration slight and infrequent.” http://www.env.gov.bc.ca/soils/landscape/1.2climate.html

“When f i n e – g r a i n e d s o i l s f r e e z e , t h e p h a s e change of t h e p o r e w a t e r t o i c e i n c r e a s e s t h e m o i s t u r e s u c t i o n a t t h e f r e e z i n g p l a n e d r a w i n g i n w a t e r f r om u n f r o z e n a r e a s . The s u c t i o n l e v e l d e p e n d s l a r g e l y on p o r e s i z e whi ch i s i n t u r n r e l a t e d t o t h e s i z e and g r a d a t i o n of s o i l p a r t i c l e s . The r e s u l t i n g f r o s t h e a v i n g p r e s s u r e i s s i m i l a r l y r e l a t e d t o t h e s o i l p a r t i c l e s i z e s . A s t h e o v e r – b u r d e n p r e s s u r e i n c r e a s e s , t h e r a t e a n d amount of f r o s t h e a v e d e c r e a s e s . When f r o z e n ground thaws t o o q u i c k l y f or the me l t w a t e r t o e s c a p e , a l o s s of s t r e n g t h o c c u r s and t h i s may l e a d t o s e r i o u s probl ems . It i s most i m p o r t a n t t o know i n a d v a n c e i f f r o s t a c t i o n p r o b l ems w i l l o c c u r e i t h e r d u r i n g t h e c o n s t r u c t i o n p e r i o d o r d u r i n g t h e e x p e c t e d l i f e of a s t r u c t u r e . A p r o p e r e v a l u a t i o n t a k e s i n t o a c c o u n t t h e s e v e r i t y o f f r e e z i n g o r t h a w i n g e x p e c t e d , g r o u n d w a t e r a n d d r a i n a g e c o n d i t i o n s , s o i l t y p e and v a r i a b i l i t y a s w e l l a s t h e i m p o r t a n c e of u n i n t e r r u p t e d p e r f o rma n c e of t h e s t r u c t u r e . The c o s t of m i t i g a t i v e m e a s u r e s and t h e c o s t of ma i n t e n a n c e must be weighed a g a i n s t t h e s t a n d a r d of p e r f o rma n c e a n d s a f e t y r e q u i r e d , p a r t i c u l a r l y i n t h e c a s e of a i r p o r t runways, and a l s o t h e d i f f e r e n t i a l d i s p l a c e m e n t t h a t t h e s t r u c t u r e c a n accommodate w i t h o ut damage. A l l s u c h f a c t o r s s h o u l d be c o n s i d e r e d i n making a n o v e r – a l l a s s e s s m e n t of t h e d e g r e e of f r o s t s u s c e p t i b i l i t y t h a t i s a c c e p t a b l e .

“For frost heave to occur, three conditions must be present.
If any one is absent, the ground will not heave. These conditions are:
• sub-freezing temperatures
• moisture
• frost-susceptible soil.
Soil type affects the degree of frost heave. Well-drained sand and gravel (large soil particles) do not heave because they do not support capillary movement of water and because the spaces between the sand and gravel are too large to allow the growth of ice lenses. Clay (very fine soil
particles) is not very frost-susceptible because it blocks the movement of water. This property allows use of clay in building earthen dams. Silt is most frost-susceptible because its particles are of moderate size and support both capillary action and frost lens growth. A small amount of water in soil contributes little to frost
heave”

Thanks,

Patrick