Building on Disturbed Soil
There is a property that I frequently pass that was demolished back in November. It is now the end of winter and the area that was once a house, has been an open pit for four months—exposed to the harsh winter we’ve had. Most people wouldn’t see a problem with this, but there is a big problem that could cost the homeowner a lot of money down the road.
Why is building on disturbed soil a problem?
The bottom of the footing for a house needs to sit on undisturbed soil that is a minimum 48” below the surface. Why this deep? We know that in our climate, we get weather that is cold enough to push the frost deep into the ground up to 36”, give or take a few inches. If by chance the footing you place in the ground is at an elevation of 24” below the surface, it will be affected by the frost. The frost will get under the footing, lifting the home and causing the walls to heave. Sound impossible? It happens every year with our roads. In the summer, a road is smooth before the frost comes! Then the frost gets under the roads, causing the roads to lift; as a result, that same road is no longer smooth, and is now bumpy, cracked and quite possibly has potholes.
Why do we care?
The home being exposed to these elements is now going to be built on this open pit, which is now disturbed soil. How has the soil become disturbed? After winter, frost has permeated the ground, and as a result, the ground has frozen! This freezing has in turn shifted the dirt by lifting and disturbing it so much that you can no longer build on it, as the home will settle with the weight.
What are the best practices to fix this problem?
Prior to the concrete being poured, a trench footing needs to be excavated to get to an undisturbed soil depth. A Soils Engineer needs to inspect the soil and provide a report. This report must then be sent to the Municipality to accompany the building permit.
Here’s a look at the rough costs associated with the fix.
- Soils Engineer costs: $1,500.00
- Extra deep excavation and the removal of all the extra fill with dump trucks: $3,000.00 (In this case there is not enough room on site to store the fill so it all gets trucked off then brought back for back fill. Normal excavation might have been fine, depending on lot size.)
- Extra deep footing to be framed and poured, standard is 6”-8” x 18”-24”. After exposure, you would have a footing that is 30” x 18”-24”: $4,000.00
- Extra to backfill and compact the fill around the footing to get a solid base interior and exterior: $3,500.00
So, after all that, an extra whopping $12,000.00 would bring this home back to the elevation it should have been at in the first place, had the area not have been left as an open pit exposed to the elements for the winter. The foundation of the house may seem unimportant, but down the road it pays to be careful from the very start.
Remember, it all starts with a solid foundation, sitting on solid ground!