Building Homes or Extensions/Stone patio foundation

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Expert: - 8/4/2009


Question

Patio Excavation
Hello Dan,
I am building a large stone patio, with stairs leading up to a sliding door off the back of our house, which is about two and a half feet above grade. The patio will be done in pavers, and the stairs done with wall block and caps. There will be four stairs leading from the main patio to the sliding door, with an intermediate landing. When the house was built, the builder provided a small deck (about 6x8 feet), built off the back sliding door, which we have torn down and dug out the deck footings. The ground beneath the deck appeared to be loose fill, topped with course gravel. Upon excavated the area, in preparation for a granular A base, the earth beneath the old deck is highly saturated with moister, essentially soft wet clay in consistency. I compacted the area with a 220 pound vibrating compactor, but even after that the ground feels soft and spongy under the feet, compressing about 1 inch when you step on it, and then springing back when you remove your foot. I assume this is because the soil is saturated with water. I was planning to use a 6 inch granular A base for the main patio, and a thicker base (about 24 inches thick) above the wet soil area . Will this be sufficient to support the stairs and landing, or do I need to do something more, such as dig deeper with an even thicker base, or attempt to dry out the wet soil? Note that I live in Ottawa, so the ground freezes in the winter.

I also have another simple question. Is it OK for the granular A base to be compacted directly against the house concrete foundation wall? Do I need to have any additional moister barrier above the vapor barrier that is already wrapped around the foundation walls?

I attached a photo for reference.


Answer
Sheldon, this is a hard one to answer without seeing the situation.  Clay soil has a high plastic index which makes the soil very hard in a dry condition and slick and gooey when wet.  Your soil sounds more like a high loam or peet type soil as you say it springs back.  It would be best to consult someone familiar with your particular soil.  A good nursery/landscape contractor in your location should be a good source.  Anyone who deals with highway pavement or asphalt subgrade should also be quite helpful.

In general terms, it sounds like you need to create a way for your soil to dry out, stabilize, or remove and replace to get an adequate base for your pavers.  A French drain and/or dry well further away from the house may lower the moisture content under your patio.  Lime, fly ash, or cement blended into the existing soil may develop an adequate base - this is quite common here to stabilize clay soils.  Some quick reading indicates you have clay and pond ash soils in your area that may be helped by stabilization.  Removing deep deposits of saturated soils and replacing with select compactable material is expensive and disruptive.

It appears you have a basement and wood framing that will be covered by your proposed patio.  Yes, make adequate provisions to prevent water/moisture instrusion.  

Dan Griffin

Expertise

I can answer almost all questions related to the total construction process. My expertise is in commercial construction, though I can field most any residential question. I have hands on experience in concrete, heavy equipment, masonry, all phases of carpentry, interior finishes, and I am fairly strong in mechanical and electrical.

Experience

I have over 20 years experience as a commercial carpenter and commercial construction superintendent. I have another 20 years experience in facility management for a major school district.

Organizations
My favorite hobby for he past 12 years has been singing bass in a The OkChorale men's barbershop chorus and the Mature Moments quartet.

Education/Credentials
I hold a Bachelor's degree in English and Math. I have completed many continuing education hours in the building trades. I hold a Master Carpenter card from the AGC, Associated General Contractors.

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