How much does site grading cost for a new addition in Ottawa's Leda clay?

If you are building an addition in the Ottawa area, there is a real chance your lot sits on clay, and some of that clay is the sensitive marine clay known as Leda clay. It changes how a contractor approaches grading and foundations, so it is worth understanding before you compare quotes.

There is no reliable fixed price for grading on Leda clay. The cost depends on the size of the footprint, how much material has to come out, whether that material can be reused, how much imported granular fill is needed, how high the water table is, and what drainage the design calls for. A quote that lists these items separately is far easier to compare than a single lump sum.

Why the soil matters. Sensitive clays can lose strength when they are disturbed, and clay and silt soils are frost-susceptible, meaning they can heave as they freeze. That is why the Ontario Building Code treats them carefully. On sensitive clay, including Leda clay, footing sizes must be designed by an engineer rather than taken from the standard tables. Footings on clay or poorly defined soils must also be at least 1.2 m deep and not shallower than the depth of frost penetration.

The geotechnical step. Because footings have to be engineered on this soil, a geotechnical investigation is usually the first real expense. The engineer looks at what is under your lot, including the water table, and gives foundation and grading recommendations for your site. Ask whether the report is included in the quote or billed separately, and check with the City's Building Code Services about what the permit review will want to see.

The grading work itself. Contractors often use smaller equipment and careful excavation methods on these sites. Disturbed clay is often a poor backfill, so imported granular fill may be needed, which adds trucking and material cost. Ask how much fill each contractor has assumed.

Drainage. Clay drains slowly, so water tends to sit against foundations. The grading plan should move surface water away from both the addition and the existing house, and the foundation drainage needs to be planned along with it. Ask your contractor and engineer how the addition's drainage will tie into the existing system.

Don't let anyone skip or shrink the grading and foundation phase to save money. Foundation problems on clay are expensive and disruptive to fix. When you talk to contractors, ask what experience they have building on clay and how they will follow the engineer's recommendations. You can compare addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and ask each for an itemized quote.

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Looking for experienced contractors? The Ottawa Construction Network connects homeowners with qualified professionals:

• Homeupgraders
• JC Carpentry
• Denys Builds Designs Renovations
• Geerts Roofing Inc
• ALTIOR CONSTRUCTION