Foundation excavation is unforgiving. Dig too shallow and the inspector fails it; dig too deep and you pay for concrete or engineered fill to bring it back; dig in the wrong place and everything downstream is wrong. Here is how to do it once.
Depth: below frost
In southern Ontario the Ontario Building Code requires footings to bear below frost depth, which for most of this region means a minimum of 1.2 metres, or four feet, below finished grade. Your building permit drawings will show the required depth for your site and your engineer or building department may require more in some soils. A full basement is typically an eight to nine foot dig; a garage or slab-on-grade addition is often a four to five foot frost wall trench instead of a full excavation.
Layout first
Set batter boards and string lines off the surveyed property lines and the permit site plan, then mark the excavation on the ground with paint, adding the over-dig. The over-dig is the working room outside the wall line, usually two to three feet, for forming, waterproofing and the drainage layer. Check the permit for setbacks before you dig; a foundation six inches over a setback line is a very expensive mistake.
Locates and utilities
Ontario One Call locates are required by law and free. On an addition, also find the existing services from the house: the water line, the sewer or septic line, hydro and gas, since they usually cross exactly where you are digging.
Soil
Clay holds a vertical wall for a while and then lets go all at once. Sand slumps immediately. Either way, nobody works in the bottom of an excavation deeper than four feet unless the walls are sloped back or shored, and the spoil pile stays well back from the edge. Keep the bottom dry; if you hit water in a Keswick or Sutton clay, you need a sump and a pump before the footings go in, and possibly an engineer.
The machine
For a garage, an addition or a small house basement, the Kubota U35 has the ten foot dig depth and the power to move clay efficiently, and it fits in a residential yard. Larger custom homes are usually dug by an excavation contractor with a bigger machine, and our contracting side can quote that. The SVL75-3 moves the spoil and does the backfill afterward.
Grade the bottom carefully
The footing bottom needs to be level, undisturbed and at the exact elevation on the drawings. Dig to within a couple of inches with the machine and finish by hand or with a flat bucket. Do not over-dig and then throw loose soil back; footings on backfill settle and crack.
When to hire an operator
If it is your first foundation, if the excavation is close to the existing house, or if the grades are tight, book an operated rental. Our operator digs to your layout and grades while you check the elevations, and the hole is right the first time. It costs less than one mistake.
Call or text 905-251-2521 with your permit drawings and we will tell you which machine and how many days.