Ground Prep and Excavation

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Prepare the site, dig to the right depth on the right ground, and be ready for the inspector.

Site preparation

Before any excavation starts, the crew lead:

• Confirms the property lines and setbacks. On a tight lot, have the pins located by a land surveyor.
• Reviews the plans for footprint, elevations and drainage, using the current revision.
• Locates utilities: public through the one-call service, and private (septic, well, buried power, irrigation) yourself.
• Plans access for trucks, excavators and concrete, including the pump truck if one is needed.
• Sets staging areas, spoil pile locations and material zones.
• Puts temporary drainage and erosion control in place.

Inspectors and neighbours both ask the same thing: where is the water going? Have the answer before you dig.

Utility coordination

• Locate before you dig. Call your province's one-call service, allow at least 3 business days, and trace private lines yourself. See Permits and Approvals for numbers.
• Hand-expose near the marks. The locate report lists hand-expose zones. Keep machines out of those zones until the line is exposed by hand or vacuum. Hand tools can still cut a line, so use a flat blade and never a pointed probe.
• Get the servicing rules. Cover, separations and bedding come from the municipality and the utility owner, not from a national number.
• Protect crossings and keep access clear for future work.

Access, safety and site control

• Safe routes for workers and equipment, away from the dig.
• Stable ramps, and ladders in trenches as your provincial OHS regulator requires.
• Clear walk paths with no debris.
• Spoil piles kept back from the edge. Several provinces say at least 1 m. See Spoil Pile Management.
• Fencing or barriers around open excavations, especially near public walks and neighbours.
• Signs for hazards and restricted areas.

Kinds of excavation

Footing trenches. Full basements and crawl spaces. Slab-on-grade over-excavation. Utility trenches. Driveway and sidewalk subgrade. Retaining wall excavations. Sump pits and drainage trenches. The rules for depth, spoil and exits apply to all of them.

The bearing surface

The footing needs ground that can carry the load and will not move. The model code is clear on the basics (NBC 9.12, BC 2012 text checked):

• Foundation excavations go down to undisturbed soil (9.12.2.1).
• Remove topsoil and vegetable matter under the building. The bottom of every excavation is free of all organic material (9.12.1.1).
• Excavations are kept free of standing water (9.12.1.2).
• The bottom is kept from freezing for the whole construction period (9.12.1.3).

In practice that means:

• Identify the soil. See Soil Types and Bearing.
• Strip topsoil, roots and stumps.
• Over-excavate soft, loose or disturbed areas and replace with compacted granular fill, as the engineer specifies.
• Never place a footing on frozen ground, ground softened by water, or loose backfill.
• Have the engineer or geotechnical consultant confirm the bearing surface if the drawings require it.

How deep?

Depth comes from the drawings, the frost depth and the code. It is never a guess.

The model code gives a minimum foundation depth that depends on soil, drainage and whether the space is heated (NBC 9.12.2.2 and Table 9.12.2.2). In the table, rock has no minimum, coarse-grained soil with good drainage has no minimum, and silt and clay often have to go below the depth of frost penetration, with 1.2 m shown for clay. Your local authority or engineer sets the frost depth for your site.

The excavation workflow

1. Strip topsoil. Remove grass, roots and organic material. Topsoil is not load-bearing. Stockpile it separately for landscaping.
2. Dig to depth. Use an excavator, skid steer or hand tools to suit the job. Check depth often with a laser and grade rod. Dig evenly across the footprint and stop short of final grade so the last bit is trimmed by hand. Do not over-dig. Backfilling the extra with loose soil is worse than leaving the original ground.
3. Square and level. Check corners and edges with the laser. Check depth at several points. Remove high spots and loose soil. Leave the floor flat and consistent.
4. Keep the walls safe. Slope, bench or support the walls as your provincial OHS regulation requires. Keep spoil and equipment back from the edge. Keep a safe way out within reach of every worker. Re-inspect after rain, frost and any change in the soil.
5. Clean the base. Remove loose soil, rocks and mud by hand. Compact granular base where it is called for. Add gravel for drainage or stability only as specified.
6. Manage water. Pump out standing water and let silty water settle before it leaves the site. Slope the dig to a low point for pumping. Keep surface water out of trenches.
7. Get ready for the inspector. The dig is clean and clearly defined, shows footing locations and depths, has visible benchmarks and drainage intent, is safe to enter and measure, and has the approved drawings on site.

Worked example: setting the depth with a laser

The drawings put the underside of the footing at elevation 98.60. The benchmark is 100.00. You set the laser and put the rod on the benchmark: it reads 1.35 m.

1. Height of instrument. 100.00 + 1.35 = 101.35. That is the elevation of the laser beam.
2. Target rod reading at the footing bottom. 101.35 − 98.60 = 2.75 m.
3. Check the dig. The rod on the bottom reads 2.62 m. That is 2.75 − 2.62 = 0.13 m too high, so dig 130 mm more.
4. Over-dig check. A reading of 2.90 m means 150 mm too deep. Do not backfill it with loose soil. Call the engineer or building department for the fix.
5. Record it. Write the benchmark, the reading and the date, and photograph the rod on the bottom.

The same job in feet

Many grade rods read in feet, tenths and hundredths of a foot, not inches. The benchmark is 100.00 ft and the rod on it reads 4.40 ft. The footing bottom is at 95.50 ft.

1. Height of instrument. 100.00 + 4.40 = 104.40 ft.
2. Target rod reading. 104.40 − 95.50 = 8.90 ft.
3. Check the dig. The rod on the bottom reads 8.60 ft. That is 8.90 − 8.60 = 0.30 ft too high. 0.30 × 12 = 3.6 in, so dig about 3 5/8 in more.
4. Tenths to inches. Tenths of a foot are not inches. 0.1 ft is 1.2 in, and 0.5 ft is 6 in. Multiply the decimal by 12.

Common mistakes

• Digging before the locates are back.
• Over-digging and filling with loose soil. The footing then bears on soil that can settle.
• Pouring on a bottom that froze or got soaked.
• Copying a frost depth from another town.
• No plan for water. It collapses trench walls and fails inspections.
• Spoil piled at the edge. It adds weight where the wall is weakest.