During the readiness to construct footings inspection the thumb pressure test is to be carried out on undisturbed soil . . .

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## Thumb Pressure Test for Soil Classification: What the Building Code Requires

The thumb pressure test for classifying clay and silt soils must be carried out on **undisturbed soil in the wall of a test pit**, not on disturbed or remoulded material.

### What the Test Involves

Building codes across Canada describe two simple field classification methods for soils under footings:

**For sand or gravel (the picket test):** A 38 mm x 38 mm (1-1/2" x 1-1/2") picket bevelled at 45° is pushed into the soil by a person of average weight. The material is classified as:
- "Dense or compact" if the picket cannot be pushed more than 200 mm (8") into the soil
- "Loose" if the picket penetrates 200 mm (8") or more

**For clay and silt (the thumb pressure test):** The soil is classified as:
- "Stiff" if it is difficult to indent by thumb pressure
- "Firm" if it can be indented by moderate thumb pressure
- "Soft" if it can be easily penetrated by thumb pressure

This test must be carried out on undisturbed soil in the wall of a test pit.  The same requirement appears consistently in the National Building Code of Canada 2025 , the Quebec Construction Code , and the Vancouver Building By-Law .

### Why This Matters at the Footing Inspection Stage

The classification directly determines the maximum allowable bearing pressure assigned to the soil. Under the Ontario Building Code, for example, stiff clay is rated at 150 kPa (3,130 psf), firm clay at 75 kPa (1,565 psf), and soft clay at only 40 kPa (835 psf) . Quebec carries identical values . If the inspector's thumb pressure test reveals a lower bearing capacity than assumed in design, footings may need to be widened or redesigned entirely before construction proceeds.

### Key Practical Point

Because the test must be performed on **undisturbed** soil, it cannot be done on spoil or material that has been excavated and rehandled. The inspector must expose a clean face in the pit wall and test the in-place material there. The distinction between disturbed and undisturbed soil is critical: remoulded clay can exhibit very different strength from its in-situ condition, and a test on loose material would produce misleading, non-conservative results.

During the readiness to construct footings inspection the thumb pressure test is to be carried out on undisturbed soil . . .

Thumb Pressure Test for Soil Classification: What the Building Code Requires

What the Test Involves

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