Soil Types and Bearing
Reviewed by a working tradesman with 17+ years in the field.
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Recognize the common soils, know what each does under a footing, and know when to stop and call an engineer.
Tap your level and the boxes below open to the advice for you. Remembered on this device.
By the end you can
- Name the common soils and what each does under a footing.
- Do a squeeze and a ribbon test and know what they do and do not tell you.
- Spot a soft, wet, organic or filled bearing surface and stop before forming.
- Explain over-excavation and granular replacement.
- Prepare a bearing surface for the inspector.
Never enter a wet or flooded trench at any depth, or any trench over 1.2 m (4 ft) that is not protected. Never tell anyone the soil is good. If the bottom is soft, wet, organic, frozen or changes along its length, stop and ask before you form. Never place a footing on fill, organics or frozen ground.
Two soil languages on one site
Soil decides footing depth and width, compaction, drainage and how the building behaves over time. A foundation designer describes soil by type and bearing capacity: clay, sand, gravel, till, fill. Your provincial OHS regulator classifies the same ground by how it behaves in a trench. Ontario, for example, uses soil Types 1 to 4 under O. Reg. 213/91 (s. 226), and the type guides how the walls must be sloped or supported. Other provinces have their own systems: ask your regulator.
These are different systems. A good bearing soil can still be a bad trench class, especially when it is wet.
Common soil types
- Holds water and drains poorly
- Can swell when wet and shrink when dry
- Bearing varies a lot: stiff clay can be good, soft or wet clay can be poor
- Susceptible to frost action
- Often needs drainage measures, and sometimes over-excavation and replacement
Builder insight: Clay is not always bad, but it punishes bad drainage. Manage the water around it.
- Drains well
- Little cohesion, so trench walls slump
- Compacts well in thin lifts
- Can lose strength when saturated and loosened
Builder insight: Workable, but compact it properly and keep the walls supported.
- Generally good bearing and drainage when well graded and dense
- Low frost susceptibility when clean
Builder insight: Often the best natural bearing soil, but gravel with lots of fines or loose pockets is not the same thing.
- Fine particles
- Turns soft and unstable when wet
- Erodes easily and is highly frost susceptible
Builder insight: Silt looks harmless and then turns to soup. Keep it dry and protect the bottom of the dig.
- A mix of clay, silt, sand, gravel and stones, common across many parts of Canada
- Dense, undisturbed till can bear well
- Boulders and variable pockets are common
Builder insight: Dense till can be excellent, but it varies from one end of a footing to the other. Look at the whole trench bottom.
- Topsoil, roots, peat and decomposing material
- No reliable bearing
- Must be removed from under footings
Builder insight: Even small pockets of organics settle. Remove all of it.
- Soil that has been placed or dumped, often poorly compacted
- Can contain debris, buried waste or soft pockets
- Treat as unsuitable for bearing unless an engineer says it was placed and tested
Builder insight: If you do not know where it came from, assume the worst.
- Placed on purpose, in controlled lifts, to a specification
- Compacted and tested
- Lifts are typically about 150 to 200 mm (6 to 8 in) loose thickness, but follow the engineer's spec
Builder insight: Engineered fill is only engineered if there is a test report to prove it.
Bearing capacity
Bearing capacity is how much load the soil can carry without settling or failing. In Canada it is stated in kilopascals (kPa), and the figure for your site comes from the geotechnical report or the engineer. A single number per soil type is not safe: dense till, soft clay and loose sand each vary enormously with density, moisture and depth.
For houses on ordinary ground, Part 9 of the building code lets footings be sized one of two ways: from the Part 9 footing tables, using the maximum allowable bearing pressures the code lists for each soil, or from pressures worked out by a subsurface investigation (NBC 9.4.4.1, BC 2018 text checked). Two other code rules are worth knowing:
- If a weaker layer lies within twice the footing width below the bearing surface, the design is governed by the weaker layer (9.4.4.2).
- For gravel, sand or silt, the allowable pressure is halved when the water table is within one footing width below the bearing surface (9.4.4.3).
The local building department decides when a geotechnical report is needed.
This page does not print typical bearing values for each soil, because one number per soil is not safe. The code table lists firm clay at 75 kPa, but the conditions behind that figure depend on your code edition. Get the figures, and the rule for when a report is required, from your code edition and your building department.
If bearing is in doubt, the inspector or designer may require:
- Over-excavation and granular replacement
- Compaction testing
- A geotechnical engineer to inspect the bearing surface
- A revised footing design
Identifying soil on site
Field tests give you a first read. They do not replace a geotechnical report on anything structural or anything you are unsure about.
- Hand squeeze. A damp handful that holds its shape leans toward clay or silt. Sand falls apart.
- Ribbon test. Squeeze a damp ball between thumb and finger and push out a ribbon. See the picture.
- Drainage. Clay holds water, sand drains, silt turns to mush.
- Smell. A rotten, earthy smell means organics. Remove them.
- Colour. Dark soil often means organics. Beyond that colour is unreliable. Do not call grey clay or tan sand on colour alone.
- Pocket probe. A bar that sinks easily into the trench bottom means soft ground. Stop and get it looked at.
Over-excavation and replacement
If the soil is unsuitable it comes out and is replaced with engineered granular fill.
- Remove soft, loose, wet or organic soil.
- Dig down to firm, undisturbed ground.
- Check the width of the over-dig with the engineer. It normally extends beyond the footing edge.
- Place granular fill in lifts (about 150 to 200 mm / 6 to 8 in loose) and compact each one.
- Have the bearing surface and fill checked before forming.
How far the replacement extends beyond the footing and what material to use come from the engineer. The lift thickness is a typical field range, not a code value.
Drainage by soil type
- Clay: needs a good drainage system and careful grading.
- Sand: drains, but can shift and wash out.
- Gravel: good for drain tile and base layers.
- Silt: keep it dry or it collapses.
- Organic soil: remove it entirely.
Preparing for inspection
- Bearing surface clean, exposed and level
- All topsoil and organics removed
- Free of loose soil, mud and standing water
- Footing depth visible with a stake or measurement
- The geotechnical report or engineer's field review ready, if required
Inspectors ask: what soil is this, has all the topsoil been removed, is the bearing stable, and how are you handling the water?
If the bottom of the trench is soft, wet or changes along its length, stop and ask before you form. A phone call to the engineer costs far less than a footing that has to come out. Take dated photos of the bearing surface before you cover it.
Rain turns clay and silt to mud and makes sand collapse. Frost heaves fine soil, and frozen ground can look solid and then settle when it thaws. Never place concrete on frozen ground. Heat dries the surface and hides soft ground below.
Worked example: why soft soil means a wider footing
Teaching numbers only. Real footing sizes come from the code tables or the engineer, and loads are factored.
A footing carries 40 kN for every metre of wall. How wide must it be on firm clay at 75 kPa?
- Pressure is load over area. Area needed per metre of wall = 40 kN ÷ 75 kPa = 0.53 m².
- Width. Over a 1 m length, that is a footing about 0.53 m wide.
- Now halve the soil. On soil at 37.5 kPa the same load needs 40 ÷ 37.5 = 1.07 m. Half the strength needs twice the width.
- Why you care. Soft soil under a footing drawn for firm soil is overloaded. That is why you stop and call when the ground is weaker than the drawings assume.
- Replacement fill. If the engineer has you remove 300 mm of soft soil under a 10 m run of footing, 0.9 m wide, that is 10 × 0.9 × 0.3 = 2.7 m³ in the ground. Order extra, since fill is ordered loose and compacts down.
A damp ball of soil makes no ribbon and falls apart. What is it, roughly?
The dark, smelly soil in the bottom of the trench looks solid. Can the footing sit on it?
Who decides whether the bearing surface is good enough?
Gravel or sand with the water table close to the footing: what does the code do about it?
The trench bottom is firm in the morning, and soft by the afternoon after rain. What do you do?
- Must-haveDigging and probing:
- Must-haveField tests: a water bottle for wetting samples for the squeeze and ribbon tests, and clean containers for samples the engineer may want
- Must-haveMeasuring:
- Rent itBearing prep: a plate compactor
- Must-haveBearing prep: a hand tamper, and granular fill when the engineer specifies it
- Must-haveRecords:
- Must-haveHard hat (CSA Z94.1), safety glasses (CSA Z94.3) and green-triangle footwear (CSA Z195)
- Must-haveA trench ladder
- Rent itTrench protection that your provincial regulation requires before anyone enters
Home DIY
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The model code rule is the same everywhere: footings bear on suitable undisturbed soil, rock or compacted granular fill. How trench soil is classified is provincial.
- BC: BC Building Code 2024 Part 9. Seismic and liquefaction concerns on parts of the coast and in river deltas are an engineering matter. Trench soil classification: check WorkSafeBC Part 20.
- ON: O. Reg. 213/91 soil Types 1 to 4 (s. 226) set how walls are sloped or supported (ss. 234 to 242). Check the sections for the slope for each type.
- MB: the Manitoba guide says slope 1:1 after the first metre for excavations over 1.5 m.
- NB, NS: NB slopes no steeper than 1:1 within 1.2 m of the bottom. In NS an engineer certifies slopes steeper than 1:1.
- YT, NU: YT needs an engineer over 6 m or in permafrost. Nunavut permafrost foundations need thermosyphon documentation where it applies.
- AB, SK, QC, PE, NL, NWT: check with your regulator and local authority.
Full comparison in Canada by Province.
- BC Building Code 2018, Division B, 9.4.4 (bearing pressure, weaker layers, water table; same article structure as NBC)
- BC Building Code 2012, Division B, 9.12 (organic material, standing water, freezing)
- NSW Environment, Assessing the texture of your soil (ribbon lengths)
- Ontario, Excavation hazards on construction projects (soil type under s. 226)
- Manitoba, Excavation work guide
- Current NBC, CSA and provincial OHS text are paid or JavaScript-only in places. Check the clause numbers in your province's edition.
Last reviewed October 2026. General information, not engineering or legal advice.