Ground carries a building safely only if the pressure under the footing (load divided by footing area) stays below what that soil can bear, so a soil half as strong needs a footing twice as wide. Reject sites where soil is soft, organic, filled, wet or prone to swell and shrink, because uneven settlement cracks walls long after the building is finished.
How to judge, at the siting stage, whether ground can carry a building, which soils fail, and why the field investigation comes before the design.
Soil Fails by Squeezing Unevenly, Not Usually by Collapsing
The pressure on the ground under a footing is the load divided by the footing’s area. The soil has a limit on that pressure. Beyond it the footing punches into the ground; well below it, the soil still compresses a little, and if it compresses unevenly the building tilts and cracks. In practice uneven settlement is the common failure. Measuring the actual bearing value is the job of Soil Bearing Capacity; this page is about deciding which sites deserve that effort and which to drop outright.
Soils to Distrust at the Siting Stage
- Soft, organic or peaty ground — compresses a lot and unevenly. Wet, dark, spongy, smelling of rot.
- Fill and dumped ground — loose and varied, may contain voids and debris.
- Expansive clay — swells when wet and shrinks when dry, lifting and dropping footings with the seasons. Signs: deep cracks across dry ground, and sticky, greasy clay in wet weather. This is a judgment from experience of the region; confirm with local buildings.
- Ground near the water table or with seeping water — water reduces strength.
- Steep or filled slopes — risk of slip. The carrier lists landslide risk above 15% grade.
- Soil that changes across the footprint — half the building on rock and half on clay settles unevenly.
What You Need
- A shovel — test pits at the proposed foundation depth.
- A steel rod — to probe for soft layers. See Soil Bearing Capacity for its field tests.
- A look at old structures — a long-standing stone wall or building on similar ground without cracks is evidence the soil is adequate.
Method: Screen the Site
Step 1: Look at what already stands Old walls, wells and buildings nearby show how the ground has behaved over decades.
Step 2: Dig test pits to foundation depth and below Several, across the footprint. Note layers, moisture, firmness at the bottom, and whether the walls of the pit stand.
Step 3: Probe Push a rod down at many points. A spot where it sinks easily where neighboring spots do not is a soft pocket.
Step 4: Frost depth Footings in cold climates must sit below the depth where the ground freezes, or frost heave will lift them. Find the local frost depth from local building history; do not guess.
Worked Example
Assumptions, stated: a compacted-earth wall 0.45 m thick and 2.5 m high with an assumed density of 1,800 kg/m3, load per metre of wall length (roof not included).
- Wall mass per metre = 0.45 x 2.5 x 1 x 1,800 = 2,025 kg.
- Weight = 2,025 x 9.81 = 19,865 N, about 19.9 kN per metre.
- On a footing 0.6 m wide, pressure = 19.9 / 0.6 = 33 kPa.
- On a footing 0.9 m wide, pressure = 19.9 / 0.9 = 22 kPa.
Widening the footing from 0.6 m to 0.9 m cuts the pressure by one third. If your test shows the soil can carry only half as much as you assumed, you must make the footing twice as wide to keep the same pressure. Roof and floor loads add to this. The allowable value comes from the field tests on the soil-bearing page, with its safety factors, never from this example.
Common Mistakes
- Testing only the surface — soft layers lie below.
- Wide footing as the only cure — a wide footing spreads the load but does not stop settlement on a thick soft layer; move the site.
- Ignoring water — drainage near the footing is as important as its width. See Slope & Drainage Assessment.
Safety
Pits and trenches collapse
Never work in an unshored trench deeper than your waist, particularly in wet or sandy soil, and keep heavy loads and spoil away from the edge. A person trapped in collapsed soil can be unable to breathe and cannot be freed without help.
What’s Next
- Learn next: Soil Bearing Capacity — field tests and safety factors
- Combine with: Stone Foundation — building the footing once the ground is chosen