Topic 2 of 6 · Form 5
Soils and foundation design
Rock, gravel, sand, silt, clay and peat; properties: bearing capacity, permeability, shrinkage and swelling (expansive "black cotton" clay soils cause cracking), compressibility and drainage.
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Soils
Rock, gravel, sand, silt, clay and peat; properties: bearing capacity, permeability, shrinkage and swelling (expansive "black cotton" clay soils cause cracking), compressibility and drainage.
Loads
Dead loads (self-weight of structure), imposed loads (people, furniture), and wind loads.
Foundation types
Strip, wide strip, trench fill, pad, raft and pile foundations; chosen according to soil, loads, water table and cost.
Strip foundation design
Width = load per metre run ÷ safe bearing capacity of soil.
Worked example: A wall transmits 60 kN per metre run onto soil with a safe bearing capacity of 100 kN/m². Find the minimum foundation width.
Width = 60 ÷ 100 = 0.6 m (600 mm)
Rules of thumb: thickness at least equal to the projection beyond the wall and not less than 150 mm (often 200–300 mm); depth below topsoil and below seasonal moisture movement.
Problems
Differential settlement, heave in expansive clays, and building on fill or slopes. Remedies include deeper foundations, reinforced strips, rafts and piles.
Concrete quantities
Worked example: Find the volume of concrete for a strip foundation 24 m long, 0.6 m wide and 0.25 m deep.
24 × 0.6 × 0.25 = 3.6 m³
Check yourself
- Why are expansive clay soils a problem?
- Which foundation suits weak soil over a large area?
- A wall load is 45 kN/m and the soil bearing capacity 75 kN/m². Find the foundation width.
Show answers
- They swell when wet and shrink when dry, cracking foundations and walls.
- A raft foundation (or piles if firm strata are deep).
- 0.6 m
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