Foundations
Foundations transfer structural loads safely into the ground. Learn shallow foundations (isolated, combined, strip, raft), deep foundations (piles, drilled shafts), footing sizing, pile capacity from shaft and base resistance, group effects, and foundation selection.
A building is only as good as its foundation. Foundations spread column and wall loads so the soil is neither overstressed nor settles too much — or reach down through weak soil to stronger strata with piles.
Choosing between shallow and deep foundations depends on loads, soil profile, water table, neighbouring structures and cost.
An analogy
Snowshoes (shallow foundations) spread your weight on the surface; walking poles pushed through snow onto the ground beneath (piles) transfer load to firmer layers.
Shallow foundations
Isolated pad footings under columns, strip footings under walls, combined footings when columns are close or at property lines, and rafts (mats) covering the whole footprint for heavy loads or weak soils. Founded typically 1–3 m deep, below frost and topsoil.
Deep foundations
Piles transfer load by shaft friction and end bearing. Types: driven precast/steel, bored cast-in-situ, CFA, micropiles, helical. Used when surface soils are weak, loads are large, uplift or lateral loads exist, or scour/excavation threatens shallow foundations.
Design checks
Bearing/geotechnical capacity, settlement (total and differential), structural design of footing (punching shear, bending) or pile (compression, tension, buckling), group efficiency and negative skin friction (downdrag) from settling fill.
Testing and advanced systems
Pile load tests (static, dynamic PDA, bi-directional O-cell) verify capacity. Piled rafts share load between raft and piles for tall buildings. Ground improvement (stone columns, vibro-compaction, preloading with drains, grouting, jet grouting) can make shallow foundations viable.
Formulas
- Footing area:
A = (P + self-weight) / q_a - Pile capacity:
Q_u = Q_s + Q_b - Pile in clay (α-method):
Q_s = α c_u · (π D L), Q_b = 9 c_u · A_b - Safe pile load:
Q_safe = Q_u / FS (FS ≈ 2.5)
Worked example: Sizing a pad footing
Given: Column service load 1200 kN; q_a = 200 kPa, allow 10 % for self-weight
Find: Square footing size.
- A = 1.1 × 1200/200 = 6.6 m²
- B = √6.6 = 2.57 m → use 2.6 m square
Answer: 2.6 m × 2.6 m
Worked example: Bored pile in clay
Given: D = 0.5 m, L = 15 m; c_u = 50 kPa, α = 0.7
Find: Ultimate and safe capacity.
- Q_s = 0.7 × 50 × π × 0.5 × 15 = 825 kN
- Q_b = 9 × 50 × 0.196 = 88 kN
- Q_u = 913 kN → Q_safe = 913/2.5 = 365 kN
Answer: ≈ 365 kN safe
Common mistakes
- Wrong: Choosing foundations without site investigation. — Right: Always base design on boreholes and tests — soil varies.
- Wrong: Ignoring differential settlement between foundation types. — Right: Avoid mixing piles and pads under one structure without joints.
Applications
- Buildings: Pads and rafts for low-rise; piled rafts for towers.
- Bridges & marine: Large-diameter bored piles and caissons resisting scour and lateral loads.
- Industrial: Machine foundations designed for dynamic loads and vibration.
Key takeaways
- Shallow first; deep when needed.
- Footing area = load/q_a.
- Pile Q_u = Q_s + Q_b; α c_u π D L in clay.
- Always base design on site investigation and verify by testing.
Going further
- β-method: Effective-stress shaft friction for piles in sand and long-term clay behaviour.
- Lateral pile analysis: p–y curves model soil springs for laterally loaded piles (bridges, offshore).
- Pile groups: Group efficiency and block failure checks; settlement of groups exceeds single piles.