Shallow vs deep foundations: when to use which

One-line orientation

Choose a foundation by comparing the structural load with the soil near the surface. Use a shallow foundation when good bearing soil is close to grade. Use a deep foundation when the load must pass through weak soil to a firm stratum. This is the anchor for every other foundation card.

Key points

• The decision driver = bearing capacity + load.
  • Competent soil near the surface that can carry the load → shallow.
  • Near-surface soil too weak, or load too heavy → deep.
• Shallow (spread) foundations:
  • Used where soil has sufficient bearing capacity at shallow depth, close to ground level.
  • Spread the load onto the soil immediately beneath the structure.
  • Families: strip/wall, isolated/column, combined, cantilever/strap, mat, plus slab-on-grade.
• Deep foundations:
  • Used when shallow foundations are not feasible — poor soil or heavy loads.
  • Transfer load down to a deeper, more stable soil layer or rock.
  • Piles — generally slender members used singly or in groups; they may be driven or drilled and transfer load by end bearing, shaft friction, or both.
  • Caissons or drilled shafts — generally large cast-in-place shafts formed in drilled holes; they may socket into rock or use a belled bottom.
• Common installation contrast: piles are often driven; caissons or drilled shafts are generally cast in drilled holes. Some piles are also drilled.

Confusions / comparison

	Shallow (spread)	Deep (piles / caissons)
Use when	Competent soil near surface	Weak upper soil and/or heavy load
Load path	Spread onto near-surface soil	Carried down to firm deeper stratum/rock
Depth	Close to ground level	Extends well below grade
Typical members	Strip, isolated, combined, strap, mat	Piles (driven or drilled), caissons/drilled shafts
Pile resists by	—	End bearing or friction
Caisson install	—	Drilled and filled with concrete (belled option)

Related

→ Shallow foundation types (this topic): the spread-footing family in detail · Mat vs floating (this topic): the large-area shallow options · Slab-on-grade & FPSF (this topic): the at-grade case · Grade beam vs tie beam (this topic): elements that work with piles/piers.