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Deep foundation types and use of shallow foundations

Short answer

Deep foundations include pile, pier, well or caisson, drilled shafts, and under-reamed piles. Shallow foundations are used as follows: strip under walls, isolated under single columns, combined for close columns, strap for edge columns, raft for weak soil and heavy loads, grillage for very heavy loads, and stepped footing on slopes.

List types of deep foundation and critically illustrate on the use of various shallow foundations with sketches.

Types of deep foundation

  1. Pile foundation: Slender columns driven or bored into the ground; end-bearing, friction, or both; materials: timber, concrete (precast, cast-in-situ), steel.
  2. Pier foundation: A large-diameter shaft of masonry or concrete carrying heavy loads to firm strata.
  3. Well (caisson) foundation: A hollow cylinder or box sunk to the required depth, used for bridges.
  4. Drilled shafts and under-reamed piles.

Use of various shallow foundations

TypeWhere it is used
Strip / wall footingUnder load-bearing walls on firm soil
Isolated spread footingUnder single columns of RCC frames where soil is good
Combined footingColumns close together or near the property line
Strap footingAn edge column near the boundary, where an isolated footing would be eccentric
Raft (mat)Weak soil, heavy loads, basements; when footings would cover over half the area
GrillageVery heavy loads on soil of low bearing capacity, with less depth
Stepped footingSloping ground
 Combined:  [col1]  [col2]       Raft: |c|c|c|
            [___________]              ____________
 Strap:  [F1]=====strap=====[F2]

Shallow foundations are cheap and quick; when the soil near the surface is weak, or when depth is needed against scour or uplift, deep foundations are chosen.

✓ Answer checked by Claude Opus 5.5 (high effort)

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