Choosing the right type of foundation is a fundamental part of structural design. The foundation is the part of a structure that transfers the load of the building onto the soil over a large enough area that the bearing pressure stays within the soil's ultimate bearing capacity, and that settlement of the structure remains within an acceptable limit.
Foundations fall into two broad categories — shallow and deep — named after the depth of soil at which they bear. When the width of a foundation is greater than its depth, it is called a shallow foundation. When the width is smaller than the depth, it is called a deep foundation.
Shallow vs. deep foundations
| Feature | Shallow Foundation | Deep Foundation |
|---|---|---|
| Definition | Width of the foundation is greater than its depth. | Width of the foundation is smaller than its depth. |
| Soil requirements | Used when the bearing capacity of the soil is high at shallow depth. | Used when topsoil is compressible or marshy, or strong soil strata lie at greater depth. |
| Load handling | Economical when the structural load is low. | Designed to transmit heavily concentrated or massive loads to deep soil strata. |
| Primary examples | Isolated footing, wall footing, combined footing, raft foundation. | Pile foundation, pier foundation. |
Types of shallow foundation
Isolated spread footing
Isolated spread footing is the most common shallow foundation type, used to spread a concentrated column or pillar load over an area of soil the load-bearing capacity can tolerate. It is typically used for ordinary buildings, with a separate footing placed directly under each column base to transfer load to the soil.
Isolated footings can be rectangular, square, or circular, and reinforced or unreinforced depending on the load. Footing size is estimated by dividing the total column base load by the allowable bearing capacity of the soil; rectangular footings are used where the foundation experiences moments from horizontal forces.
Sub-types of spread footing include:
- Single pad footing
- Sloped footing for a column
- Stepped footing for walls
- Stepped footing for a column
- Wall footing without a step
- Grillage foundation
Isolated footings are economical when the structural load is low, columns are not closely spaced, and the bearing capacity of the soil is high at shallow depth.
Wall footing or strip footing
Wall footing — also called continuous or strip footing — distributes the load of load-bearing walls to the ground so the soil's bearing limit is not exceeded. The footing runs continuously along the length of the wall, typically at 2–3 times the wall width, and is generally constructed in stone, brick, or concrete.
Strip footing is economical when the loads to be transmitted are of small magnitude, or when the wall sits on dense sand and gravel.
Combined footing
Combined footing is used when columns are placed close enough together, or close enough to the property line, that individual isolated footings would overlap or cross the boundary. It merges the footings of two or more columns into a single foundation element.
Combined footings are the economical choice when columns sit near the property line, when isolated footings would otherwise cross it, when columns are closely spaced, or when one side of the footing is dimensionally restricted.
Cantilever or strap footing
Strap (or cantilever) footing is similar in purpose to combined footing. Here, individual footings under each column are built separately and connected by a strap beam rather than merged into one slab, which lets the interior footing carry an eccentric load without extending the footing across a boundary.
Raft foundation or mat foundation
Raft or mat foundations are used where shallow isolated footings or piles are not suitable — typically where soil bearing capacity is inadequate and the structural load needs to be spread over a large area. A raft foundation is built as a slab (often with T-beams) covering the entire footprint of the structure, so the whole basement floor acts as the foundation.
Raft foundations are economical when the structure includes a basement, columns are closely spaced, or the soil is weak enough that the load must be spread across a large area.
Types of deep foundation
Pile foundation
Piles are the most common deep foundation type, used to transmit structural loads down to soil strata that shallow foundations cannot reach or cannot bear.
Common pile sub-types include:
- End-bearing piles
- Friction piles
- Soil compactor piles
- Timber piles
- Concrete piles
- Steel piles
Pile foundations are economical when soil with adequate bearing capacity lies at greater depth, when the foundation carries a heavily concentrated load, or when the topsoil is marshy or compressible.
Pier foundation
Pier foundations are underground structures designed to carry loads too massive for shallow foundations, and are typically used in multi-storey buildings. Piers are the economical choice when sound rock strata lie beneath a decomposed rock layer, when the topsoil is stiff clay that resists pile driving, and when a heavy load must be transferred into the soil.
Verifying foundation choice with finite element analysis
Choosing between shallow and deep options is a soil-behaviour question as much as a structural one — bearing capacity, settlement, and how load spreads through the ground all decide which foundation type is economical for a given site. Hand calculations and empirical charts approximate this well for a well-defined, uniform bearing layer, but layered strata or marginal bearing capacity call for a numerical check.
OPTUM G3 computes bearing capacity through rigorous upper- and lower-bound Limit Analysis and settlement through elastoplastic analysis, with automatic adaptive meshing that refines around the zones of highest strain. That gives geotechnical engineers a numerical basis for the foundation-type decision before construction begins, rather than relying on an empirical bearing-capacity formula alone.
The tool for this
3D geotechnical finite-element analysis.
Compute bearing capacity by Limit Analysis and settlements by elastoplastic analysis, with automatic adaptive meshing.
FAQs
What are the two main types of foundations used in construction?
The two main categories are shallow foundations and deep foundations, classified by the depth of the soil they bear on relative to the width of the foundation.
What is an isolated spread footing?
Isolated spread footing is a shallow foundation type used to spread a concentrated column load to the soil. It is one of the most common foundation types for ordinary buildings.
When is a raft or mat foundation used?
Raft foundations are used when the soil is weak, columns are closely spaced, or the structure includes a basement. They use a slab spread over the entire footprint to distribute loads.
What is the difference between wall footing and strap footing?
Wall footing (or strip footing) supports load-bearing walls continuously along their length. Strap footing (or cantilever footing) connects independent column foundations with a strap beam to handle eccentric loads or property line restrictions.
Why are pile foundations used?
Pile foundations transmit loads to deeper, stronger soil strata when shallow foundations cannot bear the load — commonly for heavy loads or marshy soil conditions.
What is a pier foundation?
A pier foundation is an underground structure designed to support massive loads in multi-storey buildings, typically where sound rock strata lie beneath a decomposed rock layer.
