Types of Foundations: Shallow and Deep Systems Explained

What A Foundation Must Do

A foundation transfers structural actions into soil or rock while keeping movement, rotation, and structural demand within acceptable limits. Vertical compression is only one part of that job. Foundations may also resist uplift, lateral load, overturning moment, cyclic loading, downdrag, scour, frost action, expansive-soil movement, and construction-induced ground disturbance.

The first division is between shallow and deep foundations. A shallow foundation transfers most of its load near the ground surface through a footing or mat. A deep foundation transfers load farther into the profile through shaft resistance, toe resistance, or both. Depth alone does not make the decision; load level, ground conditions, movement tolerance, groundwater, constructability, risk, and cost do.

Shallow Foundation Types

Isolated spread footings

An isolated footing supports one column, pier, or concentrated load. Square footings are efficient for nearly concentric loads. Rectangular footings are useful where column geometry, property lines, or moment direction requires different plan dimensions.

Applications include low- to mid-rise buildings, industrial frames, equipment supports, and bridge substructures where competent bearing material occurs at practical excavation depth. Limitations include excessive settlement on compressible soil, large footing area at high loads, interference between adjacent footings, and sensitivity to eccentricity near property boundaries.

Continuous strip or wall footings

A strip footing supports a wall or a line of closely spaced columns. The design is often treated per unit length, with required width obtained from line load divided by allowable service pressure. Strip footings work well for bearing walls and repetitive framing on relatively uniform ground.

They are less forgiving where soil conditions change sharply along the wall. Differential movement, local soft zones, utility trenches, and stepped grades deserve explicit review.

Combined and strap footings

A combined footing supports two or more columns on one slab. A strap footing uses separate pads connected by a stiff beam that redistributes moment. These systems are common when an exterior column sits near a property line and a centered isolated footing cannot be constructed.

The plan geometry should place the resultant column load near the centroid of the effective bearing area if a reasonably uniform service pressure is intended. The strap beam is a structural load-transfer element and is not normally assumed to bear on soil unless designed for that interaction.

Mat or raft foundations

A mat supports many columns or walls over a large portion of the building footprint. Mats are used where individual footings would cover much of the plan area, where differential settlement control is important, or where a basement slab can be integrated into the foundation system.

Mats distribute load but do not eliminate settlement. Their analysis may require soil-structure interaction, staged construction, variable subgrade stiffness, punching shear, flexure, waterproofing, and thermal or shrinkage considerations.

Deep Foundation Types

Driven piles

Driven piles are prefabricated structural elements installed by impact, vibration, or pressing. Common forms include steel H-piles, pipe piles, prestressed concrete piles, and timber piles. Installation displaces or penetrates the ground and can provide useful installation resistance data.

Driven piles can be economical for large quantities and soft deposits, but noise, vibration, ground heave, obstructions, pile damage, and access may limit their use. Capacity can change with time because of setup or relaxation.

Drilled shafts and bored piles

Drilled shafts are cast-in-place deep elements formed in an excavated hole. They can carry large axial and lateral loads and can be socketed into rock or intermediate geomaterial. Large diameter reduces element count but makes construction quality especially consequential.

Groundwater, unstable excavation walls, slurry control, base cleanliness, concrete placement, reinforcement positioning, and integrity verification are central design and construction issues.

Augered cast-in-place and continuous-flight-auger piles

These piles are installed by drilling with a continuous-flight auger and pumping grout or concrete as the auger is withdrawn. They can reduce noise and vibration compared with driven piles and can achieve efficient production in suitable soils.

Installation monitoring is essential. Loss of grout pressure, incorrect withdrawal rate, necking, inclusions, reinforcement installation difficulty, and variable ground response can affect performance.

Micropiles

Micropiles are small-diameter drilled and grouted elements with substantial steel reinforcement. They are particularly useful for underpinning, seismic retrofit, restricted access, low headroom, difficult drilling, and sites where low vibration is required.

Their limitations include specialty construction, relatively high unit cost, reliance on grout-to-ground bond, and the need for careful connection and load-test design.

Helical piles

Helical piles are steel shafts with one or more helical plates installed by rotation. They can be useful for light structures, underpinning, uplift resistance, and rapid installation where soil conditions suit the system.

Torque correlations are empirical and product- and soil-dependent. Buckling, corrosion, hard layers, cobbles, installation alignment, minimum embedment, and group effects require evaluation.

The Common Mechanics

For shallow foundations, average service contact pressure begins with:

For a deep foundation in compression, the geotechnical resistance is commonly organized as:

These equations organize load transfer; they do not select the soil parameters, design format, resistance factors, safety factors, or settlement criteria.

A Practical Selection Screen

• Favor shallow foundations when competent ground is accessible, predicted movement is tolerable, excavation is practical, and footing size remains efficient.
• Consider a mat when separate footings overlap, differential movement needs redistribution, or the building layout benefits from one integrated slab.
• Consider deep foundations when weak or compressible deposits are thick, loads are high, scour or uplift is important, or movement criteria cannot be met economically with shallow systems.
• Consider micropiles or other specialty systems when access, vibration, underpinning, or difficult drilling dominates the decision.
• Compare total installed cost and risk, not only concrete or steel quantity.

References And Further Reading

• FHWA GEC 6, Shallow Foundations, FHWA-IF-02-054.
• FHWA GEC 10, Drilled Shafts: Construction Procedures and Design Methods, FHWA-NHI-18-024.
• FHWA GEC 12, Design and Construction of Driven Pile Foundations, FHWA-NHI-16-009 and FHWA-NHI-16-010.
• FHWA Micropile Design and Construction Reference Manual, FHWA-NHI-05-039.
• USACE EM 1110-1-1905, Bearing Capacity of Soils.

FAQ

Is a mat foundation shallow or deep?

A mat is normally classified as a shallow foundation even when it is placed at basement level. Its behavior can still involve a deep zone of influence and meaningful soil-structure interaction.

Are piles always founded on rock?

No. Many piles develop most of their resistance through shaft interaction in soil. Others combine shaft and toe resistance, and some are driven or drilled to rock.

Which foundation type is best?

There is no universal best type. The best project solution satisfies strength, serviceability, durability, construction, schedule, and risk requirements at reasonable whole-project cost.