Foundation Types: Shallow Footings, Mats, and Deep Pile Systems

Every structural load a building generates must ultimately be transferred into the earth. The foundation is the interface between the superstructure and the ground, and its design is governed not by the building alone but by the capacity and deformation characteristics of the soil or rock it sits on. Choosing the wrong foundation type can result in excessive settlement, differential movement that cracks finishes and frames, or outright bearing failure. The geotechnical engineer who investigates and characterizes the site is an essential partner in every foundation decision.

Soil Bearing Capacity and Allowable Bearing Pressure

The fundamental question in shallow foundation design is whether the soil beneath the footing can support the imposed pressure without shear failure or excessive settlement. Ultimate bearing capacity is the pressure at which the soil fails in shear; it is calculated using the Terzaghi or Meyerhof bearing capacity equations, which incorporate soil cohesion (c), friction angle (phi), and foundation geometry factors. For a square footing on a cohesionless sand, the simplified Terzaghi expression is:

q_ult = 1.3 c N_c + q N_q + 0.4 γ B N_γ

where q is the overburden pressure at footing depth, γ is the soil unit weight, B is the footing width, and N_c, N_q, N_γ are bearing capacity factors dependent on friction angle.

Allowable bearing pressure is the ultimate capacity divided by a factor of safety, typically 2.5 to 3.0, and further limited by settlement considerations. For many urban sites, settlement limits (not shear failure) control the allowable bearing pressure. Common allowable pressures range from 1,500 to 2,000 psf (72 to 96 kPa) for loose to medium sands, 2,000 to 4,000 psf for stiff clays, and 5,000 to 15,000 psf or higher for dense gravels and rock. The geotechnical report specifies the design value the structural engineer uses to size footings.

Spread Footings, Combined Footings, and Wall Footings

When soil has adequate bearing capacity within a few feet of the surface, shallow foundations are economical. The most common type is the isolated spread footing: a reinforced concrete pad beneath a single column. Footing area is set by dividing the column load by the allowable bearing pressure. For a 200-kip column on soil with 3,000 psf allowable bearing, the required area is roughly 67 square feet, yielding a footing of approximately 8.2 feet square.

Footing depth must satisfy two requirements beyond bearing: first, the bottom of the footing must be below the frost depth for the site. In the upper Midwest this may be 42 to 48 inches; in mild coastal climates, 12 to 18 inches suffices. IBC Table 1809.5 and local amendments govern. Second, the depth must be adequate to resist punching shear around the column and one-way shear across the footing, both calculated per ACI 318-19 Chapter 22.

Wall footings (continuous strip footings) support load-bearing walls. They are designed per linear foot of wall, with width sized by bearing pressure and reinforcement sized to resist the bending caused by soil reaction acting as an upward load on a cantilever projecting from each side of the wall.

Combined footings support two or more columns on a single footing when columns are close together or when an exterior column is so near the property line that a symmetric spread footing cannot be used. A strap footing (connected footing) uses a grade beam to link an eccentric exterior footing to an interior footing, balancing the moment caused by eccentricity without widening the exterior footing into restricted space.

Mat Foundations: When the Whole Floor Becomes a Footing

When soil bearing capacity is low relative to column loads, or when individual spread footings would overlap or cover more than about 50 percent of the building footprint, a mat (raft) foundation becomes economical. A mat is a continuous reinforced concrete slab, typically 3 to 8 feet thick for heavy buildings, that covers the entire building footprint. By spreading the total building load over the full area, it reduces the bearing pressure on the soil to levels a weaker stratum can support.

Mat design uses a soil-structure interaction model. In the simplified Winkler approach, the soil is represented by a series of independent springs with stiffness equal to the modulus of subgrade reaction (k_s, in units of force per length cubed). The mat is analyzed as a plate on an elastic foundation. For more accurate results, finite element models with soil modeled as volume elements or coupled springs are used when differential settlement must be tightly controlled, as in semiconductor fabrication plants, server farms, or precision measurement facilities.

Perimeter mat edges require additional attention: the soil contact pressure concentrates at edges and corners of mats on cohesive soils, while sand mats see the opposite distribution (higher pressure at center). This affects both bearing check and reinforcement design.

Deep Foundations: Piles and Drilled Shafts

When competent bearing strata lie too deep for shallow foundations to reach economically, deep foundations transfer load to stronger material at depth. The two dominant deep foundation types are driven piles and drilled shafts (caissons).

End-bearing piles derive most of their capacity from tip resistance on a dense bearing stratum: dense sand, gravel, or rock. The pile tip stress at ultimate capacity can be estimated using Meyerhof's formula or, more commonly today, from CPT (cone penetration test) correlations. Precast concrete piles, H-piles (HP shapes per ASTM A572 Gr. 50 or A36), and steel pipe piles are all commonly used as end-bearing elements.

Friction piles develop capacity along their shaft through side friction (skin friction) in cohesive soils or interface friction in cohesionless soils. In soft to medium clays, the alpha method estimates unit skin friction as a fraction of undrained shear strength (s_u). In sands, the beta method uses an effective stress approach. Many piles combine both mechanisms: some shaft friction plus end bearing.

Pile capacity must be verified. Static load tests (ASTM D1143) apply known loads to a sacrificial test pile and measure settlement. Dynamic load testing (ASTM D4945) using a Pile Driving Analyzer (PDA) correlates hammer blow count to capacity during driving and can be supplemented with CAPWAP signal matching. Engineers typically specify one static load test per project for major pile programs, with dynamic testing used for QA on production piles.

Drilled shafts (bored piles or caissons) are formed by drilling a hole and casting concrete in place, often with a steel reinforcing cage. They can reach very large diameters (3 to 10 feet or more) and depths exceeding 100 feet, making them suited for bridge abutments, transmission towers, and high-rise building cores. Drilled shafts in rock develop extremely high end-bearing capacity; a shaft bearing on hard limestone or granite may carry 10,000 kips or more on a single element.

Pile groups and group efficiency: Individual piles are almost always arranged in groups beneath pile caps. When piles are spaced too closely (less than about three pile diameters center to center), the stress bulbs of adjacent piles overlap and the group capacity is less than the sum of individual capacities. The group efficiency factor eta accounts for this reduction and must be computed for both bearing and settlement analysis.

Settlement Limits and Geotechnical Reports

ASCE 7-22 and IBC 2021 require that building foundations be designed to limit differential settlement to levels compatible with the structural framing system. Steel moment frames are relatively flexible and can tolerate angular distortion (differential settlement divided by span) of about 1/300 to 1/500. Brittle masonry facades may require limits of 1/600 or tighter.

The geotechnical investigation report is the foundation engineer's primary source document. A complete report includes boring logs with SPT (Standard Penetration Test) N-values or CPT tip resistance profiles, laboratory test results for index properties and shear strength, groundwater conditions, seasonal fluctuation estimates, and design recommendations covering allowable bearing, settlement estimates, pile capacity, and any special hazards such as liquefaction potential.

Liquefaction occurs when saturated loose sands temporarily lose effective stress and behave like a viscous liquid under earthquake shaking. Shallow foundations on liquefiable soils can experience catastrophic bearing failure. Mitigation strategies include ground improvement (vibro-densification, deep soil mixing), deep foundations extending through the liquefiable layer to stable bearing strata, or site-specific risk assessment where the liquefaction hazard is limited. ASCE 7-22 Chapter 11 and ASCE 7-22 Chapter 12 together with local geologic maps identify sites where liquefaction investigation is required.