Foundation Systems: Shallow and Deep Foundations, Selection Criteria, and Soil Response

Evaluating shallow and deep foundation systems, their selection criteria based on soil conditions, load requirements, and environmental factors, and understanding how soil behavior drives foundation design decisions for buildings.

ARE · Project Planning & Design

Overview

Every building rests on a foundation, but choosing the right one is far from automatic. The structural system you select for a project only works if the ground beneath it can handle the loads being transferred. That decision hinges on soil conditions, building loads, environmental hazards, and cost.

Shallow foundations, such as spread footings, continuous wall footings, and slabs on grade, transfer loads to competent soil near the ground surface. Deep foundations, including driven piles, drilled piers, and caissons, bypass weak upper soils and deliver loads to stronger strata or bedrock further down.

On the ARE, you need to evaluate which foundation type fits a given set of project constraints. Soil bearing capacity, frost depth, seismic risk, flood zones, and expansive clays all factor into that evaluation. The wrong choice can mean differential settlement, frost heave damage, or outright structural failure.

Foundation selection is never made in isolation. The architect commissions a geotechnical investigation early in design, reviews the engineer's recommendations, and coordinates with the structural engineer to ensure the foundation system integrates with the building's structural configuration and the constraints of the site.

This topic covers the major foundation types, the soil conditions that drive selection, and the trade-offs architects weigh when coordinating with geotechnical engineers during the planning and design phase.

Essential

Foundations do two things: they transfer building loads into the ground, and they anchor the structure against lateral and uplift forces. How well they accomplish both depends entirely on the soil beneath them. ## Shallow Foundations Shallow foundations rest on competent soil near the surface. They work when the upper soil layers have adequate bearing capacity to support the building without excessive settlement. Spread footings are the most common shallow foundation. A continuous wall footing runs beneath load-bearing walls; an isolated column footing (also called a pad footing) supports individual columns or piers. Footing width depends on the load it carries and the soil's allowable bearing pressure. Standard residential footing widths follow excavation equipment increments: 12, 16, or 24 inches. The minimum footing thickness should not be less than the distance the footing extends outward from the wall face, or 6 inches, whichever is greater.

Professional

An architect working on a three-story mixed-use building in a Midwestern city receives the geotechnical report. The upper 8 feet of soil is a medium-stiff clay with an allowable bearing pressure of 3,000 psf, underlain by dense glacial till at 4,000 psf. The frost depth is 42 inches. The structural engineer recommends spread footings bearing on the clay, with footings sized for the column loads. The footing design is straightforward. The heaviest column carries 120 kips. At 3,000 psf allowable bearing, the required footing area is 120,000 / 3,000 = 40 square feet, yielding a roughly 6.3-foot square footing. The geotechnical engineer confirms settlement will be less than 1 inch. Spread footings win on cost and constructability. Consider a different scenario: a four-story residential building on a coastal site in Zone AE. The upper 15 feet is loose sand susceptible to scour during flooding.

TLDR

Shallow foundations (spread footings, slabs on grade, mats) work when competent soil is near the surface and loads are manageable.
Deep foundations (driven piles, drilled piers, helical piles) bypass weak upper soils to reach stronger strata or bedrock.
Bearing capacity determines footing size: required area = load / allowable bearing pressure.
Frost depth sets the minimum footing depth, ranging from 12 inches (AFI 250) to 60 inches (AFI 4,000). FPSFs allow 12-inch depth with perimeter insulation.
Expansive soils call for drilled piers below the active zone, not shallow footings.
Liquefaction on seismic sites demands deep foundations with grade beams tying individual footings together.
Driven piles: min 8-inch butt diameter, 10-12 inches typical; load transfer via skin friction, end bearing, or both.
IBC Chapter 18 governs foundations; geotechnical investigation required per Section 1803.
Footings over 3 ft thick: min reinforcement ratio 0.0015, No. 4 bars at 12-inch max spacing.
The geotechnical report is the single most important document driving foundation selection.

ELI5

Think of a building like a person standing on a beach. If the sand is firm, you can stand with flat shoes (that is a shallow foundation). If the sand is soft and mushy, you will sink unless you wear stilts that push down to the hard rock below (that is a deep foundation). Shallow foundations sit close to the surface. The most common type is a spread footing, which is like a wide flat shoe that spreads your weight over a bigger area so you do not sink. If you weigh more, you need a bigger shoe. There is also the slab on grade, which is like standing on a giant concrete raft that covers the whole floor. Deep foundations go way down into the earth. Driven piles are long poles (wood, concrete, or steel) hammered deep into the ground until they hit something solid.

AREProject Planning & Design

Foundation Systems: Shallow and Deep Foundations, Selection Criteria, and Soil Response

2 min read213 words

Why Foundation Selection Shapes Everything Above It

Want to track your progress and access more study tools?

Create a free account