Foundation Systems

The foundation is the structural interface between the building and the ground. It transfers all loads — gravity, wind, seismic — from the superstructure (the building above ground) into the soil or rock below. A foundation must do this without exceeding the soil's bearing capacity, without causing excessive settlement, and without allowing the building to slide or overturn.

This lesson covers the major types of foundation systems, from simple spread footings for houses to deep pile foundations for high-rise buildings, the factors that govern foundation selection, and the construction considerations for each type.

Shallow foundations transfer loads to the soil near the ground surface. They are used when competent soil (with adequate bearing capacity) exists at or near the surface. The depth of a shallow foundation is typically less than or equal to its width.

Spread Footings (Isolated Footings)

A spread footing is a pad of reinforced concrete, wider than the column it supports, that spreads the column load over a sufficient area so the soil bearing pressure does not exceed the allowable bearing capacity.

Design principle: If a column load is P (pounds) and the allowable bearing capacity is qa (psf), the required footing area is:

A = P / qa

Example: A column carries 200,000 lbs (200 kips). The allowable bearing capacity is 4,000 psf. Required footing area = 200,000 / 4,000 = 50 sq ft → a 7'-2" × 7'-2" square footing (rounding up to the next 2 inches).

Spread footings are typically:

• Square for interior columns
• Rectangular when space constraints prevent a square footing
• 12–36 inches thick, depending on the load and soil conditions
• Reinforced with two layers of rebar (each way) in the bottom

Construction considerations:

• The bottom of the footing must bear on undisturbed natural soil or properly compacted fill
• The excavation must be clean — no loose soil, water, mud, or debris at the bearing surface
• Rebar must be properly placed on chairs (supports) to maintain the correct cover (distance from concrete surface to rebar)
• Concrete must meet the specified strength and be properly placed and cured
• Footings must be placed below the frost depth to prevent frost heave

Continuous (Strip) Footings

A continuous footing is a long, narrow footing that supports a bearing wall or a line of closely spaced columns. Residential foundation walls and basement walls typically sit on continuous footings. A typical residential continuous footing is:

• 16–24 inches wide (twice the wall width is common)
• 8–12 inches thick
• Reinforced with longitudinal rebar (2 bars minimum) to span across soft spots

Combined Footings

A combined footing supports two or more columns on a single footing. Used when columns are too close together for individual spread footings, or when an exterior column is at the property line and a spread footing would extend beyond the property boundary. The combined footing is sized and reinforced to keep the soil pressure uniform.

Mat (Raft) Foundations

A mat foundation is a large, thick concrete slab that supports the entire building — all columns and walls bear on a single continuous footing. Mat foundations are used when:

• Individual spread footings would be so large that they overlap or nearly overlap (when column loads are heavy relative to bearing capacity)
• The soil has low bearing capacity but a mat can spread the load over the entire building footprint
• Differential settlement between columns must be minimized (the mat's stiffness equalizes settlement)
• Hydrostatic uplift (buoyancy from high water table) must be resisted (the building's weight on the mat must exceed the uplift force)

Mat foundations are typically 3–8 feet thick, heavily reinforced with two layers of rebar each way (top and bottom), and require large volumes of concrete. They are common for high-rise buildings, industrial facilities, and any building on poor soil where deep foundations are not economically justified.

Deep foundations transfer loads through weak or compressible surface soils to a strong bearing layer (dense sand, stiff clay, or rock) at depth, or they develop sufficient friction along their embedded length to support the load. Deep foundations are used when:

• Surface soils are too weak or compressible for shallow foundations
• Loads are very heavy (high-rise buildings, bridges)
• Scour, erosion, or undermining could remove soil from around shallow foundations
• Expansive soils require isolation of the structure from surface soil movements

Driven Piles

Driven piles are prefabricated structural elements (steel, concrete, or timber) driven into the ground by a pile driver — a machine that repeatedly drops a heavy weight (hammer) on the pile head, forcing it into the soil. The pile is driven until it reaches a specified penetration depth or achieves a specified resistance (blow count per inch of penetration, called the pile driving criteria).

Types of driven piles:

• Steel H-piles: Wide-flange H-shaped steel sections. Very strong, can be driven to great depths (100+ feet) and through dense soils. Can be spliced with full-penetration welds for very long piles. Most common in commercial and heavy construction.
• Steel pipe piles: Hollow or concrete-filled steel pipes. Available in large diameters (12–48 inches). Can be driven open-ended (soil enters the pipe) or closed-ended (with a plate or conical tip).
• Precast concrete piles: Reinforced or prestressed concrete piles, typically square or octagonal in cross-section, 10–24 inches across. Very durable in marine and corrosive environments.
• Timber piles: Round, pressure-treated wood logs, typically 30–65 feet long. The most economical pile type for light to moderate loads. Limited by length and load capacity.

Pile capacity comes from two sources:

• End bearing: The pile tip rests on a hard layer (rock, dense sand) and transfers load directly through the tip. End-bearing piles behave like columns resting on a strong foundation.
• Skin friction: Friction between the pile surface and the surrounding soil resists the downward load. Friction piles develop their capacity from the accumulated friction along their length. Piles in clay rely heavily on skin friction.

Most piles develop capacity from both end bearing and skin friction.

Drilled Shafts (Caissons)

Drilled shafts (also called caissons, drilled piers, or bored piles) are constructed by drilling a hole in the ground, placing a reinforcing steel cage, and filling the hole with concrete. They differ from driven piles in that they are constructed in place rather than prefabricated and driven.

Advantages of drilled shafts:

• Can be constructed to very large diameters (3–12 feet), supporting very heavy loads on a single shaft rather than a group of piles
• No vibration or noise from driving (important in urban areas near existing structures)
• The drilled hole can be inspected before concrete placement — the engineer can verify bearing on the intended soil or rock layer
• The bottom can be enlarged (belled) to increase end-bearing area

Disadvantages:

• Require specialized drilling equipment
• Quality depends on construction technique (maintaining hole stability, proper concrete placement)
• Cannot be easily driven through boulders or other obstructions

Construction considerations:

• Casing (steel tubes) may be needed to keep the hole open in unstable soils
• Slurry (bentonite or polymer) may be used to stabilize the hole walls in soft or water-bearing soils
• Concrete must be placed using a tremie (a pipe lowered to the bottom of the hole) when water or slurry is present to prevent segregation
• The reinforcing cage must be properly positioned and supported to maintain correct concrete cover

Helical Piles (Screw Piles)

Helical piles are steel shafts with one or more helical (screw-shaped) plates welded near the tip. They are screwed into the ground using a hydraulic torque motor. The relationship between installation torque and load capacity is well-established, allowing the pile to be "proof-tested" during installation.

Helical piles are commonly used for:

• Residential foundations (especially in areas with expansive soils or high water tables)
• Foundation repair (underpinning existing structures)
• Light commercial structures
• Temporary structures and equipment foundations

Advantages: Fast installation, no spoil (excavated soil), immediate loading, removable.

The choice of foundation system depends on:

Factor	Influence on Selection
Soil conditions (type, strength, depth to competent layer)	Primary factor — determines if shallow or deep foundations are needed
Building loads (magnitude, type)	Heavier loads may require deep foundations or mat foundations
Groundwater level	High water tables complicate excavation and may require dewatering or deep foundations
Frost depth	All foundations must extend below frost depth
Adjacent structures	Vibration from pile driving can damage nearby buildings; drilled shafts may be preferred
Site access	Large equipment (pile drivers, drill rigs) requires adequate access
Cost	Shallow foundations are least expensive when soil conditions permit
Settlement tolerance	Sensitive structures may require deep foundations or mats to limit settlement
Seismic requirements	Seismic zones may require deeper foundations or special detailing
Construction schedule	Driven piles and helical piles are faster than drilled shafts

After completing this module, you should be able to:

• Identify the types of loads acting on structures (dead, live, wind, seismic, snow, impact)
• Explain the concept of load path from roof to foundation
• Describe the difference between tension, compression, shear, bending, and torsion
• Identify basic structural elements (beams, columns, walls, trusses, foundations)
• Explain the function of lateral force resisting systems (bracing, shear walls, moment frames)
• Perform simple load calculations for tributary areas
• Describe common foundation types and their applications

Previous: Module 3: Soils & Earthwork Fundamentals Next: Module 5: Residential Construction Methods