Bridge Foundation Types

Why This Matters

Every bridge you'll analyze in this course ultimately transfers its loads to the ground—and how that transfer happens determines whether a structure stands for centuries or fails catastrophically. Foundation selection isn't just about soil mechanics; it's about understanding the interplay between load magnitude, soil conditions, water presence, and construction constraints. You're being tested on your ability to match foundation types to site conditions and explain why one solution works where another would fail.

The concepts here connect directly to geotechnical analysis, structural load paths, and construction methodology—all core exam topics. When you encounter foundation problems, think in terms of load transfer mechanisms, depth requirements, and site-specific challenges. Don't just memorize that drilled shafts go deep; know when you'd choose them over driven piles and why that distinction matters for bridge performance.

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Shallow Foundation Systems

When competent soil exists near the surface, engineers can distribute loads across a wide area without drilling deep. The key principle: increase bearing area to reduce contact pressure below the soil's allowable capacity.

Spread Footings

• Distributes structural loads over a large area—reduces soil pressure to prevent bearing capacity failure and excessive settlement
• Constructed from reinforced concrete with dimensions calculated using $q = \frac{P}{A}$ where allowable soil pressure governs design
• Most cost-effective option when stable soil exists within 3-5 feet of the surface, making them the default choice for favorable sites

Shallow Foundations (General Category)

• Located near ground surface—typically defined as foundations where depth $D_f$ is less than or equal to foundation width $B$
• Includes spread footings and mat foundations, selected based on column spacing and load distribution requirements
• Faster construction timeline and lower costs compared to deep alternatives, but limited to sites with adequate bearing capacity near surface

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Deep Foundation Systems

When surface soils lack adequate bearing capacity or when structures must resist significant lateral forces, foundations extend to competent strata below. The mechanism: transfer loads through shaft friction, end bearing, or both to reach stable soil or bedrock.

Pile Foundations

• Transfers loads to deeper stable strata through long, slender structural elements—either by end bearing on rock or friction along the shaft
• Two installation methods: driven piles (displacement) or drilled piles (replacement), selected based on soil type and vibration restrictions
• Essential for weak or compressible surface soils where shallow foundations would experience unacceptable settlement or bearing failure

Drilled Shafts (Caissons)

• Large-diameter concrete shafts (typically 2-12 feet) drilled and cast-in-place to reach bedrock or stable bearing strata
• Superior lateral load resistance due to large cross-section and moment capacity—critical for bridge piers subject to wind, seismic, or vessel impact forces
• Preferred for heavy concentrated loads where a single drilled shaft can replace a group of driven piles, simplifying the pile cap design

Micropile Foundations

• Small-diameter piles (typically 5-12 inches) grouted into drilled holes, transferring load primarily through grout-to-ground bond
• Minimal vibration and disturbance during installation—ideal for sites near existing structures or with limited overhead clearance
• Primary application in retrofit projects where existing foundations need strengthening or where access constraints prevent conventional equipment

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Bridge Substructure Elements

Piers and abutments aren't foundation types per se—they're the structural elements that connect the superstructure to the foundation system below. Understanding their function clarifies why different foundations suit different locations.

Pier Foundations

• Vertical intermediate supports that carry superstructure loads at points between abutments—each pier requires its own foundation system
• Must resist both vertical and lateral loads including dead load, live load, wind, stream flow, ice, and potential vessel collision
• Foundation type varies by location: spread footings on land with good soil, deep foundations in water or poor soil conditions

Abutment Foundations

• End supports that also retain approach embankment soil—must resist horizontal earth pressure in addition to vertical bridge loads
• Designed for combined loading: vertical reactions from superstructure plus lateral forces from retained soil and live load surcharge
• Often integrated with wing walls that extend laterally to contain the embankment and prevent erosion at the bridge ends

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Water Crossing Construction Methods

Building foundations in rivers, lakes, or marine environments requires specialized techniques to manage water during construction. The challenge: create dry conditions at depth while maintaining stability against hydrostatic pressure.

Cofferdam Foundations

• Temporary watertight enclosures constructed in water to allow foundation work in dry conditions—removed after permanent foundation is complete
• Types include sheet pile, cellular, and earth cofferdams, selected based on water depth, soil conditions, and required work area
• Critical for bridge piers in rivers where dewatering allows conventional concrete placement and inspection of bearing surfaces

Pneumatic Caissons

• Pressurized work chambers where air pressure equals water pressure to exclude groundwater during deep excavation below water level
• Workers excavate inside the chamber while the caisson sinks under its own weight, allowing construction to significant depths (historically 100+ feet)
• Largely replaced by modern drilled shaft techniques but still relevant for understanding historical bridges and specialized applications

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Classification by Depth

Foundation classification often comes down to a fundamental question: can we bear on surface soils, or must we go deep? This binary decision drives cost, schedule, and construction methodology.

Deep Foundations (Category Overview)

• Defined by load transfer to strata well below the surface—includes piles, drilled shafts, caissons, and micropiles as specific systems
• Required when surface soils have inadequate bearing capacity, high compressibility, or susceptibility to scour that would undermine shallow foundations
• Selection among deep foundation types depends on load magnitude, soil profile, equipment access, and environmental constraints

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Quick Reference Table

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Self-Check Questions

1. A bridge site has soft clay extending 40 feet below grade with dense sand beneath. Which two foundation types would be most appropriate, and what load transfer mechanism would each use?

2. Compare the functions of pier foundations versus abutment foundations—why does the abutment's dual role affect foundation design?

3. An FRQ describes a river crossing where the contractor must construct pier foundations in 15 feet of water. What temporary construction method would you specify, and what permanent foundation type might it facilitate?

4. When would you select micropiles over conventional driven piles? Identify at least two site conditions that favor micropile installation.

5. A designer must choose between a group of driven piles and a single drilled shaft for a bridge pier in a seismic zone. What advantages does the drilled shaft offer for lateral load resistance, and how does this relate to the shaft's geometry?