Bridge foundations are crucial for structural stability and load distribution. This section covers shallow and deep foundation types, including spread footings, piles, drilled shafts, and caissons. Each type has unique applications based on soil conditions and bridge requirements.
Selecting the right foundation involves considering geotechnical factors, environmental impact, and construction feasibility. We'll explore load transfer mechanisms, capacity calculations, and group effects to understand how different foundations support bridge structures effectively.
Bridge Foundation Types
Shallow vs Deep Foundations
- Bridge foundations categorized into shallow and deep foundations based on soil conditions and structural requirements
- Shallow foundations used when competent soil found at relatively shallow depths
- Include spread footings and mat foundations
- Deep foundations employed when suitable bearing strata located at greater depths or significant lateral loads present
- Encompass pile foundations, drilled shafts, and caissons
Pile Foundation Types
- Pile foundations classified into driven piles and cast-in-place piles
- Driven piles include concrete, steel, or timber piles
- Offer rapid installation and immediate load-bearing capacity
- May face limitations in urban areas (noise and vibration concerns)
- Cast-in-place piles provide flexibility in length and integration with pile caps
- Potential concrete integrity issues if not properly executed
Drilled Shafts and Caissons
- Drilled shafts (bored piles) large-diameter cylindrical foundations reaching great depths
- Suitable for various soil and rock conditions
- Offer versatility but may face challenges in water-bearing soils
- Caissons used for deep foundations in water-bearing soils or water-crossing bridges
- Types include open caissons and pneumatic caissons
- Excel in water-crossing applications but complex and expensive to construct
Foundation Selection Factors
Geotechnical and Environmental Considerations
- Geotechnical properties of site heavily influence foundation type selection
- Soil strength, stratification, and groundwater conditions
- Environmental impact and regulatory requirements may restrict certain foundation types
- Especially in sensitive ecosystems or protected areas
- Presence of existing structures, utilities, or infrastructure may limit options
- May require special construction techniques (underpinning, micropiles)
Structural and Construction Factors
- Structural loads from bridge superstructure and environmental factors considered
- Vertical and lateral loads (dead load, live load, wind load, seismic load)
- Site accessibility and available construction equipment influence feasibility
- Remote locations may limit options (barge-mounted equipment, helicopter transport)
- Project budget and timeline constraints affect foundation choice
- Balance cost-effectiveness with long-term performance
- Consider equipment mobilization costs and construction duration
Long-term Considerations
- Expected lifespan of bridge structure factored into foundation selection
- Design life (typically 75-100 years for major bridges)
- Long-term maintenance considerations evaluated
- Accessibility for inspections and repairs (cofferdams for underwater foundations)
- Potential for future expansion or modification of the bridge
- Foundations designed to accommodate possible widening or strengthening
Foundation Advantages vs Disadvantages
Shallow Foundations
- Spread footings offer simplicity in construction and lower costs
- Limited to sites with competent soil at shallow depths
- Restricted load-bearing capacity compared to deep foundations
- Mat foundations distribute loads over large area, reducing differential settlement
- Cost-prohibitive for smaller structures
- Require extensive excavation and dewatering in some cases
Deep Foundations
- Pile foundations provide high load-bearing capacity and reach deep, competent strata
- May require specialized equipment (pile drivers, cranes)
- Costly for deep installations (offshore platforms, long-span bridges)
- Drilled shafts offer versatility in various soil conditions and achieve high capacities
- Challenges in water-bearing soils (slurry techniques, casing)
- Require careful quality control during construction (concrete placement, rebar cage alignment)
- Caissons excel in water-crossing applications and reach great depths
- Complex to construct and typically more expensive than other foundation types
- Require specialized workforce and equipment (compressed air work, tremie concrete)
Load Transfer Mechanisms and Capacity
Shallow Foundation Mechanics
- Shallow foundations transfer loads primarily through base resistance
- Rely on bearing capacity of soil directly beneath foundation
- Bearing capacity of spread footings calculated using classical theories
- Terzaghi's method: qult=cNc+qNq+0.5γBNγ
- Meyerhof's method: incorporates shape and depth factors
- Factors affecting shallow foundation capacity
- Soil shear strength parameters (cohesion, friction angle)
- Foundation geometry (width, depth, shape)
- Groundwater conditions
Deep Foundation Mechanics
- Deep foundations transfer loads through combination of skin friction and end-bearing
- Relative contribution varies based on soil conditions and foundation geometry
- Pile foundations derive capacity from:
- Skin friction in cohesive soils (clay, silt)
- Combination of skin friction and end-bearing in granular soils (sand, gravel)
- Capacity estimation methods for piles:
- Static analysis (α-method, β-method, λ-method)
- Dynamic analysis (wave equation analysis, dynamic load testing)
- Load testing (static load test, rapid load test, statnamic test)
- Drilled shafts rely more heavily on end-bearing in rock formations
- Skin friction significant in soil deposits
- O'Neill and Reese method for capacity estimation in soils
- Caisson load transfer similar to large-diameter drilled shafts
- Added complexity of dealing with water pressure during construction
Group Effects and Efficiency
- Group effects considered when multiple piles or shafts used
- Capacity of group not simple sum of individual capacities
- Overlapping stress zones reduce efficiency
- Group efficiency factors applied to account for interaction
- Converse-Labarre formula: η=1−90°θ(mnm(n−1)+n(m−1))
- Where η efficiency factor, θ pile spacing angle, m rows, n columns
- Minimum pile spacing typically 2.5-3 times pile diameter
- Balances group efficiency with practical construction considerations