9.3 Geotechnical considerations in bridge substructure design

Geotechnical considerations are crucial for bridge substructure design. Soil properties, groundwater conditions, and problematic soils impact foundation choices and stability. Understanding these factors helps engineers select appropriate foundation systems and design safe, durable bridge substructures.

Soil-structure interaction affects how bridges respond to loads and earthquakes. Advanced analysis techniques, like finite element modeling, help optimize designs. Proper geotechnical investigation and interpretation of soil data are essential for selecting the right foundation type and ensuring long-term bridge performance.

Foundation Soil Properties and Substructure Design

Geotechnical Properties and Soil Classification

• Geotechnical properties of soil encompass physical characteristics (particle size distribution, specific gravity) and engineering properties (shear strength, compressibility, permeability) influencing substructure design decisions
• Soil classification systems categorize soils based on engineering properties and behavior
  • Unified Soil Classification System (USCS) groups soils into coarse-grained (gravel, sand) and fine-grained (silt, clay) categories
  • AASHTO system classifies soils primarily for highway construction purposes
• Bearing capacity of soil determines maximum load safely applied to foundation without causing shear failure or excessive settlement
  • Calculated using formulas such as Terzaghi's bearing capacity equation: $q_{ult} = cN_c + qN_q + 0.5γBN_γ$
• Soil compressibility affects potential settlement of bridge substructures, leading to structural damage and serviceability issues if not properly accounted for in design
  • Measured through consolidation tests and expressed as compression index (Cc) or coefficient of volume compressibility (mv)

Problematic Soils and Groundwater Considerations

• Presence of expansive soils, collapsible soils, or liquefiable soils significantly impacts substructure design and requires specialized foundation solutions
  • Expansive soils (montmorillonite clay) swell when wet and shrink when dry, causing foundation movement
  • Collapsible soils (loess) suddenly decrease in volume when saturated, leading to foundation settlement
  • Liquefiable soils (loose saturated sands) lose strength during earthquakes, causing foundation failure
• Groundwater conditions influence soil behavior and must be considered in substructure design
  • Depth of water table affects effective stress and soil strength
  • Seasonal fluctuations in groundwater levels can cause cyclic loading on foundations
  • Presence of artesian pressure may require special dewatering techniques during construction

Geotechnical Investigation Reports for Design

Field and Laboratory Testing Methods

• Geotechnical investigation reports include borehole logs, laboratory test results, and recommendations for foundation design based on site-specific soil conditions
• Standard Penetration Test (SPT) results and Cone Penetration Test (CPT) data estimate soil strength parameters and classify subsurface materials
  • SPT N-value correlates with soil density and strength (N<4 very loose, N>50 very dense for sands)
  • CPT provides continuous soil profile and measures tip resistance, sleeve friction, and pore pressure
• Laboratory testing results provide critical information on soil strength, stress-strain behavior, and compressibility characteristics
  • Triaxial tests measure soil strength parameters (c, φ) under different stress conditions
  • Direct shear tests determine shear strength of soils, particularly for slope stability analysis
  • Consolidation tests assess soil compressibility and predict settlement behavior
• Geophysical investigation methods provide additional insights into subsurface conditions and soil properties
  • Seismic refraction surveys measure seismic wave velocities to determine soil/rock layering
  • Electrical resistivity surveys detect variations in soil conductivity, useful for identifying contamination or groundwater

Interpretation and Application of Geotechnical Data

• Interpretation of geotechnical reports requires understanding of soil mechanics principles and ability to correlate field and laboratory data with engineering design parameters
• Geotechnical recommendations often include allowable bearing capacities, estimated settlements, and suggested foundation types based on site-specific conditions
  • Allowable bearing capacity typically derived by applying factor of safety to ultimate bearing capacity (FS = 2-3 for static loads)
  • Estimated settlements calculated using elastic theory or consolidation theory depending on soil type
• Incorporation of geotechnical findings into design process involves selecting appropriate foundation systems, determining foundation depths, and establishing design criteria for substructure elements
  • Foundation selection based on soil strength, compressibility, and depth to bedrock
  • Foundation depth determined by considering frost depth, scour potential, and bearing capacity requirements
  • Design criteria may include maximum allowable settlement, minimum factor of safety against bearing capacity failure, and seismic performance requirements

Soil-Structure Interaction in Bridge Substructures

Static and Dynamic Soil-Structure Interaction

• Soil-structure interaction (SSI) refers to interdependent behavior of foundation soil and bridge substructure elements under static and dynamic loading conditions
• Stiffness and strength characteristics of foundation soil influence distribution of forces and moments within substructure elements, affecting their design and performance
  • Soil springs used in structural models to represent soil-structure interaction (Winkler foundation model)
• Dynamic soil-structure interaction significantly alters response of bridge system to earthquake ground motions
  • Period elongation due to soil flexibility can increase or decrease seismic forces on structure
  • Radiation damping in soil can reduce structural response in some cases
• Lateral earth pressures acting on retaining walls and abutments influenced by soil-structure interaction effects
  • Wall flexibility affects magnitude and distribution of earth pressures (active, at-rest, passive conditions)
  • Soil-wall friction reduces lateral earth pressures and increases wall stability

Advanced Analysis and Special Considerations

• Numerical modeling techniques assess complex soil-structure interaction problems and optimize substructure design
  • Finite element analysis allows for detailed modeling of soil-structure system
  • Time-history analysis captures nonlinear behavior during seismic events
• Potential for scour around bridge piers and abutments impacts long-term stability and performance of substructure
  • Local scour depth estimated using empirical equations (HEC-18 methodology)
  • Countermeasures include riprap protection, sheet pile cutoffs, or deep foundations
• Effects of cyclic loading on soil behavior and potential for soil degradation or liquefaction considered in evaluation of soil-structure interaction
  • Cyclic stress ratio (CSR) compared to cyclic resistance ratio (CRR) to assess liquefaction potential
  • Degradation of soil stiffness and strength under repeated loading can lead to increased deformations and reduced capacity

Foundation Systems and Substructure Design

Shallow and Deep Foundation Design

• Selection of foundation type (shallow or deep) based on soil conditions, structural loads, and performance requirements
  • Shallow foundations (spread footings, mat foundations) suitable for competent soils near surface
  • Deep foundations (piles, drilled shafts) used for weak soils or high loads
• Shallow foundation design involves determining footing dimensions, embedment depth, and reinforcement requirements
  • Footing size based on allowable bearing capacity and structural loads
  • Embedment depth considers frost penetration, scour potential, and bearing capacity
  • Reinforcement designed for flexure, shear, and punching shear (ACI 318 requirements)
• Deep foundation design includes selection of pile/shaft type, determination of length and capacity, and consideration of group effects
  • Pile types include driven piles (steel H-piles, concrete piles) and cast-in-place piles
  • Pile capacity estimated using static analysis methods (α-method, β-method, λ-method)
  • Group effects accounted for using efficiency factors or detailed analysis (e.g., GROUP software)
• Load transfer mechanisms in deep foundations evaluated to ensure adequate capacity and performance
  • Skin friction developed along pile shaft through soil-pile adhesion
  • End bearing capacity at pile tip dependent on soil strength and pile diameter

Settlement Analysis and Specialized Design Considerations

• Settlement analysis for immediate and long-term (consolidation) settlement crucial for ensuring serviceability and structural integrity
  • Immediate settlement calculated using elastic theory (Schmertmann method for sands)
  • Consolidation settlement estimated using one-dimensional consolidation theory (Terzaghi's theory)
• Slope stability analysis essential for designing bridge abutments and approach embankments
  • Limit equilibrium methods (Bishop's method, Morgenstern-Price method) used to calculate factor of safety
  • Seismic slope stability assessed using pseudo-static analysis or Newmark sliding block method
• Design of retaining structures accounts for lateral earth pressures, hydrostatic pressures, and potential surcharge loads
  • Lateral earth pressure theories (Rankine, Coulomb) used to calculate pressures on walls
  • Stability checks include overturning, sliding, and bearing capacity
• Consideration of construction methods and their impact on surrounding soil and existing structures
  • Vibrations from pile driving may cause settlement of adjacent structures
  • Excavation support systems (sheet piles, soldier piles) required for deep foundations in urban areas
  • Dewatering techniques (wellpoints, deep wells) used to control groundwater during construction