🏔️Intro to Geotechnical Science Unit 13 – Geotechnical Earthquake Engineering

Key Concepts and Terminology

• Seismic waves energy that propagates through the Earth's crust and interior during an earthquake
• Body waves (P-waves and S-waves) travel through the Earth's interior
  • P-waves (primary waves) compress and expand material in the direction of wave propagation
  • S-waves (secondary waves) cause shearing motion perpendicular to the direction of wave propagation
• Surface waves (Rayleigh waves and Love waves) travel along the Earth's surface
• Seismic hazard probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold
• Peak Ground Acceleration (PGA) maximum ground acceleration that occurred during earthquake shaking at a specific location
• Spectral acceleration (SA) maximum acceleration in an earthquake on an object (building, bridge, etc.) during an earthquake
• Liquefaction phenomenon in which saturated soil loses strength and stiffness in response to applied stress, causing it to behave like a liquid

Seismic Wave Propagation

• Seismic waves generated by sudden slip on a fault or other fracture in the Earth's crust
• Waves propagate outward from the source of the earthquake at varying speeds
• P-waves fastest seismic waves (travel at 5 to 8 km/s) and first to arrive at a given location
• S-waves slower than P-waves (travel at 3 to 5 km/s) and arrive after P-waves
  • Cannot travel through liquids (e.g., Earth's outer core)
• Surface waves slowest seismic waves but cause the most damage due to their large amplitudes and long duration
• Seismic wave propagation affected by the Earth's internal structure and composition
  • Waves refract (bend) and reflect at boundaries between different materials (e.g., crust-mantle boundary)
• Seismic wave attenuation decrease in wave amplitude as waves travel away from the source due to energy dissipation

Soil Behavior During Earthquakes

• Soil response to seismic loading depends on soil type, density, saturation, and confining pressure
• Loose, saturated sands and silts most susceptible to liquefaction during earthquakes
  • Liquefaction can lead to loss of bearing capacity, settlement, and lateral spreading
• Dense, unsaturated soils less likely to liquefy but may experience cyclic softening and degradation of shear strength
• Clays generally less susceptible to liquefaction but can experience cyclic degradation of shear strength
• Strain-softening behavior reduction in soil shear strength with increasing shear strain
  • Can lead to large deformations and instability in slopes and foundations
• Pore water pressure generation during cyclic loading can reduce effective stress and lead to liquefaction
• Soil-structure interaction (SSI) important consideration in earthquake-resistant design
  • SSI effects can modify the seismic response of structures and affect foundation performance

Site Response Analysis

• Site response analysis evaluates the effect of local soil conditions on ground motion characteristics
• Involves modeling the propagation of seismic waves through the soil profile at a specific site
• 1D equivalent linear analysis most common method for site response analysis
  • Assumes soil layers are horizontally stratified and extend infinitely in the horizontal direction
  • Accounts for nonlinear soil behavior using strain-compatible soil properties (shear modulus and damping)
• Input motion applied at the base of the soil profile (bedrock) and propagated upward through the soil layers
• Output includes acceleration time histories and response spectra at the ground surface
• Results used to develop site-specific design ground motions for structures
• 2D and 3D site response analyses may be necessary for complex site conditions (e.g., basins, topographic effects)
• Uncertainties in soil properties and input motions should be considered in site response analysis

Liquefaction and Ground Failure

• Liquefaction occurs when saturated, loose granular soils lose strength and stiffness due to increased pore water pressure during seismic loading
• Liquefaction can lead to various types of ground failure:
  • Flow liquefaction sudden loss of strength leading to large deformations and flow-like behavior
  • Cyclic mobility accumulation of shear strains during cyclic loading, leading to lateral spreading and settlement
• Factors influencing liquefaction susceptibility include soil type, relative density, saturation, and confining pressure
• Liquefaction potential evaluated using in-situ tests (SPT, CPT) and laboratory tests (cyclic triaxial, cyclic simple shear)
• Liquefaction triggering assessed using empirical correlations based on case histories and laboratory tests
• Consequences of liquefaction include bearing capacity failure, settlement, lateral spreading, and sand boils
• Mitigation measures for liquefaction include ground improvement techniques (e.g., vibro-compaction, stone columns) and foundation design (e.g., deep foundations, mat foundations)

Seismic Hazard Assessment

• Seismic hazard assessment quantifies the probability of exceeding a certain level of ground motion at a specific site
• Deterministic Seismic Hazard Analysis (DSHA) considers a single earthquake scenario based on the maximum credible earthquake (MCE)
  • MCE determined from historical seismicity, fault characteristics, and regional tectonics
• Probabilistic Seismic Hazard Analysis (PSHA) accounts for all possible earthquake scenarios and their associated probabilities
  • Incorporates uncertainties in earthquake location, magnitude, and ground motion prediction
• PSHA results expressed as hazard curves, which show the annual probability of exceeding different levels of ground motion
• Deaggregation identifies the relative contributions of different earthquake scenarios to the total seismic hazard at a site
• Seismic hazard maps display the spatial distribution of ground motion parameters (e.g., PGA, SA) for a given probability of exceedance
• Seismic hazard assessment provides input for risk analysis and earthquake-resistant design of structures

Earthquake-Resistant Foundation Design

• Foundation design critical for ensuring the stability and performance of structures during earthquakes
• Shallow foundations (e.g., spread footings, mat foundations) suitable for sites with good soil conditions and low liquefaction potential
  • Must be designed to resist sliding, overturning, and bearing capacity failure during seismic loading
• Deep foundations (e.g., piles, drilled shafts) necessary for sites with poor soil conditions, high liquefaction potential, or heavy structural loads
  • Provide lateral and vertical support, and transfer loads to competent soil or rock layers
• Raft foundations (mat foundations) can be used to distribute loads and reduce differential settlement in soft soils
• Seismic isolation systems (e.g., elastomeric bearings, friction pendulum bearings) can be used to decouple the structure from the ground motion
  • Reduces seismic forces and deformations in the structure
• Energy dissipation devices (e.g., viscous dampers, hysteretic dampers) can be incorporated into the foundation system to absorb seismic energy
• Performance-based design approach considers the desired performance level of the structure and foundation system under different earthquake scenarios
• Soil-foundation-structure interaction (SFSI) analysis may be necessary to capture the coupled response of the soil, foundation, and structure

Practical Applications and Case Studies

• Seismic design codes and guidelines (e.g., IBC, ASCE 7, Eurocode 8) provide requirements for earthquake-resistant design of structures and foundations
• Microzonation studies develop detailed seismic hazard maps for urban areas, considering local soil conditions and site effects
• Seismic retrofit of existing structures involves strengthening foundations and structural elements to improve earthquake resistance
• Liquefaction mitigation techniques (e.g., soil densification, gravel drains, soil mixing) have been successfully implemented in various projects
  • Examples include Port of Oakland (California), Kobe Port (Japan), and Christchurch (New Zealand)
• Seismic isolation and energy dissipation systems have been used in numerous buildings, bridges, and industrial facilities worldwide
  • Notable examples include the San Francisco International Airport Terminal, the Bolu Viaduct (Turkey), and the Christchurch Women's Hospital (New Zealand)
• Failure case studies, such as the Niigata earthquake (1964) and the Kobe earthquake (1995), have provided valuable lessons for liquefaction assessment and mitigation
• The Mexico City earthquake (1985) highlighted the importance of site effects and soil-structure interaction in seismic design
• Recent earthquakes, such as the Chile earthquake (2010) and the Tohoku earthquake (2011), have demonstrated the effectiveness of modern seismic design practices and the challenges posed by extreme events