🏔️Intro to Geotechnical Science Unit 13 – Geotechnical Earthquake Engineering
Geotechnical Earthquake Engineering explores how seismic waves affect soil behavior and structures. It covers key concepts like liquefaction, site response analysis, and seismic hazard assessment. Understanding these principles is crucial for designing earthquake-resistant foundations and mitigating potential ground failures.
This field combines geology, seismology, and civil engineering to improve building safety in earthquake-prone areas. By studying soil mechanics, wave propagation, and structural dynamics, engineers can develop better strategies for earthquake-resistant design and retrofit existing structures to withstand seismic forces.
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