ESP 179- Winter 2012 Geology and Soils January 24, 2012 Instructor: Trevor Macenski
Outline Review CEQA Checklist Questions Seismic Hazards in CA Alquist-Priolo Earthquake Fault Zoning Act Seismic Hazards Mapping Act Common Geologic Hazards Impact Analysis Approach Sample Discussion and Analysis
Geology and Soils Appendix G Checklist: Geology and Soils Expose people or structures to substantial adverse effects Alquist-Priolo Fault Zone Seismic ground Shaking Ground failure/Liquefaction Landslides Loss of top soil Lateral Spreading Expansive Soils Soils and Septic System
Appendix G: Checklist Questions Projects that have large
Seismic Hazards Pose a substantial danger to property and human safety. Seismic hazards present in California include: Ground rupture along faults Strong seismic shaking Liquefaction, Lateral Spreading, Lurching Landsliding Slope failure Hazardous Minerals Tunamis
Fault Rupture Fault rupture is a seismic hazard that affects structures sited above an active fault. The hazard from fault rupture is the movement of the ground surface along a fault during an earthquake. Creep= Slower moving fault action Earthquakes- Strike-slip Normal ThrustInfrastructure often can’t handle Creep or Earthquakes
Ground Shaking Ground shaking depends on several variables: Earthquake magnitude Epicenter distance Local geology and Soils Thickness Seismic wave-propagation properties of unconsolidated Materials Groundwater conditions Topographic setting Sample Modeling
Richter vs. MMI Richter Scale Measurement comparison between earthquakes Evaluates magnitude used for scientific comparison Only accounts for the actual slip that is generated Actual damage= Propagation of seismic or ground waves as a result of initial failure Loose materials tend to amplify ground waves, while hard rock can quickly attenuate them, causing little damage to overlying structures.
Richter vs. MMI Modified Mercalli Intensity (MMI) Scale provides a useful qualitative assessment of ground shaking. The MMI Scale is a 12-point scale of earthquake intensity: Local effects experienced by people Structures Earth materials. Each succeeding step on the scale describes a progressively greater amount of damage at a given point of observation.
Alquist-Priolo EarthquakeFault Zoning Act Passed in 1972 Intent: to mitigate the hazard of surface faulting to structures for human occupancy Purpose: Direct result of the 1971 San Fernando Earthquake Extensive surface fault ruptures that damaged numerous homes, commercial buildings, and other structures.
Alquist-Priolo Cont. Requires the State Geologist to establish regulatory zones (known as Earthquake Fault Zones) around the surface traces of active faults and to issue appropriate maps. The maps are distributed to all affected cities, counties, and state agencies for their use in planning and controlling new or renewed construction. If an active fault is found, a structure for human occupancy cannot be placed over the trace of the fault and must be set back from the fault (generally 50 feet). Project will require site specific geotech mitigation
Seismic Hazards Mapping Act Passed in 1990 Intent: Directs the Department of Conservation, California Geological Survey to identify and map areas prone to liquefaction, earthquake-induced landslides and amplified ground shaking. Purpose: The SHMA was passed by the legislature following the 1989 Loma Prieta earthquake.
SHMA Cont. Geologists in the Seismic Hazard Zonation Program gather existing geological, geophysical and geotechnical: Produce the Seismic Hazard Zone Maps. Designate as Zones of Required Investigation (ZORI) Prone to liquefaction and earthquake–induced landslides. The SHMA requires site-specific geotechnical investigations within ZORI’s To identify and evaluate seismic hazards and formulate mitigation measures prior to permitting
Liquefaction Liquefaction is a process by which sediments below the water table temporarily lose strength during an earthquake and behave as a viscous liquid rather than a solid. Liquefaction is restricted to certain geologic and hydrologic environments, primarily recently deposited sand and silt in areas with high groundwater levels. The process of liquefaction involves seismic waves passing through saturated granular layers, distorting the granular structure and causing the particles to collapse. This causes the granular layer to behave temporarily as a viscous liquid rather than a solid, resulting in liquefaction. USGS has maps to identify areas: MAP
Lateral Spreading and Lurching Lateral spreading is lateral ground movement, with some vertical component, as a result of liquefaction. The soil rides on top of the liquefied layer Lurching is the movement of the ground surface toward an open face when the soil liquefies. An open face could be a graded slope, stream bank, canal face, gully, or other similar feature.
Landslide Hazard Mapping Act Produced from 1986 through 1995, as directed by the now-repealed Landslide Hazard Mapping Act. Online Resource: MAPs
Landslides Categorized as either 1) rock, 2) soil, or 3) rock and soil. Rock= refers to hard or firm bedrock that was intact and in place prior to slope movement. Soil=means unconsolidated particles or poorly cemented rock or aggregates. Soil is also distinguished further on the basis of texture: Debris (coarse fragments) Earth (fine fragments)
Landslide Classification 1) Falls: Masses of soil or rock that dislodge from steep slopes and free-fall, bounce, or roll downslope. 2) Topples: Move by the forward pivoting of a mass around an axis below the displaced mass. 3) Spreads: Commonly induced by liquefaction of material in an earthquake, move by horizontal extension and shear or tensile fractures. 4) Slides: Displace masses of material along one or more discrete planes. Rotational: Sliding the slide plane is curved and the mass rotates backwards around an axis parallel to the slope Translational: Sliding the failure surface is more or less planar and the mass moves parallel to the ground surface 5) Flows : mobilize as a deforming, viscous mass without a discrete failure plane.
Hazardous Minerals of CA Asbestos-Inhalation of asbestos fibers may cause cancer. Most commonly, asbestos occurrences are associated with serpentinite and partially serpentinized ultramafic rocks. Mercury- Mercury from historic mercury mines or gold mines has entered a number of watersheds in California. Radon- Gas is a naturally-occurring, radioactive gas that is invisible and odorless. It forms from the radioactive decay of small amounts of uranium and thorium naturally present in rocks and soils.