Chapter 7 - Topography/Geology/Soils/Seismic
- What Does the Topic Include?
- Laws, Regulations, and Guidance
- Description of Project Alternatives
- Geology of the Project Area
- Geologic and Seismic Impacts
- Mitigation of Impacts
- Coordination with Other Agencies, Groups, or Consultants
- Conclusions and Recommendations
- Report Preparers' Qualifications
- Further Guidance on Geological Reports
This chapter includes information and requirements for describing geologic, soil and seismic conditions in the vicinity of the project area as well as an analysis of the potential environmental impacts of project alternatives on these conditions and the potential impacts of geotechnical conditions on the transportation facility.
This section summarizes the primary statutes and regulations applicable to geology, soils, and seismic issues. In the event that the property is owned or administrated by a federal or state agency, that agency should be contacted in order to ascertain specific requirements that they may have relative to these issues.
Federal Laws and Regulations
The National Environmental Policy Act (NEPA) (42 USC 4321), and other related regulations, require that all actions sponsored, funded, permitted, or approved by federal agencies undergo planning studies in order to ensure that environmental considerations such as impacts to the earth are given due weight in project decision-making. Federal implementing regulations are found in 23 CFR 771 (FHWA) and 40 CFR 1500-1508 (CEQ).
State Laws and Regulations
The California Environmental Quality Act (CEQA), and the CEQA Guidelines require similar consideration for state and local actions. A CEQA checklist specific to the Department has been developed and contains specific questions regarding the project's potential to expose people or structures to seismic, landside, subsidence, and tsunami hazards, etc., and also contains questions on mineral resources.
Local Ordinances and Regulations
Critical/Sensitive Area Ordinances (CAO/SAO)
These laws protect locally designated critical and/or sensitive areas, which include geologically hazardous areas. Local sensitive or critical area ordinances may identify locales susceptible to erosion, landslides, earthquakes, or other geologic events, which pose a threat to health and safety when incompatible development is sited in areas of significant hazard. Unless the local laws conflict with state law, Caltrans must be consistent with the requirements of local CAO/SAO regulations. (This sentence needs to be qualified since State agencies don't have to follow local ordinances all the time.) Local planning departments should be contacted to determine the location or descriptive criteria of geologically hazardous areas that may impact the project alternatives.
Other Local Ordinances
Local ordinances also regulate building and clearing/grading. For projects outside the right of way, Caltrans must comply with these ordinances. In this vein, counties are also generally the administrators for compliance with the California Surface Mining and Reclamation Act (SMARA) - see the California Department of Conservation website for further details on both SMARA and practitioner qualification requirements.
In addition to the above information, useful sources of information include the U.S. Department of Agriculture (e.g., Forest Service and Natural Resources Conservation Service), U.S. Department of the Interior (e.g., Geological Survey and Bureau of Land Management) staff and maps, college and university staff, California State Office of Planning and Research, Caltrans Transportation Laboratory Geologists, and the Seismic Hazard Element of local General Plans.
The following term definitions establish a baseline for the subsequent discussion.
- Active fault: A fault that has moved within late-Quaternary time (the last 750,000 yrs). Note that this definition is broader than that used by the California Department of Conservation, California Geological Survey (CGS), which defines an active fault as one that has moved within Holocene time (the last 11,000 yrs).
- Deterministic seismic hazard analysis: Seismic parameters are estimated based on the size of the maximum credible (magnitude) earthquake expected. The value obtained is essentially time-independent. This method is used by Caltrans to assess the seismic hazard at most structures. See also probabilistic seismic hazard analysis, below.
- Differential settlement: The uneven lowering of different parts of an engineered structure, often resulting in damage to the structure.
- Regulatory earthquake fault zones: Areas along faults defined as active by CGS, typically one-quarter mile or less in width, where special studies are required to determine if there is a surface rupture hazard. Caltrans’ broader definition of active faults results in other areas that also need to be addressed for surface rupture. A site near a fault defined as active by Caltrans criterion also requires a review of surface rupture potential.
- Fault creep: Slow ground displacement occurring without accompanying earthquakes.
- Liquefaction: The loss in the shearing resistance of a cohesionless soil, caused by an earthquake wave. The soil is turned into a fluid mass.
- Maximum credible earthquake: The maximum intensity earthquake that is assumed to occur closest to the site. This earthquake is also described as the maximum magnitude earthquake, or maximum earthquake.
- Magnitude: A measure of the strength of an earthquake or the strain energy released by it.
- Probabilistic seismic hazard analysis: Seismic parameters are estimated using several significant seismic sources, the likelihood of occurrence within a given time frame, and the uncertainty of the estimate. Caltrans uses probabilistic methods for important bridges and certain seismic retrofit projects.
- Recurrence interval: The average time interval between earthquake occurrences of equal magnitude on the same fault.
- Rotational slide or slump: Landslide movement due to forces that cause a concave upwards surface in the mass.
- Seiche: A wave oscillation of the surface of water in an enclosed basin initiated by an earthquake.
- Setbacks: The minimum horizontal distance slopes shall be set back from site boundaries according to Chapter 70 of the Uniform Building Code. Also applies to the minimum horizontal distance required from faults to structures (see CGS Special Publication 42, pp. 27 and 29).
- Settlement: The gradual downward movement of an engineered structure due to compression of the soil below the structure foundation.
- Soil creep: The gradual, steady downhill movement of soil and loose rock material.
- Subsidence: A localized mass movement that involves the gradual downward settling or sinking of the Earth's surface.
- Tsunami: A water wave of local or distant origin that results from large-scale displacements associated with large earthquakes, major submarine slides, or volcanic eruption.
- Translational slide: Landslide movement that occurs predominantly along planar or gently undulating surfaces.
Geologic reports should include information on the regional and local geologic setting, topography, significant landforms, soil types and thickness of soil or depth to bedrock, geologic hazards, soil/rock types, geologic structures, groundwater conditions, and other relevant properties, such as erosion potential and mineral economic resources. Suggested reference materials include, but are not limited to:
- U.S. Department of Agricultural (USDA) or National Forest Service maps should be consulted for information on soil types in the project vicinity, or from other sources if available.
- The Caltrans California Seismic Hazard Map (1996; currently being revised) should be consulted for information on locations of major earthquake faults, the maximum credible (magnitude) earthquake, and expected ground motion at a particular site.
- The CGS (www.consrv.ca.gov/CGS/) should be consulted for additional information in earthquake faults and seismic hazards if needed, mineral resources, and oil, gas, and geothermal resources.
(Also see "Further Guidance on Geologic Reports" at end of this chapter)
Project impacts include those associated with both the construction and operation of the project. Project-specific information is required for environmental documents and determinations (NEPA EIS/EA/CE and/or CEQA EIR/ND/CE), permits, and technical reports. This information also includes an assessment of the probable environmental impacts and mitigation options for each project alternative. A separate report discussing the geology, soils, topography and seismicity is generally needed when any of the following conditions exist:
- Geologic and soils-related hazards (e.g., unstable rock/soil masses, highly erosive soils) are likely to be identified within or near the project area, and the project is likely to impact or be impacted by these hazards.
- Geologic and soils-related economic mineral resources (e.g., cut/fill, borrow, aggregate, topsoil) are likely to be excavated and utilized by the project in a quality or manner which is likely to have environmental impacts, and these impacts and associated mitigation options are not adequately addressed in other reports (e.g., air quality, siltation/water quality).
- The project site lies within an area subject to a major earthquake that would result in ground surface rupture and/or strong ground shaking that could damage building structures or cause ground settlement, liquefaction, slope failures, or disturbance of surface water bodies.
The geologic report generally contains the following major sections:
- Description of Project Alternatives
- Geology of the Project Area
- Geologic and Seismic Impacts
- Mitigation of Impacts
- Coordination with other Agencies, Groups or Consultants
- Conclusions and Recommendations
- Report Preparer's Qualifications
The summary identifies geology of the project area, environmental impacts, hazards, and mitigation recommendations in non-technical terms. It should be suitable for incorporation into the environmental document or determination (NEPA EIS/EA/CE and/or CEQA EIR/ND/CE), for presentation at public hearings, and for use by management and policy groups in decision-making. The report should cover geologic considerations for future foundation analysis and geotechnical engineering recommendations for the proposed project area in the design/construction phase.
This section presents a brief description of project alternatives identified during the scoping process. Descriptions should be consistent with those in other reports.
This section describes the approach used to determine environmental impacts, hazard areas, economic resources, and other report findings and conclusions. The descriptions should include data and information sources, field methods such as exploratory drilling, sampling and laboratory testing of soil/rock properties, analysis techniques and tools, and decision criteria. It should be as succinct as possible. Detailed descriptions, where necessary, should be included in an appropriate appendix.
This section describes the existing conditions with respect to geology and soils in the vicinity of the project area based on a review of existing data, field reviews, evaluation of risks/impacts relative to proposed alternatives in order to help determine when there is an impact related to geologic conditions, and summaries of important geotechnical concerns for each alternative.
Topic areas include the following:
- Geologic Setting: Describe key structures, landforms, and geologic units.
- Physiography and Topography: Range of elevations, drainage, structure, climate, and rainfall.
- Surface and Groundwater: Describe nearby surface water bodies, drainage basins, groundwater depths or elevations, gradient, flow direction, and aquifer thicknesses.
- Soils: Identify and describe soil types and their properties such as layer thickness, grain size, distribution, permeability, run-off characteristics, susceptibility to erosion, shear strength, shrink-swell potential, compaction potential, settlement potential (long-term and differential), and sedimentation methods.
- Aerial Photographs: May provide information about site geology and geologic structures, such as faults. Current and out-of-print U.S. Geological Survey topographic and other agency maps should be reviewed for relevant information as well.
- Geologic Hazards: Identify areas that are susceptible to one or more of the following types of hazards:
- Landslide hazards: Types of movements and depths, such as shallow or deep-seated, translational or rotational landslides, slumps, debris flows, earth flows, mass wasting, etc.
- Seismic hazard: Potential for strong ground shaking, surface rupture, fault creep, and/or liquefaction. Note that Caltrans currently uses deterministic seismic hazard analysis methods to estimate most expected seismic hazards. Probabilistic methods are used only for important bridges and certain seismic retrofit projects.
- Volcanic hazards
- Other geologic hazards (e.g., subsidence, rock fall): In much of the state, hazard areas have been identified in the process of developing local critical or sensitive area ordinances. Contact the appropriate local planning departments to obtain the most current information. In some localities, hazard areas are not delineated on maps, but are defined in terms of landscape characteristics (e.g., slope, geologic unit, field indicators); in these instances, hazard areas should be mapped by identifying where the defining characteristics apply to the project area.
- Economic Resources/mineral hazards: Describe source areas (existing and potential) for construction materials (e.g., borrow, aggregate, topsoil) or mineral resources (mines or mills) in the vicinity of the project.
Stormwater impacts to soil erosion, permeability and percolation characteristics, fertility, and vegetation (these aspects are discussed at length in the Stormwater SER chapter)
This section describes the predicted environmental impacts of project alternatives on geologic and soil conditions, hazards, and economic resources, as well as predicted impacts of identified geologic hazards on project alternatives. Impacts to be considered include direct (construction and operational), indirect, and cumulative.
Describe mitigation measures, commitments and monitoring procedures, methods of analysis, geotechnical information and recommendations relevant to selection of the preferred alternative; describe measures that will reduce impacts related to geologic conditions, and the rationale for the various mitigation measures that may occur during the right of way and construction phases. In general, the hierarchical approach of avoid, minimize and then mitigate should be considered. Adequate study, design, and good construction practices should be evaluated for each project alternative with respect to geologic and seismic hazards and appropriate mitigation measures discussed. Mitigation measures include, but may not be limited to, appropriate setbacks, grading, slope stability measures, earthquake design methods, drainage and soil stabilization controls, such as vegetative cover, filtration devices, and detention basins.
This section identifies agencies and other organizations involved with, or contacted during, the development of the report. These agencies/organizations include, but are not limited to: Federal Highway Administration, U.S. Geological Survey, U.S. Department of Agriculture, California Geological Survey, local planning agency departments, and engineering firms/private consultants.
Permit requirements pertaining to geology and soils are addressed in the following sections:
For geologic and topographic features, an important federal law is the Historic Sites Act of 1935, which establishes a national registry of natural landmarks and protects "outstanding examples of major geologic features."
- Easement over Public Land
- Surface Mining Reclamation Act (SMARA; generally administered by counties)
- Other State approvals (soil borings, geotechnical investigations)
- Clearing, grading, building permits
This section should describe the general range of geologic hazards relative to the intended land use or development for all project alternatives. This includes the feasibility of the proposed project with respect to project alternatives and any need for further studies. The conclusions should include descriptions of geologic hazards on proposed structures at the site, the feasibility of the alternatives and risk evaluations, avoidance measures, data limitations, and other recommendations as appropriate for the project site.
The person(s) who prepare the reports on geology, soil, and seismicity that are used to support the environmental documents/determinations must be a California Registered Professional Geologist (PG), or a Registered Professional Engineer (PE), or persons working under the direct supervision of either of these registered professionals. The report(s) that support environmental documents/determinations must be signed and stamped by the registered professional(s).
Technical reports, memoranda, data summaries, or other documentation developed to support the report should be placed in one or more appendices after the main body of the report.
Further Guidance on Geologic Reports
Conservation Division of Mines and Geology, 1986a, Guidelines to geologic/seismic reports: DMG NOTE 42.
- California Department of Conservation Geological Survey (CGS), Guidelines for reviewing geologic reports: CGS NOTE 41.
- California Department of Conservation Geological Survey (CGS), 1998, Guidelines for evaluating the hazard of surface fault rupture: CGS NOTE 49 (PDF).
- California Department of Conservation Geological Survey (CGS), 2001, Guidelines for preparing geological reports for regional-scale environmental and resource management: CGSNOTE 52 (PDF).
- California Department of Conservation Geological Survey (CGS), 2014, Regional Geologic Hazards and Mapping Program - Alquist-Priolo Earthquake Fault Zoning.
- California Department of Conservation Geological Survey (CGS), Guidelines for evaluating and mitigating seismic hazards in California, 2008, Pub ID: SP 117A (PDF).
- Caltrans Seismic Hazard Map (Based on Maximum Credible Earthquake), 1996, by Caltrans, scale 1:750,000.