18E.90.060 – Appendix B Geological Assessments – Fault Rupture Hazard Area Geotechnical Report
A. A geotechnical report shall, at a minimum, include the following:
1. The first page of the document shall be labeled identifying the submittal as a "Fault Rupture Hazard Geotechnical Report," and will include all mandatory items listed in PCC 18E.90.030 B.4. The report shall be prepared under the responsible charge of an appropriately licensed geotechnical professional.
2. The geological assessment for fault rupture hazards shall include the minimum requirements specified in PCC 18E.90.030 B.4.
3. The following topics should be considered and addressed in detail where essential to support opinions, conclusions, and recommendations in any geologic report on faults. It is not expected that all the topics or investigative methods would be necessary in a single investigation. In specific cases, it may be necessary to extend some of the investigative methods well beyond the site or property being investigated.
a. Purpose and scope of investigation; description of proposed development.
b. Geologic and tectonic setting. Include seismicity and earthquake history.
c. Site description and conditions, including dates of site visits and observations. Include information on geologic units, graded and filled areas, vegetation, existing structures, and other factors that may affect the choice of investigative methods and interpretation of data.
d. Methods of investigation.
(1) Review of published and unpublished literature, maps, and records concerning geologic units, faults, groundwater barriers, and other factors.
(2) Stereoscopic interpretation of aerial photographs and other remotely sensed images to detect fault-related topography (geomorphic features), vegetation and soil contrasts, and other lineaments of possible fault origin. The area interpreted usually should extend beyond the site boundaries.
(3) Surface observations, including mapping of geologic and soil units, geologic structures, geomorphic features and surfaces, springs, deformation of engineered structures due to fault creep, both on and beyond the site.
(4) Subsurface investigations.
(a) Trenching and other excavations to permit detailed and direct observation of continuously exposed geologic units, soils, and structures; must be of adequate depth and be carefully logged (Taylor and Cluff, 1973; Hatheway and Leighton, 1979; McCalpin, 1996b).
(b) Borings and test pits to permit collection of data on geologic units and groundwater at specific locations. Data points must be sufficient in number and spaced adequately to permit valid correlations and interpretations.
(c) Cone penetrometer testing (CPT) (Grant and others, 1997; Edelman and others, 1996). CPT must be done in conjunction with continuously logged borings to correlate CPT results with on-site materials. The number of borings and spacing of CPT soundings should be sufficient to adequately image site stratigraphy. The existence and location of a fault based on CPT data are interpretative.
(5) Geophysical Investigations. These are indirect methods that require a knowledge of specific geologic conditions for reliable interpretations. They should seldom, if ever, be employed alone without knowledge of the geology (Chase and Chapman, 1976). Geophysical methods alone never prove the absence of a fault nor do they identify the recency of activity. The types of equipment and techniques used should be described and supporting data presented (California Board of Registration for Geologists and Geophysicists, 1993).
(a) High resolution seismic reflection (Stephenson and others, 1995; McCalpin, 1996b).
(b) Ground penetrating radar (Cai and others, 1996).
(c) Other methods include: seismic refraction, magnetic profiling, electrical resistivity, and gravity (McCalpin, 1996b).
(6) Age-dating techniques are essential for determining the ages of geologic units, soils, and surfaces that bracket the time(s) of faulting (Pierce, 1986; Birkeland and other, 1991; Rutter and Catto, 1995; McCalpin, 1996a).
(a) Radiometric dating (especially 14C).
(b) Soil-profile development.
(c) Rock and mineral weathering.
(d) Landform development.
(e) Stratigraphic correlation of rocks/minerals/fossils.
(f) Other methods – artifacts, historical records, tephrochronology, fault scarp modeling, thermoluminescence, lichenometery, paleomagnetism, dendrochronology, etc.
(7) Other methods should be included when special conditions permit or requirements for critical structures demand a more intensive investigation.
(a) Aerial reconnaissance overflights.
(b) Geodetic and strain measurements.
(c) Microseismicity monitoring.
e. Conclusions.
(1) Location and existence (or absence) of hazardous faults on or adjacent to the site; ages of past rupture events.
(2) Type of faults and nature of anticipated offset, including sense and magnitude of displacement, if possible.
(3) Distribution of primary and secondary faulting (fault zone width) and fault-related deformation.
(4) Probability of or relative potential for future surface displacement. The likelihood of future ground rupture seldom can be stated mathematically, but may be stated in semiquantitative terms such as low, moderate, or high, or in terms of slip rates determined for specific fault segments.
(5) Degree of confidence in and limitations of data and conclusions.
f. Recommendations.
(1) The recommended increase from the standard buffer distance (50 feet) of proposed structures from fault rupture hazard areas. The recommended buffer distance generally will depend on the quality of data and type and complexity of fault(s) encountered at the site and the proposed land use type (i.e. occupancy). In order to establish an appropriate buffer distance from a fault located by indirect or interpretative methods (e.g., borings or cone penetrometer testing), the area between data points also should be considered underlain by a fault unless additional data are used to more precisely locate the fault. Additional measures (e.g., strengthened foundations, engineering design, and flexible utility connections) to accommodate warping and distributive deformation associated with faulting (Lazarte and others, 1994).
(2) Risk evaluation relative to the proposed development.
(3) Limitations of the investigation; need for additional studies.
g. References.
(1) Literature and records cited or reviewed; citations should be complete.
(2) Aerial photographs or images interpreted – list type, data, scale, source, and index numbers.
(3) Other sources of information, including well records, personal communications, and other data sources.
h. Illustrations. The following illustrations should be provided:
(1) A location map that identifies site locality, significant faults, geographic features, regional geology, seismic epicenters, and other pertinent data; 1:24,000 scale is recommended.
(2) A site development map that shows site boundaries, existing and proposed structures and limits of the proposed project area, graded areas, streets, exploratory trenches, borings geophysical traverses, locations of faults, and other data; recommended scale is 1:2,400 (1 inch equals 200 feet), or larger.
(3) A geologic map that shows the distribution of geologic units (if more than one), faults and other structures, geomorphic features, aerial photo graphic lineaments, and springs; on topographic map 1:24,000 scale or larger; can be combined with h(1) or (2).
(4) Geologic cross-sections, if needed, to provide three-dimensional picture.
(5) Logs of exploratory trenches and borings that show details of observed features and conditions (note: these should not be generalized or diagrammatic). Trench logs should show topographic profile and geologic structure at a 1:1 horizontal to vertical scale; scale should be 1:60 (1 inch = 5 feet) or larger.
(6) Geophysical data and geologic interpretations.
i. Appendix. Attach any supporting data not included above (e.g., water well data, photographs, aerial photographs).
4. The geotechnical report shall be prepared under the responsible charge of an appropriately licensed geotechnical professional and be signed, sealed and dated by the geotechnical professional(s) (as defined in PCC 18.25.030).
5. The Department may request a geotechnical professional to provide additional information in the geotechnical report based upon existing conditions, changed conditions, or unique circumstances occurring on a case by case basis.