C.2. - GEOLOGIC HAZARDS REPORT

C.2.1.   General Requirements

These guidelines outline the minimum information that shall be presented and evaluated in a Geologic Hazard Report. These guidelines are not intended to be a rigid framework of requirements, nor a specific format for each report submitted for County review. The level of detail and emphasis may vary due to unique or particular geologic conditions or due to the type of project.

C.2.2.   Report Guidelines

A.

Overall Project Information. A general project description shall be included with each report and in a clear manner present at a minimum the following list of items:

• Size and location of the project;

• Existing site land uses;

• Proposed site land uses; and,

• Identification of the person who prepared the study and his/her qualifications for conducting the study.

B.

Study Overview. A general overview of the report objectives, contents, methods, level of investigation, and findings should be provided:

• State the objective(s) and level of investigation for the study;

• Cite the previous publicly available geologic reports which were reviewed or referenced in the course of preparing the geologic hazard study and indicate the author(s), firm, and date of each report;

• List the methods of investigation as well as professional firm(s) who participated;

• If the level of investigation varies within the subject area, describe in the text and show on the maps areas of concentration or exclusion;

• Describe the general physiographic setting of the project and its relationship to local topographic features;

• Describe the general geologic setting of the project and indicate any lithologic, tectonic, geomorphic, or soils problems specific to the area; and,

• Describe the general surface and groundwater conditions.

C.

Site Evaluation Techniques. The most appropriate site evaluation techniques shall be determined by the geologist/geotechnical engineer based on site conditions and the activities being proposed for the site. However, the list below presents typical methodologies and techniques used to evaluate the potential geological hazards of a site. Based on their applicability to the site being investigated, the information listed under each technique shall be included with the submitted report unless otherwise indicated to not be required. A detailed explanation is necessary for exclusion of minimum requirements listed.

1.

Extent. State the extent and method of surface and subsurface geologic studies.

2.

Geologic Mapping. The geologic mapping shall:

• Show important details corresponding to the size, extent and degree of the investigation on the project topographic map;

• Show the abundance of distributions of earth materials and structural elements exposed or inferred in the subject area. Observed and inferred features or relationships should be so designated on the geologic map;

• Portray all geologic information at a scale that is appropriate for readability and interpretation. When different sources are used. "Tie-points" between the sources shall be presented, and,

• Indicate the geologic base map use, date, and significant additions and modifications to previous work.

3.

Aerial Photographs and Remote-Sensing Imagery. When aerial photographs or imagery are used, the report shall:

• Present the source(s) of photographs or images, if available;

• Indicate data and scale of photographs or imagery; and,

• Highlight critical points and their relationship to the project.

4.

Geophysical Investigations. Geophysical investigation information shall:

• Indicate the type and objectives of the geophysical investigation(s) (if any), quality of the data, and limitations of geophysical techniques;

• Describe the information used to correlate the geophysical data and geologic conditions; and,

• Display the geophysical data on the topographic/geologic maps and cross section.

5.

Drill-Hole Data. Drill-hole data information shall:

• Describe the specific investigative methods, tests conducted, drilling, and date of investigation;

• Show the location of all borings on the topographic or geologic map; and,

• Show boring logs, geophysical logs, or profiles obtained in the investigation.

6.

Test Pits and Trenches. The test pit and trench information shall:

• Describe the location and dimensions of all pits and trenches and date of investigation;

• Indicate the location of all excavations on the topographic/geologic map and profiles;

• Provide a large scale descriptive log with sufficient detail; and,

• Show sample location if supplemental laboratory tests were conducted.

7.

Field and Laboratory Tests. The field and laboratory tests information shall:

• Describe the type of any tests conducted in the field or laboratory;

• Describe the sample method and test procedures; and,

• Show the test results on the data work sheets or on summary tables.

8.

Monitoring Programs. The monitoring program information shall:

• Describe the type, objectives, and location of all monitoring programs in the subject area; and,

• State the monitoring period, the firm(s) or individuals responsible for the care and disposal of the installations.

D.

Geologic Descriptions. As appropriate for site conditions, the report should provide the following general geologic description information.

1.

Bedrock Units.  

• Rock type and bedding attitude or foliation;

• Age of and correlation with recognized formations;

• Dimensional characteristics such as thickness and extent;

• Distribution and extent of the weathered zone;

• Physical characteristics; and,

• Response of bedrock materials to natural processes.

2.

Surficial Deposits.  

• Distribution, occurrence, and age;

• Identification of material types and sources;

• Dimensional characteristics such as thickness and extent;

• Surface expression and relationships with present topography;

• Physical and chemical characteristics; and,

• Distributions and extent of altered zones.

3.

Geomorphic Features. Geomorphic features resulting from landslides, earthflows, debris flows, mudflows, rockfalls, debris avalanches, fault scarps, soil creep, erosion scarps, avalanched paths, and subsidence phenomenon.

• Locations and distribution;

• Dimensional characteristics;

• Age of feature and history of activity;

• Recurrence interval for geomorphic process; and,

• Physical characteristics.

4.

Structural Features. Structural features include joints, faults, shear zones, folds, schistocity, and foliation.

• Occurrence, distribution, and proximity to site;

• Dimensional and displacement characteristics of faults;

• Orientation and changes in orientation;

• Physical characteristics such as brecciation, slickensides, gouge zones, sand boils, sag ponds, springs alignment;

• Disrupted drainages, or ground-water barriers;

• Nature of offset(s) and timing of movement(s); and,

• Absolute or relative age of latest movement.

5.

Surface Drainage.  

• Distributions;

• Relation to topography (drainage patterns);

• Relation to areas of vegetations;

• Relation to geologic features;

• Source permanence, and variation in amount of surface water;

• Evidence of earlier occurrence of water localities now dry;

• Estimated peak flows an physiographic flood plain of drainages;

• Probable maximum or 100-year flood limits, including flash and debris floods; and,

• Outfall.

6.

Ground Water.  

• Distributions and occurrence;

• Hydraulic gradients;

• Relation to topography;

• Relations to geologic features; and,

• Seasonal variations.

E.

Geologic Interpretation. The report shall include interpretations and detailed descriptions of the following:

1.

Geologic Hazards. Geologic hazards include landslides, avalanche, rockfall, mudflows, debris flows, radioactivity, etc.

• Geomorphic and structural features/processes present in the area;

• Man-induced features/processes;

• Age and activity of the features/processes;

• Natural contentions affecting the features/processes;

• Susceptibility to man-induced changes;

• Potential impact of hazard(s) and risk to project;

• Amenability of adverse conditions for adequate mitigation;

• Long-term lateral and vertical stability of earth materials; and,

• Impact of project on materials stability.

2.

Geologic Constraints. Geologic constraints include expansive soil or rock, potentially unstable slopes, high groundwater levels, soil creep, hydrocompaction, shallow bedrock, erosion, etc.

• Soil, surface and ground water, and geomorphic conditions;

• Man-induced conditions;

• Activity of conditions

• Effect of natural or man-induced changes;

• Potential impact of conditions and risk to project;

• Amenability of adverse conditions for adequate mitigation; and,

• Impact of project on long-term project stability.

F.

Relationship of Geologic Factors on the Proposed Action. This topic normally constitutes the principal contribution of the report. It involves both the affects of geologic features upon the proposed grading, construction, and land use; and the effects of these proposed modifications upon future geological processes in the area. The following checklist includes the topics that ordinarily should be considered as part of the findings, conclusions, and recommendations of the geologic reports:

1.

Compatibility. General compatibility of natural features with proposed action that should be discussed includes:

• Topography;

• Lateral stability of earth materials;

• Problems of flood inundation, erosion, and deposition;

• Problems caused by features or conditions in adjacent properties; and,

• Other general problems.

2.

Proposed Cuts. A description of proposed cuts and the relationship to the activities being proposed including:

• Prediction of what materials and structural features will be encountered;

• Prediction of stability based on geologic factors;

• Problems of excavation (e.g. unusually hard or massive rock, excessive flow of groundwater); and,

• Recommendations for reorientation or repositioning of cuts, reductions of cut slopes, development of compound cut slopes, special stripping above daylight lines, buttressing, protections against erosion, handling of seepage water, setbacks for structures above cuts, etc.

3.

Proposed Fill. A description of proposed fill and the relationship to the action including:

• General evaluation of planning with respect of canyon-filling and sidehill masses of fill; and

• Comment on suitability of existing natural materials for fill.

4.

Recommendations for Fill. Recommendations concerning the positioning of fill masses, provision for underdrainage, buttressing, special protection against erosion including:

• Recommendations for subsurface testing and exploration; and

• Cuts and test holed needed for additional geologic information.

5.

Special Recommendations. Details concerning any unique or unusual recommendations concerning the action including recommendations about:

• Areas to be left as natural ground;

• Removal or buttressing of existing slide masses;

• Flood protection;

• Problem of groundwater level fluctuation;

• Position of structures, with respect to active faults;

• Problems associated with radon gas and soil radioactivity; and,

• Problems caused by natural gases, such as methane and hydrogen sulfide, radon, and,

• Protection of existing and proposed utility facilities.

G.

Conclusions. The report shall provide detailed conclusions of the analysis including:

1.

Land Compatibility. State generally whether the intended use of the land is compatible with the investigated site conditions; and if mitigation measures are necessary. In addition, depict those areas where "no building" is recommended.

2.

Construction Planning. Discuss critical construction planning aspects that may need further consideration by the contractor including the stability of earth materials, grading plans and the need for selective location of project facilities.

3.

Geologic Basis. The report shall clearly state the geologic basis for all conclusions.

H.

Recommendations. The report shall provide recommendations to be used in later stages of the project based on this preliminary analysis of the project site including:

1.

Mitigation Procedures. Discuss the development of mitigation procedures or design changed necessary to minimize or abate any hazardous condition requires a recommendation.

2.

Long-term Project Objectives. Recommendations should focus upon the long-term stability and safety of the proposed project.