Chapter 14.80 LANDSLIDE HAZARD AREAS

Chapter 14.80
LANDSLIDE HAZARD AREAS

Sections:

14.80.010 Purpose.

14.80.020 Landslide hazard areas.

14.80.030 Landslide hazard area review procedures.

14.80.040 Landslide and erosion hazard area standards.

14.80.050 Buffer requirements.

14.80.060 Appendices.

14.80.070 Figures.

14.80.010 Purpose.

The following statements describe the purpose of this chapter is to:

A. Protect human life and health.

B. Regulate uses of land in order to avoid damage to structures and property being developed and damage to neighboring land and structures.

C. Identify and map active landslide hazard areas.

D. Minimize the ill effects on wetlands and critical fish and wildlife habitat that can result from landslides.

E. Establish permit requirement and review procedures for development proposals in areas with potential landslides. (Ord. 02-200 § 2).

14.80.020 Landslide hazard areas.

A. Landslide Hazard Areas Indicators. Landslide hazard areas are areas potentially subject to mass movement due to a combination of geologic, seismic, topographic, hydrologic, or manmade factors. Landslide hazard areas can be identified by the presence of any of the following indicators:

1. Areas of historic failures, including areas of unstable, old and recent landslides or landslide debris within a head scarp.

2. Areas with active bluff retreat that exhibit continuing sloughing or calving of bluff sediments, resulting in a vertical or steep bluff face with little or no vegetation.

3. Areas with the following characteristics:

a. Slopes steeper than 20 percent with a vertical relief of 20 feet or more (see EMC 14.80.070(A), Figure 14.80-1); and

b. Hillsides that intersect geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and

c. Springs or groundwater seepage.

4. Slopes that are parallel or sub-parallel to planes of weakness, such as bedding planes, joint systems, and fault planes in subsurface materials.

5. Areas exhibiting geomorphological features indicative of past slope failure, such as hummocky ground, back-rotated benches on slopes, etc.

6. Areas with tension cracks or ground fractures along and/or near the edge of the top of a bluff or ravine.

7. Areas with structures that exhibit structural damage such as settling and cracking of building foundations or separation of steps or porch from a main structure that is located near the edge of a bluff or ravine.

8. The occurrence of toppling, leaning, bowed, or jackstrawed trees that are caused by disruption of ground surface by active movement.

9. Areas with slopes containing soft or liquifiable soils.

10. Areas where gullying and surface erosion have caused dissection of the bluff edge or slope face as a result of drainage or discharge from pipes, culverts, ditches, and natural drainage courses.

11. Areas where seeps or springs or indicators (e.g., vegetation type) of a shallow groundwater table are observed on or adjacent to the face of the slope.

12. Any area with a slope of 40 percent or steeper and with a vertical relief of 15 or more feet, except those manmade slopes created under the design and inspection of a geotechnical professional or slopes composed of competent bedrock. For the purposes of determining whether a slope is considered to be a landslide hazard area, the horizontal and vertical distance between the top and toe of slope are utilized (see EMC 14.80.060, Appendix D, and EMC 14.80.070(A), Figure 14.80-1).

13. Areas that are at risk of mass movement due to seismic events.

B. Potential Landslide Hazard Areas. Potential landslide hazard areas (see EMC 14.80.070(B), Figure 14.80-2), as depicted on the Critical Areas Atlas – Landslide Hazard Areas Map, are those areas where the suspected risk of slope instability and landslide is sufficient to require a geological assessment to assess the potential for active landslide activity. Potential landslide hazard areas are determined by using the following criteria:

1. Areas identified on the city topographic maps as having slopes greater than 20 percent with a vertical relief of greater than 20 feet and any adjacent areas within a distance of 65 feet (see EMC 14.80.070(C), Figure 14.80-3).

2. Areas that possess one or more of the landslide hazard area indicators (stratigraphy, groundwater conditions, etc.) as set forth in subsection (A) of this section and any adjacent area within a distance of 65 feet.

C. Landslide Hazard Area Categories. Landslide hazard areas shall be classified into categories, which reflect each landslide hazard areas past landslide activity, and the potential for future landslide activity based on an analysis of slope instability. Landslide hazard areas shall be designated as follows:

1. Active Landslide Areas. A composite of the active landslides and/or unstable areas, including that portion of the top of slope and slope face subject to failure and sliding as well as toe of slope areas subject to impact from down slope run-out, identified and mapped during a geological assessment of a site. An active landslide hazard area exhibits one or more of the following:

a. Areas of historical landslide movement on a site which have occurred in the past century including areas identified on the Coastal Zone Atlas of Washington, Volume VII, Pierce County as Urs (unstable recent slide).

b. Unstable areas that exhibit geological and geomorphologic evidence of past slope instability or landsliding or possess geological indicators (stratigraphy, ground water conditions, etc.), as set forth in subsection (A) of this section, that have been determined through a geological assessment process to be presently failing or may be subject to future landslide activity. The impact of the proposed development activities must be considered in defining the extent of the active areas.

c. Interim areas are located between areas identified through the geological assessment process as an active landslide hazard area. Interim areas will be considered part of the active landslide hazard area if the required top of slope or toe of slope landslide hazard area buffer encompasses the area (see EMC 14.80.070(D), Figure 14.80-4).

2. Stable Areas. Areas that have been identified as potential landslide hazard areas, but through the geological assessment process meet one of the following conditions. Such stable areas shall continue to be considered critical areas for all purposes under this code including but not limited to density calculations and the application of SEPA.

a. No indicators as set forth in subsection (A) of this section actually exist that indicate the potential for future landslide activity to occur;

b. A slope stability analysis has proven that there is no landslide potential; or

c. Adequate engineering or structural measures have been provided through the submittal of a geological assessment – geotechnical report that mitigates the potential for a future landslide to occur as a result of current or cumulative development activity. (Ord. 04-240 § 1; Ord. 02-200 § 2).

14.80.030 Landslide hazard area review procedures.

A. General Requirements.

1. The city's critical areas atlas provides an indication of where active and potential landslide hazard areas are located within the city. The actual presence or location of an active landslide hazard area and/or additional potential landslide hazard areas that have not been mapped, but may be present on or adjacent to a site, shall be determined using the geological assessment procedures established in this chapter.

2. The department will complete a review of the Critical Areas Atlas – Landslide Hazard Area Map and other source documents for any proposed regulated activity to determine whether the site is, or may be, located within an active or potential landslide hazard area. Identification of an active or potential landslide hazard area may also occur as a result of field investigations conducted by department staff.

3. When the department's maps or sources indicate that the site for a proposed regulated activity is or may be located within an active or potential landslide hazard area, the department shall require the submittal of a geological assessment as outlined in subsection (B) of this section (see EMC 14.80.070(E), Figure 14.80-5).

4. Unless otherwise stated in this chapter, the critical protective measure provisions contained in EMC 14.10.080 shall apply.

B. Geological Assessment. A geological assessment is a site investigation process to evaluate the on-site geology affecting a subject property.

1. Geological assessments shall be submitted to the department for review and approval together with a landslide hazard area application and associated fee.

2. A geological assessment shall include a field investigation and may include the use of historical air photo analysis, review of public records and documentation, and interviews with adjacent property owners, etc.

3. The geological assessment shall include the following information and analysis:

a. A determination of which areas on the site or within the vicinity of the site meet the criteria for an active landslide hazard area and stable area as set forth in EMC 14.80.020(C)(1) and (2).

b. Consider the run-out hazard of landslide debris to the proposed development that starts upslope (whether part of the subject property or on a neighboring property) and/or the impacts of landslide run-out on down slope properties.

c. The geological assessment shall include a detailed review of the field investigations, published data and references, data and conclusions from past geological assessments, or geotechnical investigations of the site, site-specific measurements, tests, investigations, or studies, as well as the methods of data analysis and calculations that support the results, conclusions, and recommendations.

4. Geological assessments shall be prepared, signed, and dated by a geotechnical professional (as defined in EMC 14.10.060 and established in this chapter) and the format shall be pre-approved by the department.

5. An engineering geologist shall complete a field investigation and geological assessment to determine whether or not an active landslide hazard area exists within 300 feet of the site (see EMC 14.80.070(E), Figure 14.80-5).

a. The geological assessment shall be submitted in the form of a geotechnical letter when the engineering geologist finds that no active landslide hazard area exists within 300 feet of the site. The geotechnical letter shall meet the requirements contained in EMC 14.80.060, Appendix A.

b. The geological assessment shall be submitted in the form of geotechnical verification when the engineering geologist finds that an active landslide hazard area exists, but is located more than 300 feet away from the proposed project area. The geotechnical verification shall meet the requirements contained in EMC 14.80.060, Appendix B.

c. The geological assessment shall be submitted in the form of a geotechnical report when the engineering geologist finds that an active landslide hazard area exists within 300 feet of the proposed project area or when a geotechnical professional determines that mitigation measures are necessary in order to construct or develop within a potential landslide hazard area. The geotechnical report shall meet the requirements contained in EMC 14.80.060, Appendix C.

6. Geological assessments that do not contain the minimum required information or comply with the landslide hazard area standards set forth in EMC 14.80.030 will be returned to the geotechnical professional for revision.

7. The department shall review the geological assessment and either:

a. Accept the geological assessment; or

b. Reject the geological assessment and require revisions or additional information.

8. When the geological assessment has been accepted, the department shall issue a decision on the landslide hazard area application.

9. A geological assessment for a specific site may be valid for a period of up to five years when the proposed land use activity and surrounding site conditions are unchanged. However, if any environmental conditions associated with the site change during that five-year period, the applicant may be required to submit an amendment to the geological assessment. (Ord. 02-200 § 2).

14.80.040 Landslide and erosion hazard area standards.

A. Active Landslide Hazard Areas. Any development, encroachment, filling, clearing or grading, building structures, impervious surfaces, and vegetation removal shall be prohibited within active landslide hazard areas and associated buffers except as specified in the following standards:

1. Stormwater Conveyance. Stormwater conveyance shall be allowed when it is conveyed through a high-density polyethylene stormwater pipe with fuse-welded joints and when no other stormwater conveyance alternative is available. The pipe shall be located on the surface of the ground and be properly anchored so that it will continue to function in the event of an underlying slide.

2. Utility Lines. Utility lines will be permitted when no other conveyance alternative is available. The line shall be located above ground and properly anchored and/or designed so that it will continue to function in the event of an underlying slide.

3. Roads, Bridges, and Trails. Roads, bridges, and trails shall be allowed when all of the following conditions have been met:

a. Mitigation measures are provided that ensure the roadway prism and/or bridge structure will not be susceptible to damage from landslide-induced ground deformation or impact/coverage by landslide debris. Mitigation measures shall be designed for static and seismic loading conditions in accordance with the most recent version of the American Association of State Highway and Transportation Officials (AASHTO) Manual.

b. The road is not a sole access for a development.

c. The removal or disturbance of vegetation, clearing or grading shall be prohibited during the wet season (November 1st to May 1st).

B. Landslide Hazard Management Areas. All regulated activities may be allowed in areas located within 300 feet of an active landslide hazard area subject to the following standards:

1. The department reviews and approves a geological assessment – geotechnical report and determines that the potential landslide hazard area is stable.

2. The proposed development is located outside of an active landslide hazard area and any required buffer, as set forth in EMC 14.80.050.

3. The proposed recommendations and mitigation measures contained within the geotechnical report are adequate to reduce or mitigate risks to health and safety.

4. The proposed development shall not decrease the factor of safety for landslide occurrence below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. Analysis of dynamic (seismic) conditions shall be based on a minimum horizontal acceleration as established by the current version of the International Building Code.

5. The removal and disturbance of vegetation, clearing or grading shall be limited to the area of the approved development and shall not be allowed during the wet season (November 1st through May 1st) unless adequate provisions for wet season erosion have been addressed in the geotechnical report and approved by the department.

6. Surface drainage from developed areas, including downspouts and runoff from paved or unpaved surfaces up slope, shall not be directed through an active landslide hazard area or its associated buffer unless it is conveyed in conformance with the provisions in EMC 14.80.030(A)(1).

7. Stormwater retention facilities, including infiltration systems utilizing perforated pipe, are prohibited unless the slope stability impacts of such systems have been analyzed and mitigated by a geotechnical professional and the impacts have been determined to be negligible.

8. The proposed development shall not create a need for larger landslide hazard area buffers and setbacks on neighboring properties unless approved through a notarized written agreement with the affected property owner(s).

9. The proposed development shall be sited far enough from regressing slope faces to project 120 years of useful life for the proposed structure(s) or infrastructure.

10. Any proposed lots must be completely located outside any identified active landslide hazard areas or their associated buffers.

11. Stable landslide hazard areas that are directly adjacent to any riparian areas, or wetlands, may be subject to additional buffer requirements and standards as set forth in Chapter 14.40 EMC, Critical Fish and Wildlife Habitat Areas, or wetlands as set forth in Chapter 14.30 EMC, Wetlands. (Ord. 02-200 § 2).

14.80.050 Buffer requirements.

A. Determining Buffer Widths.

1. The buffer width shall be measured on a horizontal plane from a perpendicular line established at the edge of the active landslide hazard area limits (both from the top and toe of the slope) (see EMC 14.80.070(F), Figure 14.80-6).

2. A buffer of undisturbed vegetation shall be required for an active landslide hazard area. The required buffer width is the greater amount of the following distances:

a. Fifty feet from all edges of the active landslide hazard area limits;

b. A distance of one-third the height of the slope at the top of the active landslide hazard area and a distance of one-half the height of the slope at the bottom of an active landslide hazard area; or

c. The minimum distance recommended by the geotechnical professional, measured from the edges of the active landslide hazard area.

B. Modification of Buffer Widths. The department may require a larger buffer width than the buffer distance, as determined in subsection (A) of this section, if any of the following are identified:

1. The adjacent land is susceptible to severe erosion and erosion control measures will not effectively prevent adverse impacts.

2. The area has a severe risk of slope failure or downslope stormwater drainage impacts. (Ord. 02-200 § 2).

14.80.060 Appendices.

A. Geological Assessment – Landslide Hazard Geotechnical Letter.

B. Geological Assessment – Landslide Hazard Geotechnical Verification.

C. Geological Assessment – Landslide Hazard Geotechnical Report.

APPENDIX A

GEOLOGICAL ASSESSMENT – LANDSLIDE HAZARD GEOTECHNICAL LETTER

A. A geotechnical letter shall include the following:

1. The letter shall be labeled identifying the submittal as a “Landslide Hazard Geotechnical Letter.”

2. The date when the geological assessment was performed. The date when the letter was prepared.

3. The parcel number(s) of the site.

4. Site address, if the city has assigned one.

5. A brief description of the project (including the proposed land use) and a description of the area to be developed. The appropriate professional preparing the geotechnical letter shall provide conclusions and recommendations as to slope stability for the proposed development.

6. A paragraph that states the following specific language:

I meet the qualifications contained in EMC 14.10.060 to prepare a landslide hazard geological assessment. I understand the requirements of the current landslide hazard area Chapter 14.80 EMC and the definitions of the applicable terms contained within EMC 14.10.060. I have performed a landslide hazard geological assessment, conducted a field investigation, and researched historic records on or in the vicinity of the above referenced site and determined that no active landslide hazard area exists within 300 feet of the site.

7. The name, mailing address, and telephone number of the engineering geologist who performed the geological assessment and prepared the letter.

8. The name, mailing address, and telephone number of the property owner.

B. The engineering geologist who prepared the letter shall stamp the letter with his or her license stamp/seal.

C. Geotechnical letters shall be in conformance with a format that is pre-approved by the department.

APPENDIX B

GEOLOGICAL ASSESSMENT – LANDSLIDE HAZARD GEOTECHNICAL VERIFICATION

A. A geotechnical verification shall include the following:

1. The first page of the document shall be labeled identifying the submittal as a “Landslide Hazard Geotechnical Verification.”

2. The date when the geological assessment was performed. The date when the verification document was prepared.

3. The parcel number(s) of the site.

4. Site address, if the city has assigned one.

5. A detailed description of the project (including the proposed land use) and a description of the area to be developed.

6. A description of the surface and subsurface geology, hydrology, soils, and vegetation at the site and a list of the landslide hazard area indicators, as set forth in EMC 14.80.020(A), that were found on or in the vicinity of the site.

7. A summary of the results, conclusions, and recommendations resulting from the geological assessment of the landslide hazards on or in the vicinity of the site. This summary shall address all of the information required in EMC 14.80.030(B).

8. An accurate site plan drawn at a scale of one inch equals 20 feet, one inch equals 30 feet, one inch equals 50 feet (or other scale deemed appropriate by the department) is required. The department may require that the site plan information listed below be based on a field survey by a licensed surveyor. The site plan shall include:

a. The limits/location of the active landslide hazard area(s) set forth in EMC 14.80.020(C)(1).

b. The limits/location of the required landslide hazard buffer based upon the requirements set forth in EMC 14.80.050(A).

c. The location of any existing and proposed structures, utilities, on-site septic systems, wells, and stormwater management facilities.

d. The full geographical limits of the proposed project area (area to be developed).

e. Dimension the closest distance between the identified active landslide hazard area boundary and the project area.

f. Existing topography on the site presented in two-foot contours.

g. Property lines for the site.

h. North arrow and plan scale.

9. A paragraph that states the following specific language:

I meet the qualifications contained in EMC 14.80.030 to prepare a landslide hazard geological assessment. I understand the requirements of the current landslide hazard area Chapter 14.80 EMC and the definitions of the applicable terms contained within EMC 14.10.060. I have performed a landslide hazard geological assessment, conducted a field investigation, and researched historic records on or in the vicinity of the above referenced site and determined that no active landslide hazard area exists within 300 feet of the proposed project area.

10. The name, mailing address, and telephone number of engineering geologist who performed the geological assessment and prepared the verification document.

11. The name, mailing address, and telephone number of the property owner.

B. The engineering geologist who prepared the verification document shall stamp the verification with his or her license stamp/seal.

C. Geotechnical verifications shall be in conformance with a format that is pre-approved by the department.

APPENDIX C

GEOLOGICAL ASSESSMENT – LANDSLIDE HAZARD GEOTECHNICAL REPORT

A. At a minimum, a geotechnical report shall include the following:

1. The first page of the document shall clearly identify the submittal as a “Landslide Hazard Geotechnical Report.”

2. The date when the geological assessment was performed. The date when the geotechnical report was prepared.

3. The parcel number(s) of the site.

4. Site address if the city has assigned one.

5. A detailed description of the project (including the proposed land use) and a description of the area to be developed.

6. A description of the surface and subsurface geology, hydrology, soils, and vegetation of the site and a list of the landslide hazard area indicators, as set forth in EMC 14.80.020(A), that were found on or in the vicinity of the site.

a. The limits/location of the active landslide hazard area(s) set forth in EMC 14.80.020(C)(1). Delineation of the active landslide hazard area limits shall differentiate between areas of historic landslide activity and adjacent unstable areas.

b. The limits/location of the required landslide hazard buffer based upon the requirements set forth in EMC 14.80.050(A).

c. The limits/location of any potential landslide hazard areas that have been designated as stable areas in accordance with EMC 14.80.020(C)(2)(c).

d. The location of any existing and proposed structures, utilities, on-site septic systems, wells, and stormwater management facilities.

e. The full geographical limits of the proposed project area (area to be developed).

f. Location and unique identifier of geotechnical borings, CPT soundings, or other surveys or explorations used to characterize subsurface conditions.

g. Extent of cross-section(s) used to evaluate the three-dimensional subsurface geologic and groundwater conditions at the site.

h. Extent of cross-section(s) used in the evaluation of slope instability.

i. Existing topography on the site presented in two-foot contours.

j. Property lines for the site.

k. North arrow and plan scale.

9. Subsurface characterization data must be provided. The data shall be based on both existing and new information that may include soil borings, test pits, geophysical surveys, or other appropriate subsurface exploration methods, development of site-specific soil and/or rock stratigraphy, and measurement of groundwater levels including variability resulting from seasonal changes, alterations to the site, etc.

a. Conventional geotechnical boring data shall be reported as a graphic log utilizing the following standards:

i. The vertical scale of the graphic log shall be such that five feet of drilled depth is scaled to range of one inch to two inches (1:60- or 1:30-scale), and shall include vertical columns that record depth in one-foot increments, SPT value and incremental blow counts, a graphic pattern representation of the soil type encountered during drilling, and sample descriptions and other comments regarding drilling.

ii. The graphic log shall have a header on the first page that includes a unique identifier for the boring, the times and dates of the start and completion of drilling, the manufacturer and model of the drilling rig, the company name of the drilling contractor, the name(s) of the site geologist(s) or engineer(s) overseeing the drilling activities, the details of the method used to advance the borehole (e.g., four-inch i.d. hollow-stem auger), the type of drilling fluid used to stabilize the borehole, verification that the SPT followed all applicable ASTM standards including a description of the sampler, hammer weight, drop height, the type of hammer used to perform the SPT, number of turns of rope if a cathead is used to raise the hammer, condition of rope (i.e., new, used, frayed, oily, etc.), and the depth of static groundwater measured immediately prior to abandonment of the boring and the time and date of this measurement.

iii. All subsequent pages of the graphic log shall have the unique identifier for the boring, the times and dates of the start and completion of drilling, and the number of the page and the total number of pages comprising the log.

iv. Each SPT value will be reported in the appropriate column showing the blow counts recorded at each six-inch interval, and the sum of the blow counts between penetration distances of six inches to 18 inches, unless refusal conditions (50 or more blows with less than six inches of sampler penetration) are met anywhere in this interval. At refusal, the blow count shall be recorded as the number of blows with the corresponding sampler penetration, in inches.

v. SPT tests shall be performed every five feet during drilling, at a minimum. Additional undisturbed samples, collected following ASTM standards for undisturbed soil sampling, cannot be substituted for SPT testing.

vi. The soil sample descriptions will include the total length of the recovered sample, the soil color, odor, the density or consistency (loose to very dense, very soft to very stiff), degree of water saturation (dry, moist, wet, saturated), and dilatancy. For granular (sand and gravel) soils, the description shall include a physical description of the soil sample, including size distribution (poorly or well graded), angularity, composition, amount and plasticity of the fines fraction. For fine soils (silt and clay), the description shall include a qualitative estimate of the proportion of the silt and clay size particles (e.g., silty clay, clay with some silt, etc.), plasticity, and amount and type of organic material. The sample description shall include a description of any bedding, laminations, slickensides, or other textural or deposition features, including contact between dissimilar soil types. The sample description shall also include a field classification of the soil sample using the Unified Soil Classification System where the classification is expressed in lower case letters (e.g., sp, ml, etc.). The sample classification shall be expressed in upper case letters (e.g., SP, ML, etc.) where subsequent laboratory testing has been performed. This column of the graphic log will also include any other information relevant to the subsurface investigation, such as loss of drilling fluid, heaving, churning of the drill in gravelly soils, etc.

b. CPT sounding data shall be reported as a graphic log utilizing the following standards:

i. The vertical scale of the graphic log shall be such that five feet of penetrated depth is scaled to range of one inch to two inches (1:60- or 1:30-scale), and shall include vertical columns that record depth in one-foot increments.

ii. The graphic log shall have a header on the first page that includes a unique identifier for the boring, the times and dates of the start and completion of the CPT sounding, the manufacturer and model of the CPT system, the company name of the CPT service contractor, the name(s) of the site geologist(s) or engineer(s) overseeing the CPT sounding, and any comments regarding the conduct of the testing, reaction of the CPT system during sounding, etc.

iv. The graphic log shall display, at a minimum, a continuous depth plot of the uncorrected tip resistance, the friction (sleeve) resistance, the friction ratio, and the measured pore pressure with an overlay of the calculated hydrostatic pore pressure. These curves shall be plotted so as to show the full variation of the measured quantities within the depth range of the sounding, and each curve shall have a visible scale with the minimum and maximum ranges labeled.

v. All of the CPT data recorded for each sounding shall also be provided in either electronic or hardcopy format. Electronic data will be presented in an ASCII text file format.

c. Geotechnical borings or CPT soundings will be advanced to a depth sufficient to characterize geologic conditions within and below the existing or potential landslide mass.

d. Other methods used for subsurface characterization shall be assigned a unique identifier, and the basic data presented in appropriate graphical and/or tabular format.

e. The three-dimensional subsurface conditions at the site shall be presented using one or more cross-sections showing location and depth penetration of geotechnical borings, CPT soundings, or other subsurface characterization methods, interpretation of the geometry of major soil units, and projected location of the static groundwater surface determined from the subsurface exploration. The cross-sections shall be presented at a scale of one inch equals 20 feet, one inch equals 30 feet, one inch equals 50 feet (or other scale deemed appropriate by the department). Each cross-section shall have a legend with a description of the various major soil units.

10. Soil strength and index properties (i.e., unit weight, cohesion, etc.) shall be provided for each soil unit interpreted from the subsurface characterization of the site, and shall be presented in tabular format. Justification for the presented values of these soil parameters shall be based on one or more of the following approaches:

a. Back analysis based on pre-landslide stability conditions.

b. Laboratory measurement of strength or other index properties made on soil samples.

c. Correlation of soil strength index properties to other geotechnical indices (e.g., SPT blow counts, etc.), where the correlation relations are documented (e.g., published literatures, in-house empirical data set, etc.).

d. Soil strength and indices based on generic values must provide a clear justification for their use.

11. A detailed description of any prior grading activity, soil instability, or slope failure.

12. Assessments and conclusions regarding slope stability for both the existing and developed conditions shall be presented and documented. These assessments and conclusions shall include:

a. Determination of the potential types of landslide failure mechanisms (e.g., debris flow, rotational slump, translational slip, etc.) that may affect the site.

b. Quantitative stability evaluation of slope conditions of the various failure mechanisms using state-of-the-practice modeling techniques. Limiting equilibrium methods of analysis shall state the stability conditions as a factor of safety. The most unstable failure geometry(ies) shall be presented in the form of a cross-section(s), with the least stable failure geometry for each failure mechanism clearly indicated. The stability evaluation shall also consider dynamic (earthquake) loading, and shall use a minimum horizontal acceleration as established by the current version of the International Building Code.

c. An analysis of slope regression rate shall be presented in those cases where stability is impacted or influenced by erosional processes (e.g., wave cutting, stream meandering, etc.) acting on the toe of the slope.

13. Mitigation recommendations using engineered measures to protect the proposed structure(s) and any adjacent structures, infrastructure, adjacent wetlands, or critical fish and wildlife habitat from damage or destruction as a result of proposed construction activities shall be designed by a professional engineer. The geotechnical report shall contain:

a. Design plans and associated design calculations for engineered structures or drainage systems (e.g., structural foundation requirements, retaining wall design, etc.).

b. Recommendations and requirements pertaining to the handling of surface and subsurface runoff in the developed condition.

c. Identification of necessary geotechnical inspections to assure conformance with the report mitigation and recommendations.

d. Proposed angles of cut and fill slopes, site grading requirements, final site topography (shown as two-foot contours), and the location of any proposed structures, on-site septic systems, wells, and stormwater management features or facilities associated with the development detailed within the body of the report and shown on a site map at the same scale as that required in subsection (A)(8) of this appendix.

e. Soil compaction criteria and compaction inspection requirements.

f. An analysis that indicates how the proposal meets the standards outlined in EMC 14.80.040.

g. Structural foundation requirements and estimated foundation settlement shall be provided if structures are proposed.

h. Lateral earth pressures.

i. Suitability of on-site soil for use as fill.

j. Mitigation measures for building construction on each lot for short plats, large lots, or formal plats such that additional geotechnical professional involvement is minimized during building construction.

B. The geotechnical report shall be prepared by an engineering geologist and shall be cowritten by both an engineering geologist and professional engineer where both geological interpretations and engineering analyses and designs are necessary or prudent in the mitigation of the landslide hazard.

C. The geotechnical professional(s) who prepared the geotechnical report shall stamp the report with his or her license stamp/seal.

D. 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.

E. Geotechnical reports shall be in conformance with a format that is pre-approved by the department. (Ord. 05-247 § 1; Ord. 02-200 § 2).

14.80.070 Figures.

A. Figure 14.80-1, Landslide Hazard Indicators.

B. Figure 14.80-2, Potential Landslide Hazard Areas.

C. Figure 14.80-3, Potential Landslide Hazard Areas – Slopes Greater than 20 Percent.

D. Figure 14.80-4, Interim Areas Between Landslide Hazard Areas.

E. Figure 14.80-5, Landslide Hazard Area Review.

F. Figure 14.80-6, Required Buffers for Active Landslide Hazard Areas.

(Ord. 02-200 § 2).