§ 24.35.160. Geologic hazards—Additional requirements for geologic assessments in landslide hazard areas.

In addition to the requirements in TCC 24.35.130, the following shall be required for geologic assessments in landslide hazard areas, as specified:

A.

The geological assessment for a landslide hazard area shall be prepared by a geotechnical professional who is licensed as a professional engineer or engineering geologist with a minimum of four years of relevant professional employment, as determined by the director.

B.

The geological assessment shall be submitted in the form of a geotechnical letter when the geotechnical professional finds that no landslide hazard area exists within three hundred feet of the development proposal site. A geotechnical letter shall, at a minimum, include the following:

1.

The letter shall be labeled, identifying the submittal as a "landslide hazard geotechnical letter" and shall include all mandatory elements listed in TCC 24.35.130.

2.

The geotechnical letter shall be prepared under the responsible charge of a geotechnical professional(s) and be signed, sealed and dated by the geotechnical professional(s).

C.

The geological assessment shall be submitted in the form of a geotechnical evaluation when the geotechnical professional finds that a landslide hazard area exists, but is located more than three hundred feet away from the proposed project area, and in their opinion, will not impact the subject site. A geotechnical evaluation shall, at a minimum, include the following:

1.

The cover letter for the document shall clearly identify the submittal as a "landslide hazard geotechnical evaluation" and shall include all mandatory elements listed in TCC 24.35.130.

2.

A site plan, as defined in TCC 24.35.130(H)(9).

3.

A description of the surface and subsurface geology, hydrology, soils, and vegetation on the site, conclusions and recommendations regarding the effect of geologic conditions on the proposed development, the effect of the proposed development on geologic conditions, and opinions and recommendations on the stability or instability of the site over the life of the project.

D.

The geological assessment shall be submitted in the form of a geotechnical report when the geotechnical professional finds that 1) a landslide hazard area exists within three hundred feet of the proposed project area; or 2) a landslide hazard area is located more than three hundred feet away from the proposed project area, but in their opinion, will impact the subject site. A geotechnical report shall, at a minimum, include the following:

1.

The cover letter for the document shall clearly identify the submittal as a "landslide hazard geotechnical report" and shall include all mandatory elements listed in TCC 24.35.130.

2.

A site plan, as defined in TCC 24.35.130(H)(9).

3.

A description of the surface and subsurface geology, hydrology, and soils on the site; a list of the landslide hazard indicators; conclusions and recommendations regarding the effect of geologic conditions on the proposed development and the effect of the proposed development on geologic conditions;

4.

Assessment of the role of existing vegetation on maintaining slope stability on site;

5.

Subsurface characterization data must be provided. The data shall be based on both existing and new information that may include soil borings (SPT or other appropriate driven sample collection methods), test pits, geophysical surveys, or other appropriate subsurface exploration methods, as approved by the director, development of site-specific soil and/or rock stratigraphy, and measurement of groundwater levels including variability resulting from seasonal changes, alterations to the site, and other factors as determined by the director.

6.

The geotechnical or boring data shall provide sufficient information for the geotechnical professional to determine slope stability. A written explanation shall be provided and include the logs outlining how the work was performed (equipment, company, drillers, etc.), weather, classification systems, and other information that incorporates all of the variables related to project performance.

7.

The soil classification shall meet the requirements of Title 14 TCC, Buildings and Construction.

8.

The three-dimensional subsurface conditions at the site shall be included in the report.

9.

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.

10.

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

11.

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.

Evaluation 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 as determined by the director. 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 Title 14 TCC, Buildings and Construction.

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.

12.

Mitigation recommendations using engineered measures and any relevant best available science 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 required by the review authority.

e.

Soil compaction criteria and compaction inspection requirements.

f.

An analysis that indicates how the proposal meets the standards outlined in TCC 24.15.030-24.15.240.

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.

(Ord. No. 14773, § 3(Att. B), 7-24-2012)