17A.30.030 Soil Engineering – Stability

17A.30.030 Soil Engineering – Stability.

A. When an applicant proposes to clear, grade, cut, or fill on existing slopes steeper than 33 percent and ten feet or more in height, or when an applicant proposes to construct slopes steeper than 50 percent and ten or more feet in height, or is directed by the Manual, the County may require any of the following:

1. The applicant retain a geotechnical professional to submit design and construction recommendations.

2. The applicant retain a geotechnical professional and/or testing company to perform special inspections of the proposed clearing, grading, and filling.

3. The applicant retain a geotechnical professional to prepare a stability report.

4. The geotechnical professional's recommendations be incorporated into the construction plans, construction specifications, and if applicable, the subdivision documents.

5. The applicant retain a geotechnical professional to review construction plans and specifications or subdivision documents for conformance with the geotechnical professional's recommendations and requirements.

6. The applicant retain a professional engineer to prepare an abbreviated plan in accordance with Section 3.1 of the Manual.

B. At a minimum, a stability report shall include the following:

1. The first page of the document shall clearly identify the submittal as a "Stability Report."

2. The date when the stability report was prepared.

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

4. Site address if one has been assigned by the County.

5. A detailed description of the project and a description of the project area.

6. A description of the surface and subsurface geology, hydrology, soils, and vegetation of the site.

7. An accurate site plan drawn at a scale of 1" = 20', 1" = 30', 1" = 50' (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 location of any existing and proposed structures, utilities, on-site septic systems, wells, and stormwater management facilities.

b. The full geographical limits of the proposed project area.

c. Extent of cross-section(s) used in the evaluation of slope stability.

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

e. Property lines for the site.

f. North arrow and plan scale.

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

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

8. 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, 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:

(1) The vertical scale of the graphic log shall be such that 5 ft. of drilled depth is scaled to range of 1" to 2" (1:60- or 1:30-scale), and shall include vertical columns that record depth in 1 ft. increments, SPT value or equivalent value, and incremental blow counts, a graphic pattern representation of the soil type encountered during drilling, and sample descriptions and other comments regarding drilling.

(2) 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., 4" i.d. hollow-stem auger), the type of drilling fluid used to stabilize the borehole, indication that the SPT was completed in accordance with applicable ASTM standards or other appropriate driven sample collection methods are specified including a description of the sampler, hammer weight, drop height, the type of hammer used to drive the sampler, 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.

(3) 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.

(4) Each SPT value or equivalent value will be reported in the appropriate column showing the blow counts recorded at each 6" interval, and the sum of the blow counts between penetration distances of 6" to 18," unless refusal conditions (50 or more blows with less than 6" 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.

(5) SPT tests shall be performed every 5 feet during drilling, at a minimum.

(6) 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:

(1) The vertical scale of the graphic log shall be such that 5 ft. of penetrated depth is scaled to range of 1" to 2" (1:60- or 1:30-scale), and shall include vertical columns that record depth in 1 ft. increments.

(2) 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.

(4) 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.

(5) 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 1" = 20', 1" = 30', 1" = 50' (or other scale deemed appropriate by the Department). Each cross-section shall have a legend with a description of the various major soil units.

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, 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.

10. Assessments and conclusions regarding slope stability for the 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 Building Code as adopted by Title 17C PCC.

c. Static and dynamic factors of safety for the developed conditions.

d. Potential landslide hazard buffer and setback impacts to neighboring properties.

11. Mitigation recommendations using engineered measures to protect any proposed structure(s) and any adjacent structures, infrastructure, downstream receiving waters, adjacent wetlands, or critical fish and wildlife habitat from damage or destruction as a result of proposed construction activities. The recommendations shall contain:

a. Requirements pertaining to the handling of surface and subsurface runoff in the developed condition.

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

c. Proposed angles of cut and fill slopes, site grading requirements, final site topography (shown as 2' 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 number 7. above.

d. Soil compaction criteria and compaction inspection requirements.

e. Structural foundation requirements and estimated foundation settlements if structures are proposed.

f. Lateral earth pressures.

g. Suitability of onsite soil for use as fill.

h. Erosion protection measures.

i. Permanent stabilization measures.

C. The stability report must be prepared under the responsible charge of a geotechnical professional.

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

E. The Department may request a geotechnical professional to provide additional information in the stability report based upon existing conditions, changed conditions, or unique circumstances occurring on a case by case basis.

F. Stability reports shall be in conformance with a format that is pre-approved by the Department.