Chapter 17A.30
GRADING Revised 3/18

Sections:

17A.30.010    Excavation Standards. Revised 3/18

17A.30.020    Fill Standards.

17A.30.030    Soil Engineering – Stability. Revised 3/18

17A.30.040    Creation of Building Pads within Subdivisions.

17A.30.050    Surface Mines.

17A.30.010 Excavation Standards. Revised 3/18

A.    Cut Slopes.

1.    Slopes shall be no steeper than is safe for the intended use and shall not be steeper than 2 horizontal to 1 vertical, or as recommended by a soils Engineer.

2.    The catch point of the top of the slope shall be set back from the site boundary line in accordance with the following table, unless a retaining wall is designed by the Engineer and constructed for the project.

B.    Setback from Property Lines.

Cut Depth

Setback Distance

Under 5 Feet

2 Feet

5 – 20 Feet

Height/2

Over 20 Feet

10 Feet

C.    Cut slopes shall be constructed in a manner such that a factor of safety of at least 1.5 is achieved for static conditions and at least 1.2 for seismic dynamic conditions. Analysis of dynamic (seismic) conditions shall be based on a minimum horizontal acceleration as established by the most current version of the Building Code as adopted by Title 17C PCC.

D.    The proposed cut slopes shall not create the need for landslide hazard buffers or setbacks on neighboring properties unless approved through a notarized written agreement with the property owner.

E.    Interior cut slopes for surface mining projects which are part of an active mining operation are exempt from the 2:1 maximum cut slope angle requirement as long as access to the mine is controlled via fencing or other security measures to discourage entrance by unauthorized persons. Interior slopes are those slopes that do not impact the theoretical slope face that is created by applying a 2 horizontal to 1 vertical angle and the required setbacks to the property line. Cut slopes along the mine's property boundaries are allowed to be steeper than 2:1 if the applicant can show through the submittal of a stability study that there will not be a negative impact to neighboring properties in the form of stability impacts, increased setback requirements or increased buffer distances.

F.    Refer to the Pierce County Stormwater Management and Site Development Manual, Volume 2, for additional excavation techniques.

(Ord. 2017-28s § 3 (part), 2017; Ord. 2006-103s § 5, 2006; Ord. 2004-56s § 1 (part), 2004; Ord. 99-24S § 2 (part), 1999)

17A.30.020 Fill Standards.

A.    Fills which are intended for building sites shall be constructed in conformance with the requirements of the latest edition of the I.B.C., as adopted by Pierce County, and an assignment of allowable soil-bearing pressures will be under the jurisdiction of the Pierce County Building Official in accordance with the I.B.C.

B.    Slopes shall be no steeper than is safe for the intended use and shall not be steeper than one and one half horizontal to one vertical, or as recommended by a soils Engineer. Fill sites must be approved by the Engineer as suitable locations for the proposed fill.

C.    The ground surface for fills over five feet in height shall be prepared by removing vegetation, noncomplying fill, topsoil, and other unsuitable materials; scarifying to provide a bond with the new fill; and, where existing slopes are steeper than five horizontal to one vertical, by benching into competent material as determined by the Engineer. The bench under the toe of a fill on a slope steeper than 5 horizontal to 1 vertical shall be at least 10 feet wide or as recommended by a soils Engineer.

D.    Except as permitted by the County, no material other than earth material shall be buried or placed in fills. Placement of other than earth material is regulated by State statutes or Federal laws, and additional permits may be required.

E.    Fills shall be constructed using earth materials, compaction methods, and construction techniques so that stable fills are created.

F.    The toe or catch point of fill slopes shall be set back from the site boundary line in accordance with the following table unless a retaining wall is designed by the Engineer and constructed for the project:

Fill Depth

Setback Distance

Under 5 Feet

2 Feet

5 – 40 Feet

Height of Fill/2

Over 40 Feet

20 Feet

G.    Slopes constructed with fill shall be created in a manner such that a factor of safety of at least 1.5 is achieved for static conditions and at least 1.2 for seismic dynamic conditions. Analysis of dynamic (seismic) conditions shall be based on a minimum horizontal acceleration as established by the most current version of the Building Code as adopted by Title 17C PCC.

H.    The proposed fill slopes shall not create the need for landslide hazard buffers or setbacks on neighboring properties unless approved through a notarized written agreement with the property owner.

I.    Interior fill slopes for surface mining projects which are part of an active mining operation are exempt from the 1.5:1 maximum fill slope angle requirement as long as access into the mine is controlled via fencing or other security measures to discourage entrance to unauthorized persons. Interior slopes are those slopes that do not impact the theoretical slope face that is created by applying a 1:5 horizontal to 1 vertical angle and the required setbacks to the property line.

(Ord. 2008-59s § 2 (part), 2008; Ord. 2004-56s § 1 (part), 2004; Ord. 99-24S § 2 (part), 1999)

17A.30.030 Soil Engineering – Stability. Revised 3/18

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.

(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)    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.

(Ord. 2017-28s § 3 (part), 2017; Ord. 2008-59s § 2 (part), 2008; Ord. 2004-56s § 1 (part), 2004)

17A.30.040 Creation of Building Pads within Subdivisions.

A grading plan with existing and proposed contours must be provided for all short plats, large lots, or formal plat that identifies the earthwork required for the construction of the subdivision. The storm drainage runoff patterns that will result from the proposed grading plan must be consistent with the stormwater design contained within the project's drainage report. Grading per the approved plans must be completed and accepted for the entire subdivision prior to issuance of building permits on individual lots except where the subdivision was approved to be constructed in phases. The grading plan must include grading of individual lots when it is necessary to ensure consistency with the storm drainage runoff patterns contained within the project's drainage report. Approved grading plans may or may not include grading of building pads at the discretion of the developer. (Ord. 2004-56s § 1 (part), 2004)

17A.30.050 Surface Mines.

A.    For surface mines, the following items must be constructed prior to creating impacts, but no later than three years from the date of site development permit issuance:

1.    Permanent/long-term stormwater facilities.

2.    Other facilities necessary to control stormwater quality or quantity (such as parking lot pavement).

3.    Access driveway/road connecting the operation/processing area to the off-site private or public roads.

B.    Construction of additional permanent/long-term stormwater facilities that will not be needed until a later phase in the mining activity, or modifications to the original stormwater facility to accommodate a future phase in the mining activity, shall require the submittal of a new site development permit application. Construction of the additional stormwater facilities or modifications to the original facilities must be completed in the timeframes stated above.

C.    The storm drainage controls (temporary swales, temporary berms, etc.) that are necessary to control stormwater runoff in areas being mined must be identified in the Maintenance and Source Control Report. The long-term stormwater pollution controls that are necessary for this continuous, earth disturbing activity must also be identified in the Maintenance and Source Control Report.

(Ord. 2015-48s § 1 (part), 2015; Ord. 2004-56s § 1 (part), 2004; Ord. 99-24S § 2 (part), 1999)