HomeMy WebLinkAboutGeoTech Information - BLD Engineering / Geo-tech Reports - 10/5/1998 COMMUNITY DEVELOPMENT
Solid Waste Planning Utilities
[ N T : E R
MEM-0
O F F I C - E
DATE: OCTOBER 51 1998
TO: DEPARTMENT OF COMMUNITY DEVELOPMENT - PLANNING
DIVISION
FROM: DIRECTOR OF COMMUNITY DEVELOPMENT-
SUBJECT: GEOTECHNICAL INFORMATION
In September 1996 1 forwarded to each of you a memorandum clarifying-
some items related to geotechnical assessments and geotechnical.reports:
The areas of concern included-the Interim Resource Ordinance, Section
17.0f-100 E.3 (Landslide Hazard Areas) and Section 17.01.0. E.1.b
(Critical Aquifer'Recharge Area).
am taking this opportunity to replace the September 26, 1998
MEMORANDUM and replace with the following updated POLICY;
POLICY
1. A Geotechnical Report shall be prepared for slopes 40% or
greater by a licensed civil engineer with specialized knowledge
of geological engineering, except that areas composed of
consolidated rock without indications of earth movement, such
as rock falls, may be reviewed by a geological assessment to
determine if a geological report is warranted.
2. Areas with slopes between 15 percent (8.5 degrees) and 40
percent (21.6 degrees) will require a Geological Assessment,
and may further require a Geotechnical Report, upon
analysis of the following by the Director (the applicant can
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October 5, 199$
decide to complete a geotechnical report if he/she would rather
do so without first preparing a geological assessment):
(1) Lot size and use;
(2) Overall height of slope and maximum planned cut or
fill (requires grading plan);
(3) Soil types and history of sliding in the vicinity. (from
the geological assessment);
(4) Groundwater conditions, including depth to wafer
and quantity.of surface seepage (from the
geological assessment);
(5) Approximate depth to hard or dense competent soil,
e.g. glacial till or outwash sand (from:geological
assessment);
(6) Impervious surfaces and drainage schemes
(requires development/grading plan);
(7) Wastewater treatment (requires on-site sewage
disposal system approval from Mason County
Department of Health);
(8) Potential off-site impacts, including adjacent
-properties, roadways, etc.
3. For slopes less than 15 % a Geological Assessment or
Geological Report will be required if warranted.
4. Areas with any visible signs of earth movement such
as debris slides, earthflows, slumps and rockfalls, or
areas within 300 feet of previously mapped or
recorded landslides will require a Geotechnical
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October 5, 1998
Report. For sites more•than 300 feet but within 660 feet
of these areas, will require a G.eotechnical Assessment or
Geotechnical Report will be required (preparation to be
determined at the discretion of the Director of Community
Development as warranted)
5. Areas within 150 feet of over steepened.or otherwise potentially
unstable slopes as a result of stream incision, stream bank
erosion, and undercutting by wave action will require a
Geotechnical Report.
6. The Geologic Assessment shall be prepared at the discretion of
the Director by either a licensed civil engineer with specialized
knowledge of geotechnica(/geological engineering or a
practicing engineering geologist with special knowledge of the
local conditions. The Geotechnical Report shall be prepared at
the discretion of the Director by a licensed civil engineerwith
specialized knowledge of geotechnical/geological engineering.
The Geotechnical Report may also be prepared by a practicing
engineering geologist with special knowledge of the local
conditions, provided the work is performed under the
supervision of a licensed civil engineer who will stamp the
report and attest to the competency-of the engineering geologist
to perform landslide evaluations in accordance with the
prevailing standard of practice.
7. A Geological Assessment shall include but not be limited to the
following:
(1) A discussion of geologic conditions in the general
vicinity of the project, and specific soil types at the
project site. Soil type identifications shall be
consistent with terminology used in the Coastal
Zone Atlas (Washington Department of Natural
Resources, 1980) or in applicable U.S. Geologic
Survey maps (e.g. Geological Map of North Central
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October 5, 1998,
Mason County, by R.J. Carson, 1976, U.S. Geologic
Survey OFR 76-2). Use of Soil Conservation
Service soil layer terminology is considered
inappropriate for this assessment.
(2) A discussion of the ground water conditions at the
site, including the depth to water and the quantity. of
surface seepage.
(3) The approximate depth to hard or dense competent
soil, e.g. glacial fill or outwash sand.
(4) A discussion of the history of landslide activity in the
vicinity, available in the-Coastal Zone Atlas, the map
of"Relative Slope Stability-of the Southern Hood
Canal Area, Washington" by M. Smith and R.J.
Carson, 1977; and the landslide records on file with
the Mason County Department of Community
.Development.
(5) An opinion on the potential.for landslide activity at
the site in*light of the proposed development.
8. A Geotechnical Report shall include but not be limited to the
following:
(1 ) A discussion of general geologic conditions, specific
soil types, groundwater conditions, and history of
landslide activity in the vicinity as required for the
Geologic Assessment described above.
(2) A site plan which identifies the important
development and geologic features.
(3) Locations and logs of exploratory holes or probes.
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October 5, 1998
(4) .A minimum of one cross section at a scale which
adequately depicts the subsurface profile, and
which incorporates the details of proposed grade
changes.
(5) A description and results of slope stability analysis
performed for both static and seismic loading
conditions.
(6) Appropriate restrictions on placement of drainage
features, septic drain fields and compacted fills and
footings, including recommended setbacks from
shoreline bluffs and the tops of other slopes on the
property.
(7) A detailed clearing and grading plan which
specifically identifies vegetation to be removed, a
schedule for vegetation removal and replanting, and
the method of vegetation.removal.
(8) A detailed temporary erosion control plan which
identifies the specific mitigating measures to be
implemented during construction to protect the
slope from erosion, landslides and harmful
construction methods.
(9) An analysis of both on-site and off-site impacts of
the proposed development.
(10) Specifications of development conditions such as,
vegetative management, drainage, and buffer
widths.
17.01.100 LANDSLIDE HAZARD AREAS
The purpose of the Landslide Hazard Section is to identify areas that present potential dangers to public health and
safety, to prevent the acceleration of natural geological hazards, and to neutralize the risk to the property owner
or adjacent property owners from development activities.
A. CLASSIFICATION
The following shall be classified as potential Landslide Hazard Areas:
1. Areas with any indications of earth movement such as debris slides, earthf lows, slumps and rock
falls (see figure F.100); or
2. Areas with artificial oversteepened or unengineered slopes, i.e. cuts or fills.
3. Areas with slopes containing soft or potentially liquefiable soils.
4. Areas unstable as a result of stream incision, stream bank erosion, and undercutting by wave
action.
5. Slopes greater than 15% (8.5 degrees) except areas composed of consolidated rock and having
either of the following:
a. Steep hillsides intersecting geologic contacts with a relatively permeable sediment
overlying a relatively impermeable sediment or bedrock; and
b. Springs or groundwater seepage; or
The following criteria may be used as a guide by the County to indicate areas that have a higher likelihood
of meeting the classification criteria above:
1. The areas identified on the Mason County Soil Survey Map as having slopes greater than 15%.
2. The areas identified on the Coastal Zone Atlas, Volume 9, of Mason County, Washington as:
a. Unstable - "U"
b. Unstable Old Slides - "UOS"
C. Unstable Recent Slides - "URS"; or
3. The areas identified as Class 2, 3, 4, or 5 of the map of "Relative Slope Stability of the Southern
Hood Canal Area, Washington", Washington State Department of Natural Resources, Division of
Earth Resources, 1977.
B. DESIGNATION
1. Lands of Mason County meeting the classification criteria for Landslide Hazard Areas are hereby
designated, under RCW 36.70A.060 and RCW 36.70A.170, as critical areas requiring immediate
protection from incompatible land uses.
2. Upon an application for development on either mapped or unmapped lands, the Director shall
determine if a hazard exists on a particular site based on:
a. Information supplied by the applicant in the form of a geotechnical report,
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b. Actual physical observation of the site,
C. Existing County Hazard Area maps, or
d. Other means determined to be appropriate.
If the presence of a hazard is determined,the boundaries of the hazard and associated buffers shall
then be delineated (top, both sides, and toe) on a geologic map of the site.
C. LAND USES
1. Exempt Uses
a. The growing and harvesting of timber, forest products and associated management
activities in accordance with the Washington Forest Practices Act of 1974, as amended,
and regulations adopted pursuant thereto; including, but not limited to, road construction
and maintenance; aerial operations; applications of fertilizers and pesticides; helispots; and
other uses specific to growing and harvesting timber forest products and management
activities, except those Forest Practices designated as "Class IV -General Forest Practices"
under the authority of the "1992 Washington State Forest Practices Act Rules and
Regulations", WAC 222-12-030;
b. Those activities and uses conducted pursuant to the Washington State Surface Mining
Act, RCW 78.44 and its Rules and Regulations, where State law specifically exempts local
authority;
C. Existing and ongoing agriculture, aquaculture, floriculture, horticulture, general farming,
dairy operating under best management practices.
2. Permit Required Uses
Permits are required for all new construction, grading and other uses subject to Section 17.01.050,
and any Class IV Conversion Permit pursuant to the State Forest Practices Act which involves
conversion to a Permit Required Use, and are within a Landslide Hazard Area or its buffer. Permit
Required Use may require a Geotechnical Report, see Section 17.01.100.E.
D. DEVELOPMENT STANDARDS
Any land use on Landslide Hazard Areas or their buffers shall conform to the following standards:
1. Grading
a. No grading shall be performed in landslide or slide prone areas prior to obtaining a grading
permit subject to approval, by the Director, based on recommendations contained in the
geotechnical report with slope stability analysis and grading recommendations.
b. Clearing, grading and other construction activities shall not aggravate or result in slope
instability or surface sloughing.
C. Undergrowth shall be retained to the extent feasible.
d. Clearing methods which minimize soil disturbance shall be used.
e. No fill, dead vegetation (slash/stumps), or other foreign material shall be placed within a
Landslide Hazard Area.
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f. Fills for bulkheads constructed for landslide stabilization measures approved under the
Shoreline Master Program shall be consistent with recommendations specified in a
Geotechnical Report.
2. Vegetative Management
a. There shall be minimum disturbance of trees and vegetation in order to minimize erosion
and stabilize Landslide Hazard Areas. Limbing trees for view purposes is preferred over tree
removal.
b. Vegetation removal on the slopes of banks between the ordinary high-water mark and the
top of the bank shall be minimized due to the potential for erosion.
3. Drainage
a. Surface drainage, including downspouts, shall not be directed onto or within 50 feet above
or onto the face of a Landslide Hazard Area or its associated buffer. If drainage must be
discharged from the top of a Landslide Hazard Area to below its toe, it shall be collected
above the top and directed to below the toe by tight line drain and provided with an energy
dissipating device at the toe.
b. Stormwater retention and detention systems, including percolation systems utilizing buried
pipe or french drain, are strongly discouraged unless a licensed civil Jenaineer shall certify:
(1) The systems will not affect slope stability, and
(2) The systems were installed as designed.
4. Sewage Collection/Treatment Systems
Sewage collection and treatment systems shall be located outside of the Landslide Hazard Areas
and associated buffers, unless an approved geotechnical report specifies appropriate mitigation
measures. See Section 17.01.100.E.
5. Lot Size
For the purpose of determining lot sizes under Title 16 of the Mason County Code the Director shall
review any available information and any required Geotechnical Reports under Section
17.01.100.E, and make a decision on a case-by-case basis based on the reports.
6. Buffers
a. A 50 foot (15.25 meter) buffer of vegetation is required around the Landslide Hazard Area.
b. Upon finding substantial evidence that the proposed development is to be located near or
within a hazardous area, the Director may require a Geotechnical Report pursuant to
Section 17.01.100.E. Based on the results of the Geotechnical Report, the Director may
increase the buffer.
C. The applicant may request a reduction in the buffer. A determination shall be made by the
Director based on the results of the Geotechnical Report pursuant to Section 17.01.1 00-E.
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E. GEOTECHNICAL REPORT
1. Applicability
Every application for development within a Landslide Hazard Area or its buffer or within 660 feet
(198 meters) of the buffer shall meet the standards of Section 17.01.100.D and may require a
Geotechnical Report:
a. Areas with slopes greater than 33 percent (18.4 degrees) and are designated as a potential
Landslide Hazard Area will require an Geotechnical Report.
b. Areas with any visible signs of earth movement such as debris slides, earthflows, slumps
and rockfalls will require a Geotechnical Report.
C. Areas unstable as a result of stream incision, stream bank erosion, and undercutting by
wave action and are designated as a potential Landslide Hazard Area will require an
Geotechnical Report.
d. Slopes between 15 percent(8.5 degrees) and 33 percent (18.4 degrees); and are designated as
a potential Landslide Hazard Area may require an Geotechnical Report upon analysis of the
following factors by the Director:
(1) Lot size and use;
(2) Maximum cut slope (requires a grading plan);
(3) Soil types and properties (requires a geotechnical report);
(4) Groundwater depth and volume (requires a geotechnical investigation);
(5) Depth to glacial till (hardpan) or other competent soils (requires geotechnical
information);
(6) Impervious surfaces and drainage schemes (requires development/grading plan);
(7) Wastewater treatment (requires on-site sewage disposal system approval from
Mason County Department of Health);
(8) Potential off-site impacts (requires environmental statement dependant on scope
of project); or
2. Waiver of Report
The Director may waive some or all of the requirements for the report upon a written finding, by
a person qualified to assess landslide hazards, that the proposed development would not cause .
significant adverse impacts, or there is adequate geological information available on the area
proposed for development to determine the impacts of the proposed development and appropriate
mitigating measures.
3. Qualifications of Preparer
The Geotechnical Report shall be prepared at the discretion of the Director, by either a licensed
civil engineer with specialized knowledge of geological engineering or a practicing engineering
geologist with special knowledge to the geology of Mason County.
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4. Content of Report
a. A Geotechnical Report shall include but not be limited to the following:
(1) A description of the soil and geologic conditions of the proposed development,
including a geologic map with a cross section showing site geologic conditions at
a scale of 1 inch equals 200 feet, and opinions and recommendations of the
adequacy of the site to be developed.
(2) A detailed temporary erosion control plan which identifies the specific mitigating
measures to be implemented during construction to protect the slope from erosion,
landslides and harmful construction methods.
(3) A detailed clearing and grading plan which specifically identifies vegetation to be
removed, a schedule for vegetation removal and replanting, and the method of
vegetation removal.
(4) An analysis of both on-site and off-site impacts of the proposed development.
(5) Specifications of development conditions such as, vegetative management,
drainage, and buffer widths.
5. Administrative Determination
Any area in which the report indicates the presence of landslide hazards shall not be subjected to
development unless the report demonstrates conclusively that the hazards can be overcome, and
that the development meets all standards in Section 17.01.100.D. Hazards must be overcome in
such a manner as to prevent harm to public health, safety, and property.
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