HomeMy WebLinkAboutGeoTech Assessment for BLD2002-00179 - BLD Engineering / Geo-tech Reports - 5/14/2002 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 98584
DATE: May 141h, 2002
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Grace Miller, DCD -Planner
FROM: Alan A. Tahja, P/W- Co. Hydr. Eng. Work Order# PLG-02
SUBJ: Geo-Tech Assessment Review NAME: Franzel SFR (a, 500 E Nichole—Hrstn
BLD2002-00179
Grace,
A geotechnical assessment prepared for the proposed Jack Franzel Single Family Residence
(SFR) at 500 East Nichole Lane, Harstine Island, has been received and reviewed by Public
Works.
Though the assessment appears to dispel the need for a more detailed document in the form of a
geotechnical report, it contains details that indicate a geotechnical report should have been
developed for this location and proposed development.
Current County Critical Resource Ordinance Regulations require a 50 foot vegetated buffer
between a landslide hazard area and property development. The assessment promotes approval of
a building location 35 feet back from the present top of the bluff, and that said recommended
setback will provide a service life of 50 years. The applicant should pursue approval for a
reduction in the 50' setback as provided in the County's Critical Resource Ordinance section
17.01.100 D 6c.
Because the proposal encroaches on required setbacks, and the property includes unstable slopes
with building proposed within 35' of these identified unstable slopes, a geotechnical report
conforming to Mason County's Critical Resource Ordinance requirements should be prepared by
a licensed engineer. Support and approval of this proposal in its present form should not be
given.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel any features need further discussion or attention.
S' rely,
7�-
'Ailn A. Tah'
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Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
PO Box 12267, Olympia WA 98508-2267
Phone 360-357-7883 • FAX 360 867-9307
22 April 2002
Mr. Jack Franzel PLANNING
5805 106th NE
Kirkland, Washington 98033
Subject: Geotechnical assessment of your proposed
building site at 500 East Nichole,
Hartstene Island, Mason County Tax
Parcel Number 22026-10-90083 .
Dear Mr. Franzel:
As requested, we have conducted a reconnaissance level
survey of the above property to determine if future
soil movements would adversely affect the soils
supporting the proposed single-family residence. This
letter will present our observations and opinions
based on this survey and on our research.
Bradley-Noble has provided geotechnical consulting
services for properties on either side of this lot.
In March 1995 we prepared a report for 480 East
Nichole Lane for Mr. Brock Laffoon. In December of
1995 we prepared a report for Mr. Dale Eklund who is
constructing a residence at 570 East Nichole Lane. We
understand that you have: obtained copies of these
reports from Mason County and have reviewed our
recommendations for site development on these adjacent
lots .
We have visited your property in order to prepare this
report. At the time of our visit installation of the
septic system was underway. Falling of trees between
the top of bluff and the toe of the side cast fill
from the septic: system installation had occurred.
The project site slopes down from East Nichole Road
towards Pickering Passa(pe. Slopes in the upland area
where construction is proposed are moderate. The
marine bluff is steep to nearly vertical. Estimated
vertical relief of the marine bluff is estimated in
the field to be about 35 feet on average.
02040601
Page 1 of 4
02040601
Page 2 of 4
Two soil units are found at this site. Soil exposures
were available in the area of the septic system
installation and in the bluff face. We find at this
site a thin layer of Vashon recessional outwash soils
of sands and gravels overlying the very dense and
highly overconsolidated Vashon subglacial till . The
recessional outwash soils are permeable. The
subglacial till is considered to be impermeable due to
cementing. We expect that flows of ground water
develop on the surface of the subglacial till during
periods of heavy or prolonged rainfall .
The Coastal Zone Atlas, published by the Department of
Ecology in March 1980, Map MA-16 , presents general
information on the site geology and evaluation of the
apparent slope stability. We also reviewed geologic
information presented in the Geology of Southeastern
Mason County, Washington, by John B. Noble and Dee
Molenaar in Plate 1 of the Water-Supply Bulletin 29,
published jointly by the State of Washington's
Department of Natural Resources, Division of Geology
and Earth Resources, and the U.S . Geological Survey
for Mason County, Washington.
Map MA-16 indicates the slope as being stable in the
upland area and of intermediate stability in the bluff
and the upland area immediately adjacent to the
bluff . The area indicated for construction would be
in the area considered to be stable. We concur with
the information presented on site stability. We
consider the ravine slope to the north of the building
site to be stable, and at the proposed location of the
new residence we do not expect that erosion of the
ravine slope would have any adverse effect on the
proposed construction.
From our conversation, we understand that the
foundation for the new residence will be constructed
about 35 feet back from the present top of the bluff .
Site constraints with setback from drain fields and
general topography would make moving the structure
upslope difficult or would require a major redesign of
the project. The proposed setback is consistent with
other structures that have been constructed along this
section of Hartstene Island. We have no objection to
the construction of decks or other low-value
structures in the setback area.
02040601
Page 3 of 4
We expect that you will be excavating into the slope
in order to create a building pad for your new
residence. We also expect that the excavation will
penetrate through the recessional outwash and expose
the subglacial till . The subglacial till offers
excellent support for standard spread footings
supporting typical residential type of loadings . To
ensure uniform support, all footings should be founded
on the subglacial till .
We understand that a daylight basement will be
included in the design solution. Attention to
perimeter drainage must be addressed in your site
plans . Flows of ground water on the surface of the
subglacial till may be significant. At a minimum, you
need to include a perimeter footing drain. We also
recommend that positive drainage be provided for the
basement slab. Backfill of the foundation wall should
use clean, freely draining gravel, or a manufactured
drainage board installed per the manufacturer's
specification may be installed. This will prevent the
build-up of hydrostatic pressures against the
foundation wall .
We recommend that the basement level slab and the
second level floor framing and floor diaphragm be
installed to brace the wall prior to backfilling. You
need to be aware that the void between the forms for
the basement wall or the basement wall after it has
been cast and the cut slope are considered a trench
when they exceed four feet in height under the
Department of Labor and Industries Standards . The
slope must be sloped or shored in accordance with the
current requirements . We recommend a soil type C be
used for control .
Excavated material from the building pad excavation
must not be side-cast over the slope between the
residence and the top of the bluff . Excavated
material must be disposed of upslope of the building
pad. Placement of fill sections on slopes must be
keyed and compacted to prevent the construction of a
potentially unstable so_A mass . The side cast fill
from the septic system installation is considered to
be an uncontrolled fill section prone to future
failure. As part of your site work, this fill section
should be removed and replaced as a controlled fill
section or the ground returned to its natural contour
and revegetated.
02040601
Page 4 of 4
As part of your site development plans, you need to
address the collection and control of surface storm
water. We recommend that all roof rain leaders and
footing drains be connected into a tightline system
with the collected water carried to the ravine to the
north for disposal in an approved manner. No
uncontrolled water should be directed towards the
bluff. Concentrated flows of storm water can create
areas of localized saturation and induce slope
instability and accelerated erosion.
The geologic literature and our observations do not
indicate that this area is a landslide hazard due to
geologic conditions . We expect that the bluff will
continue to erode slowly due to sheet washing and
freeze-thaw cycles . The dense soils at the toe of the
bluff resist erosion by marine processes .
While we do not consider this site to be in a
landslide hazard area, construction on the marine
slopes of Puget Sound always involves some risk that
the structure may be adversely affected by soil
movements or erosion. This risk is reduced by
increasing the setback of the structure from the top
of the bluff . Estimating future erosional rates is
not a precise science. Erosion rates may be affected
by seismic events, climatic changes, eustatic changes
in sea level, tectonic forces, or changes to the areas
drainage patterns or ground water levels . As long as
the owner is aware that these risks do exist, we have
no objection, based on geologic factors, to the
construction of a residence at this location. We
expect that a reasonable service life for the
structure can be realized before erosion would affect
the soil support for the foundation. We expect the
service life will be at least 50 years .
If you have any questions, or if we may be of
additional service to you on this project, please
contact us at our Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL SERVICES
L`
David C. trong
Engineering Geologist
MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shehon,Washingwn 99594
DATE: June 1 P, 2002
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Grace Miller,DCD -Planner
FROM: Alan A. Tahja, P/W- Co. Hydr. Engr. Work Order# PLG-02
SUBJ: Geo-Tech Assessment Review NAME: Franzel SFR @ 500 E Nichole—Hrstn
BLD2002-00179
Grace,
A geotechnical report prepared for the proposed Jack Franzel residence at 500 E.Nichole Road
on Harstine Island has been received and reviewed by Public Works.
The report appears adequate and complete, and fulfills the County's basic requirements for a
geotechnical report.
The recommendations contained in the report should be made requirements for plan and project
approval.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel any features need further discussion or attention.
Sin a ely,
XIL A. Tahja
i
i
I
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p
Bradley-Noble Geotech cal ervices
A Division of The Bradley Group, Inc.
PO Box 12267, Olympia WA 98508-2267
Phone 360-357-7883 • FAX 360 867-9307
22 April 2002
Mr. Jack Franzel BuIL ING
5805 106th NE
Kirkland, Washington 98033
Subject: Geotechnical assessment of your proposed
building site at 500 East Nichole,
Hartstene Island, Mason County Tax
Parcel Number 22026-10-90083.
Dear Mr. Franzel:
As requested, we have conducted a reconnaissance level
survey of the above property to determine if future
soil movements would adversely affect the soils
supporting the proposed single-family residence. This
letter will present our observations and opinions
based on this survey and on our research.
Bradley-Noble has provided geotechnical consulting
services for properties on either side of this lot.
In March 1995 we prepared a report for 480 East
Nichole Lane for Mr. Brock Laffoon. In December of
1995 we prepared a report for Mr. Dale Eklund who is
constructing a residence at 570 East Nichole Lane. We
understand that you have obtained copies of these
reports from Mason County and have reviewed our
recommendations for. site development on these adjacent
lots.
We have visited your property in order to prepare this
report. At the time of our visit installation of the
septic system was underway. Falling of trees between
the top of bluff and the toe of the side cast fill
from the septic system installation had occurred.
The project site slopes down from East Nichole Road
towards Pickering Passage. Slopes in the upland area
where construction is proposed are moderate. The
marine bluff is steep to nearly vertical. Estimated
vertical relief of the marine bluff is estimated in
the field to be about 35 feet on averacre.
02040601
Page 1 of 4
l
02040601
Page 2 of 4
Two soil units are found at this site. Soil exposures
were available in the area of the septic system
installation and in the bluff face. We find at this
site a thin layer of Vashon recessional outwash soils
! of sands and gravels overlying the very dense and
highly overconsolidated Vashon subglacial till. The
recessional outwash soils are permeable. The
subglacial till is considered to be impermeable due to
cementing. We expect that flows of ground water
develop on the surface of the subglacial till during
periods of heavy or prolonged rainfall.
The Coastal Zone Atlas, published by the Department of
Ecology in March 9980, Map MA-16, presents general
information on the site geology and evaluation of the
apparent slope stability. We also reviewed geologic
information presented in the Geology of Southeastern
Mason County, Washington, by John B. Noble and Dee
Molenaar in Plate 1 of the Water-Supply Bulletin 29,
published jointly by the State of Washington's
Department of Natural Resources, Division of Geology
and Earth Resources, and the U.S. Geological Survey
for Mason County, Washington.
Map MA-16 indicates the slope as being stable in the
upland area and of intermediate stability in the bluff
and the upland area immediately adjacent to the
bluff. The area indicated for construction would be
in the area considered to be stable. We concur with
the information presented on site stability. We
consider the ravine slope to the north of the building
site to be stable, and at the proposed location of the
new residence we do not expect that erosion of the
ravine slope would have any adverse effect on the
proposed construction.
From our conversation, we understand that the
foundation for the new residence will be constructed
about 35 feet back from the present top of the bluff.
Site cons i 1:i Ez,t s 1, ith setback -from dra3.11 ie s and
general topography would make moving the structure
upslope difficult or would require a major redesign of
the project. The proposed setback is consistent with
other structures that have been constructed along this
section of Hartstene Island. We have no objection to_
the construction of decks or of er low-value
structures in the setback area.
02040601
Page 3 of 4
We expect that you will be excavating into the slope
in order to create a building pad for your new
residence. We also expect that the excavation will
penetrate through the recessional outwash and expose
the subglacial till. The subglacial till offers
excellent support for standard spread footings
supporting typical residential type of loadings. To
ensure uniform support, all footings should be founded
on the subglacial till.
We understand that a daylight basement will be
included in the design solution. Attention to
perimeter drainage must be addressed in your site
plans. Flows of ground water on the surface of the
subglacial till may be significant. At a minimum, you
need to include a perimeter footing drain. We also
recommend that positive drainage be provided for the
basement slab. Backfill of the foundation wall should
use clean, freely draining gravel, or a manufactured
drainage board installed per the manufacturer's
specification may be installed. This will prevent the
build-up of hydrostatic pressures against the
foundation wall.
We recommend that the basement level slab and the
second level floor framing and floor diaphragm be
installed to brace the wall prior to backfilling. You
need to be aware that the void between the forms for
the basement wall or the basement wall after it has
been cast and the cut slope are considered a trench
when they exceed four feet in height under the
Department of Labor and Industries Standards. The
slope must be sloped or shored in accordance with the
current requirements. We recommend a soil type C be
used for control.
Excavated material from the building pad excavation
must not be side-cast over the slope between the
residence and the top of the bluff. Excavated
material must be disposed of upslope of the building
pad. Placement of fill sections on slopes must be
keyed and compacted to prevent the construction of a
potentially unstable soil mass. The side cast fill
from the septic system installation is considered to
be an uncontrolled fill section prone to future
failure. As part of your site work, this fill section
should be removed and replaced as a controlled fill
section or the ground returned to its natural contour
and revegetated.
02040601
Page 4 of 4
As part of your site development plans, you need to
address the collection and control of surface storm
water. We recommend that all roof rain leaders and
footing drains be connected into a tightline system
with the collected water carried to the ravine to the
north for disposal in an approved manner. No
uncontrolled water should be directed towards the
bluff. Concentrated flows of storm water can create
areas of localized saturation and induce slope
instability and accelerated erosion.
The geologic literature and our observations do not
indicate that this area is a landslide hazard due to
geologic conditions. We expect that the bluff will
continue to erode slowly due to sheet washing and
freeze-thaw cycles. The dense soils at the toe of the
bluff resist erosion by marine processes.
While we do not consider this site to be in a
landslide hazard area, construction on the marine
slopes of Puget Sound always involves some risk that
the structure may be adversely affected by soil
movements or erosion. . This risk is reduced by
increasing the setback of the structure from the top
of the bluff. Estimating future erosional rates is
not a precise science. Erosion rates may be affected
by seismic events, climatic changes, eustatic changes
in sea level, tectonic forces, or changes to the areas
drainage patterns or ground water levels. As long as
the owner is aware that these risks do exist, we have
no objection, based on geologic factors, to the
construction of a residence at this location. We
expect that a reasonable service life for the
structure can be realized before erosion would affect
the soil support for the foundation. We expect the
service life will be at least 50 years.
If you have any questions, or if we may be of
additional service to you on this project, please
contact us at our Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL SERVICES
David C. trong
Engineering Geologist
z
RECEIVED
J U N 1 1 2002
GEOTECHNICAL REPORT MCCD - PLANNING
E. 500 NICHOLE LANE
HARTSTENE ISLAND
SHELTON,WASHINGTON
Prepared For:
Jack Franzel
By:
Geotechnical Testing Laboratory
Olympia,WA
With:
Spears Engineering and Technical Services
May 28,2002
t
z
GEOTECHNICAL RECEIVED
' TESTING LABORATORY
JUN 1 120
JACK FRAxzEL MCCD - PLANNING
5805100 NE
' KMKLAND, WA 98033
Re: Geotechnical Report
500 E.Nichole Lane
Hartstene Island,WA
" t INTRODUCTION
This report summarizes the results of our geotechnical engineering services for the proposed home to be located at
' 500 E.Nichole Lane on Hartstene Island, WA. The location of the site is shown relative to the surrounding area on
the Vicinity Map,Figure 1.
tOur understanding of the project is based on our discussions and site meeting with you, our review of the site and
explorations. We understand that the rectangular shaped site is to be developed as a home with associated
outbuildings, pavement areas and utilities. The site will be accessed by a driveway from Nichole Lane. Stormwater
' runoff from the site,roof and hard surfaces,will be collected and directed to on-site retention facilities. The general
layout of the site is shown on the Site Map,Figure 2.
' We further understand that very little grading is required at the site to reach design grade. In general,grading will
consist of the excavation of the house foundation in the lower portion of the site. No structural fill is anticipated.
We understand that the site will be served by local utility companies.
The western portion of the site has slopes that are greater than 50 percent. Mason County therefore requires that a
geotechnical report be prepared in accordance with the Critical Areas Ordinance.
The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for providing
geotechnical recommendations and design criteria for the project and to satisfy the requirements of the Mason County
Critical Areas Ordinance. Geotechnical Testing Lab is therefore providing geologic and hydrogeologic services for
the project. Specifically,our scope of services for this project will include the following
"�j
1. Review the available geologic, hydrogeologic and geotechnical data for the site area.
' 2. Conduct a geologic reconnaissance of the site area.
f ' 3. Explore the shallow subsurface conditions at the site by monitoring the exposed septic test pits.
4. Evaluate the landslide and erosion hazards at the site as per the Mason County Critical Areas Ordinance
' regulations.
5. Provide geotechnical recommendations for site grading including site preparation,sub-grade preparation, fill
placement criteria(including hillside grading), suitability of on-site soils for use as structural fill,temporary
rG ' and permanent cut and fill slopes,drainage and erosion control measures.
6. Provide recommendations and design criteria for the structural foundation and floor slab support, including
allowable bearing capacity, sub-grade modulus, lateral resistance values and estimates of settlement.
1001 1 Blomberg Street SW,Olympia, WA 98512
Phone#:(206)754-4612 Fax#:(206)754-4848
GEOTECIINICAL
TESTING LABORATORY
7. Provide recommendations and design criteria for design of conventional sub-gradehetaining walls, including
backfill and drainage requirements, lateral design loads,and lateral resistance values.
8. Provide our opinion regarding the feasibility of the location/construction of the stormwater outfall across a
portion of the steep bluff.
cal recommendations regarding the construction of the stormwater outfall.
9. Provide preliminary geotechni
SITE CONDITIONS
SURFACE CONDITIONS
The proposed building site is located in the lower portion of the Puget Sound glacial upland area. The site is situated
in an area of low development. The site is bounded by homesites on the northern and southern sides of the property.
The proposed layout of the site is shown on the Site Plan,Figure 2.
We conducted a reconnaissance of the site area and observed the exposed soils on the site. Elevations in the site area
range from approximately 0 feet in the west to about 100 feet in the eastern portion of the site. The majority of the
building area is located in the central and lower portion,between 5%and 15%slopes. A localized area in the
western and northern portions of the site steeps to between 30 and 45 percent. This area is outside of the proposed
development area. Site topography and slopes of 30 to 45 percent or greater are shown on the Site Plan,Figure 2.
We observed no evidence of excessive erosion. Minor surficial sloughing and soil movement was observed on the
steeper portions of the site and adjacent areas. No evidence of deep-seated slope instability was observed in the site
area at the time of our site visit.
The site is vegetated with fir,cedar and Madrona trees. A moderate to dense understory consists of grass,brush,
young alder trees, salal and Oregon grape with local areas of Scotch broom, and blackberry.
No evidence of surface water flow was observed in the site area at the time of our reconnaissance. The general
topography of the site area indicates that the site drains towards the west.
SITE GEOLOGY
The site is generally situated within the Puget Sound glacial upland. The existing topography, as well as the surficial
and shallow subsurface soils in the area,are the result of the most recent Vashon stade of the Fraser glaciation that
occurred between about 12,000 and 15,000 years ago,and weathering and erosion that has occurred since. A
description of the surficial soils is included in the"Site Soils"section of this report.
In general,Vashon glacial till underlay the upper eastern and central portions of the site. The till is absent from the
lower western portion of the site where advance sand Esperance,with gravel, is occurring near the ground surface.
SITE SOILS
1 The Mason County Soil Conservation Survey(1960)has mapped the site soils as Harstine gravelly sand in the
building area of the site.
10011 Blomberg Street SW,Olympia, WA 98512
' Phone#:(206)754-4612 Fax#:(206)754-4848
s
GEOTECHNICAL
TESTING LABORATORY
The Harstine soils typically form in sandy glacial till areas and are described as having a moderate permeability and
a low erosion hazard We observed no active erosion or slope disturbance in the site area during our reconnaissance.
SUBSURFACE EXPLORATIONS
Subsurface conditions at the site were evaluated by observing open excavations from work being performed or
anticipated The excavations extended to a depth of 4 feet below existing site ground surface. The bluff along Peale
Passage was also used for subsurface observations.
We have made a log of the subsurface conditions,and obtained representative samples,as needed. The soils
encountered were visually classified in accordance with ASTM D-2487. The log of the soil is included in the
appendix.
SUBSURFACE CONDITIONS
In general,undisturbed dense soil was observed in the open areas located in the building portion of the site. The
undisturbed soil was observed at depths of between 2 and 3 feet below the existing ground surface. The till
' thickness ranged from ten feet in the easternmost portion of the site to approximately 3 feet in the building portion of
the site.
No groundwater seepage was observed in the open excavations at the site. Based on the nature of the near surface
soils, intermittent outwash and weathered sand over undisturbed sand in the building area,seasonally perched
groundwater conditions should be expected during periods of extended wet weather.
SLOPE STABILITY
eastern and extreme western portion,along the shoreline,of the
Slopes as steep as 45 percent were observed in the
site. Since slopes of 45 percent or greater with 10 feet or more of vertical relief occur on portions of the site,Mason
County requires that a geologic hazards report be completed per the Critical Areas Ordinance.
In general,the undisturbed native soils in the east and west,or steep slope portions of the site,consist of silty sand
with variable amounts of gravel. These sandy soil materials are in a dense to very dense condition except where they
have been disturbed by weathering activity.
The near-surface weathered advance outwash materials are in a medium dense to dense condition except at the
ground surface. The surficial soils are generally in a loose to medium dense condition. These soils are generally
1 stable relative to deep-seated failure. No evidence of deep-seated landslide activity or significant erosion was
observed at the site at the time of our site visit.
{ , Weathering,erosion, and the resulting sloughing and shallow landsliding are natural processes that can affect steep
slope areas. Instability of this nature is typically confined to the upper weathered or disturbed zone,which has been
disturbed and has a lower strength.
i
' Only minor evidence of local surficial erosion, raveling and sloughing was observed in the site area at the time of our
site visit.
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t weathering typically occurs in the u 2 to 3 feet and is the result of oxidation,root penetration,wet/dry
significant 8 h'P Y PPS'
cycles,and freezetthaw cycles. Erosion in steep slope areas such as this can be reduced through proper design and
construction of the storm water systems. Erosion control recommendations for the sloping areas are provided in the
"Building Setback"and"Erosion Control"sections of this report.
' Proper planning,design and construction techniques will reduce the risk of surficial erosion or movement in these
areas. It is our understanding that this type of planning has been taken with regard to this site.
CONCLUSIONS AND RECOMMENDATIONS
GENERAL
Based on the results of our site reconnaissance and subsurface observations, and our experience in the area, it is our
opinion that the site is suitable for the proposed home. The slopes located in the building portion of the site area and
' the adjacent property are stable relative to deep-seated instability and will not be affected by the proposed home.
Proper erosion/sediment and drainage control measures will reduce or eliminate the potential for erosion in this area,
and improve slope stability.
In general,the sand and gravel soils observed at the site are suitable for use as structural fill material. The sandy
soils that occur on the site are moisture-sensitive and susceptible to disturbance when wet. Saturated soil conditions
may be associated with these soils during or following extended periods of rainfall. To reduce grading time and
construction costs,we recommend that earthwork be undertaken during favorable weather conditions.
Conventional construction equipment may be utilized for work at the site. Conventional spread footings may be
utilized at the site for support of the structure. We do recommend that roof and footing drains be installed for the
home. A vapor barrier is recommended for all slab-on-grades.
R , Pertinent conclusions and geotechnical recommendations regarding the design and construction of the proposed
development are presented below.
LANDSLIDE—EROSION HAZARD AREAS
Classification .
The Mason County Critical Areas Ordinance defines a landslide hazard area as 1)one containing slopes equal to or
greater than 30 percent with more than a 10-foot vertical relief, and 2)one containing soils described by the Soil
Conservation Service(SCS)Soil Survey as having a"severe"limitation for building site development due to slope.
The slopes located in the area of the site are in excess of 30 percent and the vertical relief is in excess of 10 feet.
Based on this,this site does meet the technical criteria of a landslide hazard.
Slope Stability
Based on our field observations, explorations and our experience with the soil types encountered on the property, we
conclude that although portions of the slopes exceed 30 percent,they are generally stable relative to deep-seated
failure in their present configuration.
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ill are expected as a result of the proposed home on the site. Development will occur in
No changes m slope stability pect P P
the generally level portions of the site based on appropriate engineering and building setback recommendations.
Grading in the building portion of the site should be conducted in accordance with geotechnical recommendations
provided herein. Site drainage will be collected and directed to the site's stormwater system.
As previously discussed,weathering,erosion,and the resulting surficial sloughing and landsliding are natural
processes that affect slope areas. Significant weathering typically occurs in the upper 2 to 3 feet and is the result of
oxidation,root penetration,wet/dry cycles and freeze/thaw cycles. These processes can be managed and the risk
' reduced through proper construction of the residence, including the stormwater system. Erosion control
recommendations in the slope and buffer areas are provided in the"Building Setback"and"Erosion Control"
sections of this report.
Building Setback
' A building setback distance from landslide hazard areas is required unless evaluated and reduced by an engineering
geologist or a licensed professional engineer. Cleaning,grading and filling within the setback area may not be
allowed because slope stability or the erosion hazard will be adversely impacted.
Based on our geotechnial evaluation of the site and our experience in the area, we recommend a building setback of
40 feet from the steep slope areas(50 percent or greater slopes). The building setback may be measured from the
' bottom of the footing to the face of the steep slope, in accordance with U.B.C. requirements.
As previously discussed,weathering, erosion and the resulting surficial sloughing and shallow landsliding are natural
processes that affect slope areas. As noted, no evidence of major surficial raveling or sloughing was observed in the
sloping portions of the site. To manage and reduce the potential for these natural processes,we recommend the
following:
'z 1. No drainage of concentrated surface water or significant sheet flow onto or near the steep slope area.
Drainage from the roof and driveway area should be collected and tightlined to a disposal area to the west of
the proposed home at the bottom of the slope. No filling within the buffer or setback zone unless retained by
retaining walls or constructed as an engineered fill.
2. No percolation of surface water within 20 feet of Building Setback.
SEISMIC—LIQUEFACTION HAZARD
According to the Seismic Zone Map of the United States contained in Figure 16-2 of the 1997 UBC(Uniform
Building Code),the project site is located within Seismic Risk zone 3.
Based on the subsurface conditions observed at the site,we interpret the site conditions to correspond to a seismic
T , Soil Profile Type Sc,for Very Dense Soil, as defined by Table 16-J(UBC). This is based on the range of SPT
(Standard Penetration Test)blow counts and/or probing with a %s-inch diameter steel probe rod. The shallow soil
conditions were assumed to be representative for the site conditions beyond the depths explored.
Based on our review of the subsurface conditions,we conclude that the site soils are not susceptible to liquefaction.
The near-surface soils are generally in a dense conditions and the static water table is located below the surface.
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Shaking of the already stiff/dense soil is not apt to produce a denser configuration and subsequently excess pore
water pressures are not likely to be produced.
EROSION CONTROL
It is our opinion that the potential erosion hazard of the site is not a limiting factor for the proposed development.
Removal of natural vegetation should be minimized and limited to the active construction areas. Temporary and
permanent erosion control measures should be installed and maintained during construction or as soon as practical
thereafter to limit the additional influx of water to exposed areas and protect potential receiving waters.
Erosion control measures should include,but not be limited to,berms and swales with ground cover/protection in
exposed areas and silt fences. Graded areas should be shaped to avoid concentrations of runoff onto cut or fill
slopes,natural slopes or other erosion-sensitive areas. Temporary ground cover/protection such as straw,wood
chips or clear plastic sheeting should be used until permanent erosion protection is established. No earthwork is
anticipated on the steep portions of the site:
EARTHWORK
Site Preparation
t
All areas to be excavated should be cleared of deleterious matter including any existing structures,foundations,
abandoned utility lines,debris and vegetation.
Based on our explorations,we estimate that stripping on the order of 4 to 6 inches will be necessary to remove the
' root zone and surficial soils containing organics. Areas with deeper,unsuitable organics should be expected in the
vicinity of depressions or heavy vegetation. Stripping depths-of up to 1.5 feet may occur in these areas. These
materials may be stockpiled and later used for erosion control and landscaping. Materials that cannot be used for
landscaping or erosion control should be removed from the project site.
t it the ex sub-grade areas should be roof-rolled to a firm and
Where placement of fill material is required, posed grad p
unyielding surface prior to placement of any fill. We recommend that trees be removed in such a manner so that a
i majority of the roots are removed,unless located on a slope. Excavations for tree stump removal in the building area
should be backfilled with structural fill compacted to the density requirements described in the"Structural Fill"
section of this report.
d
We recommend that a member of our staff evaluate the exposed sub-grade conditions after removal of vegetation and
topsoil stripping is completed and prior to placement of structural fill(if needed).
The exposed sub-grade soil should be proof-rolled and tested for compaction by nuclear densometer during dry
weather or probed with a %-inch-diameter steel rod during wet weather conditions.
9 ' Any soft, loose or otherwise unsuitable areas delineated during proof-rolling or probing should be recompacted, if
practical,or over-excavated and replaced with structural fill, based on the recommendations of our site
representative.
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Structural hill
All fill material should be placed as structural fill. The structural fill should be placed in horizontal lifts of
appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be compacted to at least 90
percent of MDD(maximum dry density as determined in accordance with ASTM D-1557/698)to within 2 feet of
1 sub-grade and 95 percent MDD in the upper 2 feet.
The appropriate lift thickness will depend on the fill characteristics and compaction equipment used. We recommend
' that the appropriate lift thickness be evaluated by our field representative during construction. We recommend that
our representative be present during site grading activities to observe the work and perform field density tests.
The suitability of material for use as structural fill will depend on the gradation and moisture content of the soil. As
the amount of fines(material passing No. 200 sieve)increases, soil becomes increasingly sensitive to small changes
in moisture content and adequate compaction becomes more difficult to achieve. During wet weather,we recommend
use of well-graded sand and gravel with less than 5 percent(by weight)passing the No.200 sieve based on that
fraction passing the%-inch sieve. If prolonged dry weather prevails during the earthwork and foundation installation
phase of construction,a somewhat higher(up to 12 percent)fines content will be acceptable.
OMaterial placed for structural fill should be free of debris,organic matter,trash and cobbles greater than 6 inches in
diameter. The moisture content of the fill material should be adjusted as necessary for proper compaction.
Suitability of On-Site Soils as hill
During dry weather construction,any non-organic on-site soil may be considered for use as structural fill, provided it
t ' meets the criteria described above in the structural fill section and can be compacted as recommended. If the material
is over-optimum moisture content when excavated, it will be necessary to aerate or dry the soil prior to placement as
structural fill.
The workability of a material for use as structural fill will depend on the gradation and moisture content of the soil.
As the amount of fines increase, soil becomes more sensitive to small changes in moisture content and adequate
compaction becomes more difficult or impossible to achieve.
If fill material is imported to the site for wet weather construction,we recommend that it be a sand and gravel
mixture such as high quality pit run with less than 5 percent fines.
CUT AND FILL SLOPES
' All job site safety issues and precautions are the responsibility of the contractor providing services and/or work. The
following cuttfill slope guidelines are provided for planning purposes.
'e Temporary cut slopes will likely be necessary during grading operations. As a general guide,temporary slopes of 1.5
to 1 (horizontal to vertical)or flatter may be used for temporary cuts in the upper 3 to 4 feet of the glacially
consolidated soils that are weathered to a loose/medium dense condition. Temporary slopes of 1 to 1 or flatter may
be used in the unweathered dense to very dense sands and gravels or till.
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These guidelines assume that all surface loads are kept at a minimum distance of at least one half the depth of the cut
away from the top of the slope and that significant seepage is not present on the slope face. Flatter cut slopes will be
3 necessary where significant raveling or seepage occurs.
We recommend a maximum slope of 2 to 1 for permanent cut and fill slopes. Where 2 to 1 slopes are not feasible,
3 shoring of the slopes should be considered. Some minor raveling may occur with time. All slopes should be seeded
as soon as practical to facilitate the development of a protective vegetative cover or otherwise protected.
FOUNDATION SUPPORT
All exterior footing elements should be embedded as least 18 inches below grade for frost protection. Where
foundation elements are located near slopes of 5 percent or more,the footings should be located a minimum of 2
times the footing width from the slope face(horizontally),and founded in medium dense or denser native soils or
properly prepared structural fill.
We recommend a minimum width of 2 feet for isolated footings and at least 16 inches for continuous wall footings.
Footings founded as described above can be designed using an allowable soil bearing capacity of 2,000 psf(pounds
per square foot)for combined dead and long-term live loads in areas of medium dense to dense soils.
The weight of the footing and any overlying backfill may be neglected. The allowable bearing value may be
increased by one-third for transient loads such as those induced by seismic events or wind loads.
Lateral loads may be resisted by friction on the base of footings and floor slabs and as passive pressure on the sides
r ' of footings. We recommend that an allowable coefficient of friction of 0.40 be used to calculate friction between the
concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent fluid density of
150 pcf(pounds per cubic foot). Factors of safety have been applied to these values.
F We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch for the
anticipated load conditions, with differential settlements between comparably loaded footings of%:inch or less. Most
of the settlements should occur essentially as loads are being applied. However, disturbance of the foundation sub-
grade during construction could result in larger settlements than predicted.
FLOOR SLAB SUPPORT
Slabs-on-grade should be supported on medium dense or dense native soils or on structural fill prepared as described
in the"Structural Fill"section of this report. We recommend that floor slabs be directly underlain by a minimum 6-
inch thickness of coarse sand and/or gravel containing less than 3 percent fines(by weight). The drainage material
should be placed in one lift and compacted to an unyielding condition.
A synthetic vapor barrier should be used for the control of moisture migration through the slab, in particular where
adhesives are used to anchor carpet or tile to the slab. A thin layer of sand may be placed over the vapor barrier and
immediately below the slab to protect the liner during steel and/or concrete placement. The lack of a vapor barrier
could result in wet spots on the slab,particularly in storage areas.
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A sub-grade modulus of 400 kcf(kips per cubic foot)may be used for floor slab design. We estimate that settlement
of the floor slabs designed and constructed as recommended will be%2 inch or less over a span of 50 feet.
Retaining Wall
Retaining walls will likely be utilized in the future on this site to retain fill material.
The lateral pressures acting on sub-grade and retaining walls will depend upon the nature and density of the soil
behind the wall. It is also dependent upon the presence or absence of hydrostatic pressure. If the adjacent exterior
wall space is backfilled with clean granular,well-drained soil(washed rock),the design active pressure may be taken
as 35 pcf(equivalent fluid density). This design value assumes a level backslope and drained conditions as described
below. The design for active pressure assumes the walls can yield 0.001 times the wall height.
Retaining walls located on or near the toe of the slope that extends up behind the wall should be designed for a lateral
pressure, which includes the surcharge effects of the steep slope in proximity of the wall. Although not expected at
' this site,the following data is provided for planning purposes.
For an irregular or composite slope,the equivalent slope angle may be determined by extending a line upwards from
the toe of the wall at an angle of 1 to 1 (Horizontal to Vertical)to a point where the line intersects the ground surface.
The surcharge effects may be modeled by increasing the equivalent fluid pressure for flat ground by the percentage
given in the following table:
tSlope Inclination:Equivalent Fluid Pressure
Slope Angle Percent Increase Equivalent Fluid Pressure
} ' Horizontal 0% 35 pcf
3H:1 V 25% 44 pcf
2H:1 V 50% 53 pcf
' 1 H:I V 75% 61 pcf
If the walls are greater than 4 feet in height,exclusive of the footing, an engineered design will be required.
Positive drainage, which controls the development of hydrostatic pressure,can be accomplished by placing a zone of
coarse sand and gravel behind the wails. The granular drainage material should contain less than 5 percent fines.
The drainage zone should extend horizontally at least 18 inches from the back of the wall. The drainage zone should
also extend from the base of the wall to within 1 foot of the top of the wall. The drainage zone should be compacted
to approximately 90 percent of the MDD. Over compaction should be avoided as this can lead to excessive lateral
pressures.
A perforated PVC pipe with a minimum diameter of 4 inches should be placed in the drainage zone along the base of
the wall to direct accumulated water to an appropriate discharge location. We recommend that a non-woven
geotextile filter fabric be placed between the drainage material and the remaining wall backfill to reduce silt
migration into the drainage zone. The infiltration of silt into the drainage zone can,with time,reduce the
permeability of the granular material. The filter fabric should be placed such that it fully separates the drainage
' material and the backfill,and should be extended over the top of the drainage zone.
a ' 10011 Blomberg Street SW,Olympia, WA 98512
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Lateral loads may be resisted by friction on the base of footings and as passive pressure on the sides of footings and
the buried portion of the walls. We recommend that an allowable coefficient of friction of 0.35 be used to calculate
friction between the concrete and the underlying soil.
Passive may be determined using an allowable equivalent fluid density of 300 pcf(pounds per cubic foot).
P Y
Factors of safety have been applied to these values.
Retaining Wall Alternatives:
Options for retaining walls other than conventional concrete retaining walls include reinforced earth systems such as
smallllarge block walls,rolled-faced walls.
Typically, reinforced-earth block wall systems are more cost effective for long term walls than the other options.
Specific design criteria for these walls can be provided by the manufacturer your request.
PAVEMENT SUB-GRADE
We recommend that pavement sub-grades be prepared in accordance with the previously described site preparation
and structural fill recommendations. The upper 2 feet of roadway sub-grade should have a density of at lust 95
percent of the MDD(ASTM D-1577)
SITE DRAINAGE
{ pavements and sidewalks should be sloped All ground surfaces,pa away from the residences and associated structures.
' Surface water runoff should be controlled by a system of curbs,berms,drainage swales,and/or catch basins,and
conveyed to the site's retention system. We recommend that conventional roof and footing drains be installed for the
home and garage. Drains should be provided behind all retaining walls. Roof dram should not be connected to the
footing drain. Footing drains should be placed so that the invert is below the bottom of the footing.
We recommend that the collected stormwater runoff at the site be directed to the site's stormwater system. Based on
the soils encountered in the,building portion of the site, it is our opinion that it is feasible to collect the runoff and
discharge the water to the bottom of the slope.
LIIflTATIONS
We have prepared this report for use by Mr. Jack Franzel and members of his design team,for use in the building of
' a home with accompanying outbuildings. The data used in preparing this report,and this report, should be provided
to prospective contractors for their bidding or estimating purposes only. Our report, conclusions and interpretations
are based on data from others and our site reconnaissance,and should not be construed as a warranty of the
subsurface conditions.
A contingency for unanticipated conditions should be included in the budget and schedule. When the design is
finalized,we recommend that the design and specifications reviewed by our firm to see that our recommendations
have been interpreted and implemented as intended.
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GEOTECHNICAL
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The scope of our services does not include services related to environmental remediation and construction safety
precautions. Our recommendations are not intended to direct the contractor's methods,techniques,sequences or
procedures,except as specifically described in our report for consideration in design.
If there are any changes in the loads,grades, locations,configurations or type of facilities to be constructed,the
conclusions and recommendations presented in this report may not be fully applicable. If such changes are made,we
should be given the opportunity to review our recommendations and provide written modifications or verifications,as
appropriate.
Respectfully submitted,
#"'Ja
Harold Parks,
Engineering Geologist
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10011 Blomberg Street SW,Olympia, WA 98512
Phone#:(206)754-4612 Fax#:(206)754-4848
1
GEOTECHNICAL
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