19-100546a s s o c i a t e d
earth sciences
i o r a t e d
April 24, 2019
Project No. 190099EO01
City of Federal Way
33325 81i' Avenue South
Federal Way, Washington 98003-6325
Attention: Mr. Jim Harris, Senior Planner
Subject: Geotechnical Peer Review
Christensen Shoreline Cabana
204 SW 292"d Street
Federal Way, Washington
File No. 19-100546-AD
Dear Mr. Harris:
At your request, Associated Earth Sciences, Inc. (AESI) recently reviewed the geotechnical
engineering report and associated documents forthe subject project. Specifically, we reviewed the
following documents:
• "Geotechnical Engineering Study, Proposed Cabana, 204 Southwest 292" d Street," prepared
by Geotech Consultants, Inc. (GCI), dated December 18, 2018, Project No. JN-18508.
• "Reduced size Boundary and Topographic Survey," prepared by Centerpointe Consultants,
Inc., P.S., dated May 21, 2008.
• "Exhibit A," prepared Centerpointe Consultants, Inc., P.S., dated July 23, 2015.
In addition to review of these documents, we also completed a brief site visit.
AESI was requested to provide third -party peer review of the project to determine if the proposal
complies with Federal Way Revised Code (FWRC) 15.10.160. The review was requested due to the
location of the subject site within a critical area.
The scope of our review was limited to an evaluation of the report with respect to compliance with
the FWRC and typical geotechnical standards of practice.
Kirkland Office 1911 Fifth Avenue I Kirkland, WA 98033 P 1425.827.7701
Mount Vernon Office 1 508 S. Second Street, Suite 101 1 Mount Vernon, WA 98273 P 1425.827.7701
Tacoma Office 1 1552 Commerce Street, Suite 102 1 Tacoma, WA 98402 P 1253.722.2992
www.aesgeo.com
Christensen Shoreline Cabana
Federal Way, Woshington Geotechnicol Peer Review
SITE AND PROJECT DESCRIPTION
The site consists of a developed, waterfront, residential parcel facing Puget Sound, located at
204 Southwest 292"d Street in Federal Way, Washington. An existing residence is located in the
relatively flat, central portion of the site. North of the residence, the topography slopes steeply
down toward the north (toward Puget Sound). Inclinations on the slope generally range from
approximately 60 to 70 percent with locally steeper areas of approximately 90 percent. The
maximum height of the slope is approximately 35 feet. An existing boathouse is located atthetoe
of the steep slope, adjacent to the beach. The boathouse is a daylight basement structure with the
south wall of the boathouse benched into the toe of the steep slope. A timber retaining wall
supports the lower portion of the steep slope adjacent to the south side of the boathouse. The
timber retaining wall is in poor condition (Photo 1). It is our understanding that current plans
include demolition of the boathouse and construction of a shoreline cabana. The cabana will be
located in roughly the same area as the existing boathouse and will consist of a small dwelling area
to be located in the southern portion of the boathouse footprint and a deck that will extend north
of the dwelling area. The steep slope classifies as a Landslide Hazard Area and an Erosion Hazard
Area under the FWRC.
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Photo 1. The timber retaining at the back of the boathouse.
April 24, 2019 ASSOCIATED EARTH SCIENCES, INC.
TJP/ms-190099EO01-2-ProiectsJ20190099�KEIWP Page 2
Christensen Shoreline Cabana
Federal Way, Washington Geotechnical Peer Review
Subsurface exploration completed by GCI included three exploration borings to depths of
approximately 8 to 11.5 feet. Boring B-1 was drilled near the northeast corner of the boathouse
near beach level, boring B-2 was drilled nearthe southeast corner of the boathouse nearthe base
of the timber retaining wall, and boring B-3 was drilled near the top of the steep slope above the
boathouse. Review of the boring logs included in the CGI report indicates that sediments
encountered in boring B-1 generally consisted of loose topsoil fill and saturated sand overlying
medium dense to very dense, very silty sand and silt. Sediments encountered in boring B-2
consisted of dense to very dense glacial till. Boring B-3, located at the top of the steep slope
encountered approximately 8 feet of fill containing pieces of creosote soaked timber overlying
dense to very dense till. Groundwater seepage was encountered in boring B-1 over the depth
interval from approximately 4 to 7.5 feet. No groundwater was encountered in the remainingtwo
borings at the time of drilling in November of 2018.
REVIEW COMMENTS
AESI reviewed the aforementioned geotechnical engineering report to determine if it meets the
criteria specified within the FWRC. Upon completion of our review, we offer the following
comments.
1. As referenced in Section 15.10.160 of the FWRC, Section 15.05.040(4)(b)(ii) states that new
development that causes risk from geologic conditions shall not be allowed. Paragraph 2 on
page 4 of the geotechnical report states that shallow landslides on the steep slope may
have impacted the existing boathouse and timber retaining wall in the past. A landslide
catchment wall is recommended to mitigate hazards associated with shallow slides. We
recommend the report also address potential risks associated with larger, deeper -seated
landslides, and mitigation of those risks, if appropriate.
2. The "Catchment Wall Criteria" section on page 8 of the report recommends an active
equivalent fluid pressure of 100 pounds per cubic foot (pcf) for catchment wall design to
account for an impact force. Impact loads for catchment wall design are typically based on
an assumed impact velocity, angle of impact, depth of slide debris, and other factors. We I
request that GCI describe what slide characteristics or assumptions were used as the basis
for the 100 pcf value.
3. As referenced in Section 15.10.160 of the FWRC, Section 15.05.040(4)(b)(iii) states that
"new development on sites with steep slopes and bluff is required to be set back
sufficiently to ensure that shoreline stabilization is unlikely to be necessary during the life
of the project as demonstrated by a geotechnical analysis." We recommend that GCI
address this in the geotechnical report.
4. Paragraph 3 on page 4 of the report discusses use of a tied -back foundation wall for the
south side of the proposed structure. This discussion appears to refer to a cast -in -place
retaining wall embedded into the toe of the steep slope. If a cast -in -place wall will be used, I
we recommend that the report address removal of the existing timber wall, temporary cut
April 24, 2019 ASSOCIATED EARTH SCIENCES, INC.
TJP/ms-190099EO01-2- ProjectsJ20190099JKEJWP Page 3
Christensen Shoreline Cabana
Federal Way, Washington Geotechnical Peer Review
slopes associated with construction of the new wall, and associated slope stability
concerns, if applicable.
5. The last paragraph on page 3 of the report recommends a bearing capacity of
3,000 pounds per square foot (psf) for the enclosed portion of the cabana, based
on footings bearing directly on the very dense glacial till. Review of boring logs B-1 and
B-2 indicate that the depth to the very dense, natural sediments varies significantly
between these two borings. Given the proximity of the proposed structure to the steep
slope, what is the estimated depth to the foundation bearing stratum across the building
area? We recommend GCI provide recommendations regarding how to compete
excavation in this area without impacting the stability of the adjacent slope (e.g. excavation
depths, slope inclinations, setbacks, etc.).
6. The last paragraph on page 3 of the report recommends a bearing capacity for the deck
footings of 1,000 psf based on support of the footings on at least 12 inches of structural fill.
What compaction specification is recommended for the structural fill placed below
foundation areas? Plate 7 shows that where structural fill will be placed below footing
areas, the area below the footing should be overexcavated down to "suitable bearing soil'
and filled to the desired foundation subgrade elevation with structural fill. Although a site
plan showing the layout of the proposed beach cabana and deck was not included in the
report, based on the project description on page 1 of the geotechnical report, it appears
that the proposed deck will be located in the vicinity of boring B-1. Boring B-1 encountered
approximately 3.5 feet of loose topsoil fill overlying loose, saturated sand to a depth of
approximately 6 feet. Please clarify what constitutes suitable bearing soil in this case. If the
12 inches of structural fill will be placed over the loose topsoil and/or saturated sand, what
is the risk of settlement under both static and seismic conditions? If the structural fill will
be placed on the medium dense to very dense sediments below six feet, how will this be
accomplished given the loose, saturated soil conditions?
7. On page 5 of the geotechnical report in "Seismic Conditions," GCI state that seismic
surcharge for retaining wall design and slope stability should include the potential effects
of the Design Earthquake per the International Building Code (IBC) and American Society of
Civil Engineers (ASCE) 7. GCI further state that the peak ground acceleration (PGA) for the
Design Earthquake is defined in ASCE 7 as 2/3 of the maximum considered earthquake
(MCE) PGA or 0.35g. The IBC and ASCE-7 do not explicitly state that 2/3 of the MCE PGA can
be used to evaluate slope stability. It is our interpretation that PGA that is factored for site
effects (not the Design Earthquake as stated by GCI) and used for liquefaction analyses
should be used for seismic slope stability evaluation.
8. An active earth pressure of 50 pcf is recommended forth e soldier pile wall based on a level
backslope (paragraph 1 under Soldier Pile Design, page 11). The soldier pile wall was
recommended as an alternative to a conventional concrete foundation wall on the south
side of the structure. The topography in this area is sloping. We recommend thatthe active
pressure used for the design of the wall be revised to reflect the topographic conditions in
this area.
April 24, 2019 ASSOCIATED EARTH SCIENCES, INC.
TJP/ms-190099E001-2-ProjectsJ20190099JKEJWP Page 4
Christensen Shoreline Cabana
Federal Way, Washington Geotechnical Peer Review
9. In the second table on page 7 of the report, an active earth pressure of 50 pcf is
recommended for retaining wall design. We request that GCI clarify what backslope
condition this value is based on.
10. The last paragraph on page 7 recommends the use of restrained soil parameters for a
distance of 1.5 times the wall height from corners or bends in the wall. Also on page 8, a
seismic surcharge pressure of 8H psf is recommended for wall design based on active earth
pressure conditions. What is the recommended seismic surcharge for at rest earth pressure
conditions?
CLOSURE
Our scope of work for this letter was limited to a review of the documents supplied to us and a
brief site visit. Our scope did not include subsurface exploration, nor does our review purport to
verify the accuracy of exploration logs or geotechnical analysis results presented within the
documents.
We trust this letter meets your current needs. Should you have any questions, please contact us at
your convenience.
Sincerely,
ASSOCIATED EARTH SCIENCES, INC.
Kirkland, Washington
4" �7t
— Timot ^I� e L. .G., L.Hg.
v{
Senior Errg+n a ing Geologist
Jy
Bruce L. Blyton,
Senior Principal Engineer
Stephen A. Siebert, P.E.
Associate Geotechnical Engineer
April 24, 2019 ASSOCIATED EARTH SCIENCES, INC.
TJP/m5-190099EO01-2- Projects1201900991KE1wP Page 5
1%
CITY OF Vn ��
Federal Way
MEMORANDUM
Community Development Department
Geotechnical Consultant
Authorization Form
Date: February 26, 2019
City: Community Development Department
33325 8`h Avenue South
Federal Way, WA 98003
Consultant: Steve Siebert
AESI
911 5`h Avenue
Kirkland, WA 98033
ssiebert0aesaeo.com, (425) 827-7701
Project: Christensen Shoreline Cabana
Geotechnical Engineering Study
204 SW 292nd Street, Federal Way, WA 98023
File No.: 19-100546-AD
Project Proponent: Robert Christensen
204 SW 292°d Street
Federal Way, WA 98023
rtchristen comcast.ne (253) 405-7388
City Staff Contact: Senior Planner Jim Harris—'im.harris ci offederalwa .coin, (253) 835.2652
Project Background: Applicant is proposing to reconstruct a structure (cabana) within a critical area
(Erosion Hazard Area), and within 200 feet of the Puget Sound shoreline.
Documents Provided: o GeoTech Consultants —December 18, 2018, Geotechnical Engineering
Study
• Reduced size Boundary and Top Survey
• Exhibit A Center Pointe Survey
Task Scope: • Review report for compliance with Federal Way Revised Code (FWRC)
critical area regulations, FWRC 15.10.160.
• Confirm GeoTech Consultants report addresses FWRC 15.10.160
requirements, or alternatively, identify any required information needed
from the applicant.
Geotech Consultant Authorization Form
Christensen Shoreline Cabana
Page 2
• Conduct site visit as necessary.
■ Provide written response to the Geotechnical Engineering Study
Conclusions and Recommendations.
■ Possible meeting with applicant's engineer.
■ Possible meeting with city staff.
Task Schedule: Provide a task cost estimate ASAP. Review work is not authorized until authorized in
writing by city.
Task Cost: Not to exceed $ 2 `J 0�Q without a prior written amendment to this
Task Authorization.
.-1
A�kCITY OF
Federal Way
Centered on Opportunity
February 27, 2019
Mr. Steve Siebert
AESI
9115 51h Avenue
Kirkland, WA 98033
siehert OaeWe .corn
CITY HALL
33325 8th Avenue South
Federal Way, WA 98003-6325
(253) 835-7000
www.cityoffederalway.com
Jim Ferrell, Mayor
FILE
Re: File No. 19-100546-00-AD; Christensen Geo -Tech Report Request for Third Party Peer Review
Christensen Geotechnical Engineering Study — Geotech Consultants 12/18/18, Federal Way
Dear Mr. Siebert:
Please find the enclosed a Geotechnical Report Review Task Authorization form and the December 18, 2018,
Geotechnical Engineering Study by Geotech Consultants for third party peer review of the proposed cabana
at 204 SW 292nd Street, Federal Way.
The primary scope of this review is to determine if the proposal complies with Federal Way Revised Code
(FWRC) 15.10.160. Please note that this code section from FWRC Chapter 15 applies to the proposal, as the
site is located within 200 feet of the Puget Sound shoreline.
City staff is requesting review pursuant to the agreed terms of the on -call contract. Please review the scope of
work on the task authorization form, enter the task cost on page 2 of the document, and return to the city.
Following the deposit of funds by the applicant, city staff will provide you with an authorization to proceed
with the scope of work.
Please contact me at 253-835-2652, or iim harris cityoffederalway.com if you have any questions regarding
this task.
Sincerely,
Harris
Senior Planner
enc: Task Authorization Form
GeoTech Consultants Geotechnical Engineering Study, December 18, 2018
Doc ID 78800
19-100546-00-AD
5� JA�, � R �.Ctjj
GEOTECHNICAL ENGINEERING STUDY
Proposed Cabana
204 Southwest 29V Street
Federal Way, Washington
RECEIVED
FEB 01 2019
TY OFCOM UNITY FEDERAL
DEVELOPMENT
This report presents the findings and recommendations of our geotechnical engineering study for
the site of the proposed cabana to be located in Federal Way.
We were provided with preliminary site plans and a topographic map. McClellan Architects
developed these plans, which are dated May 29, 2018 and Centre Point Consultants, Inc.
developed the topographic survey, which is dated May 21, 2008. Based on these plans, we
understand that the existing boathouse located on the northwestern comer of the site will be
removed and replaced with a new cabana located in roughly the same footprint as the existing
boathouse. The cabana will have a small dwelling area in the south portion of the footprint, with a
large deck extending to the north of the building. The preliminary site plans and renderings indicate
that the cabana will be one-story in height, with the main floor of the cabana closely matching the
main floor of the existing structure. The cabana will be accessed via a set of existing stairs
extending downhill from a trail that bisects a slope that is above the existing boathouse.
If the scope of the project changes from what we have described above; we should be provided
with revised plans in order to determine if modifications to the recommendations and conclusions of
this report are warranted.
SITE CONDITIONS
SURFACE
The Vicinity Map, Plate 1, illustrates the general location of the site located just south of Redondo
Beach. The irregular shaped site has approximate dimensions of 272 to 310 feet in the north -south
direction and 140 feet in the east -west direction. The subject site is bordered to the north by
Poverty Bay of the Puget Sound, to the east and west by other single-family residences of similar
construction, and to the south by Southwest 2921d Street,
The subject site is currently developed two structures. One is a fine -story house, located on its
upslope portion of the site near Southwest 292"6 Street, and the other is the boathouse, which is
located on its downslope portion adjacent to Poverty Bay. A steep slope is located between these
structures. The boathouse is one-story in height and contains a basement that daylights down to a
few feet above the beach line of Poverty Bay. The lower level of this structure is partially benched
into the slope, and an aid, failed timber retaining wall was at one point, retaining the slope above it.
The boathouse shows signs of excessive settlement that has occurred over time, indicating that the
foundation of the boathouse is in a poor Condition. The structure is accessed via a wooden
staircase that extends from a gravel trail that follows the approximate top of the slope. The
remainder of the lot is landscaped, with grass and landscaping beds covering the flatter areas, and
ivy and underbrush covering much of the slope.
The grade across the site slopes downward from south to north, with a total elevation change of 58
feet across the property. The grade drops gently to moderately on the southern portion of the
property where the house is located. There is a relatively flat area adjacent to the north of the
house, but then the grade drops steeply downward with a total elevation change of 26 to 39 feet at
GEOTECH CONSULTANTS, INC.
Robert Christensen JN 18508
Deoember 18, 2018 Page 2
inclinations ranging mostly from about 60 to 70 percent. However, a smaller portion of this steep
slope is inclined at approximately 90 percent; it is located near the top of the failed retaining wall
directly upslope of the boathouse mentioned previously. This braced timber railroad be wall lines
the south of the boathouse, and at some point, was being used to retain the cut made for the
boathouse excavation. At the base of the steep slope, the remainder of the north portion of the lot
is essentially flat, extending north to the rock and concrete bulkhead adjacent to the beach. The
steep slope between the existing house and boathouse is considered a Steep Slope Hazard Area
due to its height and inclination.
During our time onsite, we observed that the timber wall was leaning severely downslope and that
bracing had been added to the front of the wall at some point to keep the wall from failing into the
boathouse. It is apparent that, although very dense soil is mostly located at the timber wall area,
the wall was extremely under -designed for the sloping conditions above it and the timber has rotted
throughout the recent years.
SUBSURFACE
The subsurface conditions in the area of the proposed cabana were explored by drilling three test
borings at the approximate locations shown on the Site Exploration Plan, Plate 2. Our exploration
program was based on the proposed construction, anticipated subsurface conditions and those
encountered during exploration, and the scope of work outlined in our proposal.
The test borings were drilled on November 8, 2018 using a portable Acker drill. This drill system
utilizes a small, gasoline -powered engine to advance a hollow -stem auger to the sampling depth.
Samples were taken at approximate 2.5 and 5-foot intervals with a standard penetration sampler.
This split -spoon sampler, which has a 2-inch outside diameter, is driven into the soil with a 140-
pound hammer falling 30 inches. The number of blows required to advance the sampler a given
distance is an indication of the soil density or consistency. A geotechnical engineer from our staff
observed the drilling process, logged the test borings, and obtained representative samples of the
soil encountered. The Test Boring Logs are attached as Plates 3 through 5.
Soil Conditions
The three test borings were conducted at various points along the northern slope. Test
Boring 1 was located just northeast of the existing boathouse near beach level.
Approximately 4 feet of loose till and topsoil were encountered at the ground surface,
underlain by wet, native sand. This sand was initially loose, but became medium -dense and
silty at approximately 6 feet. Very dense silty sand to sandy silt was then encountered below
a depth of 7 feet, extending to the base of the test boring at 11.5 feet where auger refusal
was met on the very dense soil.
Test Boring 2 was conducted at the base of the braced timber retaining wall, just south of
the boathouse. Native, dense silty sand was revealed at a depth of approximately 2 feet
beneath the ground surface; this dense silty sand was observed to be glacially compressed
and is commonly referred to as glacial till. The glacial fill became very dense at 5 feet and
extended to the maximum explored depth of 8 feet where auger refusal was met.
Test Boring 3 was conducted upslope the boathouse adjacent to the small gravel trail. It
appears that a large piece of creosote -soaked timber was encountered that may have been
GEOTECH CONSULTANTS, INC.
Robert Christensen
December 18, 2018
JN 18508
Page 3
installed vertically on the slope. It is possible that the timber was part of an old retaining wall
used to support some fill on the downslope side of the trail. Native, very dense glacial till
was encountered beneath (or through the timber at approximately 8 feet extending to the
base of the boring at a depth of 11.5 feet where auger refusal was met.
Obstructions in the form of large cobbles and old pieces of timber were revealed by our
explorations. Debris, buried utilities, and old foundation and slab elements are commonly
encountered on sites that have had previous development.
Groundwater Condfiaons
Perched groundwater seepage was observed at a depth of 4 to 7.5 feet in Test Boring 1,
which was drilled at the beach level. This test borings were left open for only a short time
period. Therefore, the seepage levels on the logs represent the location of transient water
seepage and may not indicate the static groundwater level. Groundwater levels
encountered during drilling can be deceptive, because seepage into the boring can be
blocked or slowed by the auger itself.
It should be noted that groundwater levels vary seasonally with rainfall and other factors.
We anticipate that groundwater in the lower test boring will correspond to the Puget Sound,
while some perched groundwater could be found in more permeable soil layers, pockets
within the till and between the looser near -surface soil and the underlying denser soil.
The stratification lines on the logs represent the approximate boundaries 'between soil types at the
exploration locations. The actual transition between soil types may be gradual, and subsurface
conditions can vary between exploration locations. The logs provide specific subsurface information
only at the locations tested. If a transition in soil type occurred between samples in the borings, the
depth of the transition was interpreted. The relative densities and moisture descriptions indicated
on the test boring logs are interpretive descriptions based on the conditions observed during
drilling.
CONCLUSIONS AND RECOMMENDATIONS
GENERAL
THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A
GENERAL OVERVIEW ONLY. MDRE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE
CONTAINED IN THE REMAINDER OF THIS REPORT ANY PARTY RELYING ON THIS REPORT SHOULD
READ THE ENTIRE DOCUMENT.
The test borings conducted for this study revealed encountered competent, native, very dense soils
are the core soil of the area of the proposed cabana. These soils were revealed at shallow at
shallow depths in the area where the cabana is an enclosed structure; conventional footings
bearing on the very dense soils can be used for the enclosed portion of the structure using a
bearing capacity of 3,000 psf. However, loose soil was revealed in the test boring at the beach level
where the deck portion of the cabana is proposed. We believe that conventional footings can be
used for the deck because it is such a light and not significant structure, but they need to bear on at
least 12 inches of structural fill and have a bearing capacity of only 1,000 psf. We recommend that
the base of the excavation for the deck piers be rec:ompacted prior to placing structural fill. It will be
important that the base of the excavation where footings for the enclosed structure be cleaned of
GEOTECH CONSULTANTS, INC.
Robert Christensen JN 18508
December 18, 2018 Page 4
soil loosened and disturbed by the excavation process. This can be accomplished using a cleanout
bucket, grade bar, or flat blade shovel.
To reduce the amount of structural fill needed for the project, we recommend that the flooring
system of the cabana consist of a crawlspace that can span between the foundations of the
cabana. Additional recommendations are shown in the Building Floors section of this report.
Shallow landslides on the steep slope to the south of the development area may have impacted the
existing boathouse and timber retaining wall in the past, although poor construction and an
oversteepened excavation is the cause of the leaning timber retaining wall. In any case, shallow
landslides (of the surface soils that overlie the very dense soil) on the slope above the new cabana
are possible could reach the proposed cabana in the future; this shallow landslide possibility is not
uncommon for steep slopes in the Puget Sound region. Therefore, we recommend that the
southern foundation wall of the cabana be constructed as a catchment wall to resist the impact load
of a landslide. Additional recommendations are presented in the Landslide Catchment ball
section of this report.
Because of the constraints the steep slope, whereby a large footing cannot be constructed for the
southern foundation wall into the slope (without extensive shoring), extra lateral support for the
southern foundation wall, will likely be needed. Using permanent horizontal anchors that are locked
off to the inside face of the southern wall of the cabana is one likely option. These anchors would
be drilled through the loose surfiicial soils and become embedded into the dense core of the
southern steep slope. The horizontal anchors will likely need to consist of drilled tiebacks in order to
develop sufficient capacities. Helical anchors are another type horizontal anchor, but they will most
likely not be able to achieve adequate embedment depths into the very dense glacial till comprising
the core of the slope. If the contractor would like to explore the possibility of helical anchors, a
subcontractor that specializes in helical anchor installation should be consulted. Recommendations
regarding tied -back anchors are detailed later in this report. In lieu footings and anchors, one other
option for aateral constraint of the southern foundation would be to use a permanent cantilevered
soldier pile wall. The soldier piles could be designed to carry bath the new foundation loads of the
cabana, and to provide lateral stability to the base of the slope. Additional recommendations
regarding soldier piles are listed below in the Permanent Soldier Pile Wall section of this report.
Regardless of what wall type is used for the southern foundation wall of the cabana, we
recommend that the currently open area where the failing timber wall is located be backfilled up to
the existing ground level. LOfC-tc.k C6 Pit %S Fw 2 C 640. I (4 G
The northern slope between the main house and boathouse meets the criteria for a Steep Slope
Hazard Area per Federal Way Code section 19.120.110, subsection (2). Federal Way restricts
development on or near geologic hazard areas. However, the code allows development activities
near, or within a Steep Slope Hazard Area if a qualified professional demonstrates that the new
development will not lead to or create any adverse impacts to the stability of the slope. We believe
that the cabana can be constructed as proposed, which is essentially directly adjacent to the steep
slope because:,1) the south side of the cabana will be constructed directly upon and into the, very
dense glacial till that comprises the core of the steep slope, and 2) the southern foundation wall of
the cabana will be constructed with a catchment height that deter any surficial soil from affecting
the cabana if a shallow landslide ever were to occur. It is bur professional opinion that, if the
recommendations stated in this report are closely followed, that the new cabana will increase the
stability of the slope that is directly above if
The erosion control measures needed during the site development will depend heavily on the
weather conditions that are encountered. We anticipate that a silt fence will be needed around the
GE®TECH CONSULTANTS, INC.
Rob&rt Christensen JN 18508
December 18, 2018 Page 5
downslope sides of any cleared areas. Existing pavements, ground cover, and landscaping should
be left in place wherever possible to minimize the amount of exposed soil. Rocked staging areas
and construction access roads should be provided to reduce the amount of soil or mud carried of
the property by trucks and equipment. Trucks should not be allowed to drive off of the rock -covered
areas. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Any silty
water that accumulates in the excavation will need to be contained to prevent silty runoff from
entering the Puget Sound. Following clearing or rough grading, it may be necessary to mulch or
hydroseed bare areas that will not be immediately covered with landscaping or an impervious
surface. On most construction projects, it is necessary to periodically maintain or modify temporary
erosion control measures to address specific site and weather conditions.
The drainage and/or waterproofing recommendations presented in this report are intended only to
prevent active seepage from flowing through concrete walls or slabs. Even in the absence of active
seepage into and beneath structures, water vapor can migrate through walls, slabs, and floors from
the surrounding soil, and can even be transmitted from slabs and foundation walls due to the
concrete curing process. Water vapor also results from occupant uses, such as cooking, cleaning,
and bathing. Excessive water vapor trapped within structures can result in a variety of undesirable
conditions, including, but not limited to, moisture problems with flooring systems, excessively moist
air within occupied areas, and the growth of molds, fungi, and other biological organisms that may
be harmful to the health of the occupants. The designer or architect must consider the potential
vapor sources and likely occupant uses, and provide sufficient ventilation, either passive or
mechanical, to prevent a build up of excessive water vapor within the planned structure.
Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the
recommendations presented in this report are adequately addressed in the design. Such a plan
review would be additional work beyond the current scope of work for this study, and it may include
revisions to our recommendations to accommodate site, development, and geotechnical
constraints that become more evident during the review process.
We recommend including this report, in its entirety, in the project contract documents. This report
should also be provided to any future property owners so they will be aware of our findings and
recommendations.
SEISMIC CONSWERATIONS
In accordance with the International Building Code (BBC), i e site class within 100 feet of the
ground surface is best represented by Site Class Type C (Very Dense Soil). As noted in the USES
website, the mapped spectral acceleration value for a 0.2 second (SS) and 1.0 second period (S:)
equals 1.32g and 0.51g, respectively.
The IBC and ASCE 7 require that the potential for liquefaction (soil strength loss) during an
earthquake be evaluated for the peak ground acceleration of the !Maximum Considered Earthquake
(MCE), which has a probability of occurring once in 2,475 years (2 percent probability of occurring
in a 50-year period). The (VICE peak ground accelerations adjusted for site class effects (FPGA)
equals 0.53g. The soils beneath the site are not susceptible to seismic liquefaction under the
ground motions of the IMCE because of their dense nature.
Sections 1803.5 of the IBC and 11.8 of ASCE 7 require that other seismic -related geotachnical
design parameters (seismic surcharge for retaining wall design and slope stability) include the
potential effects of the Design Earthquake. The peak ground acceleration for the Design
GEOTECH CONSULTANTS, INC.
Robert Christensen JN 18508
December 18, 2018 Page 6
Earthquake Is defined in Section 11.2 of ASIDE 7 as two-thirds (2/3) of the ICE peak ground
acceleration, or 0.35g.
CONVENTIONAL FOUNDATIONS
As noted in the General section of this report, the proposed cabana can be placed on conventional
continuous and spread footings. The enclosed structure of the cabana can be supported on
footings bearing on undisturbed, very dense, glacial till soil, while the deck should be supported on
at least 12 inches of structural fill. See the section entitled General Earthwork and Structural Fill
for recommendations regarding the placement and compaction of structural fill beneath structures.
Prior to placing structural fill beneath foundations, the excavation should be observed by the
geotechnical engineer.
We recommend that continuous and individual spread footings have minimum widths of 16 and 24
inches, respectively. Exterior footings should also be bottomed at least 18 inches below the lowest
adjacent finish ground surface for protection against frost and erosion. The local building codes
should be reviewed to determine if different footing widths or embedment depths are required.
Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending
upon site and equipment constraints, this may require removing the disturbed soil by hand.
The following allowable bearing pressures are appropriate for footings constructed according to the
above recommendations:
Cabana footings placed directly 3,000 psf
on competent, native soil
Deck piers supported on 12 1.000 psf
inches of structural fill
Where: psf is Pounds per Square Foot.
A one-third increase in these design bearing pressures may be used when considering short-term
wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction
settlement of footings founded on competent native soil, or on structural fill up to 5 feet in
thickness, will be about one -inch, with differential settlements on the order of one -half -inch in a
distance of 20 feet along a continuous footing with a uniform load.
Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and
the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the
foundation. For the latter condition, the foundation must be either poured directly against relatively
level, undisturbed soil or be surrounded by level, well -compacted fill.
GEOTECH CONSULTANTS, INC.
Robert Christensen JN 18508
December 18, 2018 Page 7
We recommend using the following ultimate values for the foundation's resistance to lateral
loading:
T
t of Friction 0.50arth Pressure 300 pcf
! I
Where: pcf Is Pounds per Cubic Foot, and Passive Earth
Pressure is computed using the Equivalent Fluid Density.
If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will
not be appropriate. The above ultimate values for passive earth pressure and coefficient of friction
do not include a safety factor.
FOUNDA TiON AND RETAINING WALLS
Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures
imposed by the soil they retain. The following recommended parameters are for walls:
Active Earth Pressure
Passive Earth Pressure
300 pcf
Coefficient of Friction
0.5
Soil unit Weight
135 pef
Where: pcf Is Pounds per Cubic Foot, and Active and Passlva
Earth Pressures are computed using the Equivalent Fluid
Pressures.
* For a restrained wall that cannot deflect at least 0.002 times Its
height, a uniform lateral pressure equal to 10 per times the tei,aht
of the wall should be added to the above active equivalerst'guld
pressure. This applies only to walls with level back ilI.
The design values given above do not include the effects of any hydrostatic pressures behind the
walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent
foundations will be exerted on the walls. If these conditions exist, those pressures should be added
to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need
to be given the wall dimensions and the slope of the backfill in order to provide the appropriate
design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted
for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid density. Heavy
construction equipment should not be operated behind retaining and foundation walls within a
distance equal to the height of a wall, unless the walls are designed for the additional lateral
pressures resulting from the equipment.
The passive pressure given is appropriate only for a shear key poured directly against undisturbed
native soil, or for the depth of level, well -compacted fill placed in front of a retaining or foundation
wall. The values for friction and passive resistance are ultimate values and do not include a safety
factor. Restrained wall soil parameters should be utilized the wall and reinforcing design for a
GEGTECH CONSULTANTS, INC.
Robert Christensen
December 18, 2018
JIB 18808
Page 8
distance of 1.5 times the wall height from corners or bends in the walls, or from other points of
restraint. This is intended to reduce the amount of cracking that can occur where a wall is
restrained by a comer.
Catchment Wall Criteria
As noted earlier in this report, there is a potential for shallow landslides to occur in the
loose, surficial soils found on the steep slope to the south of the proposed cabana. This
potential is especially prevalent during or following times of excessive precipitation. It has
been common to mitigate the potential of the hazard of landslides in this area by
constructing a reinforced retaining (catchment) wall on the side of the development area
that is exposed to the steep slope. Such a wall would extend above the level of the
proposed development.
Based on our experience with similar projects in the area, it is our opinion that the
catchment wall should include a minimum of 4 feet of unbackfilled freeboard as the
southern wall of the cabana. This portion of the will should not have any penetrations. An
active equivalent fluid pressure of 100 pef should be used in the design of the catchment
portion of the wall to account for an impact force. It will likely be necessary to remove
accumulated material periodically. The removal of small amounts of material could be
accomplished by hand. The freeboard of the catchment wall must be maintained for the wall
to provide continued protection from landslides.
Wall Pressures Due to Seismic Forces
The surcharge wall loads that could be imposed by the design earthquake can be modeled
by adding a uniform lateral pressure to the above -recommended active pressure. The
recommended surcharge pressure is 8H pounds per square foot (psf), where H is the
design retention height of the wall. Using this increased pressure, the safety factor against
sliding and overturning can be reduced to 1.2 for the seismic analysis.
Retaining Waft Bacirflll and Water�rao�
Backfill placed behind retaining or foundation walls should be coarse, free -draining
structural fill containing no organics. This backfill should contain no more than 5 percent silt
or clay particles and have no gravel greater than 4 inches in diameter. The percentage of
particles passing the No. 4 sieve should be between 25 and 70 percent. A drainage
composite similar to Miradrain 6000 should be placed against the backfilled retaining walls.
The gravel and drainage composites should be hydraulically connected to the foundation
drain system. Free -draining backfill should be used for the entire width of the backfill where
seepage is encountered. For increased protection, drainage composites should be placed
along cut slope faces, and the walls should be backfilled entirely with free -draining soil. The
later section entitled Drainage Co.nsideradons should also be reviewed for
recommendations related to subsurface drainage behind foundation and retaining walls.
The purpose of these backfill requirements is to ensure that the design criteria for a
retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the
wall. Also, subsurface drainage systems are not intended to handle large volumes of water
from surface runoff. The top 12 to 18 inches of the backfill should consist of a compacted,
relatively impermeable soil or topsoil, or the surface should be paved. The ground surface
GE®TECH CONSULTANTS, INC.
Robert Christensen
December 18, 2018
JN 18508
Page 9
must also slope away from backfilled walls at one to 2 percent to reduce the potential for
surface water to percolate into the backfill.
Water percolating through pervious surfaces (pavers, gravel, permeable pavement, etc.)
must also be prevented from flowing toward walls or into the backfill zone. Foundation
drainage and waterproofing systems are not intended to handle large volumes of infiltrated
water. The compacted subgrade below pervious surfaces and any associated drainage
layer should therefore be sloped away. Alternatively, a membrane and subsurface collection
system could be provided below a pervious surface.
It is critical that the wall backfill be placed in lifts and be properly compacted, in order for the
above -recommended design earth pressures to be appropriate.- The recommended wall
design criteria assume that the backfill will be well -compacted in lifts no thicker than 12
inches. The compaction of backfill near the walls should be accomplished with hand -
operated equipment to prevent the walls from being overloaded by the higher soil forces
that occur during compaction. The section entitled General Earthwork and Sftuctnral Fill
contains additional recommendations regarding the placement and compaction of structural
fill behind retaining and foundation walls.
The above recommendations are not intended to waterproof below -grade walls, or to
prevent the formation of mold, mildew or fungi in interior spaces. Over time, the
performance of subsurface drainage systems can degrade, subsurface groundwater flow
patterns can change, and utilities can break or develop leaks. Wherefore, waterproofing
should be provided where future seepage through the walls is not acceptable. This typically
includes limiting cold joints and wall penetrations, and using bentonite panels or
membranes on the outside of the walls. There are a variety of different waterproofing
materials and systems, which should be installed by an experienced contractor familiar with
the anticipated construction and subsurface conditions. Applying a thin coat of asphalt
emulsion to the outside face of a wall is not considered waterproofing, and will only help to
reduce moisture generated from water vapor or capillary action from seeping through the
concrete. As with any project, adequate ventilation of basement and crawl space areas is
important to prevent a buildup of water vapor that is commonly transmitted through concrete
walls from the surrounding soil, even when seepage is not present. This is appropriate even
when waterproofing is applied to the outside of foundation and retaining walls. We
recommend that you contact an experienced envelope consultant if detailed
recommendations or specifications related to waterproofing design, or minimizing the
potential for Infestations of mold and mildew are desired.
PERMANENT TIEBACK ANCHORS FOR SOUTHERN FOUNDATION (WALL
We recommend installing tieback anchors at inclinations between 20 and 30 degrees below
horizontal. The tieback will derive its capacity from the soil -grout strength developed in the soil
behind the no-load zone. The minimum grouted anchor length should be 10 feet. The no-load zone
is the area behind which the entire length of each tieback anchor should be located. To prevent
excessive loss -of -ground in a drilled hole, the no-load section of the drilled tieback hole should be
backfilled with a sand and fly ash slurry, after protecting the anchor with a bond breaker, such as
plastic casing, to prevent loads from being transferred to the soil in the no-load zone. The no-load
section could be filled with grout after anchor testing is completed.
GEOTECH CONSULTMrT5, INC.
Robart Christensen JN 18508
December 18, 2018 Page 10
Based on the results of our analyses and our experience at other construction sites, we suggest
using an adhesion value of 2,500 psf in the glacial till to design permanent anchors. This value
applies to non -pressure -grouted anchors. Pressure -grouted or post -grouted anchors can often
develop adhesion values that are two to three times higher than that for non-pressuresrouted
anchors. These higher adhesion values must be verified by load testing.
Soil conditions, soil -grout adhesion strengths, and installation techniques typically vary over any
site. This sometimes results in adhesion values that are lower than anticipated. Therefore, we
recommend substantiating the anchor design values by load -testing all tieback anchors. At least
two anchors in each soil type encountered should be performance -tested to 200 percent of the
design anchor load to evaluate possible anchor creep. Wherever possible, the no-load section of
these tiebacks should not be grouted until the performance tests are completed. Unfavorable
results from these performance tests could require increasing the lengths of the tiebacks. The
remaining anchors should be proof -tested to at least 135 percent of their design value before being
"locked off." After testing, each anchor should be locked off at a prestress load of 80 to 100
percent of its design load.
If caving or water -bearing soil is encountered, the installation of tieback anchors will be hampered
by caving and soil flowing into the holes. It will be necessary to case the holes, if such conditions
are encountered. Alternatively, the use of a hollow -stem auger with grout pumped through the
stem as the auger is withdrawn would be satisfactory, provided that the injection pressure and
grout volumes pumped are carefully monitored.
All drilled installations should be grouted and backflled immediately after drilling. No drilled holes
should be left open overnight.
PERMANENT SOLDIER PULES
A permanent soldier pile wall is also an option for the southern foundation wall of the cabana. Its
design should be submitted to Geotech Consultants, Inc. for review prior to beginning site
excavation. We are available and would be pleased to assist in this design effort. Because the wall
is permanent, the same safety factors for static, dynamic, and catchment conditions as noted in the
Foundation and Retaining Walls section of this report.
Soldier pile walls would be constructed in front of the failed timber retaining wall, before making
planned cuts for the new foundations, by setting steel H-beams in a drilled hole and grouting the
space between the beam and the soil with concrete for the entire height of the drilled hole. We
anticipate that the holes could be drilled without casing, but the contractor should be prepared to
case the holes or use the slurry method if caving soil is encountered. Excessive ground loss in the
drilled holes must be avoided to reduce the potential for settlement on adjacent properties. If water
is present in a hole at the time the soldier pile is poured, concrete must be tremied to the bottom of
the hole.
As excavation proceeds downward, the space between the piffles should be lagged with timber, and
any voids behind the timbers should be filled with clean crushed rock, or a slurry comprised of sand
and fly ash. Treated lagging is usually required for permanent walls, while untreated lagging can
often be utilized for temporary shoring walls. Temporary vertical cuts will be necessary between the
soldier piles for the lagging placement. The prompt and careful installation of lagging is important,
particularly in loose or caving soil, to maintain the integrity of the excavation and provide safer
working conditions. Additionally, care must be taken by the excavator to remove no more soil
GECTECH CONSULTANTS, INC.
Robert Chnstsnsen JN 18508
December 18, 2018 Page 11
between the soldier piles than is necessary to install the lagging. Caving or overexcavation during
lagging placement could result in loss of ground on neighboring properties. Timber lagging should
be designed for an applied lateral pressure of 30 percent of the design wall pressure, if the pile
spacing is less than three pile diameters. For larger pile spacings, the lagging should be designed
for 50 percent of the design load.
We anticipate that permanent foundation walls will be constructed against the shoring walls.
Where this occurs, a plastic -backed drainage composite, such as Miradrain, Battledrain, or similar,
should be placed against the entire surface of the shoring prior to pouring the foundation wall.
Weep pipes located no more than 6 feet on -center should be connected to the drainage composite
and poured into the foundation walls or the perimeter footing. A footing drain installed along the
inside of the perimeter footing will be used to collect and carry the water discharged by the weep
pipes to the storm system. Isolated zones of moisture or seepage can still reach the permanent
wali where groundwater finds leaks or joints in the drainage composite. This is often an acceptable
risk in unoccupied below -grade spaces, such as parking garages. However, formal waterproofing is
typically necessary in areas where wet conditions at the face of the permanent wall will not be
tolerable. If this is a concern, the permanent drainage and waterproofing system should be
designed by a specialty consultant familiar with the expected subsurface conditions and proposed
construction.
Footing drains placed inside the building or behind backfilled walls should consist of 4-inch,
perforated PVC pipe surrounded by at least 6 inches of 1-inch-minus, washed rock wrapped in a
non -woven, geotextiie filter fabric (Miraff 140N, Supac 4NP, or similar material).
Soldier Pile MY Design
Permanent soldier pile shoring that is cantilevered or restrained by one row of tiebacks, and that
has a level backslope, should be designed for an active soil pressure equal to that pressure
exerted by an equivalent fluid with a unit weight of 50 pounds per cubic foot (pcf). This wall will also
have to be designed to include catchment as was noted earlier in this report. It is important that the
shoring design provides sufficient working room to drill and install the soldier piles, without needing
to make unsafe, excessively steep temporary cuts. Cut slopes should be planned to intersect the
backside of the drilled holes, not the back of the lagging.
Lateral movement of the soldier piles below the excavation level will be resisted by an ultimate
passive soil pressure equal to that pressure exerted by a fluid with a density of 600 pcf. This sail
pressure is valid only for a level excavation in front of the soldier pile; it acts on two times the
grouted pile diameter. Cut slopes made in front of shoring walls significantly decrease the passive
resistance. This includes temporary cuts necessary to install internal braces or rakers. The
minimum embedment below the floor of the excavation for cantilever soldier piles should be equal
to the height of the "stick-up."
The above values assume that the excavation is level in front of the soldier pile and that the bottom
of the pile is embedded a minimum of 10 feet below the floor of the excavation. For the pile end -
bearing to be appropriate, the bottom of the drilled holes must be cleaned of loosened soil. The
shoring contractor should be made aware of this if end bearing is used in design calculations, as it
will affect their installation procedures. The concrete surrounding the embedded portion of the pile
must have sufficient bond and strength to transfer the vertical load from the steel section through
the concrete into the soil.
GEOTECH CONSUL -ANTS. INC.
Robert Christensen
December 18, 2018
JN 18508
Page 12
EXCAVATIONS AND SLOPE'S
Temporary excavation slopes should not exceed the limits specified in local, state, and national
government safety regulations. Also, temporary cuts should be planned to provide a minimum 2 to
3 feet of space for construction of foundations, walls, and drainage. Temporary cuts to a maximum
overall depth of about 4 feet may be attempted vertically in unsaturated soil, if there are no
indications of slope instability. However, vertical cuts should not be made near property boundaries,
existing utilities and structures, or into the adjacent slope to the south of the cabana. Eased upon
Washington Administrative Code (WAC) 296, Part N, the upper, loose soil and underlying dense
soil at the subject site would generally be classified as Type E and A, respectively. Therefore,
temporary cut slopes greater than 4 feet in height should not be excavated at an inclination steeper
than 1:1 and 0.75:1 (Horizontal:Vertical), respectively, extending continuously between the top and
the bottom of a cut.
The above-reoommended temporary slope inclinations are based on the conditions exposed in our
explorations, and on what has been successful at other sites with similar soil conditions. It is
possible that variations in soil and groundwater conditions will require modifications to the
inclination at which temporary slopes can stand. Temporary cuts are those that will remain
unsupported for a relatively short duration to allow for the construction of foundations, retaining
walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet
weather. It is also important that surface runoff be directed away from the top of temporary slope
cuts. Cut slopes should also be backfilled or retained as soon as possible to reduce the potential
for instability. Please note that loose soil can cave suddenly and without warning. Excavation,
foundation, and utility contractors should be made especially aware of this potential danger. These
recommendations may need to be modified if the area near the potential cuts has been disturbed in
the past by utility installation, or if settlement -sensitive utilities are located nearby.
Water should not be allowed to flow uncontrolled over the top of any temporary or permanent
slope. All permanently exposers slopes should be seeded with an appropriate species of vegetation
to reduce erosion and improve the stability of the surficial layer of soil.
Any disturbance to the existing slope outside of the building limits may reduce the stability of the
slope. Damage to the existing vegetation and ground should be minimized, and any disturbed
areas should be revegetated as soon as passible. Soil from the excavation should not be placed on
the slope, and this may require the off -site disposal of any surplus soil.
DRAINAGE CONSIDERATIONS
Footing drains should be used where: (1) crawl spaces or basements will be below a structure; (2)
a slab is below the outside grade; or, (3) the outside grade does not slope downward from a
building. Drains should also be placed at the base of all earth -retaining walls. These drains should
be surrounded by at least 6 inches of 1-inch-minus, washed rock that is encircled with non -woven,
geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At its highest point, a
perforated pipe invert should be at least 5 inches below the bottom of a slab floor or the level of a
crawl space. The discharge pipe for subsurface drains should be sloped for flow to the outlet point.
Roof and surface water drains must not discharge into the foundation drain system. A typical
footing drain detail is attached to this report as Plate 6. For the best long -tern performance,
perforated PVC pipe is recommended for all subsurface drains. Clean -outs should be provided for
potential future flushing or Gleaning of footing drains.
GEOTECH CONSULTANTS, INC.
Rouen Christensen JN 18508
December 18, 2018 Page 13
As a minimum, a vapor retarder, as defined in the Slabs -On -Grade section, should be provided in
any crawl space area to limit the transmission of water vapor from the underlying soils. Crawl space
grades are sometimes left near the elevation of the bottom of the footings. As a result, an outlet
drain is recommended for all crawl spaces to prevent an accumulation of any water that may
bypass the footing drains. Providing a few inches of free draining gravel underneath the vapor
retarder is also prudent to limit the potential for seepage to build up on top of the vapor retarder.
Perched groundwater was observed during our field work. If seepage is encountered in an
excavation, it should be drained from the site by directing it through drainage ditches, perforated
pipe, or French drains, or by pumping it from sumps interconnected by shallow connector trenches
at the bottom of the excavation.
The excavation and site should be graded so that surface water is directed off the site and away
from the tops of slopes. Water should not be allowed to stand in any area where foundations,
slabs, or pavements are to be constructed. Final site grading in areas adjacent to the cabana
should slope away at least one to 2 percent, except where the area is paved. Surface drains should
be provided where necessary to prevent ponding of water behind foundation or retaining walls. A
discussion of grading and drainage related to pervious surfaces near walls and structures is
contained in the Fbundatlon and Retaining Walls section.
GENERAL EARTHWORK AND STRUCTURAL FILL
All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and
other deleterious material. It is important that existing foundations be removed before site
development. The stripped or removed materials should not be mixed with any materials to be used
as structural fill, but they could be used in non-structural areas, such as landscape beds.
Structural fill is defined as any fill, including utility backfill, placed under, or close to, a building, or In
other areas where the underlying soil needs to support loads. All structural fill should be placed in
horizontal lifts with a moisture content at, or near, the optimum moisture content. The optimum
moisture content is that moisture content that results in the greatest compacted dry density. The
moisture content of fill is very important and must be closely controlled during the falling and
compaction process.
The allowable thickness of the fill lift will depend on the material type selected, the compaction
equipment used, and the number of passes made to compact the lift. The loose lift thickness
should not exceed 12 inches, but should be thinner if small, hand -operated compactors are used.
We recommend testing structural fill as it is placed. If the fall is not sufficiently compacted, it should
be recompacted before another Taft is placed. This eliminates the need to remove the fill to achieve
the required compaction.
GEOTECH CONSULTANTS, INC.
Robert Christensen
December 18, 2018
JN 18508
Page 14
The following table presents recommended levels of relative compaction for compacted fill:
Whare: Minimum Relative Compaction Is the ratio, expressed In
parcentWr,, of the compacted dry density to the rrawd mum dry
density, as determineal In accordance with ASTM Test
Designaftron ®1557A1 JModiSed i'roc wo -
LIMITATIONS
The conclusions and recommendations contained in this report are based on site conditions as
they existed at the time of our exploration and assume that the soil and groundwater conditions
encountered in the test borings are representative of subsurface conditions on the site. If the
subsurface conditions encountered during construction are significantly different from those
observed in our explorations, we should be advised at once so that we can review these conditions
and reconsider our recommendations where necessary. Unanticipated conditions are commonly
encountered on construction sites and cannot be fully anticipated by merely taking samples in test
borings. Subsurface conditions can also vary between exploration locations. Such unexpected
conditions frequently require making additional expenditures to attain a properly constructed
project. It is recommended that the owner consider providing a contingency fund to accommodate
such potential extra costs and risks. This is a standard recommendation for all projects.
The recommendations presented in this report are directed toward the protection of only the
proposed structure from damage due to slope movement. Predicting the future behavior of steep
slopes and the potential effects of development on their stability is an inexact and imperfect
science that is currently based mostly on the past behavior of slopes with similar characteristics.
Landslides and soil movement can occur on steep slopes before, during, or after the development
of property. The owner of any property containing, or located close to steep slopes must ultimately
accept the possibility that some slope movement could occur, resulting in possible loss of ground
around the proposed cabana.
This report has been prepared for the exclusive use of Robert Christensen and his representatives,
for specific application to this project and site. Our conclusions and recommendations are
professional opinions derived in accordance with our understanding of current local standards of
practice, and within the scope of our services. No warranty is expressed or implied. The scope of
our services does not include services related to construction safety precautions, and 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. Our services
also do not include assessing or minimizing the potential for biological hazards, such as mold,
bacteria, mildew and fungi in either the existing or proposed site development.
GE®TECH CONSULTANTS, INC.
Robert Chrisfensen
December 18, 2018
9N 18508
Page 15
ADDITIONAL SERWCES
In addition to reviewing the final plans, Geotech Consultants, Inc. should be retained to provide
geotechnical consultation, testing, and observation services during construction. This is to confirm
that subsurface coondibons are consistent with those indicated by our exploration, to evaluate
whether earthwork and foundation construction activities comply with the general intent of the
recommendations presented in this report, and to provide suggestions for design changes in the
event subsurface conditions differ from those anticipated prior to the start of construction. However,
our work would not include the supervision or direction of the actual work of the contractor and its
employees or agents. Also, job and site safety, and dimensional measurements, will be the
responsibility of the contractor.
During the construction phase, we will provide geotechnical observation and testing services when
requested by you or your representatives. Please be aware that we can only document site work
we actually observe. it is still the responsibility of your contractor or on -site construction team to
verify that our recommendations are being followed, whether we are present at the site or not,
The following plates are attached to complete this report:
Plate 1 Vicinity Map
Plate 2 Site Exploration Plan
Plates 3 - 5 Test Boring Logs
Plate 6 Typical Footing Drain Detail
Plate 7 Typical Overexcavation Detail
We appreciate the opportunity to be of service on this project. Please contact us if you have any
questions, or If we can be of further assistance.
MKMIDRW:kg
Respectfully submitted,
D. Robert Ward, P.E.
FIrincipal
12/18/18
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204 Southwest 292nd Street
Federal Way, Washington
Job NO.' Date: Plate:
18508 Dec. 2018 No Scale 2
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• Perched groundwater was encountered from 4 to 7.5 feet during drilling.
GEOTECH
CONSULTANT'S, INC—
TEST BOILING LOG
204 Southwest 292nd Street
Federal Way, Washington
Job Date: LDgg d by. Plate:
18508 Nov.2018 ML 3
i . 'P g4 ` �a
e � "-Zov"V 5P�� �yGS
5
BORING 2
Destaription
Gray to gray -brown, slightly gravelly, silty SAND, fine-grained, moist, cemented,
dense (GLACIAL TILL)
No remvery
[ 88 3
r
• Test boring was terminated at 8 feet on November 8, 2018 due to
auger refusal.
10
* No groundwater was encountered during drilling.
GEOTECH
CONSULTANTS, INC.
TEST BORING LOG
204 Southwest 292nd Street
Federal Way, Washington
Job Date: Logged by: Plate:
18508 Nov_ 2018 MKM :4]
5
I
95i--•
q<0b��,Q�g �G9
9
`52
50
4"
42
11
FILL
BORING 3
o9awpliQn
Brown silty SAND with organics, fine-grained, very moist, loose (FILL)
1211 1 -with pieces of creosote soaked timber (Overstated blow counts on old post)
31
Gray -brown gravelly, silty SAND, fine-grained, moist, cemented, very dense
(GLACIAL TILL)
sm
-becomes blue -gray, increased silt content, reduced gravel content
41
* Test boring was terminated at 11.5 feet on November 8, 2018 due to
auger refusal.
* No groundwater was encountered daring drilling.
GEOTECH
CONSMTANTS, INC.
TEST BORING LOG
204 Southwest 292nd Street
Federal Way, Washington
Job I Date. I Logged by. I Plate:
18508 Nov.2018 MKM 5
Slope backfiill away from
foundation. Provide surface
drains where necessary.
Washed Rocl
(7/8" min. size)
4" min.
Tightiine Roof Drain
(Do not connect to Foct ng drain)
Rackfill
(See text for
requirements)
Nonwoven Geotextile
Filter Fabric
Possible Slab
v e o f e;V a 'Pr D'o
V0 O •O,�-
Da❑aC3 a0°0 L' o
0 o v v
4V Perforated Hard PVC Pipe
(Invert at least 6 inches below
slab or crawl space. Slope to
drain to appropriate outtall.
Place holes downward.)
Vapor Retarder/Barrier and
Capillary BreakfDrainage Layer
(Refer to Report text)
NOTES:
(1) In crawl spaces, provide an outlet drain to prevent buildup of water that
bypasses the perimeter footing drains.
(2) Refer to report tent for additional drainage, waterproofing, and slab considerations.
GEOTECH
CONSULTAMM INC.
FOOTING DRAIN DETAIL
204 Southwest 292nd Street
Federal Way, Washington
Job No: Date: Pate:
18508 1 Dec.2018 1 1 6
Unsuitable
Soils
I:::
a;.p:4:p;.rs:4.D,•A.Q.D; C.D;A:�.D,.a.O:D;•o.Q:D
ap"•DDe.p.°•pDgp'°'pD4p 0ve'. �-O'°,o'O-pD o 'TD°
D
°rip°nP o'a . •6*�P°PPa
D e
°oD°•p °'DO°•p °oD' �D°•p'°'vD°•p' pA a•p'`•aP�
np0�lDa•��D°•?O�Pc F^�'a•��Dp qj� ID
D'0-.OP 'c
m
p _ !3 D'° i p0r1n D•P r .. v • ..f-�_J o ...�1 R ..
Fill (refer to report for
gradation and compaction requirements),
See Note 2 for condition where lean
concrete is used to bac kfill the
overexcavation.
Suitable Searing Soli (Refer to report for description)
verify_ by Geotechnical Engineer prior to placing structural
Fill.
Width of Overexoavation = Footing Width (FW) + Depth of Dverexcavation
NOTES:
1. Refer to report text for additional overexcavation, foundation, and structural fill considerations.
2. where lean oonorete (minimum 1-112 sacks of cement per cubic yard) is used to bacldiq the
Merexcavation, the overexcavation must extend only 6 inches beyond the edges of the footing.
GEOTECH
CONSCT TAMS, INC.
TYPICAL FOOTING DYEREXCAVATIGN
204 Southwest 292nd Street
Federal Way, Washington
Job No: Date: Plate:
18508 Doc.
2018 7
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BOUNDARY & TOPOGRAPHIC SURVEY GONTA vuKINr
IN
PREPARED FOR: Bob & Laurie Christensen
EXHIBIT "A"
A PORTION OF THE NE1/4 OF THE NEI/4 OF SEC. 6, 711P. 21N., RNG. 4E. R K
IENG COUNTY, WASHINOTON
SOUTH LINE OF SEWER
EASEMENT FILED UNDER
Ass RECORDING NO.7206060594
10'
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J blew PA
29th Street East, J105 - Sumner, WA 98390
253-987-6924 main 253-987-7859 fax
F 'CEIVED ,
REQUEST FOk ADAHNISTRATwE DECISION
k t ZLF 19 COMMUNITY DEVELOPMENT DEPARTMENT
CITY OF
FEB 0 1 `�+ 33325 8`� Avenue South
Federal Way Federal Way, WA 98003
CEN OF FEDERAL WAY 253-835-2607; Fax 253-835-2609
COMMUNITY DEVELOPMENT WNv%V,citvoffedera1Aay.com
FILE NUMBER ! - D T Date C — !
Applicant
NAME PRIMARY PHONE
BUSINESS/ORGANIZATION ALTERNA PHONE
�zs3 Bag—?q�7
MAILING ADDRESS E-MAIL C
2OLi
CITY STATE ZIP FAX
Property Address/Location 20
II
Description of Request
_Cr-tz,+ krcc rya C.�,_y ao�� r�►wc� a rr41� x�a�
List/Describe I
CE s4Y
fSiollmRrl t@ 111
_crL Ld 1 L
For Staff Use
El Code Interpretation/Clarification
J�[ Critical Areas Letter/Analysis/Peer Review
'❑ Request for Extension (Land Use/Plat Approval)
❑ Revisions to Approved Permit
❑ Tree Removal
❑ Zoning Compliance Letter
El
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C-V--
Bulletin #079 —December 11, 2018
Page 1 of 1 k:\Handouts\Request for Administrative Decision