HomeMy WebLinkAboutPages from P_Civil_Construction_Plans_191114_v2IN COMPLIANCE WITH CITY OF RENTON STANDARDS
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November 15, 2018 | Page 3 File No. 23656-001-00
Groundwater Conditions
No groundwater seepage was observed in the test pits completed on site. Additionally, previous
explorations completed to depths of 30 feet below site grades did not encounter groundwater.
Groundwater observations represent conditions observed during exploration and may not represent the
groundwater conditions throughout the year. We anticipate that perched groundwater will exist at the
contact between the glacial till and the overlying looser fill and weathered till, and within more permeable
layers within the native glacial till. Groundwater seepage is expected to fluctuate as a result of season,
precipitation, and other factors.
CONCLUSIONS AND RECOMMENDATIONS
Based on the results of our field exploration program, laboratory testing, and engineering analysis, we
conclude that development of the proposed development can be accomplished as planned. A summary of
primary geotechnical considerations for the site development and design of the proposed development is
provided in the subsequent sections.
Summary
■ The planned townhomes site is classified as Site Class C, in accordance with the 2015 International
Building Code (IBC).
■ The planned townhomes may be supported on conventional spread footings bearing on dense to very
dense undisturbed glacial till or on structural fill placed over these soils. Footings bearing on dense to
very dense undisturbed glacial till may be designed using an allowable soil bearing value of
6,000 pounds per square foot (psf). Footings bearing on structural fill placed over undisturbed dense
to very dense glacial till may be designed using an allowable bearing value of 3,000 psf. All existing fill,
highly weathered glacial till or otherwise unsuitable soils should be removed from below foundations
prior to constructing foundations or placing structural fill. The allowable bearing value may be increased
by one-third for short duration loads such as wind or seismic events.
■ Lateral foundation loads may be resisted by passive resistance on the sides of the footings and by
friction on the base of the footings. For footings supported and surrounded by either dense native soils
or compacted structural fill, a coefficient of friction of 0.35 and a passive resistance of 350 pounds per
cubic foot (pcf) may be used.
■ A subgrade modulus of 100 pounds per cubic inch (pci) may be used for design of the slabs-on-grade
for the townhomes. Concrete slabs-on-grade should be supported on a 4-inch-thick capillary break layer
overlain by a vapor retarder (in conditioned spaces or enclosed rooms, such as mechanical or storage
space).
■ The pavement section extending to the west and north from the southeast corner of the project site
may be supported on existing fill soils provided that the upper 2 feet of the fill is placed and compacted
as structural fill. A proof roll test can also be conducted to reduce excavation costs if native soils are
encountered within the upper 2 feet and to check that the soils can perform adequately under planned
loads. If any soft spots are observed, the upper 2 feet of subgrade soils should be removed and
replaced with at least 2 feet of structural fill compacted to at least 95 percent of the maximum dry
density (MDD) per ASTM International (ASTM) D 1557. Suitable on-site soils may be used as structural
November 15, 2018 | Page 4 File No. 23656-001-00
fill under the planned road and associated hardscape provided that earthwork is accomplished during
dry weather conditions in the summer months.
Earthquake Engineering
We evaluated the site for seismic hazards including liquefaction, lateral spreading, fault rupture and
earthquake-induced landsliding. Our evaluation indicates that the site does not have liquefiable soils
present and, therefore, also has no risk of liquefaction-induced lateral spreading. In addition, the site has
a low risk of fault rupture and earthquake-induced landsliding.
2015 IBC Seismic Design Information
For the planned townhomes, we recommend the IBC 2015 parameters for average field standard
penetration resistance, site class, short period spectral response acceleration (SS), 1-second period
spectral response acceleration (S1), and seismic coefficients FA and FV presented in Table 1.
TABLE 1. 2015 IBC SEISMIC PARAMETERS
2015 IBC Parameter Recommended Value
Average Field Standard Penetration Resistance >50
Site Class C
Short Period Spectral Response Acceleration, SS (percent g) 143.2
1-Second Period Spectral Response Acceleration, S1 (percent g) 53.8
Seismic Coefficient, FA 1.000
Seismic Coefficient, FV 1.300
Liquefaction Potential
Liquefaction is a phenomenon where soils experience a rapid loss of internal strength as a consequence
of strong ground shaking. Ground settlement, lateral spreading and/or sand boils may result from soil
liquefaction. Structures supported on liquefied soils could suffer foundation settlement or lateral
movement that could be severely damaging to the structures.
Conditions favorable to liquefaction occur in loose to medium dense, clean to moderately silty sand, which
is below the groundwater level. Based on our evaluation of the subsurface conditions observed in the
explorations, it is our opinion that potentially liquefiable soils are not present at the project site.
Ground Rupture
Ground rupture from lateral spreading is associated with liquefaction. Lateral spreading involves lateral
displacements of large volumes of liquefied soil, and can occur on near-level ground as blocks of surface
soils displace relative to adjacent blocks. In our opinion, ground rupture resulting from lateral spreading at
the site is unlikely because potentially liquefiable soils are not present at the site as discussed above.
Because of the thickness of the Quaternary sediments below the site, which are commonly more than
1,000 feet thick, the potential for surface fault rupture is considered low.
November 15, 2018 | Page 5 File No. 23656-001-00
Landslides
Because site topography is relatively flat and dense to very dense glacial till deposits occur at shallow
depths, it is our opinion that landsliding as a result of strong ground shaking is unlikely at the site.
Foundations
We recommend that the buildings be supported on shallow spread footings founded on the dense to very
dense native glacial till soil encountered in the explorations, or on properly compacted structural fill
extending down to medium dense to dense glacial till. The following recommendations for the buildings are
based on the subsurface conditions observed in the explorations and the site survey.
Foundation Design
For shallow foundation support, we recommend widths of at least 18 and 24 inches, respectively, for
continuous wall and isolated column footings supporting the proposed townhomes. The design frost depth
in the Puget Sound area is 12 inches, therefore, we recommend that the footings be founded at least
18 inches below lowest adjacent finished grade.
Unsuitable soils consisting of fill, topsoil, and/or highly weathered glacial soils will vary across the site and
must be removed from below planned footings. Based on our explorations, up to 3½ feet of fill and/or
looser weathered native soils exist under the proposed north building unit (GEI-TP-1 and GEI-TP-2),
approximately 3 feet under the central building unit (GEI-TP-3 and GEI-TP-4) and 2½ feet under the south
building unit (GEI-TP-5 and GEI-TP-6). Therefore, depending on the foundation locations and depths,
overexcavation under the footings may be necessary. For foundations supported on medium dense native
glacial till or structural fill extending down to medium dense to dense native glacial till, we recommend
footings be designed using a maximum allowable bearing pressure of 3,000 psf. A maximum allowable
bearing pressure of 6,000 psf may be used in design where foundations are bearing on dense to very dense
relatively unweathered glacial till or on controlled density fill (CDF) extending down to the dense to very
dense native till. All existing fill and looser native soils should be removed from below planned footings.
These allowable bearing pressures apply to the total dead and long-term live loads and may be increased
up to one-third for short-term live loads such as wind or seismic forces.
The overexcavated areas should be backfilled with: (1) CDF having a design strength of at least 200 pounds
per square inch (psi) where 6,000 psf bearing pressures are used or, (2) imported gravel borrow where
3,000 psf is used. Where structural fill is placed below footings, the fill should extend beyond the edges of
the foundations by the depth of the overexcavation, while the CDF should extend beyond the edges of the
foundations by half the depth of the excavation.
Foundation Settlement
We estimate that the post-construction settlement of footings founded on the very dense glacial till or
structural fill extending to the medium dense to very dense till, as recommended above, will be between
½ and 1 inch. Differential settlement between comparably loaded column footings or along a 25-foot
section of continuous wall footing should be less than ½ inch. We expect most of the footing settlements
will occur as loads are applied. Loose or disturbed soils not removed from footing excavations prior to
placing concrete will result in additional settlement.
November 15, 2018 | Page 6 File No. 23656-001-00
Lateral Resistance
Lateral loads can be resisted by passive resistance on the sides of the footings and by friction on the base
of the footings. Passive resistance should be evaluated using an equivalent fluid density of 350 pcf where
footings are poured neat against native soil or are surrounded by structural fill compacted to at least
95 percent of MDD, as recommended. Resistance to passive pressure should be calculated from the
bottom of adjacent floor slabs and paving or below a depth of 1 foot where the adjacent area is unpaved,
as appropriate. Frictional resistance can be evaluated using 0.35 for the coefficient of base friction against
footings. The above values incorporate a factor of safety of about 1.5.
If soils adjacent to footings are disturbed during construction, the disturbed soils must be recompacted,
otherwise the lateral passive resistance value must be reduced.
Construction Considerations
Immediately prior to placing concrete, all debris and loose soils that accumulated in the footing excavations
during forming and steel placement must be removed. Debris or loose soils not removed from the footing
excavations will result in increased settlement.
If wet weather construction is planned, we recommend that all footing subgrades be protected using a lean
concrete mud mat or 3 inches of compacted crushed base course. The mud mat or base course should be
placed the same day that the footing subgrade is excavated and approved for foundation support.
We recommend that all completed footing excavations be observed by a representative of our firm prior to
placing mud mat, reinforcing steel, and structural concrete. Our representative will confirm that the bearing
surface has been prepared in a manner consistent with our recommendations and that the subsurface
conditions are as expected.
Footing Drains
We recommend that perimeter footing drains be installed around each building. The perimeter drains
should be installed at the base of the exterior footings. The perimeter drains should be provided with
cleanouts and should consist of at least 4-inch-diameter perforated pipe placed on a 3-inch bed of drainage
material, and surrounded by 6 inches of drainage material enclosed in a non-woven geotextile fabric such
as TenCate Mirafi 140N (or approved equivalent) to prevent fine soil from migrating into the drain material.
We recommend against using flexible tubing for footing drainpipes. The perimeter drains should be sloped
to drain by gravity, if practicable, to a suitable discharge point, preferably a storm drain. We recommend
that the cleanouts be covered, and be placed in flush-mounted utility boxes. Water collected in roof
downspout lines must not be routed to the footing drain lines.
Slab-on-Grade Floors
We expect that the lower level concrete slab-on-grade can be supported on the medium dense to very dense
native soil encountered in our explorations or on properly compacted structural fill. A subgrade modulus of
100 pci may be used for design of the slabs-on-grade at the site. We recommend that an appropriate
capillary break and vapor retarder be installed below concrete slabs to reduce the risk of moisture migration
through the floor slab. This is especially important since zones of groundwater seepage may be present at
the planned floor slab level in more permeable layers above the dense native glacial till or in looser soils
on top of the dense glacial till.
November 15, 2018 | Page 8 File No. 23656-001-00
Portland Cement Concrete Pavement
Portland cement concrete (PCC) sections may be considered for areas where concentrated heavy loads
may occur. We recommend that these pavements consist of at least 6 inches of PCC over 6 inches of CSBC.
A thicker concrete section may be needed based on the actual load data for use of the area. If the concrete
pavement will have doweled joints, we recommend that the concrete thickness be increased by an amount
equal to the diameter of the dowels. The base course should be compacted to at least 95 percent of the
MDD.
We recommend PCC pavements incorporate construction joints and/or crack control joints spaced at
maximum distances of 12 feet apart, center-to-center, in both the longitudinal and transverse directions.
Crack control joints may be created by placing an insert or groove into the fresh concrete surface during
finishing, or by saw cutting the concrete after it has initially set up. We recommend the depth of the crack
control joints be approximately one fourth the thickness of the concrete; or about 1½ inches deep for the
recommended concrete thickness of 6 inches. We also recommend the crack control joints be sealed with
an appropriate sealant to help restrict water infiltration into the joints.
Asphalt-Treated Base
If pavements are constructed during the wet seasons, consideration may be given to covering the areas to
be paved with asphalt-treated base (ATB) for protection. Light-duty pavement areas should be surfaced with
3 inches of ATB, and heavy-duty pavement areas should be surfaced with 6 inches of ATB. Thicker ATB
sections may be needed based on construction equipment loads. Prior to placement of the final pavement
sections, we recommend the ATB surface be evaluated and areas of ATB pavement failure be removed and
the subgrade repaired. If ATB is used and is serviceable when final pavements are constructed, the CSBC
can be eliminated, and the design PCC or asphalt concrete pavement thickness can be placed directly over
the ATB.
Earthwork
Based on the subsurface soil conditions encountered in the explorations, we expect that the soils at the
site may be excavated using conventional heavy-duty construction equipment. Very dense glacial till was
encountered at relatively shallow depths at the planned building locations; therefore, glacial till soils within
deeper portions of excavations may require a large excavator to accomplish the excavations. Cobbles were
observed in most of the test pits and glacial till deposits in the area commonly contain boulders that may
be encountered during excavation. Accordingly, the contractor should be prepared to deal with boulders, if
encountered.
The glacial till contains sufficient fines (material passing the U.S. standard No. 200 sieve) to be highly
moisture-sensitive and susceptible to disturbance, especially when wet. Ideally, earthwork should be
undertaken during extended periods of dry weather when the surficial soils will be less susceptible to
disturbance and provide better support for construction equipment. Dry weather construction will help
reduce earthwork costs and increase the potential for using the native soils as structural fill.
Trafficability on the site is not expected to be difficult during dry weather conditions. However, the fill and
native soils will be susceptible to disturbance from construction equipment during wet weather conditions
and pumping and rutting of the exposed soils under equipment loads may occur.
November 15, 2018 | Page 7 File No. 23656-001-00
Prior to placing the gravel layer, the subgrade should be proofrolled as described previously in the
“Earthwork” section of this report. If necessary, the building slab subgrades should be recompacted to a
firm and unyielding condition.
We recommend that concrete slabs-on-grade be constructed on a gravel layer to provide uniform support
and drainage and to act as a capillary break. The gravel layer below slabs-on-grade should consist of at
least 4 inches of clean crushed gravel with a maximum particle size of 1 inch and negligible sand or silt in
accordance with Washington State Department of Transportation (WSDOT) Standard Specification
9-03.1(4)C American Association of State Highway and Transportation Officials (AASHTO) Grading No. 67.
If prevention of moisture migration through the slab is essential, a vapor retarder such as heavy plastic
sheeting should be installed between the slab and the gravel layer. It may also be prudent to apply a sealer
to the slab to further retard the migration of moisture through the floor. We recommend that the plastic
sheet be placed over the capillary break layer.
Pavement Recommendations
Recommendations for typical pavements (asphalt and concrete) are provided in the following sections. The
City of Renton may have standard pavement sections that could apply to the site, therefore the project civil
engineer should review the City’s standards, if applicable.
Subgrade Preparation
We recommend the subgrade soils in new pavement areas be prepared and evaluated as described in the
“Earthwork” section of this report. All new pavement and hardscape areas should be supported on
subgrade soils that have been proof rolled or probed as described in the “Clearing and Site Preparation”
section of this report. If the exposed subgrade soils are loose or soft, it may be necessary to excavate
localized areas and replace them with structural fill or gravel base course. Pavement subgrade conditions
should be observed during construction and prior to placing the subbase materials in order to evaluate the
presence of zones of unsuitable subgrade soils and the need for overexcavation and replacement of these
zones.
New Hot Mix Asphalt Pavement
In light-duty pavement areas (e.g., automobile parking), we recommend a pavement section consisting of
at least a 2½ -inch thickness of ½-inch hot-mix asphalt (HMA) per WSDOT Sections 5-04 and 9-03,
over a 4-inch thickness of densely compacted crushed surfacing base course (CSBC) per WSDOT Section
9-03.9(3). In heavy-duty pavement areas (such as the driveway), we recommend a pavement section
consisting of at least a 3-inch thickness of ½-inch HMA over a 6-inch thickness of densely compacted CSBC.
The base course should be compacted to at least 95 percent of the MDD obtained using ASTM D 1557.
We recommend that proof rolling of the subgrade and compacted base course be observed by a
representative from our firm prior to paving. Soft or yielding zones observed during proof rolling may require
overexcavation and replacement with compacted structural fill.
The pavement sections recommended above are based on our experience. Thicker asphalt sections may
be needed in accordance with the City of Renton or based on the actual traffic data, truck loads, and
intended use. All paved and landscaped areas should be graded so that surface drainage is directed to
appropriate catch basins.
November 15, 2018 | Page 7 File No. 23656-001-00
Prior to placing the gravel layer, the subgrade should be proofrolled as described previously in the
“Earthwork” section of this report. If necessary, the building slab subgrades should be recompacted to a
firm and unyielding condition.
We recommend that concrete slabs-on-grade be constructed on a gravel layer to provide uniform support
and drainage and to act as a capillary break. The gravel layer below slabs-on-grade should consist of at
least 4 inches of clean crushed gravel with a maximum particle size of 1 inch and negligible sand or silt in
accordance with Washington State Department of Transportation (WSDOT) Standard Specification
9-03.1(4)C American Association of State Highway and Transportation Officials (AASHTO) Grading No. 67.
If prevention of moisture migration through the slab is essential, a vapor retarder such as heavy plastic
sheeting should be installed between the slab and the gravel layer. It may also be prudent to apply a sealer
to the slab to further retard the migration of moisture through the floor. We recommend that the plastic
sheet be placed over the capillary break layer.
Pavement Recommendations
Recommendations for typical pavements (asphalt and concrete) are provided in the following sections. The
City of Renton may have standard pavement sections that could apply to the site, therefore the project civil
engineer should review the City’s standards, if applicable.
Subgrade Preparation
We recommend the subgrade soils in new pavement areas be prepared and evaluated as described in the
“Earthwork” section of this report. All new pavement and hardscape areas should be supported on
subgrade soils that have been proof rolled or probed as described in the “Clearing and Site Preparation”
section of this report. If the exposed subgrade soils are loose or soft, it may be necessary to excavate
localized areas and replace them with structural fill or gravel base course. Pavement subgrade conditions
should be observed during construction and prior to placing the subbase materials in order to evaluate the
presence of zones of unsuitable subgrade soils and the need for overexcavation and replacement of these
zones.
New Hot Mix Asphalt Pavement
In light-duty pavement areas (e.g., automobile parking), we recommend a pavement section consisting of
at least a 2½ -inch thickness of ½-inch hot-mix asphalt (HMA) per WSDOT Sections 5-04 and 9-03,
over a 4-inch thickness of densely compacted crushed surfacing base course (CSBC) per WSDOT Section
9-03.9(3). In heavy-duty pavement areas (such as the driveway), we recommend a pavement section
consisting of at least a 3-inch thickness of ½-inch HMA over a 6-inch thickness of densely compacted CSBC.
The base course should be compacted to at least 95 percent of the MDD obtained using ASTM D 1557.
We recommend that proof rolling of the subgrade and compacted base course be observed by a
representative from our firm prior to paving. Soft or yielding zones observed during proof rolling may require
overexcavation and replacement with compacted structural fill.
The pavement sections recommended above are based on our experience. Thicker asphalt sections may
be needed in accordance with the City of Renton or based on the actual traffic data, truck loads, and
intended use. All paved and landscaped areas should be graded so that surface drainage is directed to
appropriate catch basins.
November 15, 2018 | Page 8 File No. 23656-001-00
Portland Cement Concrete Pavement
Portland cement concrete (PCC) sections may be considered for areas where concentrated heavy loads
may occur. We recommend that these pavements consist of at least 6 inches of PCC over 6 inches of CSBC.
A thicker concrete section may be needed based on the actual load data for use of the area. If the concrete
pavement will have doweled joints, we recommend that the concrete thickness be increased by an amount
equal to the diameter of the dowels. The base course should be compacted to at least 95 percent of the
MDD.
We recommend PCC pavements incorporate construction joints and/or crack control joints spaced at
maximum distances of 12 feet apart, center-to-center, in both the longitudinal and transverse directions.
Crack control joints may be created by placing an insert or groove into the fresh concrete surface during
finishing, or by saw cutting the concrete after it has initially set up. We recommend the depth of the crack
control joints be approximately one fourth the thickness of the concrete; or about 1½ inches deep for the
recommended concrete thickness of 6 inches. We also recommend the crack control joints be sealed with
an appropriate sealant to help restrict water infiltration into the joints.
Asphalt-Treated Base
If pavements are constructed during the wet seasons, consideration may be given to covering the areas to
be paved with asphalt-treated base (ATB) for protection. Light-duty pavement areas should be surfaced with
3 inches of ATB, and heavy-duty pavement areas should be surfaced with 6 inches of ATB. Thicker ATB
sections may be needed based on construction equipment loads. Prior to placement of the final pavement
sections, we recommend the ATB surface be evaluated and areas of ATB pavement failure be removed and
the subgrade repaired. If ATB is used and is serviceable when final pavements are constructed, the CSBC
can be eliminated, and the design PCC or asphalt concrete pavement thickness can be placed directly over
the ATB.
Earthwork
Based on the subsurface soil conditions encountered in the explorations, we expect that the soils at the
site may be excavated using conventional heavy-duty construction equipment. Very dense glacial till was
encountered at relatively shallow depths at the planned building locations; therefore, glacial till soils within
deeper portions of excavations may require a large excavator to accomplish the excavations. Cobbles were
observed in most of the test pits and glacial till deposits in the area commonly contain boulders that may
be encountered during excavation. Accordingly, the contractor should be prepared to deal with boulders, if
encountered.
The glacial till contains sufficient fines (material passing the U.S. standard No. 200 sieve) to be highly
moisture-sensitive and susceptible to disturbance, especially when wet. Ideally, earthwork should be
undertaken during extended periods of dry weather when the surficial soils will be less susceptible to
disturbance and provide better support for construction equipment. Dry weather construction will help
reduce earthwork costs and increase the potential for using the native soils as structural fill.
Trafficability on the site is not expected to be difficult during dry weather conditions. However, the fill and
native soils will be susceptible to disturbance from construction equipment during wet weather conditions
and pumping and rutting of the exposed soils under equipment loads may occur.
November 15, 2018 | Page 9 File No. 23656-001-00
Clearing and Site Preparation
Areas to be developed or graded should be cleared of surface and subsurface deleterious matter including
any debris, shrubs, trees and associated stumps and roots. Graded areas should be stripped of organic
soils.
The organic soils can be stockpiled and used later for landscaping purposes or may be spread over
disturbed areas following completion of grading. If spread out, the organic strippings should be in a layer
less than 1-foot thick, should not be placed on slopes greater than 3H:1V (horizontal to vertical) and should
be track-rolled to a uniformly compacted condition. Materials that cannot be used for landscaping or
protection of disturbed areas should be removed from the project site.
Undocumented fill may be present in various areas of the site and will be required to be removed under
building foundations and within the upper two feet of pavement, hardscape and slab subgrade levels.
Where existing fill and looser native soils are removed, they may be reused and recompacted as structural
fill, if conditions allow. If medium dense to dense native soils are encountered below slab subgrade,
additional excavation is not required. If old fill is encountered below slab subgrade, the fill should be
evaluated and possibly removed up to 2 feet below slab subgrade or until medium dense to dense native
soils are encountered (less than 2 feet below slab subgrade). Excavations for slab subgrade preparation
likely do not need to extend more than 2 feet below slab subgrade. The upper two feet below pavement
subgrade should also be removed and replaced as structural fill; however, if existing fill soils are suitable
and adequately compacted based on evaluations after the pavement is removed, the contractor can
perform a proof roll on the exposed surface at or below slab subgrade level, and if approved by the
geotechnical engineer, the fill may be left in place.
Subgrade Preparation
Prior to placing new fills, pavement base course materials or gravel below on-grade floor slabs, subgrade
areas should be proof rolled to locate any soft or pumping soils. Prior to proof rolling, all unsuitable soils
should be removed from below the building footprints. Proof rolling can be completed using a piece of heavy
tire-mounted equipment such as a loaded dump truck. During wet weather, the exposed subgrade areas
should be probed to determine the extent of soft soils. If soft or pumping soils are observed, they should
be removed and replaced with compacted structural fill.
If deep pockets of soft or pumping soils are encountered outside the building areas, it may be possible to
limit the depth of overexcavation by placing a non-woven geotextile fabric such as TenCate Mirafi 500X (or
equivalent) on the overexcavated subgrade prior to placing structural fill. The geotextile will provide
additional support by bridging over the soft material and will help reduce fines contamination into the
structural fill.
After completing the proof rolling, the subgrade areas should be recompacted to a firm and unyielding
condition, if possible. The degree of compaction that can be achieved will depend on when the construction
is performed. If the work is performed during dry weather conditions, we recommend that all subgrade
areas be recompacted to at least 95 percent of the MDD in accordance with the ASTM D 1557 test
procedure (modified Proctor). If the work is performed during wet weather conditions, it may not be possible
to recompact the subgrade to 95 percent of the MDD. In this case, we recommend that the subgrade be
compacted to the extent possible without causing undue heaving or pumping of the subgrade soils.
November 15, 2018 | Page 9 File No. 23656-001-00
Clearing and Site Preparation
Areas to be developed or graded should be cleared of surface and subsurface deleterious matter including
any debris, shrubs, trees and associated stumps and roots. Graded areas should be stripped of organic
soils.
The organic soils can be stockpiled and used later for landscaping purposes or may be spread over
disturbed areas following completion of grading. If spread out, the organic strippings should be in a layer
less than 1-foot thick, should not be placed on slopes greater than 3H:1V (horizontal to vertical) and should
be track-rolled to a uniformly compacted condition. Materials that cannot be used for landscaping or
protection of disturbed areas should be removed from the project site.
Undocumented fill may be present in various areas of the site and will be required to be removed under
building foundations and within the upper two feet of pavement, hardscape and slab subgrade levels.
Where existing fill and looser native soils are removed, they may be reused and recompacted as structural
fill, if conditions allow. If medium dense to dense native soils are encountered below slab subgrade,
additional excavation is not required. If old fill is encountered below slab subgrade, the fill should be
evaluated and possibly removed up to 2 feet below slab subgrade or until medium dense to dense native
soils are encountered (less than 2 feet below slab subgrade). Excavations for slab subgrade preparation
likely do not need to extend more than 2 feet below slab subgrade. The upper two feet below pavement
subgrade should also be removed and replaced as structural fill; however, if existing fill soils are suitable
and adequately compacted based on evaluations after the pavement is removed, the contractor can
perform a proof roll on the exposed surface at or below slab subgrade level, and if approved by the
geotechnical engineer, the fill may be left in place.
Subgrade Preparation
Prior to placing new fills, pavement base course materials or gravel below on-grade floor slabs, subgrade
areas should be proof rolled to locate any soft or pumping soils. Prior to proof rolling, all unsuitable soils
should be removed from below the building footprints. Proof rolling can be completed using a piece of heavy
tire-mounted equipment such as a loaded dump truck. During wet weather, the exposed subgrade areas
should be probed to determine the extent of soft soils. If soft or pumping soils are observed, they should
be removed and replaced with compacted structural fill.
If deep pockets of soft or pumping soils are encountered outside the building areas, it may be possible to
limit the depth of overexcavation by placing a non-woven geotextile fabric such as TenCate Mirafi 500X (or
equivalent) on the overexcavated subgrade prior to placing structural fill. The geotextile will provide
additional support by bridging over the soft material and will help reduce fines contamination into the
structural fill.
After completing the proof rolling, the subgrade areas should be recompacted to a firm and unyielding
condition, if possible. The degree of compaction that can be achieved will depend on when the construction
is performed. If the work is performed during dry weather conditions, we recommend that all subgrade
areas be recompacted to at least 95 percent of the MDD in accordance with the ASTM D 1557 test
procedure (modified Proctor). If the work is performed during wet weather conditions, it may not be possible
to recompact the subgrade to 95 percent of the MDD. In this case, we recommend that the subgrade be
compacted to the extent possible without causing undue heaving or pumping of the subgrade soils.
November 15, 2018 | Page 10 File No. 23656-001-00
Subgrade disturbance or deterioration could occur if the subgrade is wet and cannot be dried. If the
subgrade deteriorates during proof rolling or compaction, it may become necessary to modify the proof
rolling, compaction criteria, or methods.
Structural Fill
All fill, whether existing on-site glacial till soil or imported soil, that will support building foundations and
floor slabs, pavement and hardscape areas, or be placed in utility trenches should generally meet the
criteria for structural fill presented below. The suitability of soil for use as structural fill depends on its
gradation and moisture content.
Materials
Materials used on the project site, under buildings, pavement, hardscape areas, and to backfill utility
trenches are classified as structural fill for the purpose of this report. Structural fill material quality varies
depending upon its use as described below:
1. Structural fill placed below all building elements (except footing designed for greater than 3,000 psf
bearing pressure) and during wet weather conditions should consist of imported Gravel Borrow, as
described in Section 9-03.14(1) of the 2018 WSDOT Standard Specifications, with the additional
restriction that the fines content be limited to no more than 5 percent. On-site soils may be used as
structural fil provided it is placed during the summer months, is properly moisture conditioned to within
2 percent of the optimum moisture content, and can be compacted to at least 95 percent of the MDD.
2. CDF having a design strength of at least 200 psi should be used under all foundations designed for
greater than 3,000 psf bearing pressure.
3. Structural fill placed to construct embankment and parking areas and to backfill utility trenches may
consist of on-site fill and glacial till provided that the soils are moisture conditioned for the required
compaction. On-site till soils may be suitable for use as structural fill during dry weather conditions in
areas needing 95 percent compaction. If structural fill is placed during wet weather, the structural fill
should consist of imported gravel borrow.
4. Structural fill placed as CSBC below pavements should conform to Section 9-03.9(3) of the 2018
WSDOT Standard Specifications.
5. Structural fill placed as capillary break below slabs should consist of 1-inch minus clean crushed gravel
with negligible sand or silt in conformance with Section 9-03.1(4)C, grading No. 67 of the 2018 WSDOT
Standard Specifications.
Reuse of On-site Native Soils
The existing fill and till soils contain a high percentage of fines and will be sensitive to changes in moisture
content and difficult to handle and compact during wet weather.
The existing fill (free of organic debris) and till deposits are expected to be suitable for structural fill in areas
requiring compaction to at least 95 percent of MDD (per ASTM D 1557), provided the work is accomplished
during the normally dry season (June through September) and that the soil can be properly moisture
conditioned to within 2 percent of the optimum moisture content. Imported structural fill consisting of sand
and gravel (WSDOT gravel borrow) should be planned under all building foundation elements, especially if
construction occurs during wet weather.
November 15, 2018 | Page 11 File No. 23656-001-00
The use of existing on-site fill and till soils as structural fill during wet weather should be planned only for
areas requiring compaction to 90 percent of MDD, as long as the soils are properly protected from wet
weather, not placed during periods of precipitation, and that they can be dried if needed to achieve proper
compaction. The contractor should plan to cover and maintain all fill stockpiles with plastic sheeting if it
will be used as structural fill. The reuse of on-site soils is highly dependent on the skill of the contractor and
schedule, and we will work with the design team and contractor to maximize the reuse of on-site till soils
during the wet and dry seasons.
Fill Placement and Compaction Criteria
Structural fill should be mechanically compacted to a firm, non-yielding condition. Structural fill should be
placed in loose lifts not exceeding 12 inches in thickness when using heavy compaction equipment and
not more than 6 inches when using hand-operated compaction equipment. The actual thickness will be
dependent on the structural fill material used and the type and size of compaction equipment. Each lift
should be moisture conditioned to within about 2 percent of the optimum moisture content to achieve
proper compaction to the specified density before placing subsequent lifts. Compaction of all structural fill
at the site should be in accordance with the ASTM D 1557 (modified Proctor) test method. Structural fill
should be compacted to the following criteria:
1. Structural fill placed below floor slabs and foundations should be compacted to 95 percent of the MDD.
2. Structural fill in new pavement and hardscape areas, including utility trench backfill, should be
compacted to at least 90 percent of the MDD, except that the upper 2 feet of fill below final subgrade
should be compacted to at least 95 percent of the MDD, see Figure 3, Compaction Criteria for Trench
Backfill.
3. Structural fill placed as CSBC below pavements should be compacted to 95 percent of the MDD.
4. Non-structural fill, such as fill placed in landscape areas, should be compacted to at least 90 percent
of the MDD.
Weather Considerations
Disturbance of near-surface soils should be expected if earthwork is completed during periods of wet
weather. During dry weather, the soils will: (1) be less susceptible to disturbance, (2) provide better support
for construction equipment, and (3) be more likely to meet the required compaction criteria.
The wet weather season generally begins in October and continues through May in western Washington;
however, periods of wet weather may occur during any month of the year. For earthwork activities during
wet weather, we recommend that the following steps be taken:
■ The ground surface in and around the work area should be sloped so that surface water is directed
away from the work area. The ground surface should be graded so that areas of ponded water do not
develop. Measures should be taken by the contractor to prevent surface water from collecting in
excavations and trenches. Measures should be implemented to remove surface water from the work
area.
■ Earthwork activities should not take place during periods of moderate to heavy precipitation.
■ Slopes with exposed soils should be covered with plastic sheeting.
November 15, 2018 | Page 12 File No. 23656-001-00
■ The contractor should take necessary measures to prevent on-site soils and soils to be used as fill from
becoming wet or unstable. These measures may include the use of plastic sheeting, sumps with pumps,
and grading. The site soils should not be left uncompacted and exposed to moisture. Sealing the
surficial soils by rolling with a smooth-drum roller prior to periods of precipitation will help reduce the
extent that these soils become wet or unstable.
■ The contractor should cover all soil stockpiles that will be used as structural fill with plastic sheeting.
■ Construction traffic should be restricted to specific areas of the site, preferably areas that are surfaced
with the existing asphalt or working pad materials not susceptible to wet weather disturbance.
■ Construction activities should be scheduled so that the length of time that soils are left exposed to
moisture is reduced to the extent practical.
Routing of equipment on the existing fill and native till subgrade soils during the wet weather months will
be difficult and the subgrade will likely become highly disturbed and rutted. In addition, a significant amount
of mud can be produced by routing equipment directly on the glacial soils in wet weather. Therefore, to
protect the subgrade soils and to provide an adequate wet weather working surface for the contractor’s
equipment and labor, we recommend that the contractor protect exposed subgrade soils with sand and
gravel, crushed gravel, or ATB.
Permanent Cut and Fill Slopes
We recommend that permanent cut or fill slopes be constructed at inclinations of 2H:1V or flatter, and be
blended into existing slopes with smooth transitions. To achieve uniform compaction, we recommend that
fill slopes be overbuilt slightly and subsequently cut back to expose well compacted fill. It is our experience
that permanent cut slopes made in dense to very dense glacial till are difficult to establish vegetation on
and difficult to place and maintain topsoil on. Therefore, 3H:1V or flatter permanent cut slopes should be
considered for landscape purposes if site conditions allow for their use.
To reduce erosion, newly constructed slopes should be planted or hydroseeded shortly after completion of
grading. Until the vegetation is established, some sloughing and raveling of the slopes should be expected.
This may necessitate localized repairs and reseeding. Temporary covering, such as clear heavy plastic
sheeting, jute fabric, or erosion control blankets (such as American Excelsior Curlex 1 or North American
Green SC150) could be used to protect the slopes during periods of rainfall.
Utility Trenches
Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures
described in the 2018 WSDOT Standard Specifications or other suitable procedures specified by the project
civil engineer. The native glacial deposits and fill soils encountered at the site are generally of low corrosivity
based on our experience in the Puget Sound area.
Utility trench backfill should consist of structural fill and should be placed in loose lifts not exceeding
12 inches in thickness when using heavy compaction equipment and not more than 6 inches when using
hand-operated compaction equipment such that adequate compaction can be achieved throughout the lift.
Each lift must be compacted prior to placing the subsequent lift. The backfill should be compacted in
accordance with the criteria discussed above. Figure 3 illustrates recommended trench compaction criteria
under pavement and non-structural areas.
November 15, 2018 | Page 13 File No. 23656-001-00
Sedimentation and Erosion Control
In our opinion, the erosion potential of the on-site soils is low to moderate. Construction activities including
stripping and grading will expose soils to the erosional effects of wind and water. The amount and potential
impacts of erosion are partly related to the time of year that construction actually occurs. Wet weather
construction will increase the amount and extent of erosion and potential sedimentation.
Erosion and sedimentation control measures may be implemented by using a combination of interceptor
swales, straw bale barriers, silt fences and straw mulch for temporary erosion protection of exposed soils.
All disturbed areas should be finish graded and seeded as soon as practicable to reduce the risk of erosion.
Erosion and sedimentation control measures should be installed and maintained in accordance with the
requirements of the City of Renton.
Excavations
We anticipate that excavations are limited and will be primarily associated with footing excavations and
underground utilities. These cuts can likely be made as temporary open cut slopes depending on site
constraints. The stability of open cut slopes is a function of soil type, groundwater seepage, slope
inclination, slope height and nearby surface loads. The use of inadequately designed open cuts could
impact the stability of adjacent work areas, existing utilities, and endanger personnel.
The contractor performing the work has the primary responsibility for protection of workmen and adjacent
improvements. In our opinion, the contractor will be in the best position to observe subsurface conditions
continuously throughout the construction process and to respond to variable soil and groundwater
conditions. Therefore, the contractor should have the primary responsibility for deciding whether or not to
use open cut slopes for much of the excavations rather than some form of temporary excavation support,
and for establishing the safe inclination of the cut slope. Acceptable slope inclinations for utilities and
ancillary excavations should be determined during construction. Because of the diversity of construction
techniques and available shoring systems, the design of temporary shoring is most appropriately left up to
the contractor proposing to complete the installation. Temporary cut slopes and shoring must comply with
the provisions of Title 296 WAC, Part N, “Excavation, Trenching and Shoring.”
The excavations for the buildings and utilities will be completed primarily in loose to medium dense fill and
dense to very dense glacial till deposits. The following sections summarize the general excavation
recommendations.
Temporary Cut Slopes
For planning purposes, temporary unsupported cut slopes more than 4 feet high may be inclined at 1H:1V
maximum steepness within the dense to very dense glacial till (below a depth of about 3 feet) and 1½H:IV
maximum steepness in the overlying fill (upper 3 feet). If significant seepage is present on the cut face then
the cut slopes may have to be flattened. However, temporary cuts should be discussed with the
geotechnical engineer during final design development to evaluate suitable cut slope inclinations for the
various portions of the excavation. The contractor should scale slopes cut at 1H:1V to remove loose
materials and cobbles.
The above guidelines assume that surface loads such as traffic, construction equipment, stockpiles or
building supplies will be kept away from the top of the cut slopes a sufficient distance so that the stability
November 15, 2018 | Page 14 File No. 23656-001-00
of the excavation is not affected. We recommend that this distance be at least 5 feet from the top of the
cut for temporary cuts made at 1H:1V or flatter, and no closer than a distance equal to one-half the height
of the slope for cuts made steeper than 1H:IV.
Water that enters the excavation must be collected and routed away from prepared subgrade areas. We
expect that this may be accomplished by installing a system of drainage ditches and sumps along the toe
of the cut slopes. Some sloughing and raveling of the cut slopes should be expected. Temporary covering,
such as heavy plastic sheeting with appropriate ballast, should be used to protect these slopes during
periods of wet weather. Surface water runoff from above cut slopes should be prevented from flowing over
the slope face by using berms, drainage ditches, swales or other appropriate methods.
If temporary cut slopes experience excessive sloughing or raveling during construction, it may become
necessary to modify the cut slopes to maintain safe working conditions. Slopes experiencing problems can
be flattened, regraded to add intermediate slope benches, or additional dewatering can be provided if the
poor slope performance is related to groundwater seepage.
Drainage Considerations
We anticipate shallow groundwater seepage may enter excavations for utilities depending on the time of
year construction takes place, especially in the winter months. However, we expect that this seepage water
can be handled by digging interceptor trenches in the excavations and pumping from sumps. The seepage
water, if not intercepted and removed from the excavations, will make it difficult to place and compact
structural fill and may destabilize cut slopes.
All paved and landscaped areas should be graded so that surface drainage is directed away from the
buildings to appropriate catch basins.
Water collected in roof downspout lines must not be routed to the footing drain lines. Collected downspout
water should be routed to appropriate discharge points in separate pipe systems.
Infiltration Considerations
Sieve analyses were performed on selected soil samples collected from the test pits that were completed
as part of this study. The soil samples typically consisted of native weathered or relatively unweathered
glacial till. The design infiltration value described below is based on the results of the grain size analyses,
the United States Department of Agriculture (USDA) Textural Triangle, and the Washington State
Department of Ecology Storm Water Management Manual (2005). The grain size analyses are presented
in Appendix B.
Based on our analysis, it is our opinion that the on-site native glacial till soils have a very low infiltration
capacity. The majority of the soils across the site contain significant fines, which limits the infiltration
capacity. The results of the sieve analyses indicated that the fines content (material passing the U.S.
No. 200 sieve) typically ranges from 30 to 35 percent. Due to the density, high fines content, and relative
impermeability of the glacial till, infiltration should be assumed to be very low when designing infiltration
systems. We recommend a preliminary infiltration rate of not more than 0.2 inches per hour be used for
design of the infiltration facilities. Depending on the depth of proposed infiltration facilities, the infiltration
rate will vary; however, we recommend site specific pilot infiltration testing be performed to determine the
design infiltration rate if specific infiltration facilities are being considered.
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FILENAME:SURVEYED:VERTICAL: NAVD 1988IF NOT ONE INCHONE INCHAT FULL SCALEHORIZONTAL: NAD 1983/1991SCALE ACCORDINGLYDATUMPlanning/Building/Public Works Dept.CITY OFRENTONTR-1.01
IN COMPLIANCE WITH CITY OF RENTON STANDARDSEDMONDS AVE NE, RENTON, WATREE RETENTION PLANTOWNHOMESWILLOWCREST TOWNHOMES
CIVIL CONSTRUCTION PERMIT SUBMITTAL
STATEOFWASHINGTONNO.1.202EXPLICENSEDLANDSCAPEARCHITECT09/02/2020 BRENTM.CHASTAIN35R-408528C:19004136LUA:19-000061PR:19-000126
TED-40-4085304 Alaskan Way S., Suite 301Seattle, WA 981041CITY CORRECTIONSBC 11.6.19
07/29/19TR-1.0235LANKTREETPDCBCBC/PHC:\________\________\________\________DATE:FIELDBOOK:DRAWING NO:PAGE:SHEET: OF:SCALE:DESIGNED:DRAWN:CHECKED:APPROVED:NO.REVISIONBYDATE APPR
FILENAME:SURVEYED:VERTICAL: NAVD 1988IF NOT ONE INCHONE INCHAT FULL SCALEHORIZONTAL: NAD 1983/1991SCALE ACCORDINGLYWILLOWCREST_TOWNHOMES
Planning/Building/Public Works Dept.CITY OFRENTONTR-1.02
IN COMPLIANCE WITH CITY OF RENTON STANDARDSTREE PROTECTION1/4" = 1'-0"CROWN DRIP LINE OR OTHER LIMIT OF TREE PROTECTION AREA. SEETREE PRESERVATION PLAN FOR FENCE ALIGNMENT.6'-0"MAINTAINEXISTINGGRADE WITH THETREEPROTECTIONFENCEUNLESSOTHERWISEINDICATED ONTHEPLANS.5" THICKLAYER OF MULCH.NOTES:1- SEE SPECIFICATIONS FORADDITIONAL TREE PROTECTIONREQUIREMENTS.2- NO PRUNING SHALL BEPERFORMED EXCEPT BY APPROVEDARBORIST.3- NO EQUIPMENT SHALL OPERATEINSIDE THEPROTECTIVE FENCING INCLUDINGDURING FENCEINSTALLATION AND REMOVAL.SECTION VIEW8.5" X 11"SIGNLAMINATED INPLASTIC SPACEDEVERY 50'ALONG THEFENCE.TREEPROTECTIONFENCE: 6' HT.CHAINLINK FENCEWITH STEELPOSTSINSTALLED AT 8'O.C.NOTRESPASINGPROTECTEDTREE1FX-PL-FX-TRMT-02DATUMEDMONDS AVE NE, RENTON, WATREE RETENTION DETAILWILLOWCREST TOWNHOMES
CIVIL CONSTRUCTION PERMIT SUBMITTALTOWNHOMES1"=20'WILLOWCRESTSTATEOFWASHINGTONNO.1.202EXPLICENSEDLANDSCAPEARCHITECT09/02/2020 BRENTM.CHASTAIN304 Alaskan Way S., Suite 301Seattle, WA 98104TED-40-4085
C:19004136LUA:19-000061PR:19-000126R-408529351CITY CORRECTIONSBC 11.6.19