HomeMy WebLinkAboutSWP272850 (4)c°Pr 2 /z Dc�-
SWP-27-2800
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Tamj6ron Pointe
Geotechnical Study
June 1998
( Copy from Consultant May 2005)
From: "Collin Barrett' <CollinBarrett@paceng.corn>
To: <dcarey@ci.renton.wa.us>
Date: 05/25/2005 4:32:57 PM
Subject: Tamaron Pointe Geotechnical Study
Mr. Carey,
Here is the Geotechnical Study per your request.
Regards,
Collin Barrett
Pacific Engineering Design LLC
cbarrett@paceng.com
Phone (425) 251-8811
Fax (425) 251 8880
CC: "Jayna Paradise" <jayna@paceng.com>, "Darrin Sanford" <dsanford@paceng.com>
From:
"Jayna Paradise" <jayna@paceng.com>
To:
<Dcarey@ci.renton.wa.us>
Date:
05/27/2005 12:02-02 PM
Subject:
Tamaron Pointe
Daniel:
Attached per you request are the boring log plates 2-14 for the Tamaron
Pointe project.
Thank you,
Jayna Paradise
Pacific Engineering Design, LLC
4180 Lind Avenue SW
Renton, WA 98055
(425) 251-8811 phone
(425) 251-8880 fax
www.paceng.com
CC: "Darrin Sanford" <dsanford@paceng.corn>
i i uts ud: �2P (425)747-8561
P.2
GEOTECHNICAL ENGINEERING STUDY
Proposed Tanlaron Pointe Apartments
2100 Lake Washington Boulevard
Renton, Washington
This report presents the findings and recommendations of our geotechnical engineering study for
the site of the proposed apartment complex in Renton. The Vicinity Map, Plate 1, illustrates the
general location of the site.
We wcrc provided with a grading plan dated April 16, 1998 and developed by Pacific Engineering
Design. A copy of the Land Title Survey prepared by Hallin and Associates, and dated June 5,
1998 was also provided. The plans illustrated property boundaries, existing topography, and the
locations and finish floor elevations for the planned buildings. Based on this information, we
anticipate that the existing mobile homes will be removed, and 15 apartment buildings will be
constructed on the site. The two existing accesses off Lake Washington Boulevard will be
maintained. Several of the buildings are shown to be situated close to the crest of steep slopes
that are located along the north and west sides of the property_ Buildings 2, 4, and 6 are indicated
to be 20 to 30 feet from the toe of steep cut slopes that exist on the eastern side of the site.
Farther north, carports and paved areas abut the toes of these steep, eastern slopes.
SITE CONDITIONS
Surface
The site is a large, irregularly -shaped parcel situated immediately east of Lake Washington
Boulevard, near Coulon Park in Renton. Currently developed with the Lake Terrace mobile home
park, the property is occupied by numerous mobile homes, and several wood -framed structures.
Each of the homes has a small paved parking strip accessed from the asphalt drives that extend
through the property. In many locations the asphalt drives have broken up or have experienced
noticeable settlement The remainder of the site is covered with grass, landscaping, or gravel.
Based on our observations, the site has undergone extensive grading, likely associated with its
development for a mobile home park_ The ground surface over the majority of the site slopes
gently to moderately down toward the southwest. Along the western and northern sides of the
site are steep slopes having inclinations of 50 to 70 percent. Generally, the western slopes, which
extend down to the ditch along Lake Washington Boulevard, increase in height from south to
north. The maximum height of the western slopes is 30 feet. The steep, northern slope has a
height of up to approximately 20 feet. There are some short, steep slopes in the central portion of
the site. Along the east side of the site are very steep cut slopes. These slopes have heights of
up to 40 feet and are near -vertical in places.
During our site visits we observed a small landslide that had previously occurred on the steep,
western slope, near the proposed Building 5. This slide had apparently occurred as a shallow
slump that affected only the upper approximately 2 feet of soil on the slope.
Single-family homes are located north of the site_ To the south and southeast is the Marina
Village apartment complex.
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Subsurface
JN 98"92
Page 2
The subsurface conditions were explored by drilling 12 test borings at the approximate locations
shown on the Site Exploration Plan, Plate 2. The field exploration program was based upon the
proposed construction and required design criteria, the site topography and access, the
subsurface conditions revealed during drilling, and the scope of work outlined in our proposal.
Borings 1 through 8 were drilled on May 14, 1998, while Borings 9 through 12 were drilled on June
9, 1998. These borings were conducted using a truck -mounted, hollow -stem auger drill. Samples
were taken at 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 14.
The native soils that underlie the site consist of silty sands containing varying amounts of gravel.
Where the original topsoil was still in place, the underlying native sands were loose for a depth of
several feet. Below this, the soils were dense to very dense, and relatively unweathered. These
competent soils have been glacially -compressed. The gradation of the sands varied over the site,
and in Boring 7, sandy silt was encountered- In Borings 1, 2, 5, and 8 the soils were very dense,
native, silty sands within a few inches of the ground surface. These areas have apparently been
stripped of the looser, weathered soils during past grading.
A substantial thickness of fill was encountered in Borings 3, 4, 6, 7, and 9 through 12. This fill
consisted of silty sand with varying amounts of gravel and organics, and appears to have been
used to construct the steep slopes along the north and west sides of the property. The deepest fill
was encountered in Borings 3 and 11 to depths of 18 feet and 23 feet, respectively. The fill was
generally loose, indicating that it was not compacted at the time of its placement. The fill soils
likely originated, at least in part, from cuts made on the eastern side of the property.
The final logs represent our interpretations of the field logs. 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.
Groundwater
Groundwater seepage was observed at a- variety of depths in approximately one half of the
borings. The encountered seepage appears to primarily represent groundwater that is perched
above the dense, native soils, in the looser fill and native soils. Some groundwater may also have
originated from more permeable zones within the dense native soils. The 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. It should be noted
that groundwater levels vary seasonally with rainfall and other factors. Typically, the amount of
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localized seepage encountered in an excavation will be greatest following periods of extended
heavy precipitation.
CONCLUSIONS AND RECOMMENDATIONS
General
Based on the results of our observations and subsurface explorations, it appears that the
proposed development is feasible from a geotechnical engineering standpoint. The competent,
native soils encountered in the test borings are relatively incompressible, and are suitable to
support foundation elements for the planned apartment buildings.
No structures should be placed on the existing fill, as this soil is loose and will consolidate under
even light foundation loads. Where the dense to very dense, native soils are exposed by the
planned building excavations, the structures can be supported on conventional foundations.
Overexcavation, then backfilling the overexcavation with structural fill, could be used for buildings
where competent native soils lie within approximately 5 feet of the planned bottom of excavation.
For areas where deeper fill exists, it will likely be most economical to utilize deep foundations.
This will probably affect all of Buildings 5, 7, 9, 11, 13, and 15, and possibly portions of other
buildings. Augercast concrete piers or driven, small -diameter pipe piles will likely be the most cost
effective deep foundation options for the site. Heavily -reinforced, closely -spaced piers will be
needed for the downslope sides of Buildings 11, 13, and 15 to retain soil under the buildings in the
event of slope movement.
We suggest that test pits be excavated around the proposed buildings once the existing mobile
homes have been removed. This would allow a more detailed evaluation of the extent of deep
foundations necessary for the construction.
The steep, western and northern fill slopes will likely experience soil movement in the foreseeable
future. This slope movement could damage facilities constructed between the buildings and the
steep slopes. For this reason, we recommend that no critical utilities or structures be located
downslope of the westem buildings. If possible, in -ground irrigation systems should be avoided in
landscaped areas above the steep fill slopes. Broken and leaking sprinkler lines that go
undetected can cause slope failures in these conditions.
While comprised of very dense soils having high strengths, the cut slopes on the east side of the
property will likely experience shallow sloughing over time. This is a natural process that results
from weathering of the exposed soils_ Only carports or pavements are located within
approximately 50 feet of the out slopes, where the slopes are tallest. However, Buildings 2, 4, and
6 will be situated approximately 25 feet from the slopes, with yard area likely occupying some of
the area between the buildings and the steep slopes. We recommend that, as a minimum, a 4-
foot catchment wall be constructed at the toe of the cut slopes to collect, or at least slow, soil that
may slough off of these slopes. The Marina Village apartment complex has catchment walls
constructed of railroad ties spanning between metal beams embedded into the ground. Similar
catchment walls would be appropriate for the Tamaron Pointe project also.
Disturbance of the steep, western slopes should be avoided, wherever possible, in order to
prevent a decrease in the stability of these slopes_ Fill should not be placed within approximately
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15 feet of the crest of these slopes. Water from drains and impervious surfaces should not be
directed toward, or discharged onto, the steep, western slopes.
The on -site soils are silty and fine-grained, which makes them moisture -sensitive. Reuse of the
native silty sands as structural fill will only be possible during dry weather and if the excavated
soils are prevented from becoming wet prior to placement and compaction. Where very moist to
wet soils are encountered during earthwork, they will need to be dried prior to compaction. This is
generally only possible during hot, dry weather. Adequate compaction of all fill, including utility
backfill, for structural areas must be completed in accordance with our recommendations to limit
the potential for settlement.
Subgrades for footings and slabs should be protected with a 2- to 4-inch layer of gravel. This re-
duces subgrade disturbance due to foot traffic. Additional erosion and subgrade protection meas-
ures may be necessary, depending on the conditions encountered during construction.
The native soils have poor drainage characteristics so using them for wall backfill involves a risk
that some water may seep through walls. As a minimum, waterproofing should be provided where
there will be below -grade, occupied spaces or moisture -sensitive areas, such as storage and me-
chanical rooms. In general, the more care and expense taken during the initial drainage and wa-
terproofing installation, the fewer water problems that will develop later.
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.
Conventional Foundations
The proposed structures can be supported on conventional continuous and spread footings
bearing on undisturbed, medium -dense to very dense, native soil or on structural fill placed above
this competent, native soil. See the later sub -section entitled General Earthwork and Structural
Fill for recommendations regarding the placement and compaction of structural fill beneath
structures. We recommend that curitinuous and individual spread footings have minimum widths
of 12 and 16 inches, respectively. They should be bottomed at least 12 inches below the lowest
adjacent finish ground surface for frost protection. 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.
Overexcavation may be required below the footings to expose competent, native soil_ Unless lean
concrete is used to fill an overexcavated hole, the overexcavation-must be at least as wide at the
bottom as the sum of the depth of the overexcavation and the footing width. For example, an
overexcavation extending 2 feet below the bottom of a 3-foot-wide footing must be at least 5 feet
wide at the base of the excavation. If lean concrete is used, the overexcavation need only extend
6 inches beyond the edges of the footing.
An allowable bearing pressure of 3,UUU pounds per square foot (pso is appropriate for footings
constructed according to the above recommendations. A one-third increase in this design bearing
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pressure 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-half inch, with
differential settlements on the order of one -quarter inch in a distance of 50 feet along a continuous
footing.
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 surrounded by level, structural fill. We recommend using the
following design values for the foundation's resistance to lateral loading:
Parameter Design Value
Coefficient of Friction 0.40
-..---------- ....................................................... .......................................
Passive Earth Pressure 300 pcf
Where: (i) pcf is pounds per cubic foot, and (5) 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. We recommend a safety factor of at least 1.5 for the foundation's
resistance to lateral loading, when using the above design values.
Augercast Concrete Piers
Augercast piers are installed using continuous flight, hollow -stem auger equipment. Concrete
grout must be pumped continuously through the auger as it is withdrawn. We recommend that
augercast piers be installed by an experienced contractor who is familiar with the anticipated sub-
surface conditions.
An allowable compressive capacity of 40 tons can be attained by installing a 16-inch-diameter,
augercast concrete pier at least 10 feet into dense strata. For transient loading, such as wind or
seismic loads, the allowable pier capacity may be increased by one-third. We can provide design
criteria for different pier diameters and embedment lengths, if greater capacities are required. The
minimum center -to -center pier spacing should be three times the pier diameter.
We estimate that the total settlement of single piers installed as described above will be on the
order of one-half inch- Most of this settlement should occur during the construction phase as the
dead loads are applied. The remaining post -construction settlement would be realized as the live -
loads are applied. We estimate that ditterential settlements over any portion of the structures
should be less than about one -quarter inch.
We recommend reinforcing each pier its entire length. This typically consists of a rebar cage ex-
tending a portion of the pier's length with a full-length center bar. Each pier can be assumed to
have a point of fixity at 12 feet below the ground surface for the computation of lateral load resis-
tance. The piers that will support the downslope sides of Buildings 11, 13, and 15 should be
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spaced no further than 3 feet edge -to -edge. These piers should be designed to cantilever to a
depth of 10 feet, resisting a lateral active soil density of 40 pounds per cubic foot (pcf) acting over
the pier spacing within the cantilever portion. This is intended to retain the near -surface soil be-
neath these buildings in the event of future slope movement.
Pive Piles
As an alternative to augercast piers, 3- or 4-inch-diameter pipe piles can be used to support por-
tions of the buildings that are underlain by deep fill. Pipe piles cannot be used for the downslope
sides of Buildings 11, 13, and 15, where large lateral soil loads need to be resisted by the deep
foundations. Pipe piles driven with a 650- or 800-pound jackhammer to the following final pene-
tration rates may be assigned the following compressive capacities.
Pile Diameter
Final Driving Rate
Final Driving Rate
Allowable Bearing Capacity
(650-pound ham-
(800-pound hammer)
mer
3 inches
12 sec/inch
10 sec/inch
6 tons
4 inches
20 sec/inch
15 sec/inch
10 tons
Pile caps and grade beams may be used to transmit loads to the piles. Pile caps should include a
minimum of two piles to reduce the potential for eccentric loads being applied to the piles.
Welded, slip or threaded couplers should be used to connect subsequent pipe sections on piles
that need to be extended in length.
Due to their small diameter, the lateral capacity of vertical pipe piles is relatively small. Lateral
loads due to wind or seismic forces may be resisted by passive earth pressure acting on the verti-
cal, embedded portions of the foundation. For this condition, the foundation must be either poured
directly against relatively level, undisturbed soil or surrounded by level, structural fill. We recom-
mend using a passive earth pressure of 250 pounds per cubic foot for this resistance. If the
ground in front of a foundation is loose or sloping, the passive earth pressure given above will not
be appropriate. We recommend a safety factor of at least 1.5 for the foundation's resistance to
lateral loading, when using the above design values. If additional lateral resistance is required we
recommend driving battered piles in the same direction as the applied lateral load. The lateral ca-
pacity of a battered pile is equal to one-half of the lateral component of the allowable compressive
load, up to a maximum allowable lateral capacity of one ton. The allowable vertical capacity of
battered piles does not need to be reduced if the piles are battered steeper than 1:5
(Horizontal:Vertical).
Seismic Considerations
The site is located within Seismic Zone 3, as illustrated on Figure No. 16-2 of the 1994 Uniform
Building Code (UBC). In accordance with Table 16-J of the 1994 UBC and the 1997 UBC, the
native site soil profile is best represented by Profile Type 51 and Sc (Dense Soil), respectively.
The loose, wet fill encountered in several of the borings is potentially liquefiable during a large
earthquake. This hazard is mitigated by the use of deep foundations embedded into non -
liquefiable soils to support the affected buildings.
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Slabs -on -Grade
JN 98192
Page 7
Where undisturbed native soils are exposed, the building floors may be constructed as slabs -on -
grade. The subgrade soil must be in a firm, non -yielding condition at the time of slab construction
or underslab fill placement. Any soft areas encountered should be excavated and replaced with
select, imported, structural fill- Building floors that are underlain by loose fill or native soils should
be supported on piers, either as structural slabs or as framed floors over a crawl space.
All slabs -on -grade should be underlain by a capillary break or drainage layer consisting of a
minimum 4-inch thickness of coarse, free -draining, structural fill with a gradation similar to that
discussed later in Permanent Foundation and Retaining Walls- In areas where the passage of
moisture through the slab is undesirable, a vapor barrier, such as a 6-mil plastic membrane,
should be placed beneath the slab. Additionally, sand should be used in the fine -grading process
to reduce damage to the vapor barrier, to provide uniform support under the slab, and to reduce
shrinkage cracking by improving the concrete curing process.
Permanent Foundation 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 design parameters are for walls that
restrain level backfill:
Parameter Design Value
Active Earth Pressure ` 40 pcf
-...... .....
.--.........................---. -. ----
Wassive Earth Pressure 300 pcf
....................................................... .................... .....;............... .........--.............................
Coefficient of Friction 0.40
.................. -..................... .......................................... ..... .................. --............-..........,..
Soil Unit Weight 130 pcf
Where: (i) pcf is pounds per cubic foot, and (ii) active and passive
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 psf times the height
of the wall should be added to the above active equivalent fluid
pressure.
The values given above are to be used to design permanent foundation and retaining walls only.
The passive pressure given is appropriate for the depth of level, structural fill placed in front of a
retaining or foundation wall only. We recommend a safety factor of at least 1-5 for overturning
and sliding, when using the above values to design the walls.
The design values given above do not include the effects of any hydrostatic pressures behind the
walls and assume that no surcharge slopes or loads, such as vehicles, will be placed behind the
walls. If these conditions exist, those pressures should be added to the above lateral soil
pressures. Also, if 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
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June xx, 1998 Page 8
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 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.
Retaining Wall 13ackfill
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_ Where the on -
site silty sands, which are not free -draining, are used as wall backfill, at least 12 inches of
free -draining gravel should be placed against the walls for proper drainage.
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. 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 must also
slope away from backfilled walls to reduce the potential for surface water to percolate into
the backfill. The sub -section entitled General Earthwork and Structural Fill contains
recommendations regarding the placement and compaction of structural fill behind
retaining and foundation walls.
The shove recommendations are not intended to waterproof the below --grade walls. If
some seepage through the walls or moist conditions are not acceptable, waterproofing
should be provided. This could include limiting cold -joints and wall penetrations, and using
bentonite panels or membranes on the outside of the walls. Applying a thin coat of asphalt
emulsion is not considered waterproofing, but it will only help to prevent moisture,
generated from water vapor or capillary action, from seeping through the concrete.
Rockeries
We anticipate that rockeries may be used in the site development. A rockery is not intended to
function as an engineered structure to resist lateral earth pressures, as a retaining wall would do.
The primary function of a rockery is to cover the exposed, excavated surface and thereby retard
the erosion process_ We recommend limiting rockeries to a height of 8 feet and placing them
against only dense, competent, native soil. Where rockeries are constructed in front of
compacted fill they should be limited to 5 feet in height. The lower two-thirds of each fill rockery
should be constructed using 3- to 4-man rocks, Taller fill rockeries would require the use of
9eogrid reinforcement in the compacted backfill.
The construction of rockeries is, to a large extent, an art not entirely controllable by engineering
methods and standards. It is imperative that rockeries, if used, are constructed with care and in a
proper manner by an experienced contractor with proven .ability in rockery construction. The
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rockeries should be constructed with hard, sound, durable rock in accordance with accepted local
practice and standards. Soft rock, or rock with a significant number of fractures or inclusions,
should not be used, in order to limit the amount of maintenance and repair needed over time.
Provisions for maintenance, such as access to the rockery, should be considered in the design. In
general, we recommend that rockeries have a minimum dimension of one-third the height of the
slope cut above them. Given the existing soil conditions tiered rockeries are not recommended.
Excavations and Slopes
Excavation slopes should not exceed the limits specified in local, state, and national government
safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in
unsaturated soil, if there are no indications of slope instability. Based upon Washington
Administrative Code (WAC) 296, Part N, the existing fill and loose, native soils at the subject site
would be classified as Type B_ Therefore, temporary cut slopes greater than 4 feet in height in
these loose soils should not be excavated at an inclination steeper than 1:1 (Horizontal_Vertical),
extending continuously between the top and the bottom of a cut.
The above -recommended temporary slope inclination is based on what has been successful at
other sites with similar soil conditions. 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 dudng wet weather. The 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
contractors should be made especially aware of this potential danger.
All permanent cuts into native soil should be inclined no steeper than 2:1 (H:V). Fill slopes should
not be constructed with an inclination greater than 2:1 (H:V). To reduce the potential for shallow
sloughing, fill must be compacted to the face of these slopes. This could be accomplished by
overbuilding the compacted fill and then trimming it back to its final inclination. Water should not
be allowed to flow uncontrolled over the top of any temporary or permanent slope. Also, all
permanently exposed slopes should be seeded with an appropriate species of vegetation to
reduce erosion and improve the stability of the surficiai layer of soil.
Any disturbance to the existing steep slopes 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 possible. Soil from the excavation should not
be placed on the steep slopes, and this may require the off -site disposal of any surplus soil.
Drainage Considerations
We recommend the use of footing drains at the base of perimeter footings and all backfilled,
earth -retaining walls_ These drains should be surrounded by at least 6 inches of 1-inch-minus,
washed rock and then wrapped in non -woven, geotextile filter fabric (Mirafi 140N, Supac 4NP, or
similar material). At its highest point, a perforated pipe invert should be at least as low as the
bottom of the footing, and it should be sloped for drainage. Drainage should also be provided
inside the footprint of a structure, where (1) a crawl space will slope or be lower than the
surrounding ground surface, (2) an excavation encounters significant seepage, or (3) an
excavation for a building will be close to the expected high groundwater elevations. We can
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provide recommendations for interior drains, should they become necessary, during excavation
and foundation construction.
All roof and surface water drains must be kept separate from the foundation drain system. A
typical drain detail is attached to this report as Plate 15. For the best long-term performance,
perforated PVC pipe is recommended for all subsurface drains.
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 acid 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 (a ) building(s)
should slope away at least 2 percent, except where the area is paved. Water from roof, storm
water, and foundation drains should not be discharged onto slopes; it should be tightlined to a
suitable outfall located away from any slopes.
Pavement Areas
As exhibited by the damaged existing pavements, the subgrade soils are subject to softening
under repetitive traffic loads. To reduce the potential for this, we recommend that at least 9 inches
of imported, gravelly structural fill be placed under all pavement sections in drive lanes or entrys,
where the heaviest, most frequent loading is anticipated. The subgrade soils must be in a stable,
non -yielding condition at the time of paving, or the placement of structural fill. Structural fill or
fabric may be needed to stabilize soft, wet, or unstable areas. We recommend using Supac SNP,
manufactured by Phillips Petroleum Company, or a non -woven fabric with equivalent strength and
permeability characteristics. In most instances where unstable subgrade conditions are
encountered, 12 inches of granular, structural fill will stabilize the subgrade, except for very soft
areas where additional fill could be required. The subgrade should be evaluated by Geotech
Consultants, Inc., after the site is stripped and cut to grade. Recommendations for the compaction
of structural fill beneath pavements are given in a later sub -section entitled General Earthwork
and Structural Fill. The performance of site pavements is directly related to the strength and
stability of the underlying subgrade.
The pavement for lightly -loaded traffic and parking areas should consist of 2 inches of asphalt
concrete (AC) over 4 inches of crushed rock base (CRB) or 3 inches of asphalt -treated base
(ATB). We recommend providing heavily -loaded areas with 3 inches of AC over 6 inches of CRB
or 4 inches of ATB.. Heavily -loaded areas are typically main driveways, dumpster sites, or areas
with truck traffic.
The pavement section recommendations and guidelines presented in this report are based on our
experience in the area and on what has been successful in similar situations. Some maintenance
and repair of limited areas can be expected. To provide for a design without the need for ar)y
repair would be uneconomical.
General Earthwork and Structural Fill
.jun 1 r UU U3: 25p
(4251747-8561 p.12
Trammel Crow Residential JN 98192
June xx, 1998 Page 11
All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil,
and other deleterious material. It is extremely important that the foundation(s) and slab(s) for the
existing structures are also removed. 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 placed under a building, behind permanent retaining or
foundation walls, 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 filling 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. We recommend testing the fill as it is placed. If the fill is not
compacted to specifications, it can be recompacted before another lift is placed. This eliminates
the need to remove the fill to achieve the required compaction. The following table presents
recommended relative compactions for structural fill:
Location of Minimum
Fill Placcment Relative Compaction
Beneath footings, slabs 95%
or walkways ..................
Behind retaining walls 90%
..............................----------........- .................------......
95% for upper 12 inches of
Beneath pavements subgrade; 90% below that
level
Where: Minimum Relative Compaction is the ratio, expressed in
percentages, of the compacted dry density to the maximum dry
density, as dctennimal in accordance with ASTM Test
Designation D 1557-78 (Modified Proctor).
Structural fill that will be placed in wet weather should consist of a coarse, granular soil with a silt
or clay content of no more than 5 percent- The percentage of particles passing the No. 200 sieve
should be measured from that portion of soil passing the three -quarter -inch sieve.
LIMITATIONS
The analyses, 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 encountered In
the test borings is 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 soil conditions are commonly encountered on
construction sites and cannot be fully anticipated by merely taking soil samples in test borings.
Subsurface conditions can also vary between exploration locations. Such unexpected conditions
Jun 1 / 38 03: 26p
(425)747-8561 p.13
Trammel Crow Residential
June xx, 1998
JN 98192
Page 12
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 structures from damage due to slope movement. Predicting the effects of development
on the stability of slopes 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 must ultimately
accept the possibility that some slope movement could occur, resulting in possible loss of ground
or damage to the facilities around the proposed building.
This report has been prepared for the exclusive use of Trammel Crow Residential, and its
representatives for specific application to this project and site. Our recommendations and
conclusions are based on observed site materials, and selective laboratory testing and
engineering analyses. Our conclusions and recommendations are professional opinions derived
in accordance with current standards of practice within the scope of our services and within
budget and time constraints. 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. We recommend including this
report, in its entirety, in the project contract documents so the contractor may be aware of our
findings.
ADDITIONAL SERVICES
Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and
observation services during construction. This is to confirm that subsurface conditions 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 bo the responsibility of the contractor.
The following plates are attached to complete this report:
Plate 1 Vicinity Map
Plate 2 Site Exploration Plan
plates 3 .- 14 Test Boring Logs
Plate 15 Footing Drain Detail
We appreciate the opportunity to be of service on this project. If you have any questions, or if we
may be of further service, please do not hesitate to contact us.
•, i oo moo: C- of' t'tel- Z)l el It e -tiZ)bl P. 14
Trammel Crow Residential JN 98191
June xx. 1998 Page 13
Respectfully submitted,
GEOTECH CONSULTANTS, INC.
Marc R. McGinnis, P.E.
Associate
James R_ Finley, P.E.
Principal
MRM/JRF-alt
I
B-8
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4
2
11
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B-9 9 B-2------------
1
--
LAKE WASHINGTON BLVD.
LEGEND:
APPROXIMATE BORING LOCATIONS
PROPOSED BUILDINGS
GEOTECH
CONSULTANTS
a
SITE EXPLORATION PLAN
2100 LAKE WASHINGTON BLVD.
RENTON, WA
✓ob No. Oolt+ Sco/e P/ole+ 2
98192 j JUNE 1998
0
BORING 1
10
15
2(
2!
31
3
'T V
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No:
Date:
Logged by:
Plate:
98192
May 1998
EOP
3
5
10
15
20
25
30
35
40
°
BORING 2
Description
f ' Grass
Tan, silty,ravel) SAND with root fibers, fine
dense
- to medium -grained, moist, very
50l4" 1 ®I , I I � gravelly
16 14 Tan, slightly silty SAND with some gravel, fine- to medium -grained, moist, very
f dense
SP
SM
73/6" 1 5 11111 I I I I 1 II -_becomes gravelly and wet
* Test boring was terminated at 20.5 feet during drilling on May 14, 1998.
* Groundwater seepage was encountered at 20 feet during drilling.
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 EOP 4
5
10
15
20
25
30
35
M
S
5 00
�aab��
goo
�
4�< o-fi4
5
�G5
15 1 1
5 2
11 3
FILL
19 4
25 5
55 1 6 1 eSM
BORING 3
Grass
Description
Tan, silty, gravelly SAND with some organics, fine- to medium -grained, moist,
medium -dense (FILL)
- becomes gray, very moist, very loose
- becomes brown/black with organics, medium -dense
- becomes gray
- with fine root fibers and wood chips
Gray, silty, gravelly SAND, fine- to medium -grained, moist, very dense
Test boring was terminated at 26.5 feet during drilling on May 14, 1998.
* Groundwater seepage was encountered at 5 feet during drilling.
GEOTECH
_44 CONSULTANT'S, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 EOP 5
5
iul
oti BORING 4
0�� .1�� ati�`� tiy too Q�e G5
�5 ��ab Q �a� J5 Description
Grass and gravel surfacing over
31 1 FILL Brown/black, slightly silty SAND with some gravel and organics, moist, dense
(FILL)
18 12 IIIn�jp
II Gray, slightly silty SAND with some gravel, fine- to medium -grained, moist,
medium -dense
54/6" 3 ® Tan/brown, slightly silty, gravelly SAND, medium- to coarse -grained, moist,
very dense
SIB
SM
15 60/6" 4
I
20
25
30
35
40
* Test boring was terminated at 15.5 feet during drilling on May 14, 1998.
* No groundwater seepage was encountered during drilling.
GEOTECH
CONSULTANT'S, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 EOP 6
10
15
20
2r, V
3(
3!
BORING 5
GEOTECH
CONSULTANTS, INC.
� iwwe
Description
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged b: Plate:
98192 May 1998 EO 7
10
15
20
25
d
35
Cto7
400
BORING 6
Description
Grass
30 1 Tan/brown, silty, gravelly SAND, fine- to medium -grained, moist, medium-
_ dense (FILL)
22 ( 2 11 FILL
6 13 �f I Gray, silty, gravelly SAND with extensive organics, fine- to medium -grained,
very moist, loose (FILL)
FILL
10 14 11 1 - becomes very silty
83 15'I'�MJII Gray, silty, gravelly SAND, fine- to medium -grained, moist, very dense
* Test boring was terminated at 21.5 feet during drilling on May 15, 1998.
* Groundwater seepage was encountered at 9 feet during drilling.
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No:
Date:
Logged by:
Plate:
98192
May 1998
EOP
8
10
15
20
25
3C
3E
A
0
BORING 7
n� . ,5 G O . n.
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 EOP 9
61
10
15
20
25
35
40
N. BORING 8
oo0
Description
75 1 ®' Brown, slightly silty, gravelly SAND, fine- to medium -grained, very moist, very
dense
SP
40 2 ® SM -becomes wet
Brown, silty, gravelly SAND with orange mottling, fine- to medium -grained, moist,
81 3 ®` very dense
SM
50/5" 14
97 15
* Test boring was terminated at 18.75 feet during drilling due to auger refusal on
May 15, 1998.
* Groundwater seepage was encountered at 4 feet during drilling.
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 EOP 10
10
15
20
25
30
35
46
°- BORING 9
Description
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 DB 11
5
10
IT,
20
25
30
35
40
o� BORING 10
<4�afi4�� �G5 Description
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged : Plate:
98192 May 1998 DB I 12
ti
O
BORING 11
5 �- I I 36 11 11 FILL
Description
Grass over
Brown, gravelly SAND with some silt, dry to moist, medium -dense (FILL)
- becomes less gravelly
10 11 2' Dark brown, silty SAND with organics, wet, loose (TOPSOIL)
l 1
SM Dark gray, silty SAND, fine- to medium -grained, wet, loose
15[— I f 50/4" 13 1
F
25
30
35
40
SM
50/2" 14 s
Gray -brown, silty SAND with some gravel, very moist, very dense (Glacial Till)
* Test boring was terminated at 18 feet due to Auger refusal during drilling on
June 9, 1998.
* Groundwater seepage was encountered at 10 feet during drilling.
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 DBC 13
5
10
Z
15
20
25
30
35
40
14 1
FILL
4 2�
BORING 12
Grass
Description
Brown -gray, silty SAND with some gravel, very moist to wet, loose to medium -
dense (FILL)
Dark brown, silty SAND with organics, very moist, loose (TOPSOIL)
Dark gray, silty SAND with some gravel, very moist, loose
- becomes wet, dense
- becomes less silty
* Test boring was terminated at 26 feet during drilling on June 9, 1998.
* Groundwater seepage was encountered at 12 feet during drilling.
GEOTECH
CONSULTANTS, INC.
BORING LOG
2100 Lake Washington Boulevard
Renton, Washington
Job No: Date: Logged by: Plate:
98192 May 1998 DB 11