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CITY OF RENTON
Technical Memorandum No. 4
Geotechnical Investigation
Thunder Hills Sewer Interceptor
February 13, 2015
Prepared By:
Phil A. Haberman, P.G., P.E.G.
Senior Engineering Geologist
Gopal A. Singam, P.E.
Senior Geotechnical Engineer
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
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Contents
1.0 INTRODUCTION ............................................................................................................. 1
2.0 PROJECT DESCRIPTION ................................................................................................. 1
3.0 SITE DESCRIPTION .......................................................................................................... 1
4.0 SUBSURFACE DATA ........................................................................................................ 3
5.0 SOIL AND GROUNDWATER CONDITIONS ..................................................................... 3
6.0 GEOLOGIC HAZARDS ................................................................................................... 5
7.0 DISCUSSION .................................................................................................................. 8
8.0 RECOMMENDATIONS .................................................................................................... 8
9.0 CONSTRUCTION FIELD REVIEWS.................................................................................. 15
10.0 CLOSURE ...................................................................................................................... 16
Appendices
APPENDIX A: STATEMENT OF GENERAL CONDITIONS
APPENDIX B: FIGURES
Vicinity Map (Figure 1)
Site Plans (Figures 2 & 3)
Cross Sections (Figures 4-9)
Gravity Wall Schematics (Figures 10 & 11)
APPENDIX C: BORING LOGS
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TECHNICAL MEMORANDUM No. 4
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1.0 Introduction
In accordance with authorization by the City of Renton, Stantec has completed a
geotechnical investigation for the Thunder Hills Creek Sewer Alternatives Analysis
located in Renton, Washington. Figure 1 shows the project area.
The purpose of our investigation was to evaluate the geologic conditions in the most
critical geologic hazard areas of the existing sewer line and to provide preliminary
geotechnical recommendations and opinions for protection of the existing or new
sewer line, manholes, and access roadway.
The scope of work for the study consisted of multiple levels of field investigations and
document reviews followed by cross section development, slope stability analyses, and
engineering analyses to prepare this report. Preliminary recommendations presented
herein pertain to various geotechnical aspects of the proposed project and provide an
overview of the geologic conditions and hazards.
2.0 Project Description
The proposed alternatives include replacement of the existing sewer line that extends
along Thunder Hills Creek from approximately Grant Avenue South to Interstate 405 (I-
405), diverting flow from this basin into sewer infrastructure west of Talbot Road South, or
rehabilitating (lining) the existing line along Thunder Hills Creek.
For the alternatives located in the Thunder Hills Creek basin, construction may include
adding sanitary sewer manholes where none currently exist, creating/rehabilitating an
access roadway to the area near I-405 for maintenance equipment; and potentially
constructing retaining structures to support the existing access roadway and sewer line.
3.0 Site Description
The Thunder Hills Creek project area is located between I -405 and Grant Avenue South,
just east of the Berkshire Apartment Home development (Figure 1). For the purposes of
this study, we narrowed the site area to include the most geologically hazardous areas
within the project area. This area is located from I -405 upstream for approximately 600
feet (Figures 2 and 3). For our study, we also consider the site to include all of the steep
slopes located east and west of Thunder Hills Creek (approximately 150-200 feet wide)
in this area.
Localized steep slope areas, drainage zones, and areas where the existing sewer line
crosses Thunder Hills Creek along the alignment south of the main site area were also
reviewed. Conclusions and opinions regarding these areas, along with geotechnical
recommendations pertaining to slope stability and access roadway re-construction are
included.
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The site consists of the existing sewer interceptor alignment through the Thunder Hills
Creek valley along with a gravel improved access roadway located adjacent to the
stream for much of the alignment. The access roadway has been damaged
significantly by soil movement and erosion near Station 5+00 (Figure 3). North of this
location, the roadway is improved (partially) with quarry rock north to a flat area near I -
405.
Attempts have been made to locate the existing sewer pipe. The pipe type varies
along the alignment and the pipe depth is known only at existing manholes and has
been interpolated between manholes. Near Station 11+80, the sewer line extends
beneath Thunder Hills Creek at an estimated depth of 7 feet.
Thunder Hills Creek appears to flow continuously throughout the year and there are
numerous intermittent, seasonal streams that join the creek. The site plans show the
locations of various streams that join Thunder Hills Creek from the east, as well as
wetland areas located in and around the creek. Minor springs and seeps seasonally
add volume to the creek throughout the valley. There are numerous residential yard
drains that add stormwater to the stream. Near Station 5+30, a relatively large creek
joins Thunder Hills Creek from the southeast, approximately doubling the volume of
runoff.
South of Station 5+50, variable amounts of sediment are present in the stream channel.
North of this area, the stream has incised into the underlying sandstone (Renton
Formation). Four to 12 inch sized quarry rock is present in the stream bed and banks in
many areas. Larger quarry rock, generally ½ to 4 man sized basalt, has been used to
stabilize the stream banks and/or to prevent ongoing stream erosion. Specifically in the
vicinity of Station 5+00, large quarry rock has been used to fill the stream channel.
Rock filled gabion walls, generally 4 to 6 feet in height, are located between the access
roadway/path and Thunder Hills Creek north of Station 4+00. For the most part, the
gabion baskets have deteriorated significantly and in places the walls are somewhat
overturned. The gabion walls appear to have limited functionality as retaining
structures for the roadway and sewer line.
The slopes extending downward into the Thunder Hills Creek valley between Stations
0+25 and 5+40 are very steep, with magnitudes of 100 to 150 percent. There are
localized slope areas that are near vertical (200 percent magnitude) to overturned due
to excavation, sloughing, and/or landslide activity. Several rockeries are located along
the east and west sides of the access roadway between Stations 3+80 and 5+20. The
rockeries are comprised of 1 to 2 man sized basalt and are up to 7 feet in height. The
rockeries are loosely constructed.
There is evidence that shallow landslide activity occurs periodically along portions of
the slope west of the access roadway. Several large, but shallow, landslides have
occurred within the last several years north of Station 2+50. The slides appear to consist
of the upper colluvium (1 to 4 feet thick) sliding off of the underlying sandstone. The
slides extend upslope between 10 and 50 feet and are up to 70 feet wide.
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The site is bordered to the north by I-405, to the west by the Berkshire Apartment Homes,
to the east by undeveloped land (easements) and single family residences, and to the
south by Thunder Hills Creek, easements, and residential developments.
4.0 Subsurface Data
4.1.1 Site Investigation Program
The geotechnical field investigation program was completed on October 17th and 20th,
2014 and included drilling and sampling four hollow stem auger borings drilled by a
Stantec subcontractor using a limited access drill rig. The borings were located at or
near pre-determined locations and extended approximately 5 to 25 feet below the
existing site grades.
The soils encountered were logged in the field during the exploration and are
described in accordance with the Unified Soil Classification System (USCS). Disturbed
soil samples were obtained by using a 140 pound hammer free falling a vertical
distance of 30 inches for the borings.
The summation of hammer-blows required to drive the sampler the final 12-inches of an
18-inch sample length is defined as the Standard Penetration Resistance, or N-value for
a 140 pound hammer and 2 inch outside diameter split spoon sampler.
The uncorrected blow count is presented graphically on the boring logs in Appendix C.
The resistance, or “N” value, provides a measure of the relative density of granular soils
and the consistency of cohesive soils. Our report discussions regarding soil density as
well as engineering parameters are based on the N values.
A Stantec field representative directed the drilling program, collected disturbed soil
samples from split spoon sampler tubes, classified the encountered soils, kept a detailed
log of each auger hole, and observed and recorded pertinent site features.
The results of the drilling and sampling are presented on the boring logs enclosed in
Appendix C.
We also reviewed six boring logs from a geotechnical investigation conducted by Soil
and Environmental Engineers, Inc. (S&EE) in 2011. This report was conducted to develop
solutions to retain/protect the existing sewer line in the lower portion of the Thunder Hills
Creek valley.
5.0 Soil and Groundwater Conditions
5.1.1 Area Geology
The site lies within the Puget Lowland. The lowland is part of a regional north-south
trending trough that extends from southwestern British Columbia to near Eugene,
Oregon. North of Olympia, Washington, this lowland is glacially carved, with a
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depositional and erosional history including at least four separate glacial
advances/retreats.
The Puget Lowland is bounded to the west by the Olympic Mountains and to the east
by the Cascade Range. The lowland is filled with glacial and nonglacial sediments
consisting of interbedded gravel, sand, silt, till, and peat lenses.
The Geologic Map of King County, indicates that the site is located near the contacts
between Vashon Glacial Till and Tertiary Bedrock.
Vashon Glacial Till is typically characterized by an unsorted, nonstratified mixture of
clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are
typically dense and relatively impermeable. The poor sorting reflects the mixing of the
materials as these sediments were overridden and incorporated by the glacial ice.
Tertiary Bedrock in this area consists of the Renton Formation. The Renton Formation
includes feldspathic fine to medium grained sandstone with beds of coal,
carbonaceous siltstone, and claystone. Tertiary Bedrock locally outcrops south of I -90
and the Seattle Fault Zone due to uplift associated with seismic activity.
5.1.2 Soil Conditions
Details of the encountered soil conditions are presented on the boring logs in Appendix
C. The detailed soil description on these logs should be referred to in preference to the
generalized descriptions below.
Boring B-1
In Boring B-1, we encountered approximately 6 inches of topsoil and vegetation
underlain by approximately 5 feet of medium dense to dense, silty-fine to
medium grained sand with variable amounts of gravel and debris (Fill). This layer
was underlain by stiff to very stiff silt with variable amounts of sand and woody
debris (Fill). The silt layer was underlain by stiff silt with variable amounts of sand,
gravel, and trace amounts of woody debris (Highly Weathered Renton
Formation). These materials were underlain by medium dense, silty-sand with
clasts of weathered sandstone (Weathered Renton Formation), which continued
to the termination depth of the boring.
Borings B-2 and B-3
In Borings B-2 and B-3, we encountered approximately 10 to 12 inches of
vegetation and topsoil underlain by approximately 5 feet of medium dense, silty-
fine to medium grained sand with variable amounts of gravel (Fill). This layer was
underlain by dense to very dense, silty-fine to medium grained sand with variable
amounts of gravel (Glacial Till), which continued to the termination depths of
these borings.
Boring B-4
In Boring B-4, we encountered approximately 8 inches of angular rock underlain
by approximately 4 feet of loose to medium dense, silty-fine to medium grained
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sand with variable amounts of gravel (Fill). This layer was underlain by hard
sandstone (Renton Formation), which continued to the termination depth of the
boring (refusal).
Hand Borings
All of the hand borings, generally located on steep slopes east and west of
Thunder Hills Creek, encountered several inches of topsoil and vegetation
underlain by 1 to 3 feet of loose, silty-sand to silt with sand (Colluvium derived
from Renton Formation). These materials were underlain by hard sandstone
(Slightly Weathered and Weathered Renton Formation).
Previous Borings and Hand Borings
The borings and hand borings conducted in May, 2010 by S & EE encountered
similar materials at similar depths and densities as those in our investigation. We
have utilized some of this data to complete our slope stability analyses and
gravity wall design in the northern portion of the site, where their explorations
were focused.
Groundwater
At the time of our investigation, groundwater was encountered in Boring B-4 at
approximately 4.5 feet below the existing site grade. Groundwater was not
encountered in any of the other explorations at the date of our investigation.
We anticipate that groundwater in the Thunder Hills Creek valley is primarily
influenced by area streams and surface water runoff/infiltrating surface waters.
There are areas of the site near I-405 where surface water and groundwater is at
the same level (ground surface) and areas where groundwater is not
encountered below stream depths due to stream channel confinement within
the Renton Formation sandstone (Station 5+50 to 6+50 area).
We anticipate low to moderate seasonal seepage from the valley/channel
sidewalls contributing to the overall volume in Thunder Hills Creek. A majority of
volume contribution to the stream is from drainage conveyance from nearby
residential developments and tributary streams.
Water levels at the time of the field investigation may be different from those
encountered during the construction phase of the project.
6.0 Geologic Hazards
6.1.1 Landslide Hazard & Preliminary Slope Stability Analyses
Typically, slopes with magnitudes greater than about 40 percent and vertical relief of at
least 10 feet can be classified as geologically hazardous (steep slope/landslide
hazards). A majority of the slopes that extend into the Thunder Hills Creek valley meet
these criteria.
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During our field assessment, we traversed the subject property, where accessible. As
we conducted the traverses, we looked for any signs that would indicate past slope
failures or features indicating possible future instability.
We observed several relatively recent landslides in the area north of Station 2+50,
primarily along the steep slopes along the west side of Thunder Hills Creek. These slides
appear to be relatively shallow, consisting of colluvium sliding off of the underlying
sandstone. We did not observe evidence of deep-seated landslide activity within the
valley.
It is our opinion that the contributing causes for landslide activity in the north portion of
Thunder Hills Creek valley include excavations for access roadway construction, surface
water and spring/seep activity along the slopes, and the presence of loose colluvium
over relatively impermeable, hard sandstone at steep inclinations.
The commercially available slope stability computer program Slope/W was used to
preliminarily evaluate the local stability of the lower portion of the existing slopes west of
Thunder Hills Creek extending east toward the creek at each of six cross sections
(Figures 4 through 9). The slope stability was analyzed under static and seismic (pseudo-
static method) conditions for existing conditions and with a proposed gravity retaining
wall in place between the access roadway and creek.
The computer program calculates factors of safety for potential slope failures and
generates the potential failure planes. This software calculates the slope stability under
seismic conditions using pseudo-static methods. The stability of the described
configuration was analyzed by comparing observed factors of safety to minimum
values as set by standard geotechnical practice. A factor of safety of 1.0 is considered
equilibrium and less than 1.0 is considered failure. A typical minimum factor of safety for
global stability is 1.3 to 1.5 for static conditions and 1.1 to 1.2 for seismic conditions. In
accordance with typical engineering standards, we used a horizontal peak ground
acceleration of 0.2g.
Based on the presence of loose fill and colluvium over hard sandstone and relatively
recent slide activity, we would expect that the current factors of safety are
approximately at equilibrium or a 1.0 factor of safety for much of the upper portions of
the area slopes. Slope stability analyses are not warranted for the upper portions of the
slopes extending into the Thunder Hills Creek valley as their relative stability can be
visually assessed.
Factors that influence the relative factors of safety along the slope areas include
surface water, vegetation and root systems, colluvium/fill density and thickness, and
slope magnitude. We anticipate lower factors of safety and higher probability of
landslide activity to occur from approximately November to May when precipitation is
highest.
We conducted slope stability analyses for the lower portion of the slope west of the
access roadway extending east through the creek. Our initial analyses indicate that
adequate factors of safety currently exist in the area between the access roadway and
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Thunder Hills Creek under static and seismic conditions. It is our opinion that the sewer
line is more susceptible to movement caused by long-term stream erosion rather than
shallow landslide activity in the area.
6.1.2 Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that
the project area and directly adjacent side slopes are underlain by Alderwood and
Kitsap soils (very steep), Alderwood gravelly sandy loam (8 to 30 percent slopes) and
Beausite gravelly sandy loam (15 to 30 percent slopes). Since the project is located
within an actively incising stream environment adjacent to very steep slope areas, all
soils should be considered to have “Severe” to “Very Severe” erosion potential.
It is our opinion that soil erosion potential at this project site, if grading activities are
proposed, can be reduced through surface water runoff control and local removal of
problem soil areas (discussed in Section 8.1). Typically erosion of exposed soils will be
most noticeable during periods of rainfall and may be controlled by the use of normal
temporary erosion control measures, such as silt fences, hay bales, mulching, control
ditches and diversion trenches. The typical wet weather season, with regard to site
grading, is from October 31st to April 1st. Erosion control measures should be in place
before the onset of wet weather.
While erosion of the sandstone that underlies the site between Stations 0+00 and 5+50
will occur at a low to very low rate over the lifespan of the sewer line (80 years), large
storm events and long term erosion of the sandstone could erode the existing gabion
walls and slope between the stream and sewer line. To reduce adverse effects of soil
and slope erosion caused by Thunder Hills Creek, permanent erosion prevention systems
should be constructed. Large rock buttressing or gravity walls embedded adequately
into the unweathered sandstone should provide adequate protection for the sewer line
and access roadway.
6.1.3 Seismic Hazard
We encountered generally medium dense to very dense soils and locally soft rock at
the project site. The overall subsurface profile corresponds to a Site Class D as defined
by Chapter 20 of ASCE 7 (Table 20.3-1) and referenced in Table 1613.3.2 of the 2012
International Building Code (2012 IBC). A Site Class D applies to an overall profile
consisting of medium dense/stiff to very dense/hard materials within the upper 100 feet.
Areas of the site, including areas directly underlain by soft bedrock (sandstone), would
be considered as a Site Class C, soft rock profile. We do not anticipate the need to
utilize seismic parameters from this profile as part of the currently proposed project and
therefore they have not been included.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website
(seismic calculator) to obtain values for SS, S1, Fa, and Fv. The USGS website includes
the most updated published data on seismic conditions. The site specific seismic design
parameters and adjusted maximum spectral response acceleration parameters are as
follows:
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PGA (Peak Ground Acceleration, in percent of g)
32.24 (10% Probability of Exceedence in 50 years)
62.52 (2% Probability of Exceedence in 50 years)
SS 141.10% of g
S1 48.30% of g
Additional seismic considerations include liquefaction potential and amplification of
ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose
sand with a high groundwater table. The dense to very dense, glacially consolidated
materials and bedrock that underlie the site have a very low potential for liquefaction.
7.0 Discussion
7.1.1 General
It may not be economically feasible to construct preventative structures to eliminate
shallow landslide activity which originates higher up the steep slopes on the
proposed/existing access roadway or stream. At a minimum, we recommend
performing remedial excavation work to reduce the likelihood and adverse effects of
shallow colluvial slides on the proposed/existing access path along with select hazard
tree removal and drainage improvements below the slope.
Gravity retaining walls or rock buttresses should be constructed between the access
roadway and stream to prevent erosion/undercutting of the roadway and sewer line by
Thunder Hills Creek over the design lifespan of the sewer line (approximately 80 years).
It is our opinion that rock buttresses or gravity walls will be the most economical method
to prevent lateral soil movement and significant stream erosion in the area between
the existing (or proposed/new) sewer line and Thunder Hills Creek.
We anticipate that significant grading may be necessary in the vicinity of Station 4+70
to 5+40 to create roadway grades suitable for the maintenance equipment. In this
area, additional excavation work may be necessary to repair sloughed soils and erosion
channels. Also, gravity walls in this area may be required to support the access
roadway. Depending on grading configurations, these walls may be over 8 feet in
height. Once the final grading plans and road width/locations have been determined,
we can provide specific wall construction recommendations.
8.0 Recommendations
8.1 Site Preparation
In general, site preparation should consist of vegetation and topsoil removal from
proposed excavation/improvement areas. Based on observations from the site
investigation program and site reconnaissance work, it is anticipated that the stripping
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depth will generally be less than 12 inches where topsoil and vegetation are present.
The excavated material is not suitable as structural fill but could be used as fill material
in non-settlement sensitive areas such as landscaping. In these non-settlement sensitive
areas, the fill should be placed in maximum 12 inch thick lifts that should be
compacted to at least 90 percent of the modified proctor (ASTM D 1557 Test Method)
maximum dry density.
As needed, leaning trees and other trees designated as hazard trees located in critical
areas, may be removed during site preparation. It may be useful to leave root systems
in place depending on the location of the hazard trees. We can provide
recommendations on which trees are suitable for full removal or partial removal upon
request.
The existing roadway alignment is underlain by approximately 4 to 14 feet of fill. The fill
consists of silty-sand and silt with variable amounts of sand. Areas of woody debris and
other fill debris occur locally.
These materials are generally considered suitable for use as structural fill provided they
are within 3 percent of the optimum moisture content. It should be noted that these
materials are typically suitable for structural fill only during the summer months and are
highly moisture sensitive due to their fines content.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain
size of 3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200
Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and
should be compacted to a minimum of 95 percent of the modified proctor maximum
dry density, as determined by the ASTM D 1557 test method.
8.2 Temporary Excavations
Based on our understanding of the project, grading associated with access road and
retaining wall construction could include significant cuts and/or fills. Preliminarily,
temporary excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical)
in medium dense fill soils and 1H:1V in medium dense to dense native soils. Locally,
steeper cuts may be feasible; however, Stantec should be on site to provide specific
recommendations at that time.
If an excavation is subject to heavy vibration or surcharge loads, we recommend that
the excavation be sloped no steeper than 2H:1V and 1.5H:1V, respectively as above,
where room permits. If groundwater is encountered, lower declinations may be
required.
In general, excavations in slightly weathered sandstone may be stable up to a vertical
condition; however, we do not anticipate the need to excavate into sandstone other
than removing loose colluvium from existing slopes along the west side of the proposed
access roadway. In these areas, we recommend scraping the loose materials and any
loose sandstone that readily comes free from the rock faces only. Again, Stantec
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should be on site to observe the conditions during construction and provide location-
specific recommendations.
All temporary cuts should be in accordance with the Washington Administrative Code
(WAC) Part N, Excavation, Trenching, and Shoring. The temporary slopes should be
visually inspected daily by a qualified person during construction activities and the
inspections should be documented in daily reports. The contractor is responsible for
maintaining the stability of the temporary cut slopes and reducing slope erosion during
construction.
The temporary cut slopes should be covered with visqueen to help reduce erosion
during wet weather, and the slopes should be closely monitored until the permanent
retaining systems or slope configurations are complete. Materials should not be stored
or equipment operated within 10 feet of the top of any temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In
the case of temporary cuts, the existing soil conditions may not be completely revealed
until the excavation work exposes the soil. Typically, as excavation work progresses, the
maximum inclination of the temporary slopes will need to be re-evaluated by the
geotechnical engineer so that supplemental recommendations can be made. Soil
and groundwater conditions can be highly variable. Scheduling for soil work will need
to be adjustable, to deal with unanticipated conditions, so that the project can
proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, Stantec
should be notified so that supplemental recommendations can be made. If room
constraints or groundwater conditions do not permit temporary slopes to be cut to the
maximum angles allowed by the WAC, temporary shoring systems may be required.
The contractor should be responsible for developing temporary shoring systems, if
needed. We recommend that Stantec and the project structural engineer review
temporary shoring designs prior to installation, to verify the suitability of the proposed
systems.
8.3 Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded
sediment to wetlands, streams, lakes, drainage systems, and adjacent properties.
Erosion and sediment control measures should be implemented and these measures
should be in general accordance with local regulations. At a minimum, the following
basic recommendations should be incorporated into the design of the erosion and
sediment control features for the site:
Schedule the soil, foundation, utility, and other work requiring excavation
or the disturbance of the site soils, to take place during the dry season
(generally June through September). However, provided precautions are
taken using Best Management Practices (BMP’s), certain grading activities
can be completed during the wet season in areas of the site upstream
from the I-405 to AB-1 area (generally October through April).
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All site work should be completed and stabilized as quickly as possible.
Additional perimeter erosion and sediment control features may be
required to reduce the possibility of sediment entering the surface water.
This may include additional silt fences, silt fences with a higher Apparent
Opening Size (AOS), construction of a berm, or other filtration systems.
Any runoff generated by dewatering discharge should be treated
through construction of a sediment trap if there is sufficient space. If
space is limited, other filtration methods will need to be incorporated.
Specifically for this project, site grading should only be performed during the summer
months (late June through mid-September) when the creek is at it lower levels and
surface waters will be less prevalent. Additional erosion control measures will likely be
required between the proposed roadway and Thunder Hills Creek during construction
due to the presence of wetlands.
There are areas where loose colluvium overlies hard sandstone at steep angles
between Station 0+50 and 4+00. Depending on the roadway grading plans, access
roadway setback from the toe of the slope, and project/maintenance requirements,
some soil removal may be necessary. The removal of soils and trees from site slopes
should be observed by the geotechnical engineer as slope stability above these
locations could be adversely affected. Replacement of loose soils with quarry rock
may be warranted. In general, we recommend removal of trees and colluvium only
where necessary.
8.4 Retaining Walls
As needed, primarily in the area between Stations 1+00 to 5+50 and locally south of the
site area extending toward Grant Avenue South, gravity retaining walls or rock
buttresses may be utilized to support the access roadway and prevent lateral pipe
movements and/or erosion/undercutting of the sewer line. However, any type of
retaining structure will only be effective if they are embedded into the underlying
sandstone. In other words, fill material or soil deposits that are used to support retaining
structures could be easily eroded by the stream during storm events when flows are
high in volume and velocity, potentially causing the structures to fail (as observed with
the current gabion walls).
Based on the existing site grades, we anticipate that the gravity walls in the lower site
area will range from 4 to 8 feet in height, with a typical height of 6 feet. Tiered systems
may be utilized if there is adequate lateral space. The new gravity walls may also be
constructed between the existing gabion walls and access roadway if there is
adequate space.
There are numerous types of retaining structures that could be utilized in these areas.
We can provide alternative options upon request. We anticipate that Ultra block or
ecology block walls will be the most economical for the proposed construction.
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Gravity Retaining Walls
For 2.5 foot by 2.5 foot by 5 foot long Ultra blocks, we recommend a minimum of
1.5 feet of embedment in dense to very dense native soils or hard sandstone. A
rock breaker attachment to a trackhoe may be necessary to achieve
embedment grades. The wall keyway should be cut to create a 8-10V:1H
(Vertical to Horizontal) batter for the walls with level backfill conditions. A
maximum 4-inch thick gravel base (leveling course) should be placed and
compacted prior to block placement (1-1/4 to 1-1/2 inch minus crushed rock).
For an exposed wall height of 6 feet, the lowest level of blocks (1st row) should
be placed perpendicular to the face of the wall. The two rows above this level
should be placed parallel to the wall direction and interlocking. Figures 10 and
11 shows the layout and generalized wall profiles. The keyway, drainage system,
backfill, and block placement should be verified by the geotechnical engineer.
A minimum 1.5 feet thick drainage/backfill zone should extend behind the back
of the wall down to the base of the excavation. Mirafi N140 filter fabric (or
equivalent) should be placed between the excavated area of roadway and
drainage/backfill zone and extend over the drainage zone. This area should be
backfilled with clean angular rock, 2 to 8 inches in size. A minimum 4-inch
diameter perforated PVC pipe (Schedule 40) should be installed behind the wall
at its base. At least 6 inches of clean washed rock (1 to 2 inches in size) should
surround the pipe and the pipe should be sloped to drain from the behind the
walls utilizing perpendicular drains placed below the wall approximately every 25
feet along the length of the wall. Additional drainage systems could be installed
across the access roadway and connected to the wall drains if desired.
Stantec should be on site during all wall construction activities, including keyway
excavation work, drainage placement, block and quarry rock placement, and
slope grading.
Rock Buttresses
In lieu of constructing gravity walls, large quarry rock could be used to stabilize
eroding areas and replace failing gabion walls. The usefulness of the existing
gabion walls over time is limited. Since their construction is unknown, we
recommend removing the gabion walls prior to rock buttress placement. Much
of the angular rock backfill may be utilized between large rocks.
We recommend that quarry rock used as buttress material be 4-man sized or
larger for applications between Stations 4+50 and 5+50. North of Station 4+50,
smaller rock may be utilized. Once the location of the access roadway (width,
elevation, and setback) has been determined, we can provide specific buttress
recommendations.
In general, buttressing should consist of the following:
Loose soil removal and temporary excavation creation (generally
between 1H:1V and 2H:1V)
TECHNICAL MEMORANDUM No. 4
February 13, 2015
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Excavation of a keyway at the toe of the buttress (at least 18 inches into
underlying dense soils or weathered sandstone)
Placement of large angular basalt with interstitial quarry rock (as void fill)
Other Gravity Retaining Walls
Shorter gravity walls or rockeries (where soil conditions are suitable), on the order
of 8 feet or less, are necessary along the upslope side (east of the access
roadway south of this site area. During our field assessments, we observed
undermined cuts and local near-vertical excavations along the upslope (east)
side of the access roadway at several locations and we understand that this
roadway may be widened in the near future. General locations of necessary
gravity walls are as follows:
Location Anticipated Height
15+80 to 18+10 Up to 8 feet
19+55 to 20+65 4 feet or under
21+30 to 23+25 4 feet
24+10 to 25+60 Up to 6 feet
Additionally, short gravity walls or rockeries may be needed between Stations
12+70 and 13+10 and 18+10 and 19+00 depending on the final grading plans.
We can provide final wall recommendations during the next phase of the
project.
8.5 Utilities
Sewer line trenches should be excavated according to accepted engineering
practices following OSHA (Occupational Safety and Health Administration) standards,
by a contractor experienced in such work. The contractor is responsible for the safety
of open trenches. Traffic and vibration adjacent to trench walls should be reduced;
cyclic wetting and drying of excavation side slopes should be avoided. Depending
upon the location and depth of some utility trenches, groundwater flow into open
excavations could be experienced, especially during or shortly following periods of
precipitation.
In general, silty and gravelly fill soils, as well as fine-grained native soils were
encountered at shallow depths in the explorations at this site. At this site, these soils
have variable density and minimal cohesion and will have a tendency to cave or
slough in excavations. Shoring or sloping back trench sidewalls is required within these
soils. If sewer line excavations extend deep enough to encounter sandstone (not
anticipated), rock chipping/breaking equipment would likely be required to allow for
excavation. Excavations in sandstone should be adequately safe to remain vertical for
a significant amount of time.
TECHNICAL MEMORANDUM No. 4
February 13, 2015
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All utility trench backfill should consist of imported structural fill. Certain on site soils may
be suitable for use as backfill in landscaping areas during the summer months; however,
we should evaluate these soils at that time to determine their moisture levels.
The upper 5 feet of utility trench backfill placed in pavement areas should be
compacted to at least 95 percent of the maximum dry density based on ASTM Test
Method D1557. Below 5 feet and in landscaping areas, utility trench backfill in
pavement areas should be compacted to at least 90 percent of the maximum dry
density based on ASTM Test Method D1557. Pipe bedding should be in accordance
with the pipe manufacturer's recommendations.
The contractor is responsible for removing all water-sensitive soils from the trenches
regardless of the backfill location and compaction requirements. Depending on the
depth and location of the proposed utilities, we anticipate the need to re-compact
existing fill soils below the utility structures and pipes. The contractor should use
appropriate equipment and methods to avoid damage to the utilities and/or structures
during fill placement and compaction procedures.
8.6 Utility Protection
Between Stations 1+00 and 5+50, the proposed gravity retaining wall(s) or rock
buttresses will be effective in protecting the sewer line from stream erosion. South of
Station 5+50, the Thunder Hills Creek valley has variably incised sides (to near vertical)
and the channel is filled with sediments (to variable depths). Erosion hazard potential
and recommendations related to sewer interceptor protection south of Station 5+50
can be found in our Preliminary Erosion Hazard Evaluation report dated February 13,
2015.
The existing sewer line crosses Thunder Hills Creek near Station 11+80 at an estimated
depth of 7 feet. For sewer line locations that are at or above bottom of stream channel
elevations, additional backfill erosion measures may be necessary, depending on the
alternative that is chosen. Once an alignment and profile have been developed, we
can finalize our recommendations; however, the following options may be considered
as part of erosion protection for the sewer line:
Angular rock backfill placement (2 to 4 inch sized quarry rock)
Lean mixed concrete placement around sewer lines
Localized ecology/Ultra block wall placement along sewer lines
In general, Thunder Hills Creek could adversely affect the sewer line through severe
erosion and undercutting in localized areas. Our observations and analyses indicate
that there are areas where groundwater is not present below stream levels in the
vicinity of the existing sewer line, indicating that the channel is confined and not likely
to create issues for the sewer line.
For the alternative that includes lining of the existing sewer line, we do not anticipate
the need to add specific erosion protection other than gravity walls/buttresses between
TECHNICAL MEMORANDUM No. 4
February 13, 2015
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the access roadway and stream from Stations 0+00 to 5+50 (this site area). Analysis of
stream erosion effects on the existing sewer line south of 5+50 can be found in our
Preliminary Erosion Hazard Evaluation report.
If a new sewer line is proposed, there is a potential need for utility protection in areas
where the sewer line crosses Thunder Hills Creek, or a combination of being located
within 10 feet laterally of Thunder Hills Creek and at a depth of 3 feet below stream level
or deeper. We should review the preliminary plans in order to provide comments
regarding utility erosion protection recommendations.
8.7 Groundwater Influence on Construction
At the time of our investigation, groundwater was encountered in Boring B-4 at
approximately 4.5 feet below the existing site grade. Groundwater was not
encountered in any of the other explorations at the date and time of our investigation.
We anticipate that groundwater in the Thunder Hills Creek valley is primarily influenced
by area streams and surface water runoff/infiltrating surface waters. There are areas of
the site near I-405 where surface water and groundwater is at the same level (ground
surface) and areas where groundwater is not encountered below stream depths due to
stream channel confinement within the Renton Formation sandstone (Station 5+50 to
6+50 area).
We would expect groundwater to be locally encountered at sewer line depths south of
Station 7+00 and north of Station 5+00. If groundwater is encountered in trenches south
of Station 7+00, we would expect it to be found at or near the elevations of Thunder Hills
Creek. There may be areas where groundwater is not encountered due to channel
confinement, similar to our observations in the area of Station 6+00. When groundwater
is encountered, we would expect moderate to heavy seepage.
North of Station 5+00, we would anticipate light to moderate seepage from trench
sidewalls extending from the ground surface down to the level of the sandstone (upper
4 to 5 feet).
If temporary dewatering systems to remove groundwater are used, their design should
be the responsibility of the contractor. We should review any dewatering design prior
to their use on site.
9.0 Construction Field Reviews
Stantec should be retained to provide part time field review during construction in order
to verify that the soil conditions encountered are consistent with our design assumptions
and that the intent of our recommendations is being met. This will require field and
engineering review to:
Observe all aspects of gravity wall, rock buttress, and access roadway
construction
Monitor temporary excavations, slope stability, and grading activities
TECHNICAL MEMORANDUM No. 4
February 13, 2015
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Density testing to verify compaction of structural fills
Geotechnical design services should also be anticipated during the subsequent final
design phase to support the structural design and address specific issues arising during
this phase. Field and engineering review services will also be required during the
construction phase in order to provide a Final Letter for the project.
10.0 Closure
This report was prepared for the exclusive use of the City of Renton and their appointed
consultants. Any use of this report or the material contained herein by third parties, or
for other than the intended purpose, should first be approved in writing by Stantec.
The recommendations contained in this report are based on limited data from provided
test holes, and preliminary proposed construction. Additional exploration work is
warranted and necessary to provide engineering parameters and recommendations
for sewer line placement.
Use of this report is subject to the Statement of General Conditions provided in
Appendix A. It is the responsibility of the City of Renton who is identified as “the Client”
within the Statement of General Conditions, and its agents to review the conditions and
to notify Stantec should any of these not be satisfied.
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
APPENDIX A
Statement of General Conditions
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
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TECHNICAL MEMORANDUM NO. 4
February 13, 2015
Statement of General Conditions
USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent
and may not be used by any third party without the express written consent of Stantec
Consulting Services, Inc. and the Client. Any use which a third party makes of this report is the
responsibility of such third party.
BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report
are in accordance with Stantec Consulting Services, Inc.’s present understanding of the site
specific project as described by the Client. The applicability of these is restricted to the site
conditions encountered at the time of the investigation or study. If the proposed site specific
project differs or is modified from what is described in this report or if the site conditions are
altered, this report is no longer valid unless Stantec Consulting Services, Inc. is requested by the
Client to review and revise the report to reflect the differing or modified project specifics and/or
the altered site conditions.
STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in
accordance with the normally accepted standard of care in the state of execution for the
specific professional service provided to the Client. No other warranty is made.
INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements
regarding their condition, made in this report are based on site conditions encountered by
Stantec Consulting Services, Inc. at the time of the work and at the specific testing and/or
sampling locations. Classifications and statements of condition have been made in accordance
with normally accepted practices which are judgmental in nature; no specific description
should be considered exact, but rather reflective of the anticipated material behavior.
Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling
or test points. The extent depends on variability of the soil, rock and groundwater conditions as
influenced by geological processes, construction activity, and site use.
VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be
encountered that are different from those described in this report or encountered at the test
locations, Stantec Consulting Services, Inc. must be notified immediately to assess if the varying
or unexpected conditions are substantial and if reassessments of the report conclusions or
recommendations are required. Stantec Consulting Services, Inc. will not be responsible to any
party for damages incurred as a result of failing to notify Stantec Consulting Services, Inc. that
differing site or sub-surface conditions are present upon becoming aware of such conditions.
PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should
be reviewed by Stantec Consulting Services, Inc., sufficiently ahead of initiating the next project
stage (property acquisition, tender, construction, etc), to confirm that this report completely
addresses the elaborated project specifics and that the contents of this report have been
properly interpreted. Specialty quality assurance services (field observations and testing) during
construction are a necessary part of the evaluation of sub-subsurface conditions and site
preparation works. Site work relating to the recommendations included in this report should only
be carried out in the presence of a qualified geotechnical engineer; Stantec Consulting
Services, Inc. cannot be responsible for site work carried out without being present.
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
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TECHNICAL MEMORANDUM No. 4
February 13, 2015
APPENDIX B: FIGURES
Vicinity Map (Figure 1)
Site Plans (Figures 2 & 3)
Cross Sections (Figures 4 – 9)
Gravity Wall Schematics (Figures 10 & 11)
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TECHNICAL MEMORANDUM NO. 4
February 13, 2015
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SITE
N
VICINITY MAP
FIGURE 1
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Project
Location
Renton
WASHINGTON
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
SITE PLAN
FIGURE 2
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
PB-1
PB-2
PB-3
3015015
(In Feet)
1 inch = 30 feet
Approximate Graphic Scale
A B
C
A’
B’
C’
B-4
B-3
Exposed
Sandstone
Exposed
Sandstone
Exposed
Sandstone
Exposed Sandstone
& Near Vertical Slopes
Older Slide
Older Slide
Approximate Thunder Hills Creek Channel
Approximate Boring Location
Approximate Boring Location (P&EE)
Large Area of Exposed Sandstone
B-1
PB-1
Cross Section
A
A’
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
N
Wetland E
I-405
1+00 4+00
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
SITE PLAN
FIGURE 3
3015015
(In Feet)
1 inch = 30 feet
Approximate Graphic Scale
PB-6
PB-5
PB-4PB-3
B-1
B-2
D
D’
E
E’
F
F’
Exposed Sandstone
“New” Stream
Joining THC
Exposed
Sandstone
Approximate Thunder Hills Creek Channel
Approximate Boring Location
Approximate Boring Location (P&EE)
Large Area of Exposed Sandstone
B-1
PB-1
Cross Section
A
A’
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
N
6+00
7+004+00
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
A - A’
FIGURE 4
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
A
80
90
100
Elevation
(Feet)
110
120
130
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
C
C
RR
F
T
Approximate Sewer Line Location
A’
Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
B - B’
FIGURE 5
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
80
90
100
Elevation
(Feet)
B’
110
120
130
140
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
R
C/W
C
T
R
F
Approximate Sewer Line Location
Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
C - C’
FIGURE 6
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
100
Elevation
(Feet)
110
120
130
140
C C’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
R
C
F
C
R
T
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
D - D’
FIGURE 7
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
Elevation
(Feet)
110
120
130
140
150
D’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
R
C
F
R
C
T
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
Approximate Location of Existing Gabion Wall
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
E - E’
FIGURE 8
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
130
140
Elevation
(Feet)
150
E’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
F
W
R
R
W
T
C/W
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
F
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
CROSS SECTION
F - F’
FIGURE 9
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
Elevation
(Feet)
140
150
160
F’
C Colluvium (Primarily Loose Silty Sand (SM))
F Fill/Debris (Loose to Medium Dense Silty Sand (SM) and Silt with Sand (ML))
T Vashon Glacial Till (Generally Medium Dense to Dense Silty Sand with Gravel (SM))
W Highly Weathered Renton Formation (Generally Medium Dense Silty Sand (SM) to Stiff Sandy Silt (ML))
R Renton Formation (Slightly Weathered Sandstone)
T
W
R
F
W
T
R
Approximate Sewer Line Location Observed/Interpreted Surface/Stream Water Elevation
Interpreted Ground Surface
Ground Surface (Surveyed)
Approximate Geologic Contact
10505
(In Feet)
1 inch = 10 feet
Approximate Graphic Scale
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
GRAVITY WALL
SCHEMATIC
FIGURE 10
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Min. 4 Inch Diameter Perforated PVC Pipe
(Schedule 40, Sloped to Drain
Mirafi 140N Filter Fabric or Equivalent
1 to 2 Inch Diameter Washed Rock
2 to 4 Inch Angular Quarry Rock
1-1/4 Inch Crushed Rock Base
(4 Inch Thickness)
1 Min.
8 Max.
1.5 Ft. Min.
Not to Scale
6 Inches
Minimum
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
NOTES:
For use with 2.5’x2.5’x5’ interlocking concrete blocks
Intended for use between Thunder Hills Creek and Sewer Line/Access Roadway
Rock base to be compacted to at least 95% of the
modified proctor (ASTM D1557 Test Method)
Stantec to verify keyway, drainage, backfill, soil conditions,
and block installation during construction
Medium Dense Road Fill
(Silty Sand/Rock)
Max. 2H:1V Slope
2’ Max. Ht.
Sandstone
Thunder Hills
Creek
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
GRAVITY WALL
SCHEMATIC
FIGURE 11
11130 NE 33rd Place, Suite 200
Bellevue, WA 98004
(425) 869-9448
(425) 869-1190 (Fax)
www.stantec.com
Min. 4 Inch Diameter Perforated PVC Pipe
(Schedule 40, Sloped to Drain
Mirafi 140N Filter Fabric or Equivalent
1 to 2 Inch Diameter Washed Rock
2 to 4 Inch Angular Quarry Rock
Medium Dense Road Fill
(Silty Sand/Rock)
Sandstone
1-1/4 Inch Crushed Rock Base
(4 Inch Thickness)
1 Min.
8 Max.
NOTES:
For use with 2.5’x2.5’x5’ interlocking concrete blocks
Intended for use between Thunder Hills Creek and Sewer Line/Access Roadway
Rock base to be compacted to at least 95% of the
modified proctor (ASTM D1557 Test Method)
Stantec to verify keyway, drainage, backfill, soil conditions,
and block installation during construction
Thunder Hills
Creek 1.5 Ft. Min.
Not to Scale
6 Inches
Minimum
Max. 2H:1V Slope
2’ Max. Ht.
Thunder Hills Creek
Renton, Washington
Dec., 2014 2002003607
TECHNICAL MEMORANDUM NO. 4
February 13, 2015
APPENDIX C
Boring Logs
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Vegetation/Topsoil
SM; Medium dense to dense, silty-sand with variable amounts of gravel
and debris, dark yellowish brown to grayish brown, moist to very moist.
(Fill)
ML; Stiff to very stiff, silt with variable amounts of sand, trace gravel,
trace debris, trace woody debris, grayish brown to olive gray, moist to
very moist. (Fill)
ML; Stiff, silt with variable amounts of sand, trace gravel, trace woody
debris, olive gray, moist. (Highly Weathered Renton Formation)
SM; Medium dense, silty-sand, tan to yellow clasts of highly weathered
sandstone, moist. (Renton Formation - Slightly Weathered)
Borehole terminated at 25 feet.
SM
ML
ML
SM
7
6
4
2
16
17
3
3
3
7
11
12
2
4
3
3
4
6
2
3
5
5
4
7
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-1
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):25.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time&Depth(feet)5
10
15
20
25
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/17/14 10/17/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCSGraphicLogMETHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5
10
15
20
25BlowCountSampleHeadspacePID(units)Time
Sample ID
Topsoil/Vegetation
SM; Medium dense, silty-sand with variable amounts of gravel, trace
debris, yellowish brown, moist. (Fill)
SM; Dense, silty-sand with variable amounts of gravel, sandstone
remnants at 8.5-9 feet, yellowish brown to grayish brown, moist. (Glacial
Till)
Borehole terminated at 9 feet.
SM
SM
2
3
9
10
15
11
5
14
19
16
17
22
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-2
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):9.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time&Depth(feet)5
10
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/20/14 10/20/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCSGraphicLogMETHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5
10BlowCountSampleHeadspacePID(units)Time
Sample ID
Topsoil/Vegetation
SM; Medium dense, silty-sand with variable amounts of gravel, trace
debris, yellowish brown, moist. (Fill)
SM; Dense, silty-sand with variable amounts of gravel, yellowish brown
to grayish brown, moist. (Glacial Till)
Borehole terminated at 9 feet.
SM
SM
2
3
5
10
11
10
8
14
15
18
20
22
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-3
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):9.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time&Depth(feet)5
10
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/20/14 10/20/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCSGraphicLogMETHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5
10BlowCountSampleHeadspacePID(units)Time
Sample ID
Quarry rock
SM; Loose to medium dense, silty-sand with variable amounts of gravel,
trace debris, yellowish brown, moist to wet. (Fill)
SM; Dense to hard, slightly weathered sandstone, yellowish brown to
tan, moist. (Renton Formation)
Borehole terminated at 6 feet.
SM
SM
2
5
4
8
12
11
50
INITIAL DTW (ft):Not Encountered
WELL CASING DIA. (in):---
STARTED
B-4
EXCAVATION COMPANY:CN
EQUIPMENT:Limited Access DEPTH (ft):5.0
BORING NO.:
SAMPLING EQUIPMENT:Split Spoon CHECKED BY:GS
Time&Depth(feet)5
PROJECT NUMBER:2002003607
PROJECT:Thunder Hills Interceptor
STATIC DTW (ft):Not Encountered
LONG:LAT:COMPLETED:10/20/14 10/20/14
LOGGED BY:PH
GROUND ELEV (ft): TOC ELEV (ft):
LOCATION:Renton, WA
Description
NORTHING (ft): EASTING (ft):
PAGE 1 OF 1
USCSGraphicLogMETHOD:HSA SIZE:6
WELL DEPTH (ft):---
EXCAVATION / INSTALLATION:GEOFORM304THUNDERHILLS.GPJSTANTECENVIROTEMPLATE010509.GDT2/12/15MeasuredRecov.(feet)Depth(feet)5BlowCountSampleHeadspacePID(units)Time
Sample ID