HomeMy WebLinkAboutmem_thunderhills_o&m_Memo 7
CITY OF RENTON
Technical Memorandum No. 7
Operation & Maintenance Study
Thunder Hills Sanitary Sewer Interceptor Alternative
Analysis
November 13, 2015
Prepared By:
Stantec Consulting Services Inc.
11130 NE 33rd Place
Suite 200
Bellevue, WA 98004
425.869.9448
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Contents
1.0 INTRODUCTION ...........................................................................................................1.1
2.0 PROJECT DESCRIPTION ...............................................................................................2.1
3.0 EXISTING CONDITIONS ...............................................................................................3.2
4.0 EXISTING OPERATION AND MAINTENANCE EQUIPMENT ...........................................4.4
5.0 PRELIMINARY RECOMMENDATIONS ...........................................................................5.5
5.1 ADDITIONAL MANHOLE ................................................................................................. 5.5
5.2 RELOCATE SEWER MAIN ................................................................................................ 5.6
5.3 ACCESS ROAD IMPROVEMENTS ................................................................................... 5.6
5.4 POTENTIAL OPERATION AND MAINTENANCE EQUIPMENT ........................................ 5.7
6.0 CONCLUSIONS ............................................................................................................6.8
APPENDIX A – PROPOSED OPERATION & MAINTENANCE IMPROVEMENTS .........................6.2
THUNDER HILLS SEWER INTERCEPTOR ALTERNATIVE ANALYSIS .............................................. 6.2
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1.0 INTRODUCTION
Stantec has completed an operation and maintenance investigation for the Thunder Hills Creek
Sewer Alternatives Analysis located in Renton, Washington. The purpose of the investigation was
to evaluate the necessary operation and maintenance improvements and to provide
preliminary recommendations on how to meet these needs.
The scope of work for the study consisted of several field investigations, meeting with City staff,
product research, and categorizing existing sewer equipment. This investigation also references
the findings presented in Stantec’s Technical Memorandum (TM) No. 4 “Geotechnical
Investigation” and Technical Memorandum No. 5 “Preliminary Erosion Hazard Evaluation.”
This memorandum aims to find the best alternative to improve the functionality of the Thunder
Hills Interceptor. Specifically, it reviews several structural improvements to the project area:
adding sanitary sewer manholes to improve accessibility; re-locating sewer mains further from
the creek; creating/rehabilitating an access roadway to the area near I-405 for maintenance
equipment; and constructing retaining structures to support the existing access roadway and
creek bed.
2.0 Project Description
The Thunder Hills Interceptor was originally constructed in 1965, and serves a portion of the City’s
Rolling Hills neighborhood southeast of Renton City Hall. The portion of the interceptor between
Grant Avenue South and I-405 runs within or adjacent to Thunder Hills Creek. Over the years,
erosion created by Thunder Hills Creek has compromised the existing sewer interceptor in various
locations and has required stream bank stabilization.
Accessibility is also an issue along this stretch of the interceptor. At some point a narrow gravel
access road was constructed through the canyon. It runs from Grant Avenue South to the
Berkshire Apartments, approximately 2,300 feet. As with the interceptor, erosion from the
Thunder Hills Creek along with vegetation growth has made the road too narrow in some areas
for the vactor truck.
The City is looking to make improvements to the Thunder Hills Canyon to protect the interceptor
and access road from erosion, and to improve the sewer main’s long term viability. The City is
currently reviewing two alternatives to improve the sewer main’s long term viability. Alternative
No. 1 is to improve the existing Thunder Hills interceptor so that it can support the existing and
ultimate flows. Alternative No. 2 is to divert the upstream flows to the Talbot Hills Sewer, and only
improve the section of the interceptor that will remain in active use.
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3.0 EXISTING CONDITIONS
The Thunder Hills Interceptor runs from I-405 to Grant Avenue South through a small canyon
created by Thunder Hills Creek. The canyon is lined by residential and undeveloped land. The
Berkshire Apartment Homes are to the west of the canyon, and undeveloped land (easements)
and single family residences are to the east of the canyon. The canyon is vegetated with both
native and invasive species of deciduous trees, grasses, and brush. Small areas of designated
wetlands are sprinkled throughout the canyon.
Sewer, storm, and power utilities run through the canyon. The Thunder Hills Inceptor (sewer) has
various pipe types along the alignment. The sewer depth is known at existing manholes, and has
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been interpolated between manholes. In the exploration phase of this project, potholing of the
existing sewer main was attempted to better understand the depth of the existing sewer main.
Even with potholing, the depth of the sewer main could not be determined in some areas. Near
Station 11+80, the sewer line extends beneath Thunder Hills Creek at an estimated depth of 7
feet.
Storm runoff drains to the Thunder Hills Creek from the surrounding areas. This runoff is conveyed
to the creek through both constructed storm drains and natural seasonal streams. The Thunder
Hills Creek appears to flow continuously throughout the year.
Major power lines for the area cross the canyon around Station 15+00.
The existing access road is approximately 2,300 feet long, running south from Berkshire
Apartments (Station 5+50) to Grant Avenue South (Station 28+50). Soil movement and erosion
have destroyed the access roadway around Station 5+00. From this section to approximately
Station 0+25, slopes are very steep with magnitudes of 1:1 to 1:1.5. North of this location, the
roadway is improved (partially) with quarry rock north to a flat area near I-405.
South of Station 5+50, large sections of the roadway are overgrown with grass covering the road
and side vegetation narrowing the lane width. In other sections the Creek bed and erosion
have cut away at the access road. The road width currently ranges from 7 feet to 13 feet.
The access road has two access points: one at Grant Avenue South for the portion of the road
that parallels the eastern side of the creek from Station 12+00 to Station 28+50, and the other at
the Berkshire Apartments for the portion of the road that parallels the western side of the creek
from Station 5+50 to Station 12+00. The access road itself does not cross the creek, so vehicles
must enter and exit from the same access point.
Rock filled gabion walls are located between the access roadway/path and Thunder Hills Creek
north of Station 4+00. These gabion walls are generally 4 to 6 feet in height. The Geotechnical
Investigation noted that these 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.
Rock buttresses armor the creek bed between Station 1+65 to 1+80, Station 2+45 to 3+40, and
Station 4+65 to 5+65. These rock buttresses prevent the stream from wandering, protecting the
sewer main and nearby properties.
The Geotechnical Investigation also found evidence of shallow landslide activities that have
occurred 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 sliding off the underlying sandstone. The slides
extend upslope between 10 and 50 feet and are up to 70 feet wide.
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4.0 EXISTING OPERATION AND MAINTENANCE EQUIPMENT
Manholes are the points of access for operation and maintenance of the sewer main. From the
manholes, various pieces of equipment can be sent into the existing sewer main, and used to
inspect and maintain it. In order to operate and maintain the sewer main, the City must have
equipment that can access the manholes and that have the reach to inspect and maintain the
sewer main between manholes.
The City’s sewer department currently has one Vactor truck with a 1-inch rodder hose, one
closed circuit television (CCTV), and one recently purchased easement machine. In addition,
the City is looking into ordering a new, smaller Vactor truck. Table 4.1 summarizes the
specifications of these pieces of operation and maintenance equipment.
Table 4.1 Existing Operation and Maintenance Equipment
Equipment
Hose
Size
(inch)
Reach
(feet)
Vehicle
Length
(feet)
Vehicle
Width
(feet)
Vehicle
Height
(feet)
Vehicle Turning
Radius
(feet)
Vactor Truck with Rodder Hose 1” 600’ 40.0’ 10.5’ <13’ 57.5’
Mainline Truck with CCTV N/A 1000’ 28.0’ 8.0’ 13’ 38’
New Easement Machine
w/Extendable Tracks 1” 600’ 6.83’ 3.0’ 5.33’ Unknown
The vehicle dimensions listed in Table 4.1 create the minimum requirements for the access road
for the interceptor in order to accommodate proper inspection and maintenance. The current
Vactor truck is the widest and longest, and therefore, has the largest turning radius. Because of
this, the current Vactor will set the access road’s width and turning radius limits. The mainline
truck used for the CCTV has antennas that makes it the tallest vehicle, and therefore, will set the
vertical clearance for the access road. In order to comfortably drive these vehicles on the
access road, the access road should be at least 12 feet wide with a minimum turning radius of
60 feet and a vertical clearance of at least 13.5 feet.
Building an access road of this size is unrealistic between Station 0+00 and Station 5+00 because
of the existing topography. The most northern manhole in the Thunder Hills Interceptor, SSMH
2309, is located at approximately Station 0+00, and it cannot be accessed by the Vactor truck.
In order to access this manhole and other difficult to reach features, the City has recently
purchased an easement machine. The easement machine is a narrow, durable machine that
can access areas that a Vactor truck cannot. A 1-inch rodder hose with a 600 feet reach is
attached to the easement machine, allowing it to perform similar tasks to the Vactor truck. The
construction of a small foot path, approximately 7 feet wide, will allow an easement machine to
reach SSMH 2309.
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The hose and reach dimensions listed in Table 4.1 provide the recommended requirements for
manhole spacing to allow proper operation and maintenance of the main. The rodder hoses on
both the Vactor truck and the easement machine are 1-inch diameter with 600 feet of reach.
The CCTV has a reach of 1000 feet. Therefore, the manholes should be approximately 600 feet
apart in order to allow the rodder hoses to reach and property maintain the entire sewer main
between the manholes.
5.0 PRELIMINARY RECOMMENDATIONS
The goal of this memorandum is to suggest improvements to the Thunder Hill Canyon that would
improve the City’s ability to operate and maintain the interceptor. Specifically, the City needs
better access to the interceptor in order to properly operate and maintain it. In order to do this,
the City needs to add an additional manhole south of I-405, relocate some sections of sewer
main away from the creek, and make significant improvements to the existing gravel access
road. Plans showing these suggested improvements for Alternative No. 1 and No. 2 are included
in Appendix A.
5.1 ADDITIONAL MANHOLE
The most northern manhole in the Thunder Hills Interceptor, SSMH 2309, is located at
approximately Station 0+00 (shown on Sheet 1). Because of the state of the access road from
Station 0+00 to 5+00, SSMH 2309 is difficult to access for inspection and maintenance. The
nearest existing points of access for the system are the upstream and downstream manholes,
SSMH 2310 and SSMH 2308 respectively. The upstream manhole, SSMH 2310, is located at
approximately Station 13+00, 1,300 feet south from SSMH 2309. The downstream manhole, SSMH
2308, located across I-405, is approximately 520 feet north of SSMH 2309. Because of these
circumstances, a section of main between SSMH 2309 and SSMH 2310 is nearly impossible to
maintain.
A new manhole needs to be added around Station 6+00 to provide an additional access point
for inspecting and maintaining the sewer main. This new manhole will create a spacing of
approximately 600 feet to SSMH 2309 and approximately 700 feet to SSMH 2310.
New manholes are proposed in several other places throughout the canyon. These new
manholes are primarily to allow connection points for the relocated sewer main, but they also
provide additional points of access for inspection and maintenance of the sewer main. These
new manholes are shown on the plans at approximately Station 4+25, Station 8+00, Station
11+25, and Station 15+50. Alternative No. 1 adds two additional manholes at Station 24+10 and
Station 27+75.
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5.2 RELOCATE SEWER MAIN
Throughout the Canyon, sections of the interceptor will be abandoned and relocated because
they are too close to Thunder Hills Creek. This relocation will protect both the stream from
contamination and the sewer main from exposure and damage. The relocated pipe will be
HDPE, so the pipe can more easily match the curves of the Canyon.
Where the existing sewer main is to remain, it will be rehabbed with a lining of Cured-In-Place
Pipe (CIPP). CIPP liners are made of non-woven polyester felt with an exterior polyurethane
coating. The felt is impregnated with a catalyzed thermosetting resin mixture. To install, the liner
is threaded through the pipe, and then inflated with either air pressure or water until it presses
against the existing pipe. Once fully inflated, steam or hot water is circulated within the liner to
start the thermosetting resin to cure or harder. Once cured, the CIPP liner provides structural
support for the existing pipe. This structure support will extend the life time of the existing sewer
main. The proposed locations of the CIPP rehab between Station 0+00 to 3+30, Station 11+20 to
13+05, Station 15+50 to 20+40. Alternative No. 1 adds additional CIPP rehab from Station 20+40
to 24+10 and Station 27+80 to 28+75 (see attached plan sheets). For more information about the
CIPP rehabilitation, refer to Technical Memorandum No. 3 “Remaining Useful Life (RUL) &
Rehabilitation Analysis.”
As discussed in the previous section, additional manholes will be added to the interceptor at
approximately Station 4+25, Station 8+00, Station 11+25, Station 15+50. Alternative No. 1 also
adds manholes at Station 24+10 and Station 27+75. These manholes area primarily for
connection to the existing system, but also provide additional access points for inspection and
maintenance.
5.3 ACCESS ROAD IMPROVEMENTS
The existing gravel access road needs to be modified to allow easy access for the sewer
department’s Vactor truck. As previously mentioned, the Vactor truck is 10.5 feet wide (side
mirror to side mirror), 40.5-feet long, and has a minimum turning radius of 57.5 feet. To meet
these needs, the access road should have a width of 12 feet and a minimum radius of 60 feet. A
13.5 feet vertical clearance should also be provided to allow the mainline truck with the CCTV to
drive the access road with damage to its antennas.
The majority of the existing access road between Station 5+50 and 30+75 is already
approximately 12 feet wide (see plan sheets 5 through 8). Only minor widening will be needed,
but the entire access road will need to be resurfaced. The existing access road is asphalt
between Station 29+00 and Station 30+75; this section will need to be repaved. The rest of the
existing access road is gravel, which will also need to be resurfaced.
The majority of the access road can be widened to 12 feet without cutting into the toe of the
slope, but the plans identify the areas where it is needed. The plans also show where retaining
walls or rockeries are suggested to protect the access road from erosion. Refer to the
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recommendations made in Technical Memorandum No. 4 “Geotechnical Investigations” for
additional information about the potential slope stabilization methods.
To improve the ingress and egress of the Vactor truck along the access road, a turnaround
point, wide enough for a three point turn in the Vactor truck is proposed between Station 6+00
and Station 7+00, and an additional access point is proposed around Station 15+00. Both the
turn-around and the additional access point would require additional easements. The turn-
around would require the relocation and extension of an existing retaining wall. Appendix A,
sheets 2 through 9, show the proposed access road with these dimensions.
A narrow gravel path would be constructed from Station 0+00 to Station 5+50. This gravel path
should allow the easement machine to easily access SSMH 2309. The easement machine will be
4 feet wide. Sheets 1 and 2 of Appendix A currently show a 7 feet wide gravel path. In order to
construct this path, heavy vegetation will need to be cut back and the canyon wall would need
to be stabilized. Between Station 1+50 and 3+50, this 7-foot wide path will cross a wetland, and
mitigation efforts will be needed.
5.4 POTENTIAL OPERATION AND MAINTENANCE EQUIPMENT
The City is currently looking into purchasing a smaller Vactor truck to allow easier access to the
interceptor. The smaller Vactor truck would cost approximately $150,000 plus the cost of freight,
warranty, and surcharges. Designing the access road for this smaller Vactor truck would lead to
some construction savings by allowing a narrower access road and creating a smaller
turnaround area. A conservative estimate of the total cost savings accrued by designing the
access road for the smaller Vactor truck is approximately $50,000, about a third of the total
purchase price of the Vactor truck. However, leaving the access road designed for the larger
Vactor truck will allow the City to use the larger truck to maintain the sewer system in the event
the smaller Vactor is down. That being said, having the smaller vactor truck would still be
advantageous for the continual maintenance and operation of the interceptor.
Table 5.1 shows the dimensions of this potential smaller Vactor truck.
Table 5.1 Potential Operation and Maintenance Equipment
Equipment
Hose
Size
(inch)
Reach
(feet)
Vehicle
Length
(feet)
Vehicle
Width
(feet)
Vehicle
Height
(feet)
Vehicle
Turning
Radius
(feet)
Potential Smaller Vactor Truck N/A N/A 17.0’ 8.5’ 12.0’ 35.4’
The current access road design provides a 12-foot access road for the larger 10.5-foot wide (side
mirror to side mirror) Vactor truck. This leaves less than a foot of clearance on either side of the
road. While this is enough clearance to be drive-able, it would not be comfortable, especially
where the creek is directly alongside the road. The smaller 8.5-foot wide Vactor truck would
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leave just under 2 feet of clearance on either side of the 12-foot access road, making it a lot
more comfortable to drive and making maintenance of the interceptor easier to perform.
Accessing the lower half of the interceptor will also be significantly easier with the smaller Vactor
truck. Entering the access road from the Berkshire Apartments is currently difficult with the
existing Vactor truck; the truck has to perform a several point turn to enter the access road due
to the existing limited turning radius. Due to existing parking space requirements for the Berkshire
Apartment complex, there is not enough room at this entrance to provide a simple single point
turn for the larger Vactor truck. However, the smaller Vactor truck has a significantly smaller
turning radius, which will make it significantly easier for the smaller Vactor truck to enter the
access road.
The smaller Vactor truck will also reduce the amount of road maintenance in order to keep the
access road accessible. The lighter weight and smaller turning radius of the smaller vactor truck
will cause less wear and tear on the gravel access road than the existing Vactor truck.
We suggest purchasing the smaller Vactor truck, but continuing to design the access road
improvements for the larger Vactor truck. The smaller Vactor truck would significantly improve
the accessibility of the interceptor, making maintenance easier to perform.
6.0 CONCLUSIONS
The Thunder Hill Interceptor, constructed in 1965, needs improvements to allow the City to
continue to operate and maintain it for the long term. The City’s existing equipment has
difficulty accessing the interceptor because of the poor access road and limited manhole
access points to the system. Erosion caused by the steep canyon walls and the Thunder Hills
Creek also threatens the long term viability of the sewer interceptor.
This TM suggests adding additional sewer manholes, relocating the sewer main, and improving
the access road. In particular this TM suggests the following improvements to the Thunder Hills
Interceptor:
Add a new manhole at approximately Station 6+00 to give the Vactor truck access to
the northern portion of the interceptor.
Relocate the sewer main away from Thunder Hills Creek by creating parallel HDPE sewer
mains from Station 3+30 to 11+20 and Station 13+05 to 15+50. Alternative No. 1 adds an
additional sectionof parallel pipe fromStation 24+10 to 27+80.
Rehabilitate the existing sewer main that is to remain active by lining it with CIPP from
Station 0+00 to 3+30, Station 11+20 to 13+05, and Station 15+50 to 20+40. Alternative No.
1 adds two additional sections of CIPP rehab between Station 20+40 to 24+10 and
Station 27+80 to 28+75.
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Add new manholes for connection between the parallel main and the existing system at
approximately Station 4+25, Station 8+00, Station 11+25, and Station 15+50. Alternative
No. 1 adds two addition manholes at Station 24+10 and Station 27+75.
Resurface and widen the existing gravel access road between Station 5+50 and 29+00.
Repave and widen the existing paved entrance to the access road between Station
29+00 and 30+75.
Create a 7-foot wide gravel path between Station 0+00 and 5+50.
Create a turnaround between Station 6+00 and 7+00.
Create an additional access point to the access road around Station 15+00.
Build gravity walls or rockeries where necessary to protect the access road and/or sewer
main.
Purchase the smaller vactor truck to improve the ease of access to the interceptor.
These improvements to the Thunder Hills Interceptor, related access road, and the City’s
operation and maintenance equipment will allow the City to properly operate and maintain the
interceptor for years to come.
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Appendix A – Proposed Operation & Maintenance
Improvements
THUNDER HILLS SEWER INTERCEPTOR ALTERNATIVE ANALYSIS
By Stantec Consulting Services Inc.
Dated November 4, 2015
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Planning/Building/Public Works Dept.
CITY OF
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EXISTING SANITARY SEWER
CIPP EXISTING THUNDER HILLS
SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED LOCATIONS OF
PARALLEL SEWER PIPE
ALTERNATIVE NO. 1 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD
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EXISTING SANITARY SEWER
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SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED LOCATIONS OF
PARALLEL SEWER PIPE
ALTERNATIVE NO. 1 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD
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EXISTING SANITARY SEWER
CIPP EXISTING THUNDER HILLS
SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED NEW SEWER PIPE
UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
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EXISTING SANITARY SEWER
CIPP EXISTING THUNDER HILLS
SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED NEW SEWER PIPE
UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD
MATCH LINE - SHEET 3 OF 3
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SCALE IN FEET
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EXISTING SANITARY SEWER
CIPP EXISTING THUNDER HILLS
SEWER INTERCEPTOR
PROPOSED LOCATIONS OF
GRAVITY WALL OR ROCKERY
PROPOSED NEW SEWER PIPE
UPSIZE EX SEWER PIPE
REDIRECT EX SEWER PIPE
ALTERNATIVE NO. 2 LEGEND
PROPOSED LOCATION OF
IMPROVED GRAVEL ROAD
PROPOSED LOCATION OF
ROCK BUTTRESSES
PROPOSED LOCATIONS OF
IMPROVED PAVED ROAD