HomeMy WebLinkAbout18 - Design Report - South Wallace - Sewer
ENGINEERING REPORT
PROJECT:
2018 SOUTH WALLACE SEWER MAIN UPGRADE
City of Bozeman, Montana
February 2018
Submitted to:
City of Bozeman Engineering Department
20 East Olive Street
Bozeman, MT 59715
Montana Department of Environmental Quality
Permitting & Compliance Division
Public Water & Subdivisions Bureau
PO Box 200901
Helena, MT 59620-0901
BRIDGER ENGINEERS, INC.
2150 Analysis Drive, Ste. B Ph (406) 585-0590
Bozeman, Montana 59718 Fax (406) 582-1057
BEI
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TABLE OF CONTENTS
NOTE: This Engineering Report follows the format of Circular DEQ-2 Chapter
10, Design Standards for Public Sewage Systems, Engineering reports for minor
sewer systems and meets the design criteria of DEQ-2 Chapter 30.
10.0 GENERAL INFORMATION .................................................................................. 3
11.0 ENGINEERING REPORT ...................................................................................... 3
11.1 Problem Defined ................................................................................................. 3
11.2 Design Conditions ............................................................................................... 4
11.2.a Design Average Flow .......................................................................................... 4
11.2.b Design Maximum Day Flow ............................................................................... 4
11.2.c Design Peak Hourly Flow (PHF) ........................................................................ 4
11.2.c Hydraulic Capacity .............................................................................................. 5
11.3 Impact on Existing Wastewater Facilities ........................................................... 5
11.4 Project Description .............................................................................................. 5
11.5 Drawings ............................................................................................................. 6
11.6 Design Criteria .................................................................................................... 6
11.7 Site Information .................................................................................................. 6
11.8 Alternative Selection/Analysis ............................................................................ 6
11.9 Environmental Impacts ....................................................................................... 6
12.0 W-B PROPERTIES PEAK SEWER DEMAND CALCULATIONS ..................... 7
13.0 GRAVITY SEWER MAIN HYDRAULIC ANALYSIS......................................... 8
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10.0 GENERAL INFORMATION
The purpose of this report is to provide a summary of the design elements for the proposed
2018 South Wallace Sewer Main Upgrade. This sewer main upgrade project is being
required by the City of Bozeman Engineering Department to upgrade a portion of the City’s
wastewater collection system. The purpose of the upgrade is to replace a failing and
undersized sewer main for a new commercial/multi-family residential building being
proposed by Wallace-Babcock Properties, LLC. The portion of public sewer main to be
upgraded is located along South Wallace Avenue, roughly between the intersections of East
Main Street and East Babcock Street.
The Property Owner, Wallace-Babcock Properties, LLC, is responsible for all costs
associated with the design, permitting, construction, inspection and certification of the facility
upgrades. The facility will be owned and maintained by the City of Bozeman.
Property Owner/Developer:
Wallace-Babcock Properties, LLC
417 West Mendenhall Street
Bozeman, MT 59715-3448
Facility Owner:
City of Bozeman
Engineering Department
20 East Olive Street
Bozeman, MT 59715
This Engineering Report follows the format of Circular DEQ-2 Chapter 10, Design Standards
for Public Sewage Systems, Engineering reports for minor sewer systems and meets the
design criteria of DEQ-2 Chapter 30. It is being submitted with the MDEQ Sewer Main
Extension and Replacement Certified Checklist.
11.0 ENGINEERING REPORT
11.1 Problem Defined
The existing City of Bozeman wastewater collection system being upgraded consists of the
original sewer infrastructure constructed in 1910. The sewer main consists of a 6-inch
diameter vitrified clay pipe (VCP). TV reports of the sewer main being upgraded indicate the
presence of multiple cracks and fractures. In addition to the poor condition of the pipe, the
pipe is a 6-inch diameter pipe. The City of Bozeman is requiring that the portion of existing
6-inch sewer main receiving flow from the proposed project be increased in size to an 8-inch
main. The section to be upgraded consists of 406 lineal feet from the project’s service
connections to an existing manhole located in the intersection of Main Street and Wallace; at
which point the existing sewer main increases in size to an 8-inch diameter pipe. There are
three existing manholes within this segment of main which shall require replacement.
Additionally, there are five existing sewer service stubs which shall require new connections
to the main.
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11.2 Design Conditions
The gravity sewer main to be upgraded consists of an existing 6-inch vitrified clay pipe and
shall be upgraded to an 8-inch PVC pipe as required by the City of Bozeman for the
proposal of the Wallace-Babcock Properties, LLC new commercial/multi-family building.
This upgrade is being required due to the poor condition of the existing pipe and the
inadequate size of 6 inches. The replacement size of 8-inches is adequate for this sewer
main. The land surrounding the service area of this main consists of historically-developed
residential properties and City parkland. It is not anticipated that this main will require future
extensions or increases in capacity from an 8-inch main; as substantiated by the City’s
request for an 8-inch main.
Design criteria and calculations used to estimate flows is included at the end of this report
and have been submitted to the City of Bozeman for the proposed commercial/multi-family
building project. The proposed project is increasing the existing sewer load in the main by
2.8 gpm. Calculations to estimate this flow were performed using design criteria from the
City of Bozeman Design Standards and Specifications Policy.
The proposed sewer system components for this project include 390 L.F. of 8-inch SDR 35
PVC sewer main, (3) 48-inch diameter manholes, (2) new 4-inch PVC sewer service
connections, and (5) existing sewer service connections to be reconnected to the new main
with new in-line wye fittings. The sewer main slopes are between 1 and 1.5 percent which
exceed the minimum slope for an 8-inch line and are to be laid with uniform slope and
straight alignment between manholes. New 48-inch diameter pre-cast concrete manholes
are proposed to replace the existing manholes. The sewer main to be replaced is
sufficiently deep to prevent damage from superimposed live, dead, and frost induced loads.
11.2.a Design Average Flow
The receiving sewer main has been evaluated for future collection needs based on
assigned zoning and acreage as recommended in Table V-1 of the City of Bozeman
Design Standards and Specifications policy. The receiving sewer main serves
approximately 8 acres of B-3 zoning (at 3,000 gpd/acre) and 2.5 acres of R-2 zoning (at
980 gpd/acre). An infiltration rate of 150 gpd/acre is included per the City of Bozeman
Design Standards. The anticipated average daily flow utilizing the rates and acreages
above is 28,025 gpd (or 19.5 gpm over 24 hours).
11.2.b Design Maximum Day Flow
A maximum daily demand peaking factor of around 3 has been suggested in various
literatures. Consequently, a peaking factor of 3 was used for the maximum daily
demand factor for this analysis. The design maximum day flow is calculated as 3.0 x
28,025 gpd = 84,075 gpd (or 58.4 gpm over 24 hours).
11.2.c Design Peak Hourly Flow (PHF)
A peaking factor of five times the average daily demand has been recommended by the
Federal Housing Administration in determining the peak hourly flow. A conservative
estimate has been employed by using the maximum day demand in the calculation
instead of the average day demand. Employing a peaking factor of 5, the design peak
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hourly flow is calculated as 5.0 x 84,075 gpd = 420,375 gpd (or 292 gpm over 24 hours).
The peak hour would produce 292 gpm x 60 minutes = 17,520 gallons per hour, which is
21 percent of the maximum daily flow of 84,075 gpd.
11.2.c Hydraulic Capacity
The gravity sewer mains have been designed as 8” diameter PVC lines. The peak
hourly flow (PHF) including infiltration and inflow has been determined to be 17,520
gallons, or 0.65 cubic feet per second (cfs). The sewer design was prepared utilizing
Manning’s Equation (n=0.013). The slope of the existing sewer main is between 1 and
1.5 percent. A slope of 1% was used in the calculations. At the peak hourly flow rate of
0.65 cfs, the 8-inch diameter main will be flowing at 54% capacity, which is under the
City of Bozeman’s maximum of 75%, and is therefore adequate in size. Hydraulic
calculations are provided at the end of this report.
11.3 Impact on Existing Wastewater Facilities
No significant impacts to the City of Bozeman’s Wastewater Treatment Facility are
anticipated. The direct impact of the proposed project involves upgrading a failing portion of
the collection system. The upgraded portion and all further downstream facilities are
capable of handling the increased project load.
11.4 Project Description
Commercial/Multi-Family Residential Building:
Wallace-Babcock Properties, LLC is proposing to build a combination commercial/multi-
family residential building at the southwest corner of S. Wallace Ave. and E. Babcock St. in
Bozeman, Montana. The building shall consist of approximately 18,000 square feet of gross
commercial area and four residential units and will be served by the City of Bozeman’s
Wastewater Treatment Facility. The proposed project will generate a design flow of 6.2
gpm.
Sewer Main Upgrade:
The existing sewer main to serve this project consists of the original City sewer infrastructure
constructed in 1910. The receiving main consists of a 6-inch diameter vitrified clay pipe
(VCP) for approximately 400 feet until it changes to an 8-inch pipe at a manhole within the
intersection of Main Street. This 400-foot stretch of 6-inch main is being replaced by a new
8-inch PVC main. Existing manholes and sewer service connections within this segment will
also be replaced or upgraded.
The upstream portion of the existing sewer main to be upgraded consists of a straight
segment of gravity sewer main approximately 1,000 feet in length. The first 300 feet
immediately upstream of the upgrade consists of the original 6-inch VCP. At the intersection
of S. Wallace and Olive Street the existing 6-inch VCP terminates at a manhole. This
manhole was installed in 1994 as part of a 700-foot sewer main extension project. The
sewer main extension consists of an 8-inch PVC pipe and terminates at the end of S.
Wallace Avenue. Beyond this street/sewer main exists City parkland; therefore, future
extensions to this sewer main are not anticipated.
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The sewer main is located along the centerline of the asphalt-paved Wallace Street. The
main is approximately 10-feet deep. The proposed sewer infrastructure has been designed
to meet the criteria described in DEQ-2 Chapter 30.
Construction shall be accomplished with trench excavation methods in accordance with
MPWSS and the City of Bozeman Modifications to MPWSS. The sewer mains and
manholes will be tested according to the City of Bozeman Modifications to MPWSS and
leakage tests will be conducted using the water test.
11.5 Drawings
Construction drawings are included with this report. The drawings identify the location of the
project site and all design elements of the sewer main upgrade as required in DEQ-2
Chapter 20.
11.6 Design Criteria
The sewer main upgrade has been designed in accordance with DEQ-2 Chapter 30 and the
City of Bozeman Design Standards and Specifications Policy, Latest Edition.
11.7 Site Information
The site is located within downtown Bozeman. It is relatively flat with a grade of
approximately 1.5% to the south. The sewer main is located within the center of a straight,
paved city street. Geotechnical borings at the building site have determined soils typical of
this area, fine grained soils overlying dense gravels at approximately 5 feet. The estimated
depth to high groundwater obtained from the geotechnical borings is approximately 10 feet.
If the sewer improvements are installed during high groundwater times, dewatering may be
required.
11.8 Alternative Selection/Analysis
Due to the project’s location within the City of Bozeman, and the inadequate condition of the
receiving public sewer main, an upgrade of the inadequate portion of the City sewer main is
the only option for the project’s sanitary sewer service.
11.9 Environmental Impacts
The South Wallace Avenue right-of-way has been identified by the City of Bozeman as an
area with potential asbestos contamination. All work to be done for this project is within this
area. The Owner/Contractor shall be responsible for notifying the City of Bozeman as a part
of the street cut permit. The Owner will be responsible for hiring a Montana Accredited
Asbestos Contractor/Supervisor to inspect the excavation for the presence of asbestos-
containing material. The accredited contractor/supervisor will be present whenever new
areas are excavated. If any asbestos containing material is identified, a Montana Accredited
Asbestos Abatement Contractor/Supervisor or Worker must immediately remove, transport
and dispose of the asbestos in compliance with all applicable state and federal
environmental requirements, criteria, and limitations, including OSHA regulations.
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12.0 W-B PROPERTIES PEAK SEWER DEMAND CALCULATIONS
Average Daily Wastewater Flow (in gallons per minute)
COMMERCIAL
Typical Business Day: 8 am – 5 pm
Net Floor Area of Office Space: 9,700 sf
Use 1 employee per 200 sf: 9,700 sf ÷ 200 sf/emp. = 48.5 employees
Using MT DEQ Circular 4, Table 3.1-1:
Typical Flow (in gallons per day) for Office = 13 gpd/employee
Q Ave (Com) = 48.5 emp. x 13 gpd per emp. = 631 gpd
RESIDENTIAL
Typical Residential Hours: 6 am – 10 pm
Number of Residential Dwelling Units: 4 du
Using COB Design Standards Section V.B:
Ave. Daily Flow Rate = 2.11 people/dwelling unit & 89 gal/capita/day
Q Ave (Res) = 4 du. x 2.11 people/du x 89 gpcd = 751 gpd
Average hourly flow shall be determined for a typical 16-hour day:
Q Ave (Total) = (631 gpd + 751 gpd)÷(16 hours per day x 60 min. per hour) = 1.44 gpm
Total Peak Wastewater Flow (in gallons per minute)
POPULATION
Commercial: 48.5 employees
Residential: 2.11 people/du x 4 du = 8.4 residents
Population: 48.5 emp + 8.4 residents = 56.9 57 persons
PEAKING FACTOR
Using COB Design Standards Section V.B.6:
Peaking Factor (PF)
= Q Max ÷ Q Ave
= [18+(57/1000).5 ] ÷ [4+(57/1000).5 ] = 18.24 ÷ 4.24 = 4.3
Q Peak (Total) = Q Ave (Total) x PF = 1.44 gpm x 4.3 = 6.2 gpm (Project Total)
Subtract Flow from Existing (4) Residential Units Being Replaced
Q Peak (Increase)
= Q Peak (Total) - Q Ave (Residential) x PF
= 6.2 gpm – [751 gpd ÷ (16 x 60)] x 4.3
= 6.2 gpm – 3.4 gpm = 2.8 gpm (Project Increase)
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13.0 GRAVITY SEWER MAIN HYDRAULIC ANALYSIS
INPUTS (for a pipe MORE than half full)
CALCULATIONS
Pipe Diameter D 7.92 in
Pipe Diameter D 0.66 ft
Depth of Flow y 4.2 in
Pipe Area A full 0.34 ft2
(must have y ≥ D/2)
Pipe Radius r 0.33 ft
Manning
n 0.013
Depth of Flow y 0.35 ft
Slope
S 0.01 ft/ft
y/D 0.53
Circ Segment
Height h 0.31 ft
ANALYSIS
(h = 2r - y)
Velocity
V 3.531 ft/sec
Central Angle θ 3.020
Flow
Q 0.651 cfs
Flow Area A flow 0.184 ft2
Flow
Q 291.97 gpm
Wetted Perimeter P 1.077 ft
Percent Full
54 %
Hydraulic Radius R h 0.171 ft