little hunting creek sewer saglittle hunting creek sanitary sewer sag project 1 introduction the...

LITTLE HUNTING CREEK SEWER SAG Final Basis of Design Report

November 8, 2017

LITTLE HUNTING CREEK SEWER SAG BASIS OF DESIGN REPORT

Arcadis Fairfax County Department of Public Works and Environmental Services Little Hunting Creek Sewer Sag – Basics of Design Report

LITTLE HUNTING CREEK SEWER SAG Final Basis of Design Report

Prepared for:

Fairfax County

Department of Public Works and Environmental Services

12000 Government Center Parkway

Fairfax

Virginia 22035

Prepared by:

Arcadis

11350 Random Hills Road

8th Floor

Fairfax, VA 22030

Tel 703 351 9100

Fax 703 351 1305

Our Ref.:

027790WW.1702

Date:

November 8, 2017

This document is intended only for the use of the individual or entity for which it was prepared and may contain information that is privileged, confidential and exempt from disclosure under applicable law. Any dissemination, distribution or copying of this document is strictly prohibited.

Meredith Raetz Task Manager

Tom Grala WDCD Senior Engineer III

Arcadis Fairfax County Department of Public Works and Environmental Services Little Hunting Creek Sewer Sag – Basics of Design Report i

Table of Contents Introduction ............................................................................................................................................. 1

Project Background ................................................................................................................................. 1

Project Description ............................................................................................................................ 1

Project Justification ........................................................................................................................... 1

Available Information ......................................................................................................................... 2

Condition Inspection and Maintenance Data .............................................................................. 2

GIS Floodplain and Resource Protected Area ............................................................................ 3

As-Built Drawings ...................................................................................................................... 4

Flow Projections ........................................................................................................................ 4

Stakeholders ............................................................................................................................. 4

Potential Installation Methods ............................................................................................................ 5

Alternatives Analysis ............................................................................................................................... 5

Assumptions ..................................................................................................................................... 5

Alternatives ....................................................................................................................................... 6

Alternative 1: Pipe Bursting ........................................................................................................ 6

Alternative 2: Cured-in-Place Lining ........................................................................................... 6

Alternative 3: Open-Cut Method with Coffer Dam ....................................................................... 6

Alternative 4: New Pump Station Recommendation ................................................................... 7

Alternative 4A: New Pump Station with a New 6-inch Force Main Inside the Existing 12-inch Pipe. ............................................................................................................................... 7

Alternative 4B: New Pump Station to Tie In to Existing 30-inch Ductile Iron Pipe Force Main ...... 8

Alternative 4C: New Pump Station for a New Force Main on West Side of Street, Tie In Existing Force Main to Gravity Sewer........................................................................................... 8

Alternative 5: Jack and Bore ...................................................................................................... 8

Alternative 6: Horizontal Directional Drilling ................................................................................ 8

Alternative 7: Leave in Existing Condition .................................................................................. 9

Regulatory/Permit Considerations ..................................................................................................... 9

Cost Analysis .................................................................................................................................... 9

Recommendation .................................................................................................................................. 10

Arcadis Fairfax County Department of Public Works and Environmental Services Little Hunting Creek Sewer Sag – Basics of Design Report ii

Selected Alternative ........................................................................................................................ 10

Project Planning .............................................................................................................................. 11

References ............................................................................................................................................ 11

TABLES Table 1. Available Information .................................................................................................................. 2

Table 2. Stakeholders .............................................................................................................................. 4

Table 3. Permits ...................................................................................................................................... 9

Table 4. Cost Opinion ............................................................................................................................ 10

FIGURES Figure 1. Images from CCTV inspection. .................................................................................................. 3

APPENDICES

Appendix A Sewershed Overview Map

Appendix B Conceptual Pipe Plan and Profile

Appendix C Floodplain and Resource Protection Area Map

Appendix D Alternative Analysis Summary

Appendix E Criteria Matrix

Appendix F Alternative Site Layouts

Appendix G Cost Estimates and Present Worth Analysis

Appendix H Preliminary Project Schedule

Arcadis Fairfax County Department of Public Works and Environmental Services Little Hunting Creek Sanitary Sewer Sag Project 1

INTRODUCTION The Basis of Design Report (BODR) provides an evaluation of alternatives for the repair, rehabilitation, or replacement of the Little Hunting Creek Sanitary Sewer (LHCSS) subject pipe. The pipe crosses Little Hunting Creek, from manhole 111-1-262 to 111-1-001 between Woodland Lane and Thomas J. Stockton Parkway. Arcadis U.S., Inc. (Arcadis) evaluated several alternatives based on site visits and review of available information. A summary of each alternative considered is presented.

PROJECT BACKGROUND This section provides a description of and justification for the LHCSS project, outlines available information reviewed by Arcadis for use in the evaluation of alternatives, and summarizes an evaluation of the existing LHCSS.

Project Description The LHCSS pipe is a 12-inch diameter cast iron gravity sewer that starts at Woodland Lane and extends 789 linear feet (LF), across the creek, to Thomas J. Stockton Parkway. The pipe was constructed in 1961. Approximately 480 homes and one church are served by the sewer, which according to Fairfax County Wastewater Planning and Monitoring Division (WPMD) conveys approximately 0.146 million gallons per day (MGD) of average daily flow. LHCSS Area and sewershed overview map can be found in Appendix A.

The purpose of the Little Hunting Creek Sewer Sag Basis of Design Report is to evaluate alternatives to establish a 50 year sewer service, for the 12-inch gravity sewer discussed above, along with classifying a cost effective solution for the existing sewer sag. In addition, reduce or eliminate the perpetual maintenance efforts currently needed, avoid costly emergency repairs, and reduce risks to public health and environmental impacts.

Project Justification Fairfax County retained the services of Hazen and Sawyer to assess sewer sags throughout the County’s sanitary system and to develop rehabilitation or replacement recommendations. The findings of the assessment and recommendations were presented to Fairfax County in the Preliminary Engineering Report (PER), titled Sewer Capacity Improvements Program: Sewer Sag Assessment and Recommendations, April 24, 2015.

The PER included a prioritization of assets based on Impact and Risk of Failure. The LHCSS pipe was identified as the second-most critical in the prioritization table, with a criticality rating of 3 of 3, one being the least critical and three the most critical. A closed-circuit television (CCTV) inspection of the LHCSS pipe, performed in 2011 and referenced in the PER, identified large amounts of debris buildup as well as sags in the sewer, limiting the inspection to 197.6 LF of the planned 788.96 LF. Due to the lack of available CCTV data, the PER included a recommendation to reinspect the pipe.

In response to the PER recommendation, the Fairfax County Wastewater Collection Division (WCD) conducted a CCTV inspection in May 2015 to reassess the condition of the LHCSS pipe. The 2015 CCTV inspection successfully completed the full length of the pipe and identified three major sags. The first sag spanned from 470 feet to 500 feet, the second from 540 feet to 580 feet, and the third from 620 feet to


720 feet. Arcadis developed a conceptual pipe profile based on the water levels shown in the CCTV data (Appendix B).

The footage also revealed fat, oil, and grease (FOG) buildup throughout the pipe which led to the sewer being placed on a 90-day pressure cleaning schedule.

Available Information Fairfax County provided existing information regarding the LHCSS. The available information is summarized in Table 1.

Table 1. Available Information

Information Type Description

Condition Inspection and Maintenance Data

Provides a history of backups and sanitary sewer overflows (SSOs). Aids in providing profile of sewer sags. Includes CCTV video performed by WCD. See discussion following table.

Geographic Information System (GIS) Includes locations of sewer pipes, manholes, property boundaries, and floodplains. See discussion following table.

PER: Sewer Sag Assessment and Recommendations (Hazen and Sawyer 2015)

Identifies sewers with sags and other defects throughout Fairfax County. Results in a prioritized list of pipes to be addressed. The LHCSS pipe ranked #2 in the prioritized list.

Little Hunting Creek Pump Station PER (Dewberry 2017)

Provides an evaluation of existing conditions at the pump station, and preliminary recommendations for rehabilitation of the station.

As-Built Drawings

Pump station as-builts, 30-inch force main as-builts, and 48-inch gravity sewer line near the pump station as-built. One sheet of 12-inch sanitary sewer crossing the creek. See discussion following table.

Flow Projections According to the WPMD, there is adequate capacity in the 12-inch sewer through 2045. See discussion following table.

Map of Topographic Survey for Little Hunting Creek (July 14, 2017)

Location, depth, and extent of Little Hunting Creek, and alignment and as-built elevation of LHCS.

Other PDF maps of LHCSS area; stakeholder information. See discussion following table.

Condition Inspection and Maintenance Data The information provided identifies deficiencies and issues with the LHCSS pipe. WCD records indicate a backup on February 17, 2000. Two SSOs are on record and are dated February 13, 2000 and July 10, 2001. These incidents appear to be isolated events that could have been caused by an unusual extreme weather event, temporary blockage in the sewer line, or other anomaly. The reported back up and one of


the SSO’s occurred only several days apart and could be linked to that same unusual event. These reports are 17 years old without any further reports, backups, or SSOs since July of 2001. Suggesting the cause is not systemic to the sewer system.

Arcadis performed an evaluation of the CCTV data using the National Association of Sewer Service Companies (NASSCO) Pipeline Assessment Certification Program (PACP) standard. The evaluation showed significant grease build-up, corrosion, and debris. Images of these defects from the CCTV inspection can be seen in Figure 1. These findings are consistent with Fairfax County’s initial expectations of the pipe condition and provides additional basis for rehabilitation or replacement.

Figure 1. Images from CCTV inspection. Image A shows grease build-up on crown of pipe. Image B shows corrosion on side of pipe. Image C shows floating debris. Image D shows high water level indicating sag in sewer.

GIS Floodplain and Resource Protected Area According to the Federal Emergency Management Agency’s (FEMA’s) Flood Insurance Rate Map (FIRM), Little Hunting Creek is zoned AE. An AE zone is subject to inundation by the one percent annual chance flood event. The base flood elevation at this location is 10 feet.

Based on the GIS data provided, the total Resource Protected Area (RPA) inside the assumed project limits of disturbances is 90,758 square feet, or 2.1 acres. A RPA is an area of environmentally sensitive land that lies alongside or near a waterway draining to the Chesapeake Bay. Development in a RPA is

A B

D C


regulated by the Chesapeake Bay Preservation Ordinance. A map showing the floodplain and RPA area is presented in Appendix C.

As-Built Drawings The available as-built information for the LHCSS is limited. An as-built drawing shows that Little Hunting Creek is shallow for most of the width of the creek except for a narrow portion that is much deeper near the western end of the pipe. The bottom of the creek at this section appears to reach the crown of the pipe. The pipe has between four and eight feet of cover for most of the crossing except for this section. Further discussion is included in the Evaluation of Existing Pipe section.

Flow Projections According to the WPMD, the 12-inch sewer’s existing average daily flow is 0.146 MGD and for the year 2045 the projected average daily flow is 0.147 MGD. The sewer line has a maximum flow carrying capacity of 1.2 MGD and therefore is adequate to handle predicted flows through 2045. The 253-acre LHCSS watershed is highly developed, containing 480 residences and one church, indicating that flow rates should not increase significantly in the future.

Stakeholders Seven stakeholders were identified for the project and must be notified prior to any construction activities. Stakeholders are listed in Table 2.

Table 2. Stakeholders

Stakeholder Description Contact Information

Northern Virginia Conservation Trust

Owns property at 8610 Thomas J. Stockton Parkway

Rentz Hilyer [email protected] 703-354-5093

Friends of Little Hunting Creek

Local environmental group that organizes clean-ups in the area.

http://www.friendsoflittlehuntingcreek.org/

Little Hunting Creek, Inc. Serves the community by providing information related to governance, environmental management, and other issues of interest.

http://www.littlehuntingcreek.org/index.htm

Woodland Park Civic Association

Owns 3201 Woodland Lane, which contains manholes 111-1-002 and 111-1-262.

James Newby

Stratford Landing Citizens Association

Owns 8614 and 8612 Thomas J. Stockton Parkway, which contain three manholes associated with the project. All homes in zip code area 22038.

[email protected] Committee Chair – Jim Horn: [email protected]

Friends of Accotink Creek Not affiliated with project but requested to be included in project coordination.

http://www.accotink.org/

Mount Vernon Council of Citizens’ Associations

A nonpartisan, non-profit organization of citizens’ associations in Mt. Vernon district.

http://www.mvcca.org/

Mt. Vernon District Supervisor’s Office

Notification to Fairfax County Board of Supervisors about project

Dan Stork [email protected]



http://www.accotink.org/


Stakeholder Description Contact Information

Wessynton Homes Association

Homeowner association in the vicinity of the boat ramp/park on the west side of the creek.

Chris Revere, President [email protected]

Installation Methods Based on the available information, it is assumed that the existing pipe was installed using a dragline method. In the dragline construction method, the pipe typically is assembled in sections and pulled through a previously prepared trench. Alternatively, the pipe may have been floated into position and lowered to the bottom of the creek’s trench. It is assumed that the pipe was laid in the trench without adequate bedding support. Another possible cause of the sewer sagging could be due to unstable subgrade below the pipe. Unstable subgrade could be attributed to poor trench bottom compaction methods during installation or natural differences in the subgrade properties. It’s possible that any of these construction methods or other inappropriate pipe laying techniques could have been used resulting in the changing grades along the pipe either during or after the installation.

ALTERNATIVES ANALYSIS Arcadis performed a preliminary comparison of nine alternatives, as presented in Appendix D - Alternatives Analysis Summary. Based on this preliminary evaluation, six alternatives were removed from consideration. The remaining three viable alternatives were evaluated further, using the following criteria. Based on the outcome of the criteria evaluation, the three alternatives being evaluated further are Alternatives 3, 4A and 4B. Details of this evaluation are presented in Appendix E – Criteria Matrix.

• Homeowner Disruption – all options will be very disruptive to the immediate community during construction. This criteria focuses on homeowner disturbance after construction.

• Operation and Maintenance – inspections/maintenance. Including routine visits, equipment rehabilitation or replacement, cleaning.

• Capital Cost – cost of construction, engineering services, regulatory fees, property/easement acquisition.

• Permitting – number of permits, duration to obtain permit.

• Easements – if new easements are required and difficulty, time and cost to obtain.

• Schedule – design, public outreach, easement acquisition, bidding and construction duration.

Assumptions The following assumptions were used in alternative analysis and cost estimating:

• The LHCSS subject pipe conveys flow from 480 residences, according to available GIS information.


• The sewage design flow is assumed to be 335 GPD per residence. This number assumes that approximately half of the LHCSS sewershed is comprised of town homes/apartments and half is comprised of single-family homes, and uses the values shown in Table 10.1 of the Fairfax County Public Facilities Manual (PFM).

• The peaking factor is 5.6 – This peaking factor was chosen from the Peak Flow Curve on Plate 1-10 of the PFM. Plat 1-10 shows that an average flow of 0.16 MGD should have a peaking factor of about 5.6 applied to obtain maximum flow for design purposes.

Using the above assumptions, the average design flow is 0.161 MGD or 111 GPM and a peak design flow of 0.90 MGD or 620 GPM.

This calculated flow is a 0.015 MGD higher than the existing average daily flow of 0.146 MGD provided by the WPMD. The design flow of 0.90 MGD is 25% less than WPMD projected carrying capacity of 1.2 MGD.

Alternatives

Alternative 1: Pipe Bursting The sewer was evaluated for replacement via pipe bursting. The distance between the two manholes in this situation is too long to be able to ensure a successful pipe-bursting pull. Pursuing pipe bursting would be a risk that Arcadis would not recommend taking. Additionally, the existing pipe exhibits significant misalignment that pipe bursting would not be able to sufficiently correct. Without correcting the misalignment, the pipe will continue to be particularly prone to debris and grease build up. Furthermore, this method does not address the underlying issue of insufficient bedding and will leave it vulnerable to further sagging and possible failure. For these reasons pipe bursting was eliminated from further consideration.

Alternative 2: Cured-in-Place Lining The sewer was evaluated for rehabilitation via cured-in-place pipe (CIPP) lining. However, the 2015 PER (Hazen and Sawyer 2015) and additional Arcadis evaluation of the 2011 CCTV inspection concluded that this alternative is inadequate to repair the pipe for two main reasons. First, the existing pipe exhibits significant misalignment that CIPP would not be able to correct since the liner will take the shape of the existing pipe. Without correcting the misalignment, the pipe will continue to be particularly prone to debris and grease build up. The sags could be corrected with point repairs, but a point repair for a sewer under a creek would be exceedingly expensive. Second, this method does not address the underlying issue of improper bedding, and could introduce additional sagging and possible failure. For these reasons, CIPP lining was eliminated from further consideration.

Alternative 3: Open-Cut Method with Coffer Dam Open-cut construction of a new pipe using coffer dams is a traditional method used for construction across a shallow body of water. It allows for proper excavation, bedding, and pressure testing of the pipe. Appropriate bedding installation would provide optimal life expectancy of the pipe and, an open-cut replacement in parallel with the existing sewer would eliminate the need for bypass pumping.


The proposed location of the new adjacent sewer is shown in Appendix F. The new proposed sewer would pass through only one private property, parcel 111-06-0007A, minimizing coordination of easements and agreements. The existing sewer has a manhole connection, with the downstream pipe off the road in vegetation, which makes access for assessment and maintenance difficult. Moving the connection closer to the road and building a doghouse manhole with the downstream sewer will provide better accessibility. Also, the proposed alignment will provide a slightly more favorable flow angle into the downstream sewer.

This alternative would require a contractor qualified to perform land and water construction. Significant permitting would be required, as construction impacts include floodplains, an RPA, and wetlands, potentially requiring significant and costly restoration. The permitting process would involve a Joint Permit Application (JPA) through the US Army Corps of Engineers.

As shown in the Criteria Matrix in Appendix E this alternative was rated a 3.45 of 5, the highest of the evaluated alternatives.

A general site layout is presented in Appendix F.

Alternative 4: New Pump Station Recommendation Installing a new pump station on the west shore of the creek eliminates the need for construction in Little Hunting Creek and would reduce the amount of permitting necessary. The existing line would be abandoned in place and either capped or filled with grout on each end.

The pump station would include submersible, centrifugal, duplex or triplex pumps. The station would run the primary electric feed with a backup generator. Due to the extent of the floodplain area, the generator would need to be situated at least three feet above the ground elevation of 6.6 feet for the proposed pump station location. A block wall surrounding the pump station at an approximate height of 9 feet is recommended as well for flood prevention and to serve as a noise barrier from the generator.

Implementing a new pump station would require maintenance on an as-needed basis. Three alternatives utilizing a pump station are described below.

Alternative 4A: New Pump Station with a New 6-inch Force Main Inside the Existing 12-inch Pipe. Utilizing the existing 12-inch ductile iron pipe (DIP) to host a smaller 6-inch high-density polyethylene (HDPE) pipe would minimize the extent of permitting required. Disturbance would be maintained in the original easement, while eliminating the need for a coffer dam or open cut construction through the creek. The original DIP would protect the HDPE pipe from abrasions or punctures.

Based on the design concept assumptions stated above, and in order to have a force main sized such that it can be pulled through the existing 12-inch gravity sewer, a 6-inch diameter force main is being recommended. With an average daily flow of 110 GPM and a peak flow of 620 GPM, either a duplex or triplex pump arrangement could be used to accommodate the flows while maintaining acceptable flow velocities and head loss conditions in the force main. This determination should be made after further investigation is done regarding the actual peak flow factor.

This alternative would require cleaning the original pipe prior to inserting the smaller HDPE pipe. In addition, the severity of the sags will need further investigation. If the sags are too steep for a proper alignment, the original pipe will not be able to accommodate the HDPE.


This alternative is considered a viable solution and was rated 2.40 in the Criteria Matrix in Appendix E.


Alternative 4B: New Pump Station to Tie In to Existing 30-inch Ductile Iron Pipe Force Main Another variation of the pump station alternative is to tie in to the existing force main at the proposed pump station location. This alternative minimizes community disruption in comparison to the other alternatives and eliminates the need to cross Little Hunting Creek

This tie-in alternative assumes that the existing 30-inch force main material is DIP, and that it was constructed in 1988, as represented in Fairfax County’s GIS database.

Challenges to this option include pumping into a force main with variable head conditions from the Little Hunting Creek Pump Station. However, this alternative is still viable, and is included in the Criteria Matrix in Appendix E which rated this alternative a 2.55 of 5.


Alternative 4C: New Pump Station for a New Force Main on West Side of Street, Tie In Existing Force Main to Gravity Sewer To avoid the challenges of pumping into a force main, another option would be to run a new force main from the new pump station to a location downstream, away from the creek. The closest downstream gravity fed sanitary sewer from the new pump station is approximately 4,000 liner feet to the west. Although most of the route can be made within existing easements or within Virginia Department of Transportation (VDOT) right-of-way, 4,000 feet of a new force main sewer would be costly, disruptive to the community and logistically challenging.

Although this option eliminates the need for crossing the creek the cost, community disruption and logistical challenges render it not a viable solution. No further evaluation will be done.

Alternative 5: Jack and Bore Use of the jack and bore method minimizes construction in the creek and is sufficient for realignment and depth alignment. However, this alternative is not considered a viable option. Deep trenching would be necessary at either end of the pipe to mobilize a jack and bore operation with an alignment that is safely below the bed of the creek. In a high-water-table area such as along the banks of the creek, deep trenching would be challenging. Furthermore, if drop manholes would be utilized to connect the deeper creek crossing sewer to the upstream and downstream pipes, the result would be a siphon configuration that is not favorable to operations. Finally, the span of the LHCSS pipe may be impractical for conventional contracting jack and bore methods. Therefore, no further evaluation was performed.

Alternative 6: Horizontal Directional Drilling Horizontal directional drilling (HDD) is a trenchless method for installing underground utilities. For the LHCSS pipe, constructing a new sewer adjacent to the original using HDD would require permitting through the U.S. Army Corps of Engineers (USACE). Depending on the stabilization of bedding, a deeper profile may be required, resulting in siphon conditions (again, not favorable to operations). An adjacent


sewer would allow the original to continue to flow, eliminating the need for bypass pumping during construction.

Review of the original as-built drawings indicates that the amount of cover available to maintain the existing slope of 0.27 is problematic. The HDD access pit would require significant space, encroaching on homeowner property. Therefore, no further evaluation was performed.

Alternative 7: Leave in Existing Condition Under this alternative, the sewer would remain on a 90-day pressure cleaning cycle to prevent clogging and potential overflows, future point repairs would be needed to address deteriorated sections or joints compromised due to settling.

Regulatory/Permit Considerations Table 3 below summarizes a preliminary list of anticipated required permits. Please refer to Appendix H Preliminary Project Schedule for permit development and review periods and duration.

Table 3. Permits

Regulatory Entity Permit

Virginia Department of Environmental Quality (DEQ) Virginia Marine Resource Commission (VMRC)

Virginia Pollutant Discharge Elimination System (VPDES), CWA Sec. 404/401, VSMP-Stormwater Pollution Prevention Plan

United States Army Corps of Engineers (USACE) Nationwide Permit #6

DEQ, VMRC, USACE Tidal Water Joint Permit Application

VDOT Various

Fairfax County

Compliance with Chesapeake Bay Preservation Ordinance

DPWES Land Development Site and Building Permits

Virginia Department of Environmental Quality (DEQ) Certificate to Contruct (CTC)

United States Army Corps of Engineers Nationwide Permit #12

Cost Analysis Based on the assumptions and preliminary design concepts developed for the three alternatives being considered, preliminary project cost opinions were developed and have been summarized in Table 4 below. The cost opinions include construction cost, engineering services throughout the project, potential wetland mitigation fees, easement and property acquisition costs, and contingencies.

In addition to the initial project cost, a present worth analysis has also been completed for the three alternatives being considered, and the present worth for each alternative has been summarized in Table 4 below. The present worth values presented are calculated based on the cost of replacement, maintenance and operation costs, and salvage or residual value over a period of 50 years using a 3%


interest rate. Since the worth calculation represents costs that will be incurred, a low present worth is more favorable than a high present worth.

A more detailed breakdown of the project cost opinions and present worth analyses are included in Appendix G, along with various assumptions and explanations regarding the values used in the calculations.

Table 4. Cost Opinion

Project costs include both engineering services and construction cost.

Alternative Project Cost

Opinion

Present Worth (50 Yr)

Alternative 3: Open Cut with Coffer Dam $2,018,910 $2,035,388

Alternative 4A: New Pump Station for a New 6-inch Force Main Inside the Existing 12-inch Pipe

$1,317,000 $2,618,000

Alternative 4B: New Pump Station to Discharge to Existing 30-inch Ductile Iron Pipe Force Main

$1,308,000 $2,609,000

RECOMMENDATION The Little Hunting Creek Sewer Sag presents unique challenges for rehabilitation or replacement. Due to its geographic locations, there are many constraints presenting challenges to typical construction methods. It spans almost 800 feet across a tidal creek and is near community residents. After consideration of nine possible alternatives, three are considered viable solution for further study. After further evaluation of present worth and construction costs, Arcadis recommends Alternative 3: Open Cut with a Coffer Dam.

Selected Alternative The Open Cut Method with Coffer Dam alternative offers a long-term solution to the replacement of this important asset. The resulting gravity sewer would be sized to accommodate future flows from the collection area, and would feed into the downstream sewer in much the same way the existing sewer was intended to operate. The sewer would only require the nominal operating care that any other gravity sewer receives in the County’s system. The construction would require significant permitting, since the sewer is installed in a tidal, navigable water body with adjacent wetlands. To ensure proper support to provide a long life expectancy, the bedding of the pipe may include the use of piles, pile caps, anchoring devices, stone/gravel bedding and concrete encasement for protection within the channel of the creek. Mobilization for construction purposes would require access on both east and west shorelines of the creek. Docking and mooring of barges and other in-water equipment will need to be accommodated.


Project Planning A schedule that includes elements of design and construction identified thus far can be found in Appendix H. Note that the schedule includes the preparation of a Preliminary Engineering Memo (PEM) that will help to further define elements of the project before detailed design and permitting begins. The PEM will, in particular, help bring focus on the following elements of the project, including:

• Confirmation of the alignment of new sewer pipe

• Assessment of easements, letters of permission, and property acquisition needs both forconstruction and laydown areas, and for the permanent alignment

• Location of new manholes to minimize or eliminate the need for bypass pumping

• Selection of pipe and pipe joint materials

• Plans for geotechnical soil investigation to support construction of coffer dams and properfoundation / bedding of the pipe

• Consideration of tidal changes in water elevation

• Surveying of stream bottom to determine feasibility of using various cofferdam system

• Selection of cofferdam type

• Considerations for utilizing the annual contractors and contracts

• Considerations for utilizing a marine contractor, including best method for engaging this contractor

• Means to achieve early regulatory agency engagement

REFERENCES Dewberry. January 24, 2017. Preliminary Engineering Report: Little Hunting Creek Sewage Pump Station Rehabilitation.

Hazen and Sawyer. April 24, 2015. Preliminary Engineering Report: Sewer Capacity Improvements Program: Sewer Sag Assessment and Recommendations.

APPENDIX A

Sewershed Overview Map

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Sewershed Overview Map Appendix ALittle Hunting Creek Sewer Sag

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LegendLHCSS Pipe

Sewershed for LHCSS Pipe! Manhole

Sanitary Sewer

Building

Property Boundary

Pavement

Water0 410 820205Feet

0 21Miles

APPENDIX B

Conceptual Pipe Profile

APPENDIX C

Floodplain and Resource Protection Area Map

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Little Hunting Creek

CULPEPERRD

SEVORLN

DOEG

INDIAN

CTDUXBURY PL

THOMAS J STOCKTON PKWY

RYEGATE LN

TURBRIDGE LN

WATERFORD RD

LONDONDERRY RD

BRADGATERD

WOODLAND LN

±

0 220 440110Feet

Legend! Sewer Manhole

LINETYPEFM

GR

Water

Floodplain

RPA

Building

Appendix C Little Hunting Creek Sewer Sag

Resource Protective Area & Floodplain Map

APPENDIX D

Alternative Analysis Summary

Appendix DAlternative Analysis Summary

Alternative Pro's Con's Viable Next Steps 1 Pipe Bursting Utilizes the existing alignment

along a known easement. Employs technology suitable for older cast iron pipe.

Does not allow for the pipe to be adequately corrected in profile – pipe could still have sags.

No No further evaluation.

2 Cured in Place Pipe Lining Utilizes the existing alignment along a known easement. Employs a rehabilitation method suitable for a pipe of this diameter and material.

Does not allow for the pipe to be adequately corrected in profile – pipe could still have sags.

No

No further evaluation.

3 Open Cut Method with Coffer Dam

Utilizes a traditional method of new construction. Allows best assessment of bedding of pipe during construction. Reduces the amount of on-shore work in proximity of residences. Provides a long life expectancy.

Requires significant detailed permitting. Requires coffer-damming and related impacts in the creek. May require restoration of impacted wetlands.

Yes Requires exploring permitting challenges. Should be explored and costed-out as a viable alternative for comparison.

4 New Pump Station Recommendation

Eliminates need for work in the creek. Provides abandonment of the old line .

Adds a new pumping station that needs to be maintained. Requires constructing a new small force main as described in sub-alternatives Requires building pump station within HOA property.

Yes Should be explored and costed-out as a viable alternative for comparison May require flow metering to confirm flow and flow peaking quantities Need to consider effects of flow to the downstream pump station.

4a New Pump Station with a New 6-inch Force Main Inside the Existing 12-inch Pipe

Minimizes extensive permitting. Uses existing easement. Eliminates creek crossing construction work

Requires cleaning existing pipe prior to pulling smaller pipe through. Need to determine if sag slopes are too steep to accommodate HDPE pipe.

Yes Re-CCTV existing pipe with elevation readings to make sure pipe can act as a good conduit for a smaller pipe inside.

4b New Pump Station to Tie-In To Existing 30-inch” DIP Force Main on West Side of Creek

Reduces length of force main construction.

Eliminates creek crossing construction work.

Risk of 30” pipe integrity issues to accommodate pipe tapping.

Yes Investigate pipe integrity. If early phase of construction finds pipe is not suitable, direct contractor to alternative options (4a).

4c New Pump Station for a New Force Main on West Side of Street, Tie-In Existing Force Main to Gravity Sewer

Eliminates creek crossing construction work.

Requires community disruption along a significant distance.

No No further evalutaion.

5 Jack and Bore Minimizes any work in the creek Creates options for realignment and depth of alignment.

Too long of a reach for most contracting methods.


6 Horizontal Directional Drilling

Creates options for realignment and depth of alignment. Minimizes any work in the creek.

May require deep bedding to achieve sufficient long-term stability. The new alignment will be a siphon which could have maintenance challenges. Entire 789’ has to be fused ahead of time and requires a long lay-down area for construction HDD permit is rigorous.


7 Leave in Existing Condition Eliminates construction in the creek. No disruption to the community.

Continued maintenance for cleaning and inspection. Asset remains in poor condition and stays on the high-risk list.

No Will be developed only as a baseline alternative for cost comparison

APPENDIX E

Criteria Matrix

Little Hunting Creek SewerFinal BODR

Appendix E Criteria Matrix

Criteria Rating Weight Score CommentsHomeowner Disturbance 4 35% 1.40 Short term disruption. No long term disturbance to residence.

Operation and Maintenance 4 25% 1.00 Minimal to no maintenance for 50+ years.

Capital Cost 3 10% 0.30 Approximately $1.8 million.

Permits 2 15% 0.30 Extensive permitting estimated between 6 - 18 months.

Easements 3 10% 0.30 One easement for relocation of new pipe, all other are temporary construction easements.

Schedule 3 5% 0.15 Expected project construction duration 8 months.

Summary 100% 3.45

Criteria Rating Weight Score Comments

Homeowner Disturbance 2 35% 0.70

Proposed pump station location would block homeowners view of creek. Odor would be evident along with noise of the generator. Pump station maintence will require regular visit from Fairfax County staff.

Operation and Maintenance 2 25% 0.50 The new pump station will require regular maintenance.


Permits 3 15% 0.45 Expected permitting approximately 6 months.

Easements 2 10% 0.20 New easement and property acquisition required for new pump station and pump station access.

Schedule 3 5% 0.15 Expected project construction duration 8 months.

Summary 100% 2.40

Criteria Rating Weight Score Comments

Homeowner Disturbance 2 35% 0.70

Proposed PS location would block homeowners view of creek. Odor would be evident along with noise of the generator. Pump station maintence will require regular visit from Fairfax County staff.

Operation and Maintenance 2 25% 0.50

The new pump station will require regular maintenance. Historic grease issues suggest cleaning may be required more frequently. The check valve to the 30" force main could require extra maintenance.


Permits 4 15% 0.60 Expected permitting approximately 6 months.

Easements 2 10% 0.20 New easement and property acquisition required for new pump station and pump station access.

Schedule 3 5% 0.15 Expect project construction duration 8 months.

Summary 100% 2.55

Rating: Based on a scale of 1 (poor) - 5 (very good) is an engineering judgement of impact the project will have on the criteriaWeight: weighting is an indication of the criteria's relative importance to Fairfax County

Criteria Definitions:Homeowner Since all construction will be very disruptive to community, this criteria focuses on disturbance post construction.Maintenance Routine inspections/maintenance. PS: routine visits, normal ware and tear, O&M cost.Capital Cost Cost of construction, engineering services, regulatory fees, property/easment acquisition.Permits Number of permits, duration to obtain permits.Easements If new easements are required and difficulty, time and cost to obtain.Schedule Design, public outreach, easement acquisition, bidding and construction duration.

Alternative 3: Open Cut Method with Coffer Dam

Alternative 4A: New Pump Station for a New 6-inch Force Main inside Existing 12-inch Pipe

Alternative 4B: New Pump Station to Tie In to Existing 30" DIP Force Main on West Side of Creek

Score = Weight x Rating : It is a combination of Fairfax County priorities and engineering judgement

\\arcadis-us.com\OfficeData\Arlington-VA\projects\2779 - Fairfax County\2017 WW BOA\AR17-02 Little Hunting Creek Sewer Sag\8 - Deliverables\Final BODR\E - Criteria Matrix (final MMP) 1/1

APPENDIX F

Alternative Site Layouts

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Little Hunting Creek

SEVO

RLN

CULPEPERRD


TURBRIDGE LN

RYEGATE LN

LONDONDERRY RD

WATERFORD RD

WOODLAND LN

±

0 180 36090Feet

Legend!H Project Manhole

Proposed New Sewer

! Sewer Manhole

LINETYPEForcemain

Gravity

Building

Water

Parcel Boundary

Alternative 3Open Cut Method with Coffer Dam

Appendix FLittle Hunting Creek Sewer Sag

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TURBRIDGE

LN

RYEGATE LN

WATERFORD RD

WOODLAND LN

6-in HDPE

±

0 125 25062.5Feet

Legend!H Project Manhole

Connection to PS

Proposed Smaller FM

Existing Sewer

Proposed PS

! Sewer Manhole

LINETYPEFM

GR

Building

Water

Alternative 4A New Pump Station for a New 6-inch Force Main Inside the Existing 12-inch Pipe


Proposed Pump Station

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ÍB

WOODLAND LN

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0 50 10025Feet

LegendÍB Valve

!H Project Manhole

abandon_pipe

New PS Connection

Proposed PS! Sewer Manhole

Tie into FM

LINETYPEFM

GR

Building

Water

Alternative 4BNew Pump Station Tie In to Existing 30" Ductile Iron Pipe Force Main on West Side of Creek


APPENDIX G

Cost Estimate and Present Worth Analysis

Little Hunting Creek Sewer95% BODR Draft

Appendix GCost Estimates

Item Alt 3 - Gravity SewerAlt 4A - Pump Station

and Force Main Within Exist. 12-inch Sewer

Alt 4B - Pump Station and Force Main

Connected to Exist. Force Main

Site Work & Restoration $37,450 $123,599 $122,259

Concrete / Wet Well / MH / Piers $52,480 $104,184 $103,048

Force Main / Gravity Sewer $615,350 $43,880 -

Cofferdam & Dewatering (Creek Crossing) $557,980 - -

PS - Pump Package - $132,946 $131,506

PS - Mechanical - $135,651 $180,046

PS - Fencing - $10,758 $10,640

PS - Landscaping - $10,757 $10,640

PS - Electrical & SCADA - $161,356 $159,607

PS - Building - $190,638 $188,572

Subtotal Construction $1,263,259 $913,769 $906,320

Construction Contingency (20%) $252,652 $182,754 $181,264

Total Construction $1,515,910 $1,096,523 $1,087,584

Engineering Services $428,000 $180,000 $180,000

Wetland Mitigation Cost $50,000 - -

Easement & Property Acquisitions $25,000 $40,000 $40,000

Total Project Cost $2,018,910 $1,316,523 $1,307,584

Prepared by: C. Matthews, M. Pacini, D. Hamilton

Opinion of Probable Construction Cost

Notes:1. Cost estimate based on 2017 economy (no escalation).

\\arcadis-us.com\OfficeData\Arlington-VA\projects\2779 - Fairfax County\2017 WW BOA\AR17-02 Little Hunting Creek Sewer Sag\8 - Deliverables\Final BODR\G - Cost Opinions and Present Worth Analysis 1/5


Appendix GPresent Worth Analysis

Alternative Alt 3 - Gravity SewerAlt 4A - Pump Station

and Force Main Within Exist. 12-inch Sewer

Alt 4B - Pump Station and Force Main

Connected to Exist. Force Main

Initial Project Cost $2,018,910 $1,316,523 $1,307,584

Replacement Cost $0 $31,639 $31,639

Annual O&M Cost $0 $1,385,136 $1,385,136

Non-Annual O&M Cost $16,477 $34,299 $34,299

Salvage Value $0 $149,583 $149,583

Present Worth Value $2,035,388 $2,618,014 $2,609,075

Formulas and Assumptions:1. The present value PV given a future cost F is:

n=number of interest periodsi=interest rate

2. The present value PV given O&M costs recurring annuity is:n=number of interest periodsi=interest rate

3. Energy consumption is calculated based on the following formula:HP = horsepower Q = pump flow (gpm)TDH = total dynamic headeff = pump efficiencykW-h = kilowatt hours HP-h = horsepower hours

4. Electricity cost of $0.08 per kilowatt-hour is based on input from Fairfax County.

9. The study period of 50 years is based on Fairfax County's preference.

Prepared by: M. Pacini, D. Hamilton

Present Worth Analysis Summary

6. Interest rate 'i' is based on current national municipal bond rates for a 30-year maturity period.7. Sewer cleaning cost is based on common per diem rates for jet-vac and CCTV equipment. Cost is based on dedicating a full day to cleaning only this pipe segment. Cleaning this pipe segment as part of a larger quantity of pipe to be cleaned would result in a lower unit cost for this item.8. Salvage value calculations are based on straight line depreciation and zero value at the end of the useful life.

5. Pump parameters are assumed to be: flow is 440 gpm; efficiency = 50% for submersible sewer pumps; and run time is 6 hours per day.

PV|F= F x __1___( 1+i )ⁿ

PV|A= A x ( 1+ i )ⁿ - 1_ i ( 1+i )ⁿ

HP = Q*TDH kW-h = HP-h * 0.74573,960*eff




Initial Cost

Construction/Engineering/Land $2,018,910

Replacement Cost Year Present Cost Present Value

None - - -

Annual O&M Cost Annual Cost Present Value

None - - -

Non-Annual O&M Cost Year Present Cost Present Value

Gravity Pipe Cleaning 5 $3,400 $2,933








Gravity Pipe Cleaning 45 $3,400 $899

Gravity Pipe Cleaning 50 $3,400 $776

Residual Value Life Initial Value Value at50 Years

Gravity pipe 50 $1,515,910 $0

Total Present Worth $2,035,388

Interest Rate (i) 3.00%Life (yrs) 50

PW = initial + replacement + annual O&M + non-annual O&M - salvage

Prepared by: M. Pacini, D. Hamilton

Present Worth Analysis - Alternative 3




Initial Cost



Pumps 15 $24,000 $15,405

Pumps 30 $24,000 $9,888

Pumps 45 $24,000 $6,347


County staff labor $48,000 $1,235,029

Generator fuel $5,250 $135,081

Electricity $584 $15,026


Pump bearings and seals 10 $14,000 $10,417

Pump impellers 20 $5,000 $2,768

Miscellaneous 20 $10,000 $5,537





Salvage Value Usefule Life Initial Value Value at50 Years

Building 80 $200,000 $75,000

Concrete structures 80 $110,000 $41,250

Generator 50 $40,000 $0

Pumps (3rd replacement) 15 $50,000 $33,333



PW = initial + replacement + annual O&M + non-annual O&M - salvageElectricity Cost (kWh) $0.08

Energy consumption (kWh/day) 20Prepared by: M. Pacini, D. Hamilton

Present Worth Analysis - Alternative 4A




Initial Cost



Pumps 15 $24,000 $15,405

Pumps 30 $24,000 $9,888

Pumps 45 $24,000 $6,347


County staff labor $48,000 $1,235,029

Generator fuel $5,250 $135,081

Electricity $584 $15,026









Salvage Value Usefule Life Initial Value Value at50 Years

Building 80 $200,000 $75,000

Concrete structures 80 $110,000 $41,250

Generator 50 $40,000 $0

Pumps (3rd replacement) 15 $50,000 $33,333



PW = initial + replacement + annual O&M + non-annual O&M - salvageElectricity Cost (kWh) $0.08

Energy consumption (kWh/day) 20Prepared by: M. Pacini, D. Hamilton

Present Worth Analysis - Alternative 4B


APPENDIX H

Preliminary Project Schedule

ID Task Name Duration Start Finish

1 Design and Regulatory Permitting 494 days Fri 12/1/17 Thu 4/11/192 Design Memo 165 days Fri 12/1/17 Tue 5/15/183 NWP 6 application (for soil bores) 90 days Sat 12/16/17 Fri 3/16/184 WSSI delineations and surveys 60 days Sat 12/16/17 Wed 2/14/185 JPA development 30 days Thu 2/15/18 Fri 3/16/186 Soil bores and report 45 days Sat 3/17/18 Mon 4/30/187 SAV Survey (if required - assumes SAV

not required for soil bores)124 days Tue 5/15/18 Sat 9/15/18

8 DM County review & revisions 14 days Wed 5/16/18 Tue 5/29/189 35% Design 90 days Wed 5/16/18 Mon 8/13/1810 35% design review 7 days Tue 8/14/18 Mon 8/20/1811 70% Design 75 days Tue 8/21/18 Sat 11/3/1812 JPA submittal and review 120 days Sun 11/4/18 Tue 3/5/1913 70% design review 7 days Sun 11/4/18 Sat 11/10/1814 95% Design 75 days Sun 11/11/18 Sat 1/26/1915 VPDES/SWPPP application/review 45 days Sun 1/27/19 Tue 3/12/1916 VDOT permitting 45 days Sun 1/27/19 Tue 3/12/1917 Fairfax County E&SC permitting 45 days Sun 1/27/19 Tue 3/12/1918 DEQ CTC permitting 45 days Sun 1/27/19 Tue 3/12/1919 95% design review 7 days Sun 1/27/19 Sat 2/2/1920 Bid ready documents 30 days Wed 3/13/19 Thu 4/11/1921 Bid/Negotiate Construction 41 days Fri 4/12/19 Wed 5/22/1922 Bid documents reviewed 20 days Fri 4/12/19 Wed 5/1/1923 Receive and review bids 20 days Thu 5/2/19 Tue 5/21/1924 Notice to proceed 1 day Wed 5/22/19 Wed 5/22/1925 Public Outreach 174 days Tue 8/14/18 Tue 2/5/1926 Early notification 10 days Tue 8/14/18 Thu 8/23/1827 Design and alignment details 10 days Sun 1/27/19 Tue 2/5/1928 Construction Phase 271 days Fri 2/15/19 Tue 11/12/1929 Submittals, reviews and mobilization 30 days Thu 5/23/19 Fri 6/21/1930 Time of Year Restriction (stream activities) 136 days Fri 2/15/19 Sun 6/30/19

31 Phase 1 cofferdam install 15 days Mon 7/1/19 Mon 7/15/1932 Phase 1 gravity sewer installation 30 days Tue 7/16/19 Wed 8/14/1933 Phase 1 gravity sewer testing 2 days Thu 8/15/19 Fri 8/16/1934 Phase 1 cofferdam removal 15 days Thu 8/15/19 Thu 8/29/1935 Phase 2 cofferdam install 15 days Fri 8/30/19 Fri 9/13/1936 Phase 2 gravity sewer intallation 30 days Sat 9/14/19 Sun 10/13/1937 Phase 2 gravity sewer testing 2 days Mon 10/14/19 Tue 10/15/1938 Phase 2 cofferdam removal 15 days Mon 10/14/19 Mon 10/28/1939 Restoration and punchlist 15 days Tue 10/29/19 Tue 11/12/19

5/22

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec2018 2019

Task

Split

Milestone

Summary

Project Summary

External Tasks

External Milestone

Inactive Task

Inactive Milestone

Inactive Summary

Manual Task

Duration-only

Manual Summary Rollup

Manual Summary

Start-only

Finish-only

Progress

Deadline

Page 1

Project: Little Hunting Creek SewerDate: Wed 10/25/17

Arcadis U.S., Inc.

11350 Random Hills Road

Suites 861 & 862

Fairfax, Virginia 22030

Tel 703 351 9100

Fax 703 351 1305

www.arcadis.com

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