Memorandum. PROJECT: Postmark Emergency Storm Repair Alternative Analysis

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1 Memorandum CRW Engineering Group, LLC 3940 Arctic Blvd. Suite 300 Anchorage, AK (907) FAX TO: Mike Lee, P.E., Chief Engineer, ADOT/ANC FROM: Shane Blanchard, P.E. THRU: Mike Rabe, P.E. DATE: August 28, 2009 PROJECT: Postmark Emergency Storm Repair Alternative Analysis PROJECT DESCRIPTION ANC requested CRW to prepare storm drain alternatives to repair the failing storm pipe from the Taxiway V headwall at Gate N10 to approximately 1100 feet north of DeHavilland Avenue. The Postmark storm drain system is a vital conduit for storm flows for ANC as it receives runoff from the North Terminal area, Postmark and DeHavilland roadways. This memo presents a description of the pipe failure, the alternatives and construction cost estimates along with recommendations. EXISTING PIPE CONDITIONS In the summer of 2008 ANC Field Maintenance performed an emergency repair on a collapsed portion of the Postmark storm drain system. After the repair Field Maintenance requested the remaining sections be inspected. In October 2008 ANC contracted CRW to perform an assessment of three storm drain systems. The three systems were inspected and the report Ron 12 to 14, Runway 7R, & Postmark Drive Storm Pipe and Manhole Assessment was published in December The Postmark Drive system showed signs of pipe deformation and joint separation in the plastic pipe installed in In June 2009 the report North Tug Road and Taxiway Victor Storm Sewer Pipe and Manhole Condition Assessment Report investigated the storm drain system exiting the Taxiway V headwall for a distance of 240 feet towards Lake Hood. This system, normally closed by a sluice gate at the headwall, is showing structural failure and pipe deformation.

2 Postmark Emergency Repair Page 2 of 4 The geotechnical report Storm Drain Emergency Repairs - ANC East Airpark Phase 2 was prepared to asses possible causes of the failure and give recommendations. Possible causes of the pipe failure are: improper bedding materials not implementing the design requirements for different native soil conditions improper pipe joint sealing unsatisfactory bedding compaction overloading with unanticipated fill (snow or vehicle traffic) REPAIR RECOMMENDATIONS The geotechnical report investigated three pipe materials for use in the repair corrugated metal pipe (CMP), steel pipe, and reinforced concrete pipe (RCP). The materials were evaluated against a variety of factors, including ease of installation, impact to existing ANC facilities, and hydrological efficiency. Of these three, RCP best met the needs of the project and are presented as the new pipe material in all alternatives. Figure 1 illustrates a typical RCP installation. ROUTE ALTERNATIVES Alternatives 1 and 2 were previously presented to ANC. During a review meeting in February 2008 ANC requested 2 more alternatives be investigated to remove conflicts with the controlled access point to the AOA at Gate N10. After the previous May 2009 Technical Memorandum the storm drain system exiting the headwall eastward towards Lake Hood was inspected, and afterwards created a 5 th alternative. A description of each alternative is presented below. See Figure 2 for plan view layout of the individual alternative routes. Alternative 1 This alternative is simply a replace in kind - the existing plastic pipe is replaced by an equivalently sized RCP pipe. No change in grade is made; the RCP pipes are installed at the same depth and slope as the existing pipe. No change to the existing dual 36 inch CMP section under DeHavilland. This option has the advantage keeping the Lake Hood discharge line and not needing to reconstruct 850 feet of sanitary sewer. Disadvantages would be no increase in pipe slope, impacts to the C Parking area and the Gate N10 operations. Alternative 2 This alternative uses the same alignment as Alternative 1, but increases the overall pipe slope. The pipe sizes are also increased to 48-inches from the Taxiway V manhole to DeHavilland for the purpose of potentially being the tie-in point for the future East Airpark force main.

3 Postmark Emergency Repair Page 3 of 4 Advantages would be an increase in the hydrologic efficiency of the system and capable of handling the future East Airpark flows, along with keeping the Lake Hood discharge line. Disadvantages are the need to reconstruct 850 feet of sanitary sewer main, impacts to the C Parking area and the guard post operation. Additionally, the existing fuel line crossing the Tug Road and paralleling Taxiway V would be impacted and require adjustment/relocation. Alternative 3 This alternative consists of using a portion the existing 48 inch CMP line exiting the Taxiway V headwall that discharges to Lake Hood. This line is normally closed at the headwall. A manhole would be installed near Postmark Drive and new 48 inch RCP would run northwest along Postmark Drive to DeHavilland, where it would follow the alignment of the previous alternatives. Due to the increased alignment length and slope within the existing 48 inch CMP, the new RCP pipe would have an unsatisfactory pipe slope and was not investigated any further. Additionally, after the June 2009 inspection is was discovered that this line is failing within the lengths investigated. Finally, discharge to Lake Hood would be eliminated. Alternative 4 This alternative uses a portion of the existing 48 inch CMP exiting the headwall to both clear the existing fuel line and not impact the operations of Gate N10. A manhole is installed approximately 150 feet east of the headwall and connected to the existing CMP. New 48 inch RCP crosses Taxiway V and follows the same alignment as alternatives 1 and 2. Advantages of this alternative are increased pipe slopes, no impacts to the existing fuel line and no impacts to the operation of Gate N10. This alternative would require the reconstruction of 850 feet sanitary sewer main and have impacts to the C Parking area and the connection to Lake Hood would be eliminated. Additionally, the June 2009 inspection showed the existing 48 inch CMP to be structurally failing. Alternative 5 Similar to Alternative 4 with the addition of replacing a portion of the existing 48 CMP with RCP and slip-lining the failing 48 CPEP pipe under the Gate N10 roadway. A manhole is installed approximately 70 feet east of the headwall. New 48 inch RCP crosses Taxiway V and follows the same alignment as alternatives 1 and 2. Advantages of this alternative are increased pipe slopes and limited impacts to the operation of Gate N10. This alternative would require the reconstruction of 850 feet sanitary sewer main, have impacts to the C Parking area and the existing fuel lines, and the connection to Lake Hood would be eliminated. Plan and profile drawings for alternatives 1,2, & 5 are included.

4 Postmark Emergency Repair Page 4 of 4 COST ESTIMATES The follow table gives the estimated construction costs of the four individual alternatives. ESTIMATED ALTERNATIVE CONSTRUCTION COSTS Segment Construction Cost* Description Alternative 1 $2,100,000 Replace pipe only Alternative 2 $2,660,000 Replace pipe, increase slope Alternative 3 Not Evaluated Reroute around Postmark Alternative 4 $2,640,000 Alternative 5 $2,740,000 Use existing CMP, replace pipe, increase slope Slip-line under Gate N10 roadway, replace pipes, increase slope *Does not include cost to relocate/adjust existing fuel line. RECOMMENDATIONS In the previous May 2009 Technical Memorandum, Alternative 4 was the recommended alternative. Two issues arose that have changes this recommendation: The existing 48 CMP is failing ANC would like to maintain a connection to Lake Hood for emergency storm flows For these reasons, Alternative 2 is the preferred alternative. By increasing the pipe slope the efficiency of the system is improved and removes the constriction caused by the dual 36 CMP pipes at DeHavilland. Clearance from the fuel lines is minimal (< 1 ), which will require coordination with ASIG (see Figure 3). Detour routes will need to be provided for traffic using the Tug Road and Taxiway V while construction activities occur with the Taxiway. As for the failing 48 CMP discharging to Lake Hood, on August 10, 2009 during a review meeting on this project ANC personnel indicated that keeping system in its current state is an acceptable risk and would be corrected at a later date. Even so, the segment under the Gate N10 roadway from the headwall to the first manhole should be corrected by slip-lining or other trenchless repair to ensure that the Gate N10 roadway is not compromised.

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7 Location.dwg CRW ENGINEERING GROUP STATE OF ALASKA DEPARTMENT OF TRANSPORTATION AND PUBLIC FACILITIES Ted Stevens Anchorage International Airport ANC EMERGENCY STORM DRAIN REPAIR TECHNICAL MEMORANDUM EXISTING FUEL LINE IMPACTS SHEET FIG 2 3of 4

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