PennDOT Collaborates with Universities Along I-95 in Philadelphia to Understand SMP Maintenance, Lam and Henery, B51

Transcription

1 PennDOT Collaborates with Universities Along I-95 in Philadelphia to Understand SMP Maintenance, Lam and Henery, B51 Edwina Lam P.E., Conshohocken, PA Eric Henery P.E., Conshohocken, PA Project History Interstate 95 was constructed in the 1960 s as part of the massive Interstate Highway System initiated by President Dwight D. Eisenhower. The freeway stretches from Maine to Florida and serves as the primary north-south route on the American east coast. Within the City of Philadelphia, I-95 follows the alignment of the Delaware River, traversing over 23 miles and carrying significant traffic volume in both the northbound and southbound directions. As the highway completed its third decade in the 1990s, the Pennsylvania Department of Transportation (PennDOT) began planning and scheduling the upcoming reconstruction of the facility. The I-95 Girard Avenue Interchange was identified as one of the highest priority areas for reconstruction, due to the age of the highway and the significant percentage of deteriorating bridge structure. This specific section was featured during a segment of the CBS news program 60 Minutes, in which then-governor Ed Rendell told the story of the discovery of the structure deficiency, evaluation, and rehabilitation. The Girard Avenue Interchange section extends for approximately three miles from Race Street to Allegheny Avenue and includes complete reconstruction of the interstate, as well as reconfiguration of the interchange ramps and associated surface streets. This stretch of I-95 is one of the most heavily traveled interstates in Pennsylvania, carrying over 200,000 vehicles per day, and the interchange provides access to and from four major arterials: Girard Avenue, Delaware Avenue, Aramingo Avenue, and Richmond Street. Locally, I-95 is perceived as a barrier between Philadelphia s Riverward neighborhoods and the Delaware River and has become a focus of development and waterfront planning efforts. In addition to the roadway improvement effort, PennDOT is also focusing on enhancing the underpasses within the project area with improved lighting, landscaping, and signage to reconnect the neighborhood to the waterfront. Rendering of the proposed condition of the Girard Avenue Interchange area

2 Since the total estimated construction cost for the entire reconstruction of the I-95 Girard Avenue Interchange (I-95 GIR) is over one billion dollars, PennDOT separated the project into six individual construction contract sections. Phase 2, commonly referred to as Section GR2, is the first mainline construction section completed and, at just over 1,200 feet in length, it is the smallest construction segment. It was built in advance to use as a traffic crossover between reconstruction of the long highway viaduct structures to the north and south during future sections. Despite its small size, this section proved to be a microcosm of the urban stormwater design challenges that will be faced over the entire length of I-95 in Philadelphia. Section GR2 extends from Frankford Avenue to Palmer Street in the Fishtown neighborhood. Prior to reconstruction, it was built on embankment with three single-span bridges over city streets. To avoid taking right-of-way from the adjacent neighborhood, the sloped embankments were replaced with retaining walls. This configuration provided space for wider shoulders and continuous auxiliary lanes to increase safety and accommodate projected traffic volumes. Unfortunately, this approach left very narrow slivers of right-of-way remaining to use for a stormwater management system capable of meeting Philadelphia Water Department (PWD) and Pennsylvania Department of Environmental Protection (PADEP) requirements and regulations. The dense urban environment surrounding the project and close proximity to adjacent homes and businesses created additional challenges for the design team. The future maintenance of the facility was also a concern and required realistic assessments of the maintenance capabilities of PennDOT and other entities. The final design solution for the many stormwater challenges in Section GR2 was a series of 10 small, shallow bioswales along the project s retaining walls and embankment that are landscaped for both stormwater function and to improve the appearance of the highway. Rendering of Phase 2 stormwater management with integrated landscape Prior to the reconstruction of I-95, there were no existing stormwater management facilities to control or treat the roadway runoff. All highway runoff was discharged to the PWD combined sewer overflow (CSO) system which routed storm and sanitary flows to the local water treatment plants during dry

3 weather and smaller storms. During larger rain events, these systems overflowed directly into the adjacent waterways, which in the case of Section GR2 is the Delaware River. All I-95 reconstruction projects within Philadelphia are required to meet PADEP National Pollution Discharge Elimination Systems (NPDES) Phase II permit requirements and PWD stormwater management requirements. Due to the limited CSO capacity and mandates from the Environmental Protection Agency (EPA), PWD is considering separation of storm and sanitary flows and stormwater treatment as part of any proposed project in the city, including PennDOT s I-95 GIR reconstruction project. Design Requirement Since the project design and construction schedules span more than twenty years, stormwater regulations have significantly changed during the design phase, resulting in some of the later phases falling under different requirements than the early phases. The early project sections, including Section GR2, were initially required to treat the first 1 of runoff from the I-95 mainline and ramps for water quality and quantity using stormwater management practice (SMP) devices. This requirement was recently increased to the first 1.5 of runoff for the later design phases. While infiltration for groundwater recharge is also encouraged, it is not feasible in some locations. For example, impervious liners are installed in SMPs with closely adjacent basements. These facilities are exempted from the volume reduction requirement but must still meet the water quality requirement. Subsequent phases of the I-95 GIR project are able to separate some or all of the stormwater runoff from the combined sewer system because the scopes of work involve major reconstruction of the infrastructure between I-95 and the Delaware River, providing more feasible direct discharge connections to the river. Section GR2 did not have feasible direct discharge locations, so the project outfalls connect to the existing combined sewer systems upstream of the CSO intercepting chambers. Due to this limitation, the project was required to provide water quality treatment as well as peak rate control at each connection. Peak rate control requires the post-development peak flow rate for the 2- year storm to be reduced to match the pre-development peak flow rate for the 1-year storm. The postdevelopment peak flow rates for 5-, 10-, and 100-year storms also need to be equal to or less than the equivalent pre-development storms. In addition, there is a release rate requirement of 0.24 cubic feet per second per acre applicable to Section GR2. This requirement was tightened recently to 0.15 cubic feet per second per acre for the later phases to avoid overburdening the CSO system and ultimately the water treatment plants. PWD also prefers a 10:1 loading ratio for all SMP devices; however, a higher loading ratio can be justified if pretreatment of the runoff is provided prior to entering the SMP devices. In the I-95 GIR project, stilling basins and brick paver forebays are used to collect sediment that would otherwise clog the infiltration system in the main basin. To ensure infiltration occurs, each SMP has a 24 to 42 deep bottom layer of engineered amended soil. If the existing soils below the SMPs amended soils do not infiltrate as expected, each basin has an underdrain that can be uncapped to provide an outlet for trapped runoff as an additional protection against standing water. The underdrain will be operating initially to allow the plants to establish but is intended to be capped to promote infiltration to the water table following plant establishment. As part of an unprecedented planning and outreach program driven by PennDOT, the design team was able to engage the neighboring communities to tailor the features surrounding the highway to fit community needs. The landscaping component of the stormwater management design was one of many elements that were guided by the public design charrettes and near neighbors meetings. In many instances, the planted portions of the right-of-way function as a backyard for the highway s neighbors, providing aesthetic enhancement, a visual buffer, noise attenuation, and carbon sequestration. For the

4 entire project corridor, the design team conducted an in-depth sun/shade analysis, which informed the locations of planting zones. Areas with enough sunlight were celebrated with a carefully selected planting palette chosen for seasonal interest, tolerance to urban conditions, low maintenance requirements, and where necessary, their suitability for stormwater management. Within the SMP devices, the landscape provides the added benefit of improved water quality. As runoff is captured in the SMP, the planted material acts as a filter for suspended solids. The plants and their network of roots preserve the infiltration capacity of the amended and existing soils and continue to provide cleaning power through the uptake of pollutants and nutrients. Dense landscaping within the SMP devices providing the added benefit of improved water quality Construction on Section GR2 was completed at the end of 2015 and the bioswales have been operating for nearly two years. The facilities are generally functioning well, but there have been a share of challenges along the way. The design team has used this experience to drive the SMP design for the remainder of the I-95 GIR project, which will utilize over 70 SMP devices, including rain gardens, bioswales, and bioinfiltration basins.

5 Rendering of large scale stormwater management in the Girard Avenue Interchange with integrated landscape Lessons Learned The stormwater management design for the overall I-95 GIR project will incorporate many lessons learned from the Section GR2 bioswales. There were two major lessons learned in this construction section. First, the staging for the construction of the bioswales is critical and must be detailed explicitly in the contract documents. Heavy construction traffic and any foot traffic within the footprint of the bioswales compress the underlying soils and hinder infiltration. As an example, one of the proposed embankment slopes was proposed to be landscaped with trees and shrubs before the excavation of the SMP. Instead, the contractor installed the plants on the embankment after the excavation of the SMP, resulting in foot traffic and equipment traveling across the SMP while they installed the planting on the embankment. As a result, the amended soil was compacted which jeopardized infiltration capabilities and causing standing water to occur in the SMP after the overall landscaping was completed. To avoid this problem, the designer must consider adjacent construction tasks and the placement of these tasks in the construction schedule. Another challenge that was encountered was the stilling basins, which were designed as pre-treatment to manage the runoff from the highway. One of the stilling basins initially struggled to withstand the velocity of the flow coming down from the I-95 viaduct. The interstate is elevated about 15 to 20 feet above the surface streets so the runoff is dropped vertically in the storm sewer system along the retaining wall before hitting a 90 degree elbow and flowing out into the stilling basin. The volume and resulting velocity displaced the stones in the stilling basin. The solution was to increase the size of the stone to adequately protect the SMP and reduce the inflow velocity and to extend an apron of stone to eliminate erosion problem as the transition area to the amended soils. In addition, some of the stilling basins will be replaced to forebays in later construction phases to ensure that this erosion problem does not occur again. The stilling basins require regular maintenance to keep the accumulation of sands, cigarette butts, and other expected debris from entering the SMPs and hindering infiltration. In addition to informing the future design of the Girard Avenue Interchange, the lessons learned are also communicated to the design consultants for other I-95 reconstruction projects during regular

6 coordination meetings. A consistent design review team and a comprehensive set of corridor design guidelines also ensure that experience gained during the early stages of construction is used throughout the I-95 corridor in Philadelphia. University Monitoring Program PennDOT is partnering with local universities, Temple University and Villanova University, to establish a monitoring program for some selected Section GR2 SMP devices. The program was initiated after the construction was completed. The universities began work in 2016 to monitor and analyze the function of the bioswales constructed under Section GR2 with respect to water quality and quantity treatment. The ultimate goal for this program is to help PennDOT determine the maintenance procedures required and also estimate the life expectancy of the SMP devices. In addition, PennDOT and the universities are looking at the current design requirements as compared to the performance of the SMP devices to determine if the SMPs are over-designed or under-designed. The result of the study could be used to guide future design standards for landscaping and stormwater management related to transportation facilities in Philadelphia and the entire Commonwealth of Pennsylvania. Three Weather Stations (Top: 50 feet above the surface street level, Bottom left: next to outlet structure, Bottom Right: near SMP)

7 The universities have a several research topics that are being progressed through this monitoring program. Their research topics include the SMP design performance from a water quantity and quality analysis as runoff routes through the SMP, and as well as maintenance requirement for the SMP to optimize their performance. They are also interested in the background conditions as baseline monitoring to evaluate the pre and post development conditions. This monitoring program will provide an opportunity to understand the relationship between the urban environment and the SMP locations and design. The universities installed some major monitoring equipment to analyze the meteorological and hydrological conditions throughout the SMPs. Several weather stations were installed to evaluate meteorological conditions in some of the SMPs, as well as above the I-95 highways. For hydrological monitoring, several area velocity sensors were installed at inflow points to collect the proposed runoff going into the SMPs, pressure transducers were installed to measure the ponding within the SMPs, soil moisture sensors were installed to investigate the sub-surface storage, and weirs were installed in the outlet structures to monitor the outflow to the combined sewers. They have also installed groundwater wells within the SMP to monitor the groundwater s lateral movement throughout the SMPs. Additionally, baseline groundwater monitoring wells were also installed in order to establish a baseline model for comparison with future data. Additional monitoring equipment has been installed to monitor various aspects in order to conduct a full scale analysis for the functionality of the SMPs. Monitoring Wells for Groundwater

8 Inflow Meters Installation of Soil Moisture Sensors

9 In the future construction phases (Sections GR3 and GR4), design modifications have been made to accommodate future research needs, such as revising the outlet structure to incorporate sharp-crested weirs so that it will avoid obstruction and make it easier to measure the outflow accurately. Openings have also been included in the outlet structures to allow future equipment conduit to be connected to collection devices. The biggest modification in the future construction phase is the amended soil mix. In the original design, this material is a uniform mixture used in all of the bioswales. The universities are interested in testing several different types of amended soil mixtures at various testing sites to evaluate which mix will optimize performance and benefit the environment for the SMP in an urban setting. Various mixes will include the comparison of adding more clay into the amended soil mix creating a loamy-sand mixture. Also, some mixes will include additives, such as aluminum oxide and biochar, in order to evaluate the removal of specific pollutants that are typically generated from highway runoff and also to investigate the effects on the plant life. Proposed plantings have also been modified in the future phases to incorporate landscaping materials that have performed well in Section GR2. Researchers from Temple University have been conducting plant life analyses by taking inventory of the plants and monitoring their health in each SMP and have worked closely with the project landscape architects to identify the reasons that plants are failing in certain situations. This information has also been used to inform the proposed plantings for the future design phases. An additional focus of the university monitoring program is evaluating the effects of evapotranspiration provided by the plantings. Despite being an important factor, there is no design credit in many states to account for the evapotranspiration process. Villanova University has researched this topic in different applications throughout their university campus. The researchers are planning to apply the same approach to the I-95 GIR project site. The intent will be to demonstrate that the SMPs are significantly larger than needed for the majority of storm events and possibly influence the regulators to scale back the design requirements. Reducing the SMP footprint would significantly reduce the construction and maintenance costs for transportation projects. For more information on PennDOT s I-95 reconstruction, please visit Author Biographies Edwina Lam is the design consultant s Water Resource Task Lead for the I-95 Girard Avenue Interchange Reconstruction Project and I-95 Central and South Philadelphia Reconstruction Project. She is based in AECOM s Conshohocken, PA office. Eric Henery is the design consultant s Water Resource Project Engineer for the I-95 Girard Avenue Interchange Reconstruction Project and is based in AECOM s Conshohocken, PA office.