Design and construction challenges for the Ellicott City North Tunnel
The major flash floods of 2016 and 2018 inundated the downtown area of Ellicott City, a historic district in Howard County, MD, situated in the hills above the Patapsco River. These catastrophic floods resulted in significant property damage and, tragically, loss of life. Approximately 231 m3/s (8,170 cu ft/s (2016) and 335 m3/s (11,860 cu ft/s) (2018) surged through Main Street in Ellicott City, causing millions of dollars in property damage and the tragic loss of life (Doheny and Nealen, 2021). As of October 2021, the county has developed and is implementing a collective flood mitigation plan, known as the Safe and Sound Plan, with nine unique projects, two of which are under construction and four more are in various stages of design. The selected plan option, which includes the North Tunnel, will reduce the anticipated flood level from a 100-year event along lower Main Street from approximately several feet to less than 0.3 m (1 ft).
One major component of this Safe and Sound Plan is the Ellicott City North Tunnel, a stormwater conveyance tunnel that will capture stormwater flows from the upland watershed to be diverted directly to the Patapsco River east and downstream of Ellicott City. The proposed tunnel will be ~1,770 m (5,800 ft) long, with a minimum internal diameter of 4.5 m (15 ft). It is currently envisioned that the tunnel will slope between 0.1 percent and 0.5 percent from the inflow drop shaft adjacent to Frederick Road to the eastern outfall structure at Lot B. An intermediate diversion structure with a drop shaft is anticipated at Lot F, approximately 1,220 m (4,000 ft) along the alignment. The final design is anticipated to be complete by Q4 2022. Construction is anticipated to begin in 2023. This article describes the proposed tunnel and stormwater capture system. In addition, the major challenges are identified, and proposed solutions are explored.
The left side of Fig. 1 illustrates the proposed tunnel alignment starting at the mining site on the 8800 block of Frederick Road, running roughly parallel to Frederick Road to the Lot F site at Ellicott Mills Drive, and finally to the outfall site at the Patapsco River. The right side of Fig. 1 depicts the flood mitigation anticipated for the Ellicott City North Tunnel as determined by a hydrologic modeling study conducted as part of the Safe and Sound Plan evaluation.
Project background
Ellicott Mills was founded in 1772 by the Ellicott Brothers, Quakers who ultimately settled in a fertile river valley along the Patapsco River, not far from ports in Baltimore and farms to the west. The Ellicott Brothers eventually came to operate a number of mills along the river, generally between what is now known as Ellicott City and Elkridge, to the south. Ellicott City is a vibrant and rich historic district, with structures dating back to the 1700s. However, the same qualities that appealed to the Ellicott Brothers some 250 years ago — the location at the bottom of a river valley surrounded by steep terrain — present challenges today.
Throughout its history, Main Street and the Ellicott City Historic District have seen at least 15 significant flood events dating back to the 1700s. One noted flood in the 1800s destroyed much of what was originally Ellicott Mills, and spawned construction of most of the district as it is known today. Over the last 10 years, three flood events have affected Ellicott City. Most recently, the community has seen two major flash floods within the last five years. The most recent flash-flood events have been referred to as “top-down” flood events, in which stormwater ran from adjacent topography through the Main Street area. Topdown flooding has occurred in Ellicott City throughout history. These flood events cause significant damage, as the flood waters travel at a high velocity, collecting anything in their path. Storms in 2011, 2016 and 2018 resulted in significant damage to infrastructure and buildings. Although structures have since been repaired or replaced, what cannot be replaced are the lives lost in both the 2016 and 2018 storms.
Upon taking office in late 2018, county executive Calvin Ball announced the Ellicott City Safe and Sound Plan, a major improvement project benefitting the county, which consists of multiple facets, including flood mitigation. Prior efforts were re-evaluated, with a renewed focus on preservation and public safety. In May 2019, the county executive announced that he had selected to proceed with Option 3G7.0, a series of nine projects that collectively seek to mitigate the potential for future top down flash-flooding events.
Most notably, the plan includes the preservation of six buildings originally slated to be demolished, as well as inclusion of the North Tunnel, intended to divert flood waters from the western end of Main Street directly to the Patapsco River. To ensure the plan would meet its intended goals, the county solicited a peer review of the proposed flood mitigation strategies by the U.S. Army Corps of Engineers (USACE). Upon conclusion of their peer review, USACE noted “Overall, the team determined that the county followed a comprehensive process, that the flood risk management (FRM) measures being considered by the county are similar to those utilized during USACE FRM projects, and that the current County-selected alternative can significantly reduce flood risk to downtown Ellicott City” (USACE, 2019).
Since the inception of Option 3G7.0, the county has revised and extended the original proposed North Tunnel project to eliminate two smaller projects and improve the efficacy of the plan. Most notably, the extension of the tunnel precludes the need for demolition or partial demolition of nine residential structures throughout Ellicott City’s West End, which constitute important contributions to the character of the historic district. When completed, the plan should reduce water levels and velocities seen during the 2016 and 2018 storms along Lower Main Street to levels where nonstructural floodproofing of buildings would be effective.
Generally, the flood mitigation projects work as a system to collectively mitigate flash flooding, incorporating both stormwater retention facilities and conveyance system improvements. In order to be most effectively implemented, significant constrictions in the conveyance system need to be alleviated. The Maryland Avenue Culvert project, one of the nine aforementioned components of the Safe and Sound Plan, will provide significant additional stormwater conveyance from the Tiber/Hudson Branch to the Patapsco River while mitigating a significant constriction to water flow. The North Tunnel functions similarly; however, it captures its flow much further upstream. The plan is being primarily developed from the Ellicott City Hydrology/Hydraulic Study and Concept Mitigation Analysis (McCormick Taylor, 2017).
Finally, in conjunction with the flood mitigation projects, the county developed a master plan for Ellicott City and the surrounding watershed: the Ellicott City Watershed Master Plan (Mahan Rykiel et al., 2020). Aside from prescribing policies and implementing actions associated with the flood mitigation plans, the master plan addresses a number of other needs in the watershed, including transportation. This plan
was adopted by the County Council in January 2021.
As of November 2022, one project is substantially complete and one is nearing completion (Q1, 2023). Two others are in the late stages of design while design of a third is fully funded. The county received a Water Infrastructure Financing and Innovation Act (WIFIA) loan from the U.S. Environmental Protection Agency (EPA) to support remaining construction, including the North Tunnel Project.
Geological setting
Ellicott City is within the Piedmont Physiographic Province on the eastern edge of Howard County along the border of Baltimore County. The eastern Piedmont is generally characterized by relatively low, rolling topography, with major streams incised into narrow, steep-sided valleys (Reger and Cleaves, 2008). Thin soil deposits are underlain by igneous and metamorphic rock. The Howard/Baltimore County line is delineated by the Patapsco River, which locally runs south. The city and the project extents are underlain by the Ellicott City Granite of the Silurian Period. Ellicott City Granite is typically characterized as a uniform, medium- to coarsegrained, weakly foliated to massive granite (Edwards, 1993). Inclusions of fine-grained gneiss are common and elongated in the plane of foliation (Crowley and Reinhardt, 1980). The granite body in Ellicott City generally strikes northwest–southeast, discordant to the general strike in the Piedmont region. The granite is an intrusive igneous formation that contacts the Wissahickon schist and Baltimore Gabbro Complex. A characteristic outcrop of the Ellicott City Granite is depicted in Fig. 2, near the proposed tunnel outfall location.
A phased geotechnical investigation was conducted from 2020 to 2022 and consisted of 29 soil borings and rock cores ranging from 28 to 87 m (92 to 284 ft) below ground surface (bgs). The investigation identified topof- rock depths ranging from 1.5 to 46 m (5.0 to 150 ft) bgs. The granite bedrock is typically hard to very hard, slightly weathered to fresh, with very close to wide fracture spacing. A transition zone of highly to completely weathered rock was identified at the contact between soil and bedrock. Dip angles range from 5 to 85° with a minimum of six joint sets.
These include a subhorizontal set and five steeper sets dipping between 30 and 85deg. Rock mass classes (RMC) along the tunnel alignment vary from Class I to Class IV with corresponding descriptions of Intact to Moderately Joint, Moderately Jointed, and Blocky & Seamy (both III & IV) with various degrees of weathering and discontinuity condition. 2022a).
The soil overlying the granite bedrock typically consists of silty sands and sandy silts with some clays and gravels. Typical standard penetration test (SPT) N values range from 7 to 45 blows per ft with some values exceeding 50 blows over 15 cm (6 in.) close to top of rock. Additional test pits and UAV-based photogrammetry has been completed to further characterize the ground conditions.
Design challenges
Ground support. As the tunnel approaches the Lot F site, it will be advanced through decomposed to intensely weathered granite. Localized overbreak is expected where intensely to slightly weathered, blocky and seamy to moderately jointed rock is present in the crown and sidewalls. Ground support will be required to address the poor ground conditions at the turn under. Supplementary initial support consisting of spiles will be required for some tunnel reaches.
Tunneling method selection. The preliminary design discussions eliminated drill and blast excavation methods, mainly because of concerns with vibrations of historic structures along the project alignment and excessive noise causing disturbance to the community. A hard-rock tunnel boring machine (TBM) has been selected as the preferred excavation method. TBM specifications will be developed as part of the final design process, but a main-beam TBM with an open gripper is likely the preferred option The small outfall site will likely require the TBM to be retrieved through the tunnel back to the mining shaft — a more difficult undertaking than breaking through the end station. Additional complications will arise if steel ribs are required for support along the alignment.
Railroad crossing/outfall. The outfall of the tunnel will cross under a single-track CSX railroad line that runs parallel to the Patapsco River. The tracks are bedded in 15- cm (6-in.) compacted subbase overlying the Ellicott City granite. A staged construction approach will allow for the construction of a load transfer slab between the tracks and the tunnel prior to excavation. This will allow installation of vertical rock dowels and a reinforced concrete slab. After the load transfer slab is in place, the TBM will mine under the tracks with just 8 inches of natural cover remaining below the slab. This will allow for TBM excavation of the entire tunnel alignment without the need for alternative excavation methods at the outfall or TBM extraction from a blind heading while mitigating the risk of adverse impacts to the CSX tracks.
Historic considerations. As part of the regulatory approval processes for the Section 404 permit, the proposed project constituted an “undertaking” subject to review by the U.S. Army Corps of Engineers under Section 106 of the National Historic Preservation Act (NHPA). The undertaking included substantial review and consideration of the impacts not only of the proposed project, but of the Safe and Sound Plan as a whole. Ultimately, while it was determined that the project is necessary and the plan is sound, the plan will have an adverse impact on the historic resource. To limit the adverse impact of the project, the county and its consultant teams extended the length of the tunnel, in the process eliminating several proposed structure removals, as well as changed the originally anticipated drill-and-blast method of construction to a proposed tunnel-boringmachine method of construction. Immediately adjacent to or atop the proposed alignments are numerous historic and architecturally significant structures. While the tunnel itself is envisioned as a deep bedrock tunnel, thus limiting the potential damage to these structures, evaluation and consideration of the risks of damage are critical. During preliminary design, a preliminary construction impact assessment report (McMillen Jacobs, 2022b) was prepared, and an initial review of structures in the area of the proposed alignment was undertaken. Ultimately, through the Section 106 process, the project was found to mitigate adverse effects. The final Section 106 Programmatic Agreement includes stipulations for monitoring and other actions that must be implemented through design and, ultimately, construction.
Construction challenges
Site access. Site access restrictions will be a significant construction challenge at all three of the near-surface structures. One option for the inlet structure and drop shaft site is approximately 2 acres on undeveloped county property adjacent to homeowners-association (HOA) and historic properties. The site will span a small creek; special considerations for erosion and sediment control will be required to protect the creek from construction runoff. The site will need to accommodate truck loading areas, staging areas, a 17 ft ID drop shaft, a 40 ft ID mining shaft, ventilation/cooling/electrical equipment, deaeration equipment, a crawler crane, a TBM maintenance shop, rail assembly area and muck piles.
The Lot F site is approximately 2.4 acres on developed county property adjacent to commercial and historic properties. The site will need to accommodate dump truck loading areas, an office trailer, a muck pile, deaeration equipment, generator/compressor/ventilation equipment, and a 25 ft × 25 ft (inner) square drop shaft and 30 ft × 20 ft (inner) diversion structure.
The available work area for the outlet structure is tightly constrained by the Patapsco River to the east and the CSX rail line to the west. The contractor’s means and methods should consider the difficult access, limited staging area and steep haul roads. The construction area will also need to be isolated from the river because of elevated water surface elevations in the Patapsco River due to seasonal precipitation events and its position within the 100 year floodplain of the Patapsco River. This will require a coffer dam or other separation structure.
Impacts to Historic Ellicott City. The potential for construction traffic and other related impacts to the Historic Ellicott City represents a significant construction challenge. Generally, the launch and outlet sites are accessed by a single two-lane roadway (Frederick Road/ Main Street), which is designated as an arterial collector and has approximately 15,000 daily vehicular trips. While not a significant distance away, Baltimore National Pike/U.S. Highway 40 has the capacity to carry additional vehicular traffic, and maintaining vehicular flow through the historic district is important to its viability. With the potential of 75 or more trips of muck removal a day, planning the haul routes to limit impact to businesses that have not only been impacted by two floods but also a pandemic represents a key consideration.
Other considerations. The proximity of the Intake Structure to the Hudson Branch presents a risk of inundation during construction. Weather monitoring and evacuation protocols will need to be developed to address this project risk.
Drill and blast operations will likely be required to excavate the drop shafts at the mining site and Lot F. Vibration, noise and air overpressure will need to be considered during the blast design process to prevent community disturbances. The round thickness, drill pattern and explosive load will be adjusted to keep vibrations and noise below acceptable levels.
Tunnel hydraulics have been thoroughly evaluated during the design process. This evaluation included watershed hydrologic and hydraulic modeling, alternative development and evaluation, hydraulic transient modeling, and the construction and use of two physical models.
Sustainability
Envision framework. The Institute for Sustainable Infrastructure’s (ISI) Envision framework will be used to guide and quantify sustainable aspects of the project’s design, proposed construction and operation. Developed by ISI, an organization founded by the American Public Works Association (APWA), American Society of Civil Engineers (ASCE) and American Council of Engineering Companies (ACEC) in collaboration with the Harvard University Graduate School of Design, Envision is a comprehensive tool to help deliver infrastructure projects that address climate change, public health and safety, environmental justice and economic recovery. It is similar to LEED certification for building projects.
The Envision framework is divided into five categories, 14 subcategories and 64 credits. The five categories (Leadership, Quality of Life, Resource Allocation, Climate & Resilience, and Natural World) are structured to guide project teams to have a positive impact on community sustainability. Infrastructure projects are evaluated for applicability and level of achievement in each of the 64 credits. Additional points are available for innovation in each of the five categories. A project’s score is the percentage based on the total points awarded and the total applicable points. Award levels (Verified, Silver, Gold and Platinum) are defined at 20 percent, 30 percent, 40 percent and 50 percent achievement.
Verification pathway. The project will use Envision’s Verification Pathway A, which includes an iterative review process after 95 percent design completion. A graphic illustrating the two Envision verification pathways is provided in Fig. 3. Any award level granted to the project will require a post-construction audit to maintain the award.
Predesign checklist and anticipated award. Prior to schematic design, a predesign checklist was completed to identify key credits and required supporting documentation. The checklist was also used to predict the anticipated Envision award level for the project using many assumptions regarding applicability and level of achievement for individual credits based on preliminary design documents. The anticipated score for the Ellicott City North Tunnel is 32 percent with 245 total awarded points out of 764 total applicable points (1,000 maximum applicable points). If this score is achieved during the design review and maintained after the post-construction review, the project will be awarded the Envision Silver Award.
This award level is a realistic and beneficial goal that has recently been achieved by another project in Howard County, MD. The Biosolid Processing Facility Improvements Project at the Little Patuxent Water Reclamation Plant earned an Envision Silver award in September 2021 after a nine-year design and construction process (ISI, 2021).
Key credits. Several key credits were identified during the predesign checklist exercise. Design and construction decisions will be made with consideration of the sustainability impact of those decisions and consequently the level of achievement for related Envision credits. Selected key credits are summarized below (ISI, 2018).
QL 3.2 Preserve historic and cultural resources. This credit is intended to preserve or restore significant historic and cultural sites and related resources. A maximum of 18 points will be awarded if (1) the project team works with the community and required regulatory and resource agencies to identify historic and cultural resources, (2) the project team develops strategies to document, protect or enhance historic and cultural resources to the project, (3) the identification of historic/cultural resources extends beyond registries to identify important parts of the community culture, (4) the project team works with stakeholders to develop a sensitive design and approach, (5) the project avoids all historic/cultural resources or fully preserves/protects their character-defining features, and (6) the project enhances or restores threatened or degraded historic/cultural resources in the community, or adds a resource to a protected registry. The predesign checklist assumes that the project will score 7 out of 18 possible points for this credit.
LD 1.4 Pursue byproduct synergies. This credit is intended to critically reconsider whether traditional waste streams can be beneficially reused. A maximum of 18 points will be awarded if (1) the project team assesses the availability of either internal or external excess resources or capacity, (2) the project team identifies opportunities for byproduct synergies or reuse, (3) the project team actively pursues a byproduct synergy or reuse, (4) the project includes a long-term regularly recurring byproduct synergy/reuse throughout project operations, and (5) the project is part of a circular economy whereby the majority of operational byproducts are beneficially repurposed or the majority of operational resources consumed are beneficially repurposed. The predesign checklist assumes that the project will score 12 out of 18 possible points.
RA 1.4 Reduce construction waste. This credit is intended to divert construction and demolition waste streams from disposal to recycling and reuse. A maximum of 16 points will be awarded if (1) the project team develops a comprehensive waste management plan to decrease project waste and divert waste from landfills during construction, and (2) during construction at least 95 percent of waste materials are recycled, reused and/or salvaged. The predesign checklist assumes that the project will divert 75 percent of construction waste and score 10 out of 16 possible points for this credit.
NW 2.4 Protect surface and groundwater quality. This credit is intended to preserve water resources by preventing pollutants from contaminating surface water and groundwater as well as monitoring impacts during construction and operations. A maximum of 20 points will be awarded if (1) the project team determines the potential for surface water and/or groundwater contamination during construction and operations, (2) the project includes spill and leak prevention and response plans and avoids creating new pathways for contamination during construction and operations, (3) the project reduces the risk of quality degradation to surface water and/ or groundwater, (4) the project incorporates adequate and responsive surface water and/or groundwater quality monitoring and reporting systems, (5) the project actively eliminates at least one source of hazardous and/ or potentially polluting substances, and (6) the project improves surface water and/or groundwater quality. The predesign checklist assumes that the project will score 9 out of 20 possible points for this credit.
CR 1.1 Reduce net embodied carbon. This credit is intended to reduce the impacts of material extraction, refinement/manufacture, and transport over the project life. A maximum of 20 points will be awarded if (1) the project team determines materials that are the primary contributors to embodied carbon for the project during construction and operation, (2) the project team calculates the primary contributors to overall embodied carbon, and (3) the project team demonstrates at least a 50 percent reduction in total embodied carbon of materials over the life of the project compared to the baseline. The predesign checklist assumes that the project will demonstrate a 5 percent embodied carbon reduction and score 5 out of 20 possible points for this credit.
Conclusions
The Ellicott City North Tunnel will significantly mitigate the frequency and severity of flooding in the Historic Ellicott City. The project is not without challenges in design and construction, but the project team is well equipped to overcome them. The Envision framework for sustainable infrastructure will provide a consistent and forward-thinking basis of design for the technical and socioeconomic aspects of the project.
Acknowledgments
The authors thank Christopher Brooks and Edward Cronin for their contribution to this paper. This paper was originally presented at the 2022 North American Tunneling (NAT) Conference in Philadelphia, PA.
References
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