Building Over the Massachusetts Turnpike - Foundation Design and Construction Challenges

Marya Gorczyca, PE, Senior Principal, Jean Louis Locsin, PhD, PE, Senior Geotechnical Engineer, Damian Siebert, PE, Principal, Mary Votto, Assistant Project Manager, Mark Zablocki, PE, Project Manager; Haley & Aldrich, Inc.

Background
The completion of Lyrik Back Bay – Air Rights Parcel 12, in 2024 makes it the first successful air rights project over the Massachusetts Turnpike/Interstate I-90, in 40 years since the construction of Copley Place in the 1980s. Although many other Air Rights projects were planned, they never made it to construction. One exception is Air Rights Parcel 9, Fenway Center, which is under construction at the time of writing this article. This article describes the project constraints, geotechnical and construction challenges, and foundation systems designed and constructed to overcome them and realize a project of this size and complexity.

The Massachusetts Department of Transportation (Mass DOT) originally envisioned that air rights development parcels would reconnect neighborhoods divided by the extension of I-90 in the 1960s as illustrated in Photo 1 which shows the location of the Massachusetts Turnpike (I-90) through the Back Bay neighborhood of Boston.

Image 1- View of completed project looking east
(courtesy Samuels & Associates)

Air Rights Parcel 12 is a 77,295-square-foot (1.77-acre) parcel which consisted of the area over the east- and westbound lanes of I-90 and Massachusetts Bay Transportation Authority (MBTA) tracks, an entrance ramp onto I-90 west bound, and two small “terra firma” parcels on the north and south sides of the interstate.  The configuration of Parcel 12 prior to construction is shown in the aerial photograph below, Photo 2.

Image 2 – Aerial view of parcel prior to construction
(courtesy Elkus Manfredi Architects (EMA))

The designated developer Samuels & Associates embraced the complex engineering challenges unique to an Air Rights project and engaged and led a project team of design consultants in a collaborative design process to make the project a success.  The $750-million mixed-use development transforms the neighborhood as it covers a gaping hole in the city street network created by a depressed, open highway structure and railroad line. The development features a 20-story office building over a two-level, below-grade parking garage, a 13-story hotel, retail and restaurant space as a two-level podium on the deck over I-90, a relocated westbound I-90 access ramp, a new entrance into the MBTA Green Line Hynes Auditorium station, and public open space, including an elevated public park. The development decreases traffic noise for nearby residents and offers improved pedestrian access, a new bike lane, and sustainable design features.  

Haley & Aldrich served as the primary geotechnical and environmental consultant for Lyrik Back Bay – Air Rights Parcel 12. We also served as the support of excavation designer including a critical excavation section adjacent to the MBTA tracks. 

The project team contended with many engineering, design, and construction challenges. Though a project of this size and complexity, with so many firms and stakeholders involved, has many interesting engineering, design, and construction challenges, this article will only focus on a few key geotechnical items.  We will describe how the geotechnical challenges were met using unique and innovative solutions, to overcome the site constraints. This paper will describe: 

• The background on site and subsurface conditions
• Unique project and site challenges, and constraints
• Geotechnical solutions and learnings:
  • Hybrid deep foundation system
  • Unique Secant Pile Wall application for the below grade garage adjacent to MBTA tracks
  • Instrumentation monitoring program for track monitoring 

Site Conditions and Project Constraints
Some of the major conditions and constraints that needed to be considered in developing geotechnical solutions included:

• Previous site development and buried relic foundations/structures
• Variation in site surface grades and steeply sloping grades with the Massachusetts Turnpike and MBTA tracks depressed below the surrounding street grades on three sides, and steep slopes present on the north and south land parcels
• Need for protection of adjacent transportation infrastructure including the Massachusetts Avenue bridge to the east, MBTA Green Line tunnel and vent building on the north, east and west bound I-90 roadways running through the middle of the parcel, and MBTA tracks south of I-90, specifically to not result in ground movement or applied load changes
• Maintaining access to I-90 west bound via the Newbury Street access ramp
• Maintaining uninterrupted highway and rail operations during construction
• Work hour limitations for maintaining rail and I-90 normal operations
• Limited space available for foundation and substructure construction
• Depth to suitable foundation bearing soils and high groundwater levels
• Protection of adjacent buildings and city of Boston Streets

Conditions and locations of existing transportation infrastructure below and adjacent to the Air Rights parcel are indicated on the plan below.

Image 3 – Aerial view of Parcel 12 identifying major transportation infrastructure
(courtesy EMA)

Site and Subsurface Conditions
Site access presented challenges from the beginning, starting with subsurface investigations needed to identify the underlying soil, groundwater, and bedrock conditions and engineering parameters. Subsurface exploration required access agreements and work plans approved by the owner agencies. Limitations on access and work hours, coordination of roadway closures, and the paramount importance of site restoration within the I-90 roadway, required significant coordination, planning and phasing of the exploration program.  

It had been determined early on that based on site geologic conditions, spatial constraints and high concentrated loads that deep foundations would be required. The project sits partially on land created by filling in the tidal flats of the Back Bay to create additional developable land for the growing city of Boston in the 1800s. Consequently, the upper 25 feet of the land consists of fill and soft organic soils. In addition, a deep bedrock valley exists below the area that was filled with an up-to-110-foot-thick deposit of soft and compressible clay, referred to locally as Boston Blue Clay. Depth to bedrock ranged from approximately 125 to 175 ft below existing site grades corresponding to approximately El. -98 to  145 BCB (Boston City Base Datum). Groundwater is shallow across the area and was generally encountered between El. 4 and 8 BCB. A generalized subsurface profile across the parcel indicating relative depths and thicknesses of soil strata is provided as Figure 1. 

Image 4 - Generalized subsurface profile north- south through the parcel

Beyond understanding typical engineering properties of soil and bedrock for geotechnical design, it was critical to identify locations of buried relic structures, probe for undocumented temporary works, and confirm as-built conditions of adjacent structures. Site history research indicated that the area has been developed since the late 1800s, with parts of I-90 previously serving as a rail yard and railroad lines. It was also anticipated that relic structures from former site development remained in place after they were demolished to allow for construction of the Mass Turnpike extension in the 1960s. Subsurface investigations identified structures along Boylston and Newbury Streets that had been demolished and buried in place.

In addition, the MBTA Green Line — one of the oldest subway lines in the country — runs through a tunnel constructed in 1913, below Newbury Street on the north side of the parcel, Image 4 indicates the general depth and configuration of the MBTA green line tunnel. Test pit explorations were undertaken to identify locations of relic buried structures such as foundation walls, granite block pile caps, wood piles, and existing structure foundations. This was critical to assist in determining the scope of advanced site enabling work and requirements for mitigating the risk of encountering obstructions which would result in delays to schedule and cost impacts during construction.

Foundation Design and Construction
Although we found uniform subsurface stratigraphy across the site, the near surface conditions were highly variable. Surface grades varied by as much as 20 feet, with the highway and rail lines (El. 11 to 13 BCB) at about 20 to 25 feet below the surrounding city street grid (El. 29 to 30 BCB typ.). The depth to bedrock also varied, with the elevation of top of bedrock varying by about 50 feet across the site, impacting foundation design lengths and installation requirements. Understanding subsurface conditions was important not only for design and construction of deep foundations but for determining project cost and schedule implications. Understanding the schedule of enabling work and foundation installation in the I-90 roadway was of particular importance because construction had to adhere to the time frames negotiated with Mass DOT for mitigating impacts to the travelling public.  

To support the structures that were part of the Parcel 12 project, foundations needed to extend through the soft clay to bear in the bedrock at depths of 125 to 175 feet.  Multiple types of deep foundation systems were evaluated based on the variable conditions and multiple constraints.

Hotel Foundations North of I-90
Hotel foundations consisted of Drilled Micropiles (DMPs).  DMPs were selected to limit the number of foundation elements and size of pile caps due to property line constraints.   They could also be installed near the MBTA Green Line tunnel below Boylston Street with minimal vibration or ground movement.  The west end of the hotel was supported on a unique splayed super column referred to as the “tree column” adjacent to the I-90 WB access ramp, refer to Photo 1. The column was supported on a large mat/pile cap also on DMPs.

Central Deck and Retail Podium Foundations
Foundations and support of excavation for the deck structure over the highway needed to be installed in the limited spaces north, south and in the median between roadways and rail lines. The new tunnel walls enclosing the MBTA rails and highway lanes supporting the deck were supported on DMPs due to limits on allowed lane closures, and restricted space for access and material staging.  Since the tunnel walls and deck were considered part of the transportation infrastructure, the DMPs were designed to Mass DOT and AASHTO standards.

Image 5 – Foundation construction adjacent to tracks and highway

Office Building Foundations South of I-90
The office building being the tallest structure, with a two level below grade parking garage adjacent to the MBTA tracks and sloping topography, required the most complex foundation system. Building columns were founded on a combination of: 

1. Rock socketed drilled shafts adjacent to the MBTA tracks, 
2. Driven steel H-piles,
3. Rock-socketed drilled micropiles (DMPs) for the elevator core mat and specific columns, and
4. Load Bearing Elements (LBE) socketed into bedrock, integral with the west slurry wall.

Drilled shaft foundations supporting building column loads were incorporated into a secant pile wall by extending specific secondary secant elements, reinforced with wide-flange (WF) steel sections, to support building column loads. Following installation of the perimeter SOE/foundation walls, driven steel H-piles were installed to support the interior building columns and the elevator core mat. The secant pile wall acted as a barrier protecting the MBTA tracks from ground movement during driven pile installation. 

Additional details for the foundation system designed and installed are provided on Table 1.

Table 1 – Summary of foundation and support of excavation systems

Deep foundation construction enabling and installation of the approximately 750 deep foundation elements was undertaken over a period of 15 months between August 2020 and November 2021.  Multiple work shifts and weekend shifts were required to compress the foundation construction schedule and sequence the work such that one activity would not impact the other. Photographs representing DMP installation conditions are provided below.

Site enabling for foundation construction started in July 2020 during the Covid-19 Pandemic. With the exception of essential workers (such as construction) most of the project team was working remotely.  Masking, distancing, and quarantine protocols were in place adding to logistical challenges to protect worker and public safety. As workers or crews were impacted by positive Covid test results, quarantine requirements added disruption and resource limitations.

Support of Excavation Adjacent to MBTA Tracks
Excavation for the construction of the below grade parking garage below the office building required a stiff watertight support of excavation system to control ground movement and achieve the strict track deflection tolerances.  For below grade parking on the triangular shaped parcel to be feasible it was necessary to have the garage wall as close as allowed to the track centerline, 8.5 ft, for any permanent structures.  Installation and deflection of steel sheeting were anticipated to exceed movement tolerances. The use of concrete diaphragm wall installed by slurry trench excavation methods (slurry wall) and load bearing elements (LBEs or Barrettes), commonly used in similar cases locally, was not considered practical close to the tracks.  The potential risk of collapse and resulting consequences were significant. 

Therefore, a secant pile wall was used as the temporary support of excavation along the north side of the 16 ft deep excavation adjacent to the MBTA tracks.  The secant pile wall also served as the permanent foundation wall.  The secant piles were installed with temporary casing to protect against loss of ground while drilling through the surficial fill, soft organic soils, and marine sand layers. The wall was reinforced with a 24-inch-wide flange steel beam and was designed to resist Cooper E80 train loading.

Images 6 and 7 – View of excavation and foundations for office building

A plot indicating profile of lateral deflection with depth at the mid-point of the wall as measured by an inclinometer is provided below. The maximum lateral deflection observed during excavation was ½ inch, below predicted performance and the strict performance criteria for lateral deflection of 1 inch.

Image 8 - Inclinometer plot

Instrumentation and Monitoring Railroad Tracks
A comprehensive instrumentation and monitoring program was necessary to provide advanced warning for potential construction related impacts to adjacent infrastructure. Early in the design process it was evident that automated remote monitoring of the MBTA tracks should be performed due to the strict performance criteria established for track movement, the projects’ goal of not impacting rail operation, and the proximity of the construction to the tracks. Automated Motorized Total Station (AMTS) connected to a wireless web-based system for data collection and reporting was the primary measurement system for track monitoring. AMTS surveyed railroad monitoring points which consisted of survey reference prisms mechanically attached to the rails by a mounting bracket with each read in three orthogonal directions. 

An automated rail monitoring system was preferred over a more traditional optical survey due to personnel safety concerns working on active tracks, to minimize needing frequent access to the tracks to collect data, and to provide near “real time” alerts for advanced warning.  The automated system was not without its own limitations and lessons were learned during the monitoring program.  The automated monitoring resulted in numerous alerts, not all of which were construction related. Erroneous alerts resulted from environmental impacts, interference, occasional erroneous measurements, and ground movement during various incidental activities.  A plot of movement vs. time for a monitoring point at mid-point adjacent to the excavation is provided below and compared to movement criteria. 

Images 9 and 10 - Plot of movement vs time, vertical and lateral for track monitoring point

A communication protocol was established as it was necessary to respond with an evaluation or corrective action within 24 hours of any alerts. The monitoring system provided advance warning of potential impacts of construction activity and provided confirmation to the project team and Agency stakeholders that specific performance criteria were being met which was essential for the project. 

Conclusions
The successful design and construction process required extensive collaboration with design and construction professionals across disciplines. Multiple agencies (Mass DOT/MBTA) and other third-party reviewers had to approve design documents before construction started. Highly specialized and detailed technical analyses were necessary to demonstrate that design and installation could be completed so that these parties could make informed decisions.

The team also kept a laser focus on maintaining safety and limiting public disruptions as the project took shape. The contractor’s safety personnel addressed site constraints and potential hazards in work areas before they developed into problems, completing work without a safety incident. Additionally, the team completed nearly all work without shutting down passenger service on the MBTA or closing I-90, with the exception of temporary shutdowns and rolling roadway closures in the middle of the night for deck construction so that teams could lower and secure massive steel beams. 

In summary, overcoming the geotechnical design and construction challenges associated with site conditions and project constraints required thoughtful and creative problem solving, design excellence, careful monitoring, and safe construction practice by the entire team to achieve the project goals and realize such a successful outcome.  

Image 11 - Public park and view over the highway looking west

Credits/Project Team
The authors would like to acknowledge the contributions of significant team members below, and many others not listed, who contributed to the foundation design and construction for this complex project.

• Geotechnical and Environmental Consultant: Haley & Aldrich, Inc.
• SOE Designer: Haley & Aldrich (Construction Services Team Slurry Wall and Secant Pile Wall Design) 
• Designated Developer: P-12 Property LLC/Samuels & Associates
• Project Architect: Elkus Manfredi Architects
• Construction Manager: Suffolk 
• Structural Engineer: McNamara/Salvia Structural Engineers: 
• Civil and highway engineer, permits and planning: Vanasse Hangen Brustlin (VHB)
• Transportation planning and traffic management: Howard Stein Hudson (HSH)
• Foundation sub-contractor: Trevi ICOS Corporation (Secant Pile Wall ++)
• Foundation sub-contractor: Keller North America/GeoEngineers (Enabling Work Design)
• Site work contractor: J DeRenzo Company
• Instrumentation Specialist: GeoInstruments (Track Monitoring)
• Surveyor: Feldman Geospatial - Surveyors (Settlement Monitoring)
• Mechanical, electrical, plumbing engineer (MEP): WSP
• Owner Agency: MassDOT/MBTA 

We would also like to acknowledge the contributions of numerous former and current Haley & Aldrich staff members who contributed to the success of the project especially Scott Bamford, project manager (retired), Ryan Baker, lead senior field technician, David Palleiko, and Matt Dodson, geologist for weeks of night work monitoring test borings.