Project owner: Dana-Farber Cancer Institute, Inc.
Dana-Farber Cancer Institute, known around the world for its clinical excellence and cancer research, is consistently recognized as one of the leading cancer research organizations in the country. Between 2001 and 2008 alone, annual outpatient visits and infusions at DFCI increased from approximately 140,000 to more than 264,000; clinical trials increased by nearly 80 percent. DFCI sought to provide ample, state-of-the-art facilities for leading-edge treatment of cancer and related diseases for the expanded patient population, and to create an enhanced healing environment with a strong patient and family centered focus, improved patient safety, and support for safe staff practices.
Utilizing the last buildable site on the DFCI Longwood Campus in Boston, the new Yawkey Center for Cancer Care (YCCC) is a 500,000-square-foot building with seven stories of below-grade parking and 15 above-grade stories. The structure is situated on a 198- by 186-foot lot sandwiched between Dana-Farber’s 13-story Richard A. and Susan F. Smith Research Laboratories Building, the Medical Area Total Energy Plant (MATEP), Jimmy Fund Way, and Brookline Avenue, a busy four lane artery through the heart of the Longwood Medical Area.
To help unite the DFCI campus and to alleviate pedestrian and vehicular traffic, the design incorporates both below- and above-grade connections to the adjacent Smith Building and the Dana Building across the street. The facility represents the Institute’s forward-looking vision and according to Edward Benz, president of DFCI, "The new YCCC is Dana-Farber’s response to the need for the best 21st century cancer care and for new modes of clinical research designed to bring better treatments to patients more quickly and safely."
Working with ZGF Architects and construction manager Walsh Brothers, Inc., Simon Design Engineering (SDE) provided site engineering master planning and served as structural engineer of record for the superstructure and foundations, and in collaboration with GEI Consultants, Inc. designed an innovative foundation and excavation support system to isolate the building from ground vibrations generated by the adjacent MATEP building. The design and construction team engaged in intense collaboration, which ultimately resulted in the project being completed two months ahead of schedule and on budget.
The MATEP facility provides power, including electricity, steam and chilled water, for nine million square feet of occupied space in the medical area. There are six 32-cylinder diesel engine generators in the facility that produce low-frequency ground vibrations in the 10 hertz range, which is similar to the natural frequency of typical building elements. The low-frequency vibrations produced by generators, if left unmitigated, would have a severe impact on DFCI’s sensitive operations, and provide an unacceptable environment for clinical care.
In situ measurements of ground vibrations performed in boreholes showed that there was little attenuation of the vibrations with depth in the soil overlying the bedrock, but the vibration amplitude in the bedrock was markedly lower than in the overlying soil. Based on this data, an innovative foundation system was designed to support the building on bedrock located up to 110 ft. below ground surface, while isolating it from the surrounding soil. This was done by physically separating the below-grade parking garage floors from the perimeter foundation walls to create a free-standing structure supported on the bedrock. In effect, YCCC is a 22 story free-standing structure that starts seven stories below grade.
|Completed excavation with installation of building foundations in progress. Photo by Walsh Brothers, Inc.|
This vibration isolation design is similar to the abutting 15-year-old Smith Building, but significantly more complicated, so DFCI utilized the same engineering design team (SDE and GEI) for the YCCC.
Innovative foundation design
The vibration isolation design requires that the perimeter foundation walls for the below-grade structure be self-supporting. This was achieved by using cast-in-place concrete diaphragm walls (slurry walls) supported by permanent tieback anchors. In addition to serving as the permanent foundation walls, the anchored slurry walls provide temporary excavation support for the below-grade excavation. The slurry walls are also designed to act as a permanent groundwater cutoff for a permanent under-drainage system located below the underground structure.
The perimeter slurry walls are 3-foot-thick and are socketed into bedrock at depths ranging from 70 to 110 ft. The walls are supported laterally by 268 permanent tieback anchors that are anchored into the bedrock with design capacities ranging from 225 to 780 kips. The tieback anchors are composed of bundles of steel cables up to 150 ft. long (total of 75 miles of cable, weighing 148 tons).
|Schematic cross-section. Image by GEI Consultants, Inc.|
The slurry wall is located immediately adjacent to the MATEP facility and supports an excavation that extends 45 ft. below the MATEP foundations. The slurry wall and tiebacks had to be designed to control ground movements that could impact sensitive mechanical equipment inside the MATEP facility. A design based on movement control criteria was performed using soil-structure interaction analyses that modeled each stage of construction as well as the permanent condition. Extensive instrumentation was used to monitor ground movement impacts during construction, including inclinometers installed inside the slurry walls, high resolution digital optical surveys, an automated water level settlement monitoring system, and direct instrumentation of critical mechanical equipment.
The foundations for the YCCC building consist of spread footings, concrete piers and drilled shafts bearing on the bedrock. The different foundation types were selected to accommodate the varying (0 to 30 ft.) depth of bedrock below the bottom level of the parking garage. Where the building foundations had to penetrate soil above the bedrock, a 2-inch-thick layer of closed-cell polyethylene vibration isolation foam was used to isolate the foundations from the surrounding soil. For the drilled shafts, the vibration isolation foam was attached to the inside of a permanent steel casing.
The superstructure foundations were offset 10 ft. from the slurry wall to avoid exposing the toe of the wall and mitigate ground vibration transmission. In some locations, columns were sloped inward 10 ft. from the ground floor to the lower level, to maintain this 10-foot setback while avoiding large transfer girders at grade.
|"Steeling their Courage." This colorful display is the work of ironworkers who, in tribute to young children receiving cancer treatments, painted their names on the steel framing. Photo by Simon Design Engineering, Inc. "Steeling their Courage"|
The logistical challenges of this project were extraordinary, with no lay-down area on the tightly confined site. Creation of lay-down areas had to be incorporated into the structural design. As one example, the roof of the mechanical room along Brookline Avenue was designed to provide a construction staging platform, support a tower crane, and act as a support for earth retention.
A major concern during project planning was the need to socket the slurry wall into the hard bedrock without creating large vibrations that would disturb the nearby medical facilities. This was achieved by using a hydro-mill to perform slurry wall excavation into rock, instead of using traditional drop chisels. The hydro-mill technology cut the bedrock at a rate 10- to 15 times faster than traditional methods, saving three weeks on the schedule and preventing shutdowns due to excessive vibrations.
Perhaps the greatest construction challenge was the installation and waterproofing of tieback anchors through slurry wall penetrations located more than 40 ft. below groundwater level in soils that included a thick layer of fine "running" sand. The water pressure behind the wall can cause this fine sand to flow like a liquid, resulting in large soil losses and ground settlement. Installation of tieback anchors under these conditions required special procedures, equipment and materials to control soil loss during drilling, anchor grouting and installation of permanent waterproofing. Team members proactively addressed the issues in detail in the bid documents, contractor selection and submittal requirements to minimize project risk. An array of tools were used to address these issues, including use of permanent casing in the soil, sealed drilling collars attached to the wall penetration sleeves, dual drive head duplex drilling, heavy drill muds, drilling from elevated berms, seal grouting of rock sockets, urethane post-grouting of wall penetrations and a permanent waterproofing system with preformed water-swelling urethane seals.
Another construction challenge arose when the general contractor experienced difficulty dewatering and stabilizing the fine sand below the final excavation level for construction of building foundations. With the design engineers working "on-the-fly" with the foundation subcontractor Raito, this problem was solved by a redesign using drilled shafts. The shafts were installed by drilling uncased with slurry, seating a steel casing into the bedrock inside the slurry-supported hole, and then dewatering the casing.
The YCCC project presented major design and construction challenges that required innovative solutions from the design and construction teams. Meeting these challenges required ongoing interaction between the design team, the construction team, and an owner actively involved in managing risks associated with the proposed construction. This three-way interaction led to a successful project, completed in January, 2011.
A Q&A with Simon Design Engineering, Inc.
Q: What was the biggest structural challenge for this project and how did you overcome it?
A: The need to respect the architectural form of the building above grade and yet keep the structure isolated from the slurry wall was the most challenging. Since the building is the signature statement for DFCI, it was critical that the building take on a certain form. The building also needed to respect patients’ feelings and sensitivity to vibrations, which were being generated by the abutter. The entire team needed to work together to solve the problems. We were well aware of the vibration problems from the design of the Smith building.
Q: How did you use technology to overcome challenges?
A: The design of the slurry walls and permanent tiebacks utilized the advancements in software and computers since the previous building design. That, combined with real soil data based on the previous building, allowed optimization of the structural design of these elements. In addition we developed customized software to design the wall. The result was a significant reduction in the amount of steel used in the wall with better performance (based on the measured results).
Q: How did the project team approach and solve project challenges?
A: The main challenge was confronting the water pressures at the lower levels. The soil conditions allowed for holidays to occur in the exterior wall (both natural and manmade). These holidays revealed themselves only when disturbed, in one case after the superstructure was erected and cast. Piping of the holiday occurred and then the hole needed to be plugged and eventually patched for permanence. The team needed to work together to quickly resolve the issues to preclude soil loss through these holidays and minimize settlement of the adjacent structures.
The other significant challenge was that the soil at the bottom of the hole could not be dewatered in a timely manner and this made the original approach to provide isolated foundations not viable (impossible). We worked to develop an alternative approach to the foundation installation using drilled shafts and vibration isolating material to overcome this challenge.
David R. Shields, P.E. is a senior consultant at GEI Consultants, Inc. who specializes in geotechnical engineering. Contact him at firstname.lastname@example.org. Alan H. Simon, P.E. is the founder of Simon Design Engineering, a collaborative engineering company. Contact him at email@example.com. Michael T. Bracher, P.E. is a senior engineer at GEI Consultants Inc. who specializes in geotechnical engineering. Contact him at firstname.lastname@example.org.