Stacked Up... with Geotechnical Construction Solutions!

Scheduled to open in 2022, Boston University’s newest construction project, The Center for Computing & Data Sciences, is a new research facility that will house programs ranging from mathematics to computer science and will include the renowned Rafik B. Hariri Institute for Computing and Computational Science & Engineering. 

Figure 1: Rendering of BU Center for Computing & Data Sciences. Courtesy KPMB Architects

Once completed, the 19-story building, designed by KPMB Architects, will resemble a cantilevered stack of books overlooking the iconic Charles River, giving students and visitors stunning views of Boston’s skyline (Figure 1). The building will also be fossil-fuel free, making it one of the most energy-sustainable buildings in Massachusetts.

As noted by BU President Robert Brown on Boston University’s website, “The theme of the building is unquestionably collaboration.”  In keeping with this theme, Keller was engaged early in the project lifecycle to provide technical and constructability guidance to the project team.  Following these collaborative efforts, a “stack” of geotechnical solutions was implemented in the final design to address specific earth retention and deep foundation challenges. 

Temporary Earth Retention

The new data and sciences building’s mat foundation required excavation to depths ranging from about 25 feet at the “podium” of the building, to 42 feet below existing ground at the “tower” of the building (Figure 2).  Being part of the Design, Ownership and Construction project team early allowed for open discussion of earth retention options including considerations for cost, performance, impacts to abutters / site utilities, installation timeline and schedule.  After careful review of a variety of options, the team opted to specify an internally braced, driven steel sheet pile system.  The temporary earth retention system facilitated excavation adjacent to the heavily travelled Commonwealth Avenue and down to subgrade below area groundwater, through urban fill, organics, glaciofluvial and marine clay subsurface deposits. 

Figure 2: Completed steel sheeting and internal bracing

An important element of the earth retention solution selected was its ability to accommodate an existing telecommunications utility that could not be relocated and which crossed the earth retention in two locations; thus, interrupting continuity of the interlocking sheet piles. At these locations, jet grouting was used to provide closure as well as vertical support of the active utility. Jet grouting is a specialty geotechnical construction technique that uses high velocity fluid jets to construct cemented soil of varying geometries in the ground.  Once installed, the jet grout was excavated to facilitate subsequent bracing installation (Figure 3).  The critical communication infrastructure was eventually hung inside the temporary sheeting and before excavation below the utility began.

 Figure 3:Jet Grout utilized to provide closure at sheet pile discontinuities

Due to external site restraints, internal bracing was required to provide lateral earth support.  Three levels of large diameter pipe struts were installed and preloaded to minimize wall deflections within the deeper tower excavation; one level was required in similar fashion within the shallower podium portion of the building.  Here again, and under the pressure of a delayed construction start due to the 2020 Global Pandemic, the construction team collaborated closely in planning and choreographing a sequence of bracing installation and preloading to allow efficient excavation and meet strict schedule dates for foundation mat placement.

Given certain complexities in the earth retention design, which involves a large change in excavation depth to subgrade from one end of the site to the other, select struts were instrumented with strain gauges and tiltmeters to collect real time data to help validate design assumptions.  Though perhaps academic, the data were used to help to better understand strut movement in 4-D (time) space as well as the load experienced by the strut through the various excavation stages (Figure 4).

Deep Foundations

The 19-story structure, with one to three below grade levels, is supported by a concrete mat foundation bearing in glacial fluvial sand and marine clay deposits located below the groundwater table.  Several deep foundation techniques were also utilized to meet a variety of technical and constructability challenges for both temporary and permanent elements of the construction. 

For example, in the shallower podium section of the building, drilled micropiles were installed to provide uplift resistance against the design groundwater elevation, which was factored for flood resiliency.

In addition, the building’s award-winning cantilevered architecture required temporary foundation supports, or shoring towers, during mat foundation construction.  Driven steel friction H-piles were installed to support the temporary towers.  Staged construction and proper planning by the site team allowed early installation with pile caps and connections to be installed as needed. 

Steel H-piles were used to support the project tower crane.  For this application, end-bearing H-piles were driven within the deep glacial till and bedrock deposit extending to about 180 to 200 feet below ground surface 

 Figure 4: Driven end bearing H-pile for tower crane support

Finally, helical piles will be installed during future construction phases and will provide vertical, end bearing support and uplift anchorage for various site improvements including retaining walls and a bicycle canopy structure.

Collaboration under Global Pandemic

The year 2020 has been particularly challenging for everyone, including the construction industry, due to the Global Pandemic.  The start of this project, among many, was quickly derailed by closure of all construction sites within the City of Boston in the spring of 2020.  The project team methodically and purposefully adjusted to the unknown COVID-19 landscape, including its new rules for temperature monitoring of site personnel at check-in and social distancing and mask wearing on the construction site.  Once taken for granted, simple team collaboration efforts such as in-person site meetings, or just speaking on site, were challenged due to limitations on the number of personnel allowed on site at any one time, and mandated mask-wearing. Persistent collaborative communication, however, ensured that the project team could adapt to meet the challenge safety.

To assist with a nearly 3-month delay to start of excavation activities, the team worked closely in choreographing field operations involving installation of tension micropile, steel sheets, driven H-piles, earth retention bracing during excavation, while not sacrificing workmanship or construction quality. For example, performing micropile installation at the same time as sheeting installation required specific attention and coordination. Since both activities were performed by a single specialty contractor, and through collaboration and communication with the project team, potential vibration impacts on the grout set of the permanent tension micropiles were mitigated.  Once the sheeting was completed, the same equipment was utilized by the specialty geotechnical contractor to move into driven pile work, helping the project gain on the schedule.  Since sheeting was completed early on, excavation could proceed within the building footprint as the tower crane and temporary shoring piles were driven.

Communication Was Key to Timely Completion 

Maintaining communication between all parties involved was critical and required the skills of a seasoned project team coupled with the strength of relationships forged on previous projects.  Through this project collaboration, the team managed a Pandemic impacted schedule by leveraging technique, equipment and manpower while installing a stack of integrated geotechnical earth retention and deep foundation solutions.  Special thanks for the collaborative efforts from the entire project team, including Boston University, Welch Corp, GeoEngineers, Suffolk Construction, and Haley & Aldrich.