Construction and Integrity Testing of the Drilled Shafts on Fremont

Malcolm Drilling installed the drilled shafts from the existing grade using a Bauer BG46 drill rig with a 263-ft (80.2 metre) long Kelly bar. Since the overburden soil layers consisted of loose manmade fill, including old wooden piles, and several sand lenses to depths of about 80 feet (24.4 metres), temporary steel casing was used to stabilize these layers during construction. Drilling beyond the casing in the mostly stiff clay layers proceeded using polymer slurry (i.e. drilling support fluid). The Valley Deposits were believed to pose a significant risk to borehole stability due to their loose matrix of gravel and cobble components. Therefore, a higher-grade KB polymer was used because the product allows for immediate slurry enhancement with stabilizing additives when loose soil layers are encountered.

Arial view of project site

Since the small site (140 by 130 feet [42.7 by 39.6 metres] in plan) is in San Francisco’s financial district and at least two other major construction projects were underway throughout construction (i.e. Salesforce Transit Center and Salesforce Tower), traffic and site logistics were major drivers for all construction activities. For example:

  • Rebar cages had to be spliced multiple times over the borehole since delivery of long cages was not possible.
  • Slurry tanks had to be moved during production and covered to provide some storage capacity.
  • Large deliveries were restricted to limited windows and unscheduled events posed a significant time delay risk.
  • The drilled shaft construction was performed in two shifts, almost around the clock, in close collaboration with the general contractor.

The shafts were completed ahead of schedule and no anomalies were revealed by the integrity testing via crosshole sonic logging (CSL). The success of the drilled shaft installation is attributed to the close collaboration between the engineers, general contractor and specialty foundation contractor, as well as the focus on shaft cleanliness.

Support of Excavation

Support for the approximately 60-foot (18.3 metre) deep excavation for the 181 Fremont basement included new cutter soil mixed (CSM) walls along the south, east and west sides of the site, and the existing CDSM wall along the north side, which was in use for the construction of the below grade portion of the adjacent Salesforce Transit Center project. CSM differs from CDSM in that it employs a mixing tool with two sets of counter-rotating, vertically mounted cutter wheels to form rectangular soil-cement panels, as opposed to circular secant shafts. Four levels of internal bracing were used to restrain the soil mixed shoring walls.

Design of the CSM Wall and Bracing

The new 3.3-foot (one metre) thick cutter soil mixed (CSM) shoring walls were reinforced with W30 (W760) steel piles and were designed to act as both a stiff structural wall and to provide groundwater cutoff. Soil mixing extended to a depth of 95 feet (29 metres) below existing grade to penetrate into the relatively impermeable Old Bay Clay for a bottom seal.

The internal bracing system consisted primarily of W36 (W920) perimeter walers and 36-inch (914-millimetre) diameter pipe struts. Due to the particularly high loads at the lowest bracing level, stacked double walers were required. The struts were designed and detailed to be preloaded using hydraulic rams to reduce deflection of the shoring system and to ensure that the bracing loads were distributed as intended. Pin pile supports were provided for a heavy boxed wide flange member that was designed to carry significant axial loads across the excavation in a north-south direction.

To ensure continuity of load transfer across the adjacent transit centre site, the bracing elevations were coordinated with the bracing levels for the transit centre project, which were in place when the 181 Fremont excavation commenced. In addition, the bracing was configured to be removed progressively to minimize conflicts with the permanent basement construction. The shoring design also had to be closely coordinated with the project’s tower crane foundation; a steel grillage supported on four tightly bunched drilled shafts, and temporary work trestles that accommodated a large crawler crane and a hydraulic excavator.

Construction and Monitoring of the CSM Wall and Bracing

Compliance with the horizontal deflection limits (i.e., no more than one inch (25 millimetres) at the top level of bracing and 1.5 inches (38 millimetres) below the top level of bracing) was assessed via regular monitoring of six inclinometers embedded through the soil mix walls and extending into bedrock, and approximately 40 glass prism survey monuments mounted to the tops of the soldier piles in the CSM walls. The inclinometers were read manually at approximately two-week intervals with the calibration/interpretation of each reading corroborated by measurements from the nearby survey monuments. Monitoring of the survey monuments was performed in two-hour increments by extending an automated total station (AMTS) network that was already in place for the Salesforce Transit Center project. This survey data was made accessible in near real time to the contractors, engineers, adjacent property owners and other stakeholders through an online portal.

The specified dewatering criterion (i.e., groundwater should be lowered by no more than five feet [1.5 metres] outside the excavation) was assessed via manual reading of piezometers at two locations: one adjacent to the soil mix wall along Fremont Street and the other adjacent to the soil mix walls near the truncated southeast corner of the building footprint. Two nested piezometers were installed in each borehole, where the casing was slotted within the fill and marine sand layers.

Structure-Structure and Structure-Soil-Structure Interaction

Due to the proximity of the site to several nearby structures, major consideration was given to potential structure-structure and structure-soil-structure interaction (SSSI) phenomena. For example, to mitigate the potential for excessive movements to impact the transit center and the 199 Fremont and Townhall buildings during construction, the shoring system was designed to limit ground deformations within permissible limits. In addition, at the shared transit center shoring wall, special bracing details were employed to provide a means of controlling the response of the transit center shoring to the 181 Fremont excavation. At this location, a pair of walers with spacers were used to provide a gap where jacks could be installed, if necessary, to locally adjust the loads in the bracing system. In addition, this detail reduced the potential for longitudinal load transfer between the bracing system and the shared shoring wall.

The potential for SSSI was also considered during the seismic design of the tower. Since both the transit center trainbox and the 181 Fremont basement were cast directly against the shoring wall, there is a direct load path for the two structures to interact during an earthquake. In fact, as a condition of approval for the project proceed, SSSI analysis was required to demonstrate whether there would be an impact on the seismic performance of the adjacent transit center. To this end, a suite of bedrock-propagating ground motion time histories was applied to a large 3D finite element model that included both structures (and a simplified representation of the Salesforce Tower) embedded within the soil/rock domain.

SSSI model to examine seismic interaction between 181 Fremont Tower and Salesforce Transit Center

Summary

Very close collaboration between the owner, geotechnical engineer, structural engineer, general contractor, and specialty foundation and shoring subcontractor was critical to the successful completion of the subsurface components of the 181 Fremont Tower. Detailed planning and extra effort up-front (e.g. soil investigation, obstruction removal, work sequencing and contingency planning) benefited the construction schedule and overall project cost.

Acknowledgements

Developer Jay Paul Company
Architectural Design Heller Manus Architects
Structural Engineer Arup North America
Geotechnical Engineer Arup North America
Shoring Designer Brierley Associates
General Contractor Level 10 Construction
Specialty Foundation Contractor Malcolm Drilling

The authors would like to extend a special thanks to Stephen McLandrich (formerly Arup) who was the geotechnical engineer of record for the project.

Author info:

Kirk Ellison, Ph.D., P.E., G.E., is a senior engineer at Arup North America. He works on a variety of high-profile geotechnical projects in the building and infrastructure sectors in San Francisco and around the world, with special emphasis on seismic soil-structure-interaction analysis.

Eric S. Lindquist, Ph.D., P.E., is a managing principal and the nationwide director of engineering at Brierley Associates. He specializes in solving complex construction engineering challenges, most notably associated with support or excavation, earth retention and deep foundations, for notable public and private sector projects across the U.S.

Peter Faust, Dipl.-Ing., is vice president of business development at Malcolm Drilling Company. He has been involved in the design and construction of numerous large and complex foundation projects throughout the world.