Dana Farber Tower, Boston, MA
The Dana-Farber Research Tower is located in the Longwood Medical Area at the northwest corner of Binney Street and Deaconess road. The tower has 14 above-ground stories devoted to office and research laboratory uses and five underground parking levels. The structure occupies the entire site, with the building perimeter as close to the property line as practical. The bottom floor of the garage is at about EL. -9 feet or about 52 feet below the original grade (El. 43 ft). The research tower is abutted by existing structures on three sides of the site and by Binney Street along the southern edge (Fig. 1).
Two orthogonal cross-sections (A-A, B-B) through the center of the site are shown in Figure 2. The stratigraphy of the studied sections is typical of the subsurface of the site. The soil profiles d include the following layers: 1'-14' of miscellaneous fill, over 13' to 22' of sand, 31' to 57'-thick Boston Blue Clay with a 8'-10' yellow crust at the top of the layer, glacial till, and conglomerate bedrock. The bedrock was medium to hard, slightly weathered gray to purple, coarse-grained conglomerate with closely spaced dipping joints. The Rock Quality Designation (RQD) ranged from 28 to 40%. The depth to bedrock varied between approximately 66' and 90'. Based on the information form borings T1-T6 and from boring EC-17 drilled within the limits of the proposed tower and the slurry wall installation, the bedrock surface drops from east to west and from north to south. As discovered by the slurry wall installation the actual bedrock surface between borings was fairly irregular.
The tied back slurry wall provided temporary support during the excavation of the basement, but is not part of the permanent structure of the research tower. Instead it acts as a permanent lateral earth support system and a barrier that isolates the tower from vibrations (mainly from the adjacent MATEP power plant) in order to protect delicate (and expensive) medical experiments. The research tower itself is founded on a series of caissons that bear on the underlying bedrock (Roxbury Conglomerate).
The slurry walls are supported by six- (6) level of tiebacks to bedrock at an average inclination of 45° to the horizontal. All the tiebacks extend to bedrock and have a bonded length of about 20'. On the east wall the first two levels of tiebacks do not exist because of the existence of the MATEP utility tunnel. A prestressed concrete edge was used to provide the necessary lateral support for the east wall, supported at the north and south walls. The design thickness of the slurry wall is 3' feet, and the wall extends a minimum of 2' feet in the underlying bedrock. The slurry wall/bedrock friction was designed to be enough to counteract the lateral forces without the need for the 2' key that provides added safety.
Slurry wall deflections were very small, with a maximum value reached close to dH=0.7" (in the south wall (Fig. 3). Most slurry walls were slightly pulled back for major portions of their length although at some construction stages small inward movements were recorded. Despite the small wall movements or even the pulled back walls, the settlements were excessive in the eastern and western sides. Air drilling caused ground softening and ground losses can explain the occurrence of small wall movements and large settlements (Fig. 4). As indicated by Figure 4 the settlement troughs were typical of excavation projects. Maximum settlements were recorded near the wall and diminished with increased distance. Settlements up to dV=2.8" were measured in the eastern side within the MATEP utility tunnel. Inward east wall movements in combination with the soil losses through the tiebacks contributed to the increased settlements in the MATEP tunnel. Clearly excessive settlements were induced by soil and water losses through the anchor heads. On the slurry walls that the losses through the tiebacks were kept to a minimum the settlements were acceptable and the damage to the adjacent structures was kept to a minimum. Adjacent buildings (Jimmy Fund, and Brigham and Women's Hospital buildings) settled far less than the tunnels and with no recorded damage.
Subsequent analyses by Konstantakos, Whittle, Scharnet and Regalado et. al (2004) demonstrated that finite element simulations with inclusion of soil losses could capture the observed wall and surface deformations consistently. The authors would like to extend their gratitude to Dave Shields by GEI who were the consultants of this project and who gave invaluable insight into this project.
The strain gages placed at the bottom of the slurry wall reinforcement showed that all of the vertical forces induced on the slurry wall by the tiebacks were transferred to the base of the wall. This could be expected because the slurry wall was not allowed to settle and thus no side friction developed between the slurry wall and the retained soil. Induced moments at the bottom of the slurry wall became significant only after the excavation had reached the 5th level (out of 6 levels total) .
The calculations for this case have been checked with DeepEX
The excavation performance was benchmarked excavation with DeepEX matchning actual displacement at 1.02 cm in the last as well as other stages. The model produced a maximum bending moment of 1333 kN-m/m. By comparison, the limit-equilibrium analysis (Peck earth pressures) with the benchmarked parameters produces a wall displacement of 0.22 cm (0.1 inch), 962 kN-m/m (216-k-ft) bending moment in the wrong side. Thus in this case the limit-equilibrium method underestimates bending moments by 40%.
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