Soil Conditions and Foundation Selection The subsurface conditions at the site consisted of 23.5 ft (7.2 m) of fill starting at El. +82 ft (El. +25.0 m) (existing exterior grade). This fill consisted of sandy silt and poorly graded sand with gravel, with SPT blow counts varying in N-value from 3 to 9 blows/ft (blows/0.3 m). Below the fill layer was a 10 ft (3.05 m) deep layer of well-graded gravel and clayey sand with N-values ranging from 36 to 54, which was underlain by a disintegrated rock layer extending down to El. +20.7 ft (El. +6.3 m). The existing steel- Generalized soil profile framed structure, origi- nal ly bui l t in 1917, consists of five above- grade stories and one basement level . The structure is supported by perimeter wall and col- umn footings in addition to 42 interior spread foot- ings. The existing exterior and interior footings were bearing at about El. +76 ft (El. +23.16 m). The new basement would lower the bottom of the foundations approximately 14 ft (4.3 m) to El. +62 ft (El. +18.9 m). The additional below- grade level was estimated to increase the load on the foundat ions approxi- mately 15%. Although interior spread footings and exterior wall footings had been used to support the existing structure, the increased building load and stringent settlement requirements precluded the use of shallow foundations. Therefore, the building instead would have to be supported and stabilized by a new deep foundation system, which had to be installed prior to the excavation to the new subgrade elevation. In addition to supporting the loads of the final structure, the selected deep foundation system would be required to support the structure during excavation. Because of the load demands, stringent settlement requirements, difficult access and installation in low headroom conditions, cased micropiles were selected as the primary deep foundation system. Micropiles with a nominal outside diameter of 7 in (178 mm) were selected to support the exterior walls and columns, and micropiles with a nominal outside diameter of 9-5/8 in (245 mm) were selected to support the interior columns. The use of micropiles would not only meet the service requirements of the project, but would also be amenable to the logistical constraints present at this site. Along the perimeter of the building, an excavation support system was needed in addition to vertical support of the exterior walls and columns. Underpinning pits were selected, which would provide both the excavation support and the vertical support for the exterior of the existing building. Because of the tight settlement requirements, it was determined that the underpinning pits would need to either extend to bedrock or be supported on piles. It was determined that micropiles would be used to support the pits, which would be faster and more economical. Access Limitations The 1785 Massachusetts Avenue project had very challenging logistics. The building was very tight to the property line around the entire perimeter, so exterior staging areas were limited. Staging inside the building was also limited, as material storage would impede construction equipment and workers. It became necessary to establish an offsite staging area a few miles away from the jobsite. Materials were brought to the site on an as-needed basis, unloaded from the street and immediately brought into the building. In addition, the streets and sidewalk adjacent to the building had to remain accessible to the public throughout construction. Access into site Access into the building was initially limited to a single 5 ft x 7 ft (1.5 m x 2.1 m) basement door, which was at the bottom of a 42 in (1,067 mm) wide switch-back ramp. Materials had to be manually carried into the building through this door. There was a short, narrow alley on the east side of the building, and, at the end of this alley, a hole was cut in the side of the building and a ramp was constructed, which provided another point of access. However, the alley was too narrow and the ramp too steep to allow for vehicular traffic. Materials would be placed at the top of the ramp then dragged down the ramp and into the building. The work area was effectively an enclosed space, which necessitated constant mechanical ventilation and electric lighting. Electric and water lines needed for construction had to be run overhead to not impede the construction activities. Perimeter Support To support the 25 perimeter columns and exterior walls, and to provide support of excavation for the project, underpinning pits were dug to a depth of approximately 17 ft (5.2 m). Each underpinning pit was supported by battered micropiles, which were installed prior to digging the pits. The micropiles had a nominal outside diameter of 7 in (178 mm) and reduced the potential for differential settlements between the interior and perimeter columns. At the top of each underpinning pit, three 8 in (203 mm) deep channels were formed to create a space for 150 ton (1,335 kN) flat-body hydraulic jacks, which could be used to adjust the elevation of the perimeter columns, if necessary. DEEP FOUNDATIONS • SEPT/OCT 2017 • 15
