reinforced slopes. The soil nail design was based on the guidelines presented in Geotechnical Engineering Circular No. 7: Soil Nail Walls, FHWA-IF-03-017 (2003), AASHTO (1993) and ACI (2000). The results of the analysis provided the final wall and nail geometry. The permanent condition considered was a 90 ft (27.4 m) long 22 ft (6.7 m) high wall with a factor of safety of 1.51. The temporary condition included additional vertical excavation for the perimeter wall footings and therefore was a 27 ft (8.2 m) high wall with a factor of safety of 1.31. within the buffer zone. The Grade 75, epoxy-coated soil nails were installed 15.5 ft (4.7 m) long at a lateral distance of 8.5 ft (2.6 m) from the cemetery. Waler bars were installed at the interface of the soil nails and the shotcrete wall to prevent punching shear failure. The bars, along with the nuts and plates, were galvanized for corrosion resistance. Typically for soil nail wall installations, the nails are tied together with a shotcrete facing installed under high pressure. The shotcrete was a 4,000 psi (27.6 MPa) pump mix with a water-to-cement ratio of 0.39 to the shotcrete, so there were fewer places for voids or shadows. Lastly, in areas with uneven substrates, there was no need to increase the shotcrete thickness to cover a wire mesh. Sacrificial shotcrete test panels were formed throughout the project. Samples measuring 6 in (152 mm) high and 3.75 in (95 mm) diameter were cored from the panels for compressive testing. After three to six days, the shotcrete test results were typically over 80% of the design strength. At 28 days, the com- pressive strength test results were well in excess of the required 4,000 psi (27.6 MPa) North elevation – soil nail layout (courtesy of Berkel & Company, Contractors, Inc.) Section along soil nail wall – facing east The final wall geometry consisted of three to five rows of nails with vertical spacing of 5 ft (1.5 m) between nails and horizontal spacing of 4 ft (1.2 m). The selected nail grout was a 3,000 psi (20.7 MPa) mix with a water-to-cement ratio of 0.40 to ensure an efficient mix that would be easy to replicate in the field. Grout breaks yielded results well in excess of what was required by design. At 28 days, the compressive strength test results from the 2 in by 2 in (51 mm by 51 mm) cube samples were typically 4,200 to 5,800 psi (29 to 40 MPa) and average about 5,000 psi (34.4 MPa) — well in excess of the required 3,000 psi (20.7 MPa) design strength. The permanent wall design had to take into account drainage behind the wall, so a drainage mat and weep holes were installed within the permanent soil nail wall to relieve pore water pressure and surface water buildup. These drainage accommodations were not necessary for parts of the site that had temporary soil nail walls. The soil nail wall was ideal because only limited encroachment was required provide an efficient mixture that was still workable by the shotcrete nozzleman. To expedite construction, Novocon steel fibers were put into the shotcrete mix in lieu of using a continuous welded wire mesh. The ratio of steel fibers to concrete was 50 lbs (18.7 kg) to every 1 cu yd (0.76 cu m) of concrete. Although not often used in New York City, fiber-reinforced concrete has been used in New Zealand and Australia for over 20 years. The steel fibers are designed to enhance concrete in its hardened state. The fibers were added to the concrete mix as it was poured from the truck into the concrete pump for the shotcrete facing. Time was saved dur ing cons t ruc t ion because there was no need to install a continuous we lded wi r e me sh. Additionally, there was less steel around which to shoot design strength (typically 5,500 to 6,500 psi [37.9 to 44.8 MPa] and average about 6,000 psi [41.4 MPa]). The soil nail design was reviewed by numerous agencies, including the Dormitory Authority of the state of New York and its Department of Environmental Conservation, before construction could begin. One complication was that the soil nails had to be drilled into the buffer zone meant to protect the archaeologically- Soil nail construction – applying shotcrete facing – facing northeast DEEP FOUNDATIONS • MAR/APR 2015 • 69
