Expansion of the Dental School at the University of Medicine & Dentistry of New Jersey in Newark, NJ, included construction of a 280 foot by 120 foot, five-story, steel structure with a one-story, below-grade basement. The site was bounded on one side by the existing Dental School, and on the opposite side by 12th Avenue, a major access road for emergency vehicles en route to the adjacent University Hospital. Parallel, underground, electric and telephone duct banks ran along 12th Avenue. The telephone bank lay nearest the site, five feet below grade and within inches of the proposed building.
Subsurface conditions consisted of four to six feet of fill, underlain by natural silts and silty sands containing varying amounts of gravel, cobbles and boulders. Weathered sandstone bedrock was encountered at or above the subgrade elevation of the proposed structure.
Since excavation would be to a depth of 27 feet below grade, a temporary earth retention system was required for approximately 265 linear feet along 12th Avenue and for 125 linear feet along the west side of the site. Also, a permanent retention and underpinning system was required for 60 linear feet on the south side of the project, along the three-story Dental School, where the new building would be constructed on-line and tied in.
Preliminary excavation support system concepts included driven steel sheeting and soldier beams and lagging along 12th Avenue and the west side, and a permanent, on-line, soil nail wall along the Dental School. However, the proximity of the duct banks along 12th Avenue and the presence of rock precluded the sheeting and soldier beam options.
After evaluating the soil conditions, space limitations and the schedule, Moretrench offered a system that entailed a temporary soil nail wall along 12th Avenue and the west side of the site, and a combination of drilled-in minipiles and soil nailing along the existing Dental School. The minipiles would be drilled through the existing column footings to underpin the building and transfer the loads below the proposed subgrade. Soil nailing would be used to support and retain the soil between the existing columns.
The soil nail wall along 12th Avenue and on the west side of the site entailed three to five levels of nails installed at 15 degrees from horizontal on a five foot grid pattern. A three- inch thick layer of shotcrete, reinforced with wire mesh, was used to tie the system together and retain the soils between the nails.
However, during excavation of the first five-foot lift for construction of the permanent soil nail wall along the Dental School, unanticipated loose, cohesionless fill with poor standup time was encountered directly below the existing first floor slab. To assess the actual conditions, Moretrench installed a trial test section of shotcrete and test nails to determine if soil nailing would still be viable. The shotcrete did not adhere to the loose fill, causing sloughing of the material and "cave-in" of the test section. Additionally, the test nails did not achieve the bond values that the design assumed. Given these conditions, Moretrench offered a redesign, consisting of on-line concrete pits, tieback anchors and treated timber lagging to retain the soil between the columns. The minipiles were installed through the column footings, as planned, to transfer the column loads below the new subgrade elevation.
Soil Nail Instrumentation and Monitoring
To evaluate design assumptions and monitor the construction, a comprehensive testing program was implemented, consisting of:
- Pullout testing to verify design adhesion values
- Survey and monitoring to measure horizontal
and vertical displacements
- Strain monitoring to compare the in situ
parameters of the soil nail system with original
Although long-term monitoring was not feasible for this project, data trends indicated that in situ field stresses were similar to or less than theoretical stresses.
Moretrench's design/build approach allowed the proposed foundations to be built ahead of schedule without the need for any design modifications.