closely jointed to broken, with weathered and occasionally soil-filled joints. Rock core recoveries range from 0% to 100%, with average of about 69%. The Rock Quality Designation (RQD) ranges from 0% to 88%, with an average of only 12%. Rock core recoveries and RQD did not increase with depth as is typically encountered. As indicated on the historical maps, the Portion of 1865 Viele map with a regular truck-mounted rig. It was decided that an additional comprehensive investigation was required due to the foundation load demands of the new design, the foundations and excavation support systems envisioned for the new design, and based on a review of the rock cores from the initial investigation. The excavation for the new design would extend well into the bedrock, and the foundations very likely needed to be high- capacity deep foundations to resist the large compressive and tensile forces from the shear walls. Of particular interest in the supplemental investigation was the characteristic of the bedrock because the rock cores from the initial investigation exhibited extensive weathering that did not improve with depth. In 2013, after the existing building was demolished and the site was backfilled to street level, a supplemental investigation consisting of 12 borings was performed to further investigate the strength and permeability of the weathered rock. All additional borings were performed at the proposed foundation locations, and were advanced 15 to 75 ft (4.6 to 22.9 m) into bedrock, where the depth extended at least as deep as the assumed socket length of the deep foundations. This investigation confirmed that, in general, the subsurface conditions at the site consist of surficial fill over compressible clay, underlain by sand and silt, glacial till and decomposed rock overlying bedrock. The bedrock exhibits a profile of weathering that does not improve with depth. Bedrock Quality — Rock core samples recovered in the 2013 investigation indicate that the quality of bedrock is highly varied. It is generally described as slightly- to-highly weathered gray schistose gneiss with occasional pink pegmatite intrusions, 74 • DEEP FOUNDATIONS • MAR/APR 2017 bedrock beneath the site lies in a highly- weathered zone due to a couple of ancient faulted zones that transect the site. The bedrock underlying Manhattan commonly has faulted and jointed zones, likely dating back to the Mesozoic Era, that trend in a north-west direction. A NW trending fault that runs below the site has been offset along another faulted zone with a north-east trend. This offset creates a deep crisscrossing fault pattern across the site, and has created deep zones of fracturing within the bedrock that can be seen in the rock core photo. Bedrock Permeability — Packer permeability tests were planned in isolated zones of the bedrock for permeability testing. However, due to the poor quality of rock encountered, variable head permeability tests (both falling and rising head tests) using piezometers that were screened and sealed in the rock were performed instead. The tests indicated that between 1 x 10 and 1 x 10 cm/sec. -5 the permeability of rock at the site is -4 Design Challenges Due to the geographical constraints, difficult subsurface conditions, and the nature and sensitivity of the adjacent structures, the design of the foundation and excavation support system posed several challenges. The design team collaborated closely to implement an effective solution that considered schedule, cost and constructability. Photo of a representative rock core Historic streams ran along these fractured zones and further weathered the rock; the effect of this intense weathering can be seen in the sometimes soil-like consistency of the rock. Veins of intrusive rock are present in the rock, including pegmatite, a very hard igneous rock. The intrusions can be seen in the sporadic light pink colored zones in some of the rock cores. As rock tends to weather more at the contact zone between two different rock types, it is apparent from the rock cores that it has helped create deep interconnecting weak zones within the bedrock. High Foundation Demand — The new condominium tower was designed to occupy the entire lot, rising over 750 ft (229 m) into the New York City skyline and dwarfing all other structures in its immediate vicinity. It features full floor residences, with floors cantilevering approximately 24 ft (7.3 m) over the five- story historic building to the east. Considering that the site is only 60 ft (18.3 m) wide in the east-west direction and the cantilevered portion of the building is 24 ft (7.3 m) wide, the eccentric gravity loading, coupled with external forces such as wind,
