Study Budd Terrace Elevator

Situation

The listed General Contractor was hired for renovations to an existing seven story building. Geo-technical evaluation confirmed unsuitable soils for the construction of an elevator shaft on a typical shallow foundation. Soil borings concluded 12' to 14' of loose to firm fill was encountered. Helical piles were recommended as the lowest risk option for support of the elevator addition. The design called for fourteen (14) helical piles with an allowable compression capacity of 40 Kips per pile and an allowable uplift capacity of up to 10 Kips per pile. The geo-technical testing firm estimated pile depths ranging from 16' to 18' below existing grade; which is where partially weathered rock was identified.

Scope of Work

At the direction of the structural engineer, a test pile was first installed and load tested for confirmation as to the suitability of the selected foundation support system. Four (4) reactionary helical piles were installed around the test pile to depths of 14 feet each. The test pile was installed to a depth of 16 feet. A 12,500 ft.-lb. hydraulic drive head mounted on a Bobcat S250 skid steer loader was used to install the helical piles for this job. The limited access to the site location restricted the use of any larger equipment for installation. The load testing was overseen by the engineering firm in accordance with standard loading procedure of ASTM-D 1143-81. Deflections were measured using a single dial gauge accurate to 0.001 inch. The results of the load test indicate a total deflection of 0.42 inches and a net deflection of 0.29 inches at the design working load of 40 Kips. The testing engineering firm concluded that the Cantsink Helical Piles would be able to support the allowable working loads of 40 Kips per pile.

Following the excavation phase of the project, pile locations were marked and installed using the same equipment as the test pile. The base of the elevator pit was 5 feet below grade, which will reduce the pile depths based on soil borings at grade level. All piles were installed to depths of 10.5 feet. Final pressure reading of torque for the piles ranged from 2600 to 2750 psi for the 12,500 ft lb drive head. Pile installation was completed within a 4 hour period.

Piling System

The piling system selected was a 3.5 inch OD schedule 40 steel shaft with an 8-inch diameter lead helix and a second 14-inch diameter helix located approximately 42 inches above the lead helix. The piles were manufactured and installed by Cantsink. Pipe caps for each pile included a 10" x 10" x 1" plate sleeved into the top of pile and through bolted for uplift restraint. Working loads of 40 Kips per pile was achieved.