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Case Studies: Helical Piles Installed for Whitefish Bay High School in Whitefish Bay, WI

Sunday, September 25th, 2011 by Andrew Kratochwil

Challenge

The Whitefish Bay High School was originally constructed in 1930. Since then it has received additions in 1941, 1967 and 1991. In 2010 Whitefish Bay High School was to undergo two more additions to the field house and auditorium. The field house would be receiving a new enclosed corridor to prevent students from having to walk outside during the winter months. The auditorium was in receive new orchestra, vocal and band rooms. The field house addition was located under an existing finished second floor limiting the size of construction equipment. The auditorium addition would be constructed along the south wall in an open terrace and consisted mainly of grade beams. Since there had been so many additions the “as built” information was rather limited. It was determined that the new additions would need to be supported on deep foundations to prevent future settlement of the existing structure. Helical piles became the deep foundation of choice due to the low clearance for the field house and since multiple mobilizations would be necessary to complete the project.

Solution

The initial soils investigation included the advancement of 10 borings at the auditorium addition to depths of 25 ft. The field house addition was about 300 yards from the closet boring therefore varying geotechnical conditions were expected to be experienced there. The soil strata generally included about 8-12 ft. of uncontrolled fill with SPT N-values of 6-8 blows per ft (bpf). Medium stiff to stiff silty clay and clayey silts were observed below the fill with the last sample collected showing SPT N-values ranging from 5-6 bpf. It was determined that a deeper geotechnical investigation would be needed to determine a suitable bearing stratum for the helical piles. After the deeper borings were performed it was determined that a Model 287 with a 10”-12”-14” lead section installed to a depth of 25 ft for a 10 kip pile and 37 ft for a 22 kip could be achieved, and a Model 288 with a 10”-12”-14”-14” lead section installed to 35 ft could achieve a 30 kip pile capacity with a factor of safety of 2. Installation of the piles began at the field house addition so that work could be finished prior to winter weather. During excavation of the pile caps and grade beams unknown under ground utilities were discovered causing several piles to be moved slightly to allow for installation. Due to the project having multiple phases FSW had to mobilize to the site on six different occasions over a four week period to install the 82 helical piles.

 

Project Summary

Architect: Bray Architects

Structural Engineer: Ambrose Engineering, Inc.

Geotechnical Engineer: Terracon

Certified Pile Installer: Foundation Supportworks of WI

Products Installed: (59) Foundation Supportworks™ Model 287 Helical Piles, 10"-12"-14" Lead Section(23) Foundation Supportworks™ Model 288 Helical Piles, 10"-12"-14"-14" Lead Section

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