Providing
Value with
Driven Piles
Mountain America Credit Union campus
By Brian Tolley, Ralph L. Wadsworth Construction Company, LLC
The Mountain America Credit Union (MACU) campus
located in Sandy, Utah, contains one of the tallest office
towers constructed outside Salt Lake City’s urban core. The
campus consists of an 11-story, 327,000 sq. ft. office building that
can accommodate 1,700 employees and an attached parking structure
containing 1,800 parking stalls. These modern buildings, along
with a fitness center, a one-acre park and an outdoor amphitheater,
represent a bold and iconic space, not only for Mountain America,
but for the city of Sandy. Sandy Mayor, Tom Dolan, said the project
was “everything that we’d hoped for that area” and a major advance
in the city’s goal of building out its suburban downtown.
In September 2016, Ralph L. Wadsworth Construction
Company, LLC (RLW) was approached by the project’s owner
to provide the design and installation of the deep foundations
required to support the new buildings. At that time, the structures
were in the concept phase and the owner was looking for the best
options to accommodate the building’s high loads on a site with
relatively low bearing pressures.
The subsurface conditions encountered as part of the geotechnical
studies prepared for this project were described as being relatively
consistent, consisting of soft to medium stiff lean clays that
extended to depths ranging from 39 to 44 feet below the ground
surface, followed by dense to very dense sand that extended to
depths ranging from approximately 59 to 64 feet. Another layer of
soft to medium stiff lean clay was below that sand layer, followed by
dense to very dense sand and gravel at depths ranging from 90 to
95 feet below ground. Groundwater was noted and reported to be
at depths ranging from seven to 10 feet below grade.
RLW had performed work in the area and some aspects of the
geotechnical report contradicted its experience. Additional testing
was requested and several cone penetration tests (CPTs) were performed
in order to further assess the subsurface conditions.
Those added CPT soundings helped RLW further understand
the site. They identified the fact that the subsurface conditions
were not as consistent as the findings published in the geotechnical
report. The presence of the upper sand layer was highly
variable and although the lower sand and gravel layer was present
throughout the site, its top elevation fluctuated significantly. The
soundings also provided data that helped RLW model the potential
capacities of the different geotechnical layers.
As part of the report, the geotechnical engineer had initially
recommended 36-inch diameter ACIP piles for the tower building.
Conceptually, each ACIP pile would extend down to the lower
sand and gravel layer – approximately 90 to 95 feet below ground.
Alternatively, and as a result of the information gathered through
CPT investigation, RLW suggested driven piles.
An initial static pile capacity analysis suggested that the
required capacity could be achieved using driven 16" × 0.312" steel
pipe piles that would extend to either the lower sand and gravel
layer or to the upper sand layer, if present. The owner accepted
72 | ISSUE 6 2018