Time is Money
Savings from testing the driven pile foundation of a high-rise building
When evaluating the cost-effectiveness of a constructed
deep-foundation system, an appropriate question to
ask is, “What would an alternative viable foundation
have cost?” The difficulty when addressing this question is that
a structure’s foundation is virtually never constructed twice, two
or more different ways. The cost of the constructed foundation is
known, but the costs of alternatives are estimates.
As is typically the case, this project’s driven pile foundation
was constructed once. As is typically not the case, this project’s
driven pile foundation was designed seven times using seven different
allowable pile loads associated with various testing scenarios.
Foundation costs were evaluated for piles designed both with and
without the benefit of capacity contribution from set-up. Using
as-built costs for the driven piles, pile cap and core mat concrete
and construction-control testing, the total foundation costs for the
seven different designs using different allowable pile loads were
compared. This comparison demonstrated the savings (both time
and money) resulting from increased testing and associated higher
allowable pile loads.
Introduction
Column service loads for the high-rise building range from 1,400
to 4,200 kips. Below basement excavation level, lean clay extended
to a depth of approximately 28 feet. Underlying deposits consisted
of silty sand, to silt with sand, exhibiting increasing relative density
with depth, to a depth of approximately 115 feet, with hard lean
clay (glacial till) below.
Based on a review of the project service loads, it was determined
that the project would benefit from a program to maximize
allowable pile loads while maintaining reasonable installation
methods, including readily available pile sections and hammer
types and sizes. The project’s design team reviewed the option of
using 16-inch-diameter closed-end pipe piles, having a nominal
wall thickness of 0.50 inches. It was estimated that piles with an
allowable geotechnical load on the order of 500 to 600 kips could
reasonably be obtained utilizing a carefully planned and executed
test program, which included both dynamic and static load testing.
A 600-kip allowable load on an unfilled pipe pile results in a steel
stress of 24.7 kips per square inch, necessitating that the pipe be
filled with concrete after driving and that the concrete be credited
with contributing to the pile’s structural resistance.
This project’s foundation piles were designed using allowable
stress design (ASD). In ASD, allowable geotechnical load is equal
to ultimate geotechnical capacity1 divided by a safety factor. This
range of potential allowable pile loads was predicated on the ability
to install the 16-inch piles to long-term ultimate capacities on the
order of 1,000 to 1,200 kips and a test program scope which justified
using a safety factor of 2.0.
lainen / 123RF Stock Photo
By Van E. Komurka, P.E., D.GE, F. ASCE and Adam G. Theiss, P.E., S.E.
64 | ISSUE 3 2018