Lessons learned
The most important consideration is to
first verify the installed piles meet the
intended design requirements and the
foundation in functioning as required.
Include driving criteria minimum tip elevations
to assure the production piles are
installed to a similar tip elevation for projects
where the potential for soil relaxation was
indicated during the test pile program. A
non-instrumented set-check can be performed
quickly and easily on production
piles to determine if there is a reduction of
blow counts compared to the end of initial
driving. If a decrease in blow counts during
the set-check is observed compared to the
end of initial driving, then further evaluation
through additional PDA testing is likely
needed to verify the pile capacity, or additional
pile driving.
When evaluating piles for soil set-up,
it can be advantageous to perform shortterm
set-checks, such as 15 minutes to an
hour. When evaluating for potential soil
relaxation, we have found that a minimum
of 24 hours should pass prior to performing
a set-check to determine if a loss of
capacity is occurring. Based on our experience,
the soil relaxation is fully realized in
about 24 hours unlike soil set-up that continues
to have an increase of soil resistance
beyond 24 hours.
One of the biggest challenges we
observed on projects where soil relaxation
is occurring is the significant amount of
hard driving required to achieve the pile
penetration necessary to reach a pile tip
elevation where relaxation would not occur.
On projects where we have encountered
soil relaxation, it was common that over
5,000 hammer blows would be required
to install a single pile and several pile cushion
changes would occur. In one instance,
over 60 feet of plywood pile cushions were
used to install one bent of concrete piles.
Obviously the hard driving over the course
of a project can be costly in materials, time
consuming, increase pile damage risk and
stress the hammer.
On projects where soil relaxation is
occurring, there are things that can be done
to be more efficient when installing piles.
•• The pile type could be evaluated and
changed to a pile with less soil displacement
during driving. Changing the pile
type can be difficult once a project has
begun, so considering the potential for soil
relaxation during the design phase is ideal.
•• The hammer could be evaluated to limit
the hammer ram impact velocity. By
using a larger ram to lower the stroke
height and impact velocity, the increase
in the negative pore water pressure that
is contributing to the temporary additional
driving resistance will be limited.
•• Perform additional PDA testing, such
as 100 percent PDA, to increase confidence
in the foundation capacity to limit
factors of safety or increase LRFD resistance
factors.
•• Perform additional predrilling to a maximum
allowable depth to disturb the
dense soils to allow for easier driving
during initial installation.
•• Install larger/longer piles in phase/portions
by terminating driving operations
when driving becomes difficult and
resume at a later time when the pore
water pressure has stabilized and driving
becomes easier.
As with all installations of deep foundations,
each project site is unique with varying
soil conditions, pile types, constructability
access, capacity requirements, governing specifications,
project personnel, time constraints
and budgets. There are many things to be
considered during pile installation. Soil relaxation
should be one of the things considered
during design and construction of a driven
deep foundation system. t
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82 | QUARTER 4 2014