Static load tests are performed
on piles to determine the accuracy of dynamic load test results and to
determine if the capacity of the pile being tested has increased or decreased
after it has set in the ground for some period of time. The intent of performing the static load test
is to potentially reduce the driven length of pile by fine-tuning the pile
capacity determination. There must be a
substantial amount of piling at a structure to justify the expense of a static
load test. Generally, 10,000 feet of
piling (all of the same size and ultimate bearing value) is the amount required
before specifying a static load test.

Static load tests are not
performed very often and can be complex and must be performed properly for the
results to be useful. Always contact the
Office
of Construction Administration or the Office
of Geotechnical Engineering for assistance before performing a static load
test.

With the 2013 C&MS, the
Department adopted the ASTM
Quick Load Test method for static load testing. The duration of the static
load test using this method is generally 8 to 10 hours. Using the previous method, the test could
take longer than 24 hours. The other significant change with the 2013 C&MS
is that the Contractor is responsible for taking the readings. Previously, the Department was responsible
for taking readings during the test.

The Office
of Construction Administration or the Office
of Geotechnical Engineering must be consulted before non-performing the
static load test. Consult either of
these two offices to determine if a subsequent static load test should be
performed.

Static load tests are not
performed on piles driven to refusal on bedrock.

Most static load tests are
performed on cast-in-place piles (also called pipe piles or tube piles). The static load test will place a load on the
test pile that is twice the Ultimate Bearing Value (UBV);
therefore, the test pile may need a thicker wall than that required by 507.06
or the Contractor will have to fill the pile with concrete and allow the
concrete to cure for 5 days.

For test piles, if the wall
thickness is less than the required wall thickness given by the equation below,
then the Contractor must fill the pile with concrete and allow the concrete to
cure for 5 days before performing the static load test.

Where:

t =
Shell wall thickness (inches)

R =
Ultimate Bearing Value, UBV (pounds)

D =
Diameter of pile (inches)

If the test pile wall
thickness is equal to or greater than the wall thickness given by the above
equation, the Contractor can perform the static load test 72 hours (3 days)
after he has finished driving the test piles and anchor piles. Generally, piles with a diameter of 16 inches
or greater will meet the minimum wall thickness requirement for the test pile. Depending on the UBV,
12-inch and 14-inch piles may not be available with the minimum wall thickness
for test piles. These piles will have to
be filled with concrete before the static load test.

The test pile can be, and
usually is, a production pile. The test
pile must be vertical. Sometimes people are concerned about “failing” a
production pile. This should not be a concern.
In fact, it is desirable to “fail” the test pile as when this happens,
we determine the true capacity of the pile and achieve the most value from the
test. In the case where a test pile,
which is also a production pile, “fails” during the static load test at a
capacity lower than required, the solution is usually as simple as splicing on
more pile length and driving the pile deeper.

The standard plan note for a
static load test requires the Contractor to drive four piles, not including the
anchor piles. The first two driven piles
are test piles. Each one is driven to
the required UBV as determined by the dynamic load
test. This means both of these piles are
dynamic load tested (that is one dynamic load test item). Do not overdrive these first two piles. Perform the static load test on one of these
two piles. If the first pile is not
suitable to use for the static load test, the second pile is driven as a
backup. The dynamic load test on the
second pile gives additional data that can be used to interpret the static load
test results. The third and fourth piles
are driven to reduced blow counts 75 and 85 percent of the driving criteria.
For example, if the driving criteria from the dynamic load
test is 40 blows per foot (bpf), then the
third pile is driven to 30 bpf (0.75 × 40), and the
fourth pile is driven to 34 bpf (0.85 × 40). It is important that the third and fourth
piles be shorter than the test pile, so the person interpreting the test data
can make an accurate evaluation. The
exact method of determining shorter length is not critical. They can also be driven to 75 and 85 percent
of the length, or some other lengths shorter than the test pile. The third and fourth piles are also dynamic
load tested (one more dynamic load test item).

In some soil, piles will gain
capacity with time. This is called pile set-up or sometimes, pile freeze. The reduced capacity piles are driven to see
if the pile will gain enough capacity with time to meet the Ultimate Bearing
Value requirement. If there is some
increase in capacity, but not enough to meet the UBV,
then we can still estimate the appropriate driving criteria if we know the
amount of pile set-up. To do this, we
need to know the capacity at the end of the initial pile driving, which we can
determine from the dynamic load test.
This is why we dynamic load test the reduced capacity piles.

Production piles can be used
as anchor piles. Vertical piles are
preferred for the anchor piles, but the Contractor may use battered piles for
the anchor piles as long as the battered piles are symmetric around the test
pile. The Contractor determines the
number of piles to use as anchor piles and the required length of
penetration. Anchor piles must be at
least 8 feet (2.5 m) from the test pile, measured from center-to-center. If the anchor piles are also production
piles, they may need to be re-driven to the required driving criteria after the
static load test, depending on the test results.

Generally, the test pile
should be driven before the anchor piles, but this is not required. If the anchor piles were driven first and
there was a problem with the first test pile so that the backup test pile had
to be tested, then the Contractor would have to drive additional anchor piles
around the backup test pile.

No other production piles are
to be driven until after the results of the static load test have been interpreted.
However, the Engineer can allow the Contractor to proceed with pile driving at
his own risk, with the understanding that the Department will not pay for
piling driven deeper than required based on an evaluation of the static load
test results.

The Contractor must wait at
least 72 hours (3 days) after driving the test pile and anchor piles before
applying the test load. If the
Contractor has filled the test pile with concrete because it does not meet the
minimum wall thickness requirement, then the Contractor must wait 5 days before
applying the test load. The plan notes
may require a longer waiting period.

The test pile should be cut
off as near to the ground as practical and the jack placed along the axis of
the pile with full bearing on the required load cell and bearing plate.

The Contractor must furnish a
set of gauges or devices capable of accurately determining settlement of the
pile to 0.001 inch (0.025 mm) and a calibrated load cell for determining the
load applied. Dial gauges must have
graduations every 0.01 inch (0.25 mm) or less.

The gauges used to measure
the settlement of the pile should be placed opposite each other and should be
placed at the sides of the pile. They
should be supported from posts or fixed objects. The post or fixed objects are to be
independent of the test load set-up and at least 4 feet (1.25 m) away from the
test pile. However, the gauges should be
placed as close to the test pile as possible.
Dial gauges are furnished and should have sufficient travel to measure
up to 2 inches (50 mm). A backup system
is required in case of problems with the gauges. The backup system usually consists of a ruler
applied to the test pile, with a mirror and string line for measuring
settlement.

The primary means of
determining the applied load is a calibrated load cell. The pressure gauge on the hydraulic jack can
be used as a backup load measuring system.
The pressure gauge alone is not accurate enough for the static load
test. Also, if the jack should bind up,
the hydraulic pressure would increase while the load transmitted to the pile
would not necessarily increase.

The load is to be applied in
increments consisting of a first increment of 1/10 the UBV
of the pile (R). The Contractor records
the dial gauge readings for each gauge 1, 2, 4, 8, and 15 minutes after each
load increment is applied. Calculate the
average pile settlement from the gauge readings. Apply the next load increment after the 15
minute reading.

Maintain the test load during
each load increment. Due to settlement
of the pile, the load and pressure in the jack may decrease with time. The Contractor should run the hydraulic pump
as necessary to maintain the load on the pile.

Continue to increase the test
load until the load is twice the UBV or the pile
reaches plunging failure. Plunging
failure is defined when continuous jacking is required to maintain the test
load.

After loading is complete or
plunging failure is reached, unload the pile in five equal decrements in 15
minute intervals. Record the settlement
readings at 1 and 15 minutes after each load decrement. After the entire test
load has been removed from the test pile, record the settlement after 1 and 15
minutes.

If it is necessary to remove
and reapply the load, such as a problem with the jack or load cell, reapply the
test load using the same procedure used to apply the initial loads.

The test load Ultimate
Bearing Value (Q_{f})
is the maximum capacity of the test pile.
To determine Q_{f},
it is necessary to plot the settlement of the top of the pile versus the load
on the pile. Draw a straight line
through the zero point and the theoretical elastic settlement of the pile using
the equation in the specification. This line represents the elastic compression
of the pile (the distance that the pile compresses under the test load). Draw another line parallel to the first, but
offset by the distance given in Equation 506.1. The second line is called the Davisson
criterion line.

0.15 inch + 0.008 D ( 3.8 mm + 0.008D )

Equation
506.1 – Settlement Offset

Where:

D =
Diameter of pile (inches or millimeters)

The
point where the load-settlement curve from the static load test intersects the
Davisson criterion line is the test load Ultimate Bearing Value (Q_{f}). An example from an actual static load test is
shown in Figure 506.A.

In some cases, the slope of
the unloading portion of the curve can be used to determine the elastic
compression of the pile.

If the load-settlement curve
does not intersect the Davisson criterion line, then Q_{f} is equal to the greatest test load
applied (2R).

Figure 506.A
– Load-Settlement Curve

The standard plan note for a
static load test requires the Contractor to restrike the test pile and the two
reduced capacity piles. The note may
require re-striking the backup test pile.
The restrikes are performed at least 7 days after the piles were
driven. Each restrike test consists of
dynamically testing two piles and determining the capacity of the piles. Pile set-up is the increase in capacity for a
pile from the time it was initially driven to the time it was re-struck. Ideally, the capacity from the restrike test
on the test pile should be close to the results of the static load test.

Either the Office
of Construction Administration or the Office
of Geotechnical Engineering will review the pile driving logs, the static
load test results, the dynamic load test results, and the restrike test
results, and then recommend the driving criteria for the production piles. Wait for the recommended driving criteria
before letting the Contractor drive the rest of the production piles. However,
the Engineer can allow the Contractor to proceed with pile driving at his own
risk, with the understanding that the Department will not pay for piling driven
deeper than required based on the recommended driving criteria after the
evaluation of the static load test results.

The static load test results
will apply to the same type and size of piling, driven with the same type of
hammer, to approximately the same depth, with similar driving characteristics
as the test pile.

The pile hammer used for
driving the test pile shall be used for driving all piles represented by the
static load test. If the Contractor
subsequently finds it necessary to use a different size and type of hammer, the
Office
of Geotechnical Engineering or the Office
of Construction Administration will determine if an additional static load
test is required; any such additional static load test shall be completed at no
additional cost to the Department.

Documentation for the static
load test consists of the driving logs of the test piles and reduced capacity
piles, the dynamic load test report, the restrike test report, and the
load-settlement readings from the static load test. Identify the piles using the pile numbering
system on the plans if the piles tested are production piles.