EI 02C097
01 Jul 97
dissipation of pore water pressures and consideration of
(4) Tension test. Axial tension tests may be conducted
freeze or relaxation.
according to ASTM D 3689 to provide information on piles
that must function in tension or tension and compression.
d. Axail load tests. Axial compressive load tests should
Residual stresses may significantly influence results. A
be conducted and recorded according to ASTM D 1143.
minimum waiting period of 7 days is therefore required
The quick load test described as an option in ASTM D
following installation before conducting this test, except for
1143 is recommended for most applications, but this test
tests in cohesive soil where the waiting period should not
may not provide enough time for some soils or clays to
be less than 14 days.
consolidate and may underestimate settlement for these
soils. The standard load test takes much longer and up to
(5) Drilled shaft load test using Osterberg Cell. Load
several days to complete than the quick load test and will
tests are necessary so that the design engineer knows how
measure more of the consolidation settlement of
a given drilled shaft would respond to design loads. Two
compressible soils than the quick load test procedure.
methods are used to load test drilled shaft: the Quick Load
However, neither the standard test nor the quick test will
Test Method described in ASTM D 1143 standard, and the
measure all of the consolidation settlement. The cyclic load
Osterberg Cell Method.
test will indicate the potential for deterioration in strength
with time from repeated loads. Procedures for load tests
(a) Unlike the Quick Load ASTM test method which
are presented:
applies the load at the top of the drilled shaft, the Osterberg
cell test method applies the load to the bottom of the shaft.
(1) Quick load test. The load is applied in increments
The cell consists of inflatable cylindrical bellow with top
of 10 to 15 percent of the proposed design load with a
and bottom plates slightly less than the diameter of the
constant time interval between load increments of 2 minutes
shaft. The cell is connected to double pipes, with the inner
or as specified. Load is added until continuous jacking is
pipe attached to the bottom and the outer pipe connected to
required to maintain the test load (plunging failure) or the
the top of the cell (Figure 6-4). These two pipes are
capacity of the loading apparatus is reached, whichever
separated by a hydraulic seal at the top with both pipes
comes first.
extended to the top of the shaft. The outer pipe is used as
a conduit for applying fluid pressure to the previously
(2) Standard load test. Load is applied in increments of
calibrated cell. The inner pipe is used as a tell-tale to
25 percent of the design load and held until the rate of
measure the downward movement of the bottom of the cell.
settlement is not more than 0.01 inch/hour but not longer
It is also used to grout the space between the cell and the
than 2 hours. Additional load increments are applied until
ground surface and create a uniform bearing surface. Fluid
twice the design load is reached. The load is then removed
used to pressurize the cell is mixed with a small amount of
in decrements of 50, 100 and 200 percent of the design
water - miscible oil. The upward movement of the shaft is
load for rebound measurements. This is a proof test if no
measured by dial gauge 1 placed at the top of the shaft
further testing is performed. A preferred option of the
(Figure 6-4). Downward movement is measured by dial
standard load test is to reload the pile in increments of 50
gauge 2 attached to the top of the inner pipe above the
percent of the design load until the maximum load is
point where it emerges from the outer pipe through the
reached. Loads may then be added at 10 percent of the
hydraulic seal.
design load until plunging failure or the capacity of the
equipment is reached. This option is recommended to
(b) After drilling the shaft, the Osterberg cell is welded
evaluate the ultimate pile capacity.
to the bottom of the reinforcing cage, lifted by crane, and
inserted carefully into the hole. After proper installation
(3) Repeated load test. The standard load test is initially
and testing, the cell is grouted by pumping a carefully
performed up to 150 percent of the design load, allowing 20 minutes
monitored volume of grout through the inner pipe to fill the
between load increments. Loads are removed in decrements equal
space between the cell and the bottom of the hole. When
to the load increments after 1 hour at the maximum applied load.
the grout is set, concrete is pumped to fill the hole to the
Load is reapplied in increments of 50 percent of the design load
desired level and the casing is pulled. After concrete has
allowing 20 minutes between increments until the previous
reached the desired strength, the cell is pressurized
maximum load is reached. Additional load is then applied and
internally to create an upward force on the shaft and an
removed as described in ASTM D 1143. This test is useful to
equal and opposite downward force in end bearing. As
determine deterioration in pile capacity and displacements from
pressure increases, the inner pipe moves downward while
cyclic loads.
the outer pipe moves upward. The upward movement is a
function of the weight of the drilled shaft and the friction
6-13
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