TM 5-852-4/AFM 88-19, Chap. 4
operational deficiencies and uncertainties can be used.
sufficient time intervals to permit the plotting of accurate
For short-term, intermittent tension loading, as from
settlement vs. time and settlement vs. load data to 0.001
wind, the problem is less critical; in this case the pile may
inch. The dial gages and their supports shall be
be designed by adaptation of the procedure outlined in
completely protected from direct sunlight, and from
the previous paragraph for safety against frost heave.
precipitation and wind, by a suitable shelter. The shelter
Again, however, the designer must make sure that there
must be ventable to minimize the build-up of heat during
is no possibility of unacceptable degradation or warming
the day. As nearly uniform temperature as practicable
of the ground during the life of the facility, either from
should be sought within the shelter. Air temperatures
natural conditions or from improper practices on the part
within the shelter shall be observed at least hourly at the
of the facility operators, and a very conservative
instrumentation level. Ground temperatures with depth
approach must be taken. The decision may rest in
at test piles shall be monitored daily to establish the rate
considerable part upon how serious would be the
of freezeback following installation and the temperature
probable consequences of a failure.
conditions in the surrounding ground during the load
g. Load testing of piles in permafrost.
testing period. In winter, local heating shall be used as
(1) Pile testing in permafrost may be
necessary to avoid the possibility of frost formation on
required or desirable to obtain data needed for design, to
instruments and consequent malfunction (ordinary
verify design assumptions, and/or to evaluate various
electric light bulbs are often the most convenient source
alternative designs.
In addition to more or less
of such heat). All load increments shall be added with
conventional load settlement and extraction pile tests,
care to avoid producing impact overload.
cyclic, long term static load or lateral load tests may be
(4) For a site where little or no previous
134,168
. In addition to the
needed under some conditions
pile bearing capacity information is available, a minimum
direct load capacity value of the pile test data,
of one exploratory pile load test and one verification pile
considerable useful collateral information can be derived
load test (see below) should be performed during design
in the course of performing a pile test program. Such
investigations. Only a verification pile load test during
information may cover rates and times required for
design or at start of construction may be necessary if
freezeback; ease of driving, augering or drilling;
previous pile bearing capacity information is available for
techniques and problems of mixing and placing slurry
the construction site or the soil formation, or if the job is
backfill, and supplementary foundation soil information
small and a conservative value has been assumed for
(as from auger cuttings). The pile load tests should
design load in lieu of making detailed pile bearing
normally be performed during the facility design studies.
studies. Verification pile load tests should be made in
However, they may also be performed at the start of the
the construction stage on all major projects.
foundation construction if their function is to verify design
(5) In exploratory pile load tests the load is
assumptions, provided opportunity for design adjustment
increased progressively in relatively small increments in
exists.
order to define changes of a pile response with load with
(2) While a pile load test set-up may be
reasonable accuracy. To obtain an estimate of pile load
based on the general methods outlined in ASTM
capacity, figure 4-82 should be used along with the
118
D1143 , the following special procedures must be
procedure illustrated in steps from left to right in figure 4-
followed in testing piles in frozen ground.
83. A standard increment of 10 kips is assumed in the
(3) All vertical instrumentation supports
following discussion. Loading should be continued to
shall be 2-inches or larger pipes driven 20 or more feet
failure, normally defined to occur when the gross
into frozen soil and cased if necessary to isolate them
settlement reaches 1.5 inch, or to 2 /z times the
from frost heave. All instrumentation pipes shall be 8 or
anticipated design load. One example of such a load
more feet from test and anchor piles or load platform
test, in which a loading rate of 10 kips per day was
supports. In addition to checking observations with an
employed, has been shown in figure 4-79. Figure 4-84
engineer's level, the motion of the test pile under load
shows results of another test in which load increments
shall be frequently monitored by dial gages having 0.001
were added at the much slower rate of 10 kips every 4
inch subdivisions and having 2 or more inches of travel.
days, except for a number of loads which were held for
At least three such dials shall be used on round piles, at
longer periods, up to 12 days. Figure 4-84 has been
120 intervals, and four dials on other piles. All dials
prepared with an expanded vertical scale in order to
shall be equally spaced and equi-distant from the pile
show the deflections which occurred under the individual
center, on a common horizontal plane. The dial gage
load increments. The cumulative deflection curve AC
support beam shall be roller-supported at one end to
shows about 0.16 inch greater deflection at the
avoid bending of the beam as a result of thermal
maximum load than that (at point B) computed from the
expansion and contraction forces. During the loading
assumed load distribution pattern, shown on the figure.
and unloading of the pile the dials shall be observed at
This difference cor-
4-140
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