01 July 1997
(1) Low Resistance.
(a) Impact Hammers. Low resistance for impact hammers is generally defined when the number
of blows of the hammer is less than 14 blows/centimeter (36 blows/foot). Under these conditions, several
possible events must be watched for, including tension cracking in concrete piles, the result of excessive
energy returning from the pile toe and not going into the soil; pile run, which can be damaging to the
hammer by forcing high impact loads on the hammer frame, and also the possible loss of control of the pile;
and inability to use a diesel hammer because it cannot start in low-resistance conditions. Especially on
concrete piles, the hammer energy should be reduced during the first part of a pile' penetration to avoid
both pile and hammer damage. If moderate to high resistance was anticipated, then low resistance
suggests that the soils analysis requires reexamination. This is especially true with cal-careous soils.
Consistent low resistance on a job can also indicate that the hammer selected is too large.
(b) Vibratory Hammers. Vibratory hammers experience little difficulty in operation due to low
resistance. The largest problem with these hammers in low resistance is keeping the pile straight, especially
since a vibratory hammer is generally run free hanging from the crane, and there is little or no lateral support
for the system. If a vibratory hammer is operating at a high hydraulic pressure or amper-age during low
resistance driving, then there may be difficulty with the vibratory hammer' operation.
(2) High Resistance.
(a) Impact Hammers. High resistance for an impact hammer is any resistance above
94 blows/centimeter (240 blows/foot). Under these conditions, excessive rebound may take place,
damaging the hammer and pile top; the pile may be severely damaged by hitting an underground
obstruction; and the job may be seriously elongated, which may create contract problems. Consistently high
resistance on a job may indicate that the hammer is too small for the job.
(b) Vibratory Hammers. With vibratory hammers, the main problem with high resistance is
overloading the hammer. If the hammer operates at consistently full hydraulic pressure or amperage during
driving, the hammer is probably too small for the job, and may also overheat or fail in another fashion.
Serious mechanical failure of the hammer is also likely if the toe of the pile has contacted rock. High
resistance will also lead to excessive clamp heating and wear, and steel piling should not be driven at
penetration speeds less than 1 foot/minute.
2-2. DESIGN VERIFICATION. The design will be checked in the field prior to or during initial installation of
the piles that had been selected to be sure that the pile foundation will have adequate capacity to support the
expected loads. This field check will consist of a feasibility analysis, installation of indicator piles with pile
driver analyzer (PDA) equipment and wave equation analysis. Indicator piles will be driven prior to load
testing. Pile load tests are recommended and should be completed for economically significant projects.
Driving of indicator piles and load tests should be handled in a separate contract from the construction
project or as a minimum accomplished prior to the ordering of the bulk of the production piles. Pile lengths
should be determined based on final results from the indicator and pile load tests. Refer to chapter 6 of
TM 5-809-7 for further information on verification of the design.
a. Feasibility of Foundation Selection. The type of pile, length, and dimensions selected during the
design process will be checked to be sure that this is the most economical and suitable foundation to
support the expected loads for the soils observed during the exploration program.
(1) Examination of Exploration Data. A thorough exploration program will have been completed to
evaluate the design parameters used to determine the optimum foundation. The program should span the
full range of site conditions.
(a) Decisions to limit predesign and preconstruction site investigation efforts to save money
and time have risks. These risks could potentially result in catastrophic cost escalation/delays due to