01 July 1997
embedment depth are options that may be considered.
(c) If the penetration resistance suddenly drops and pile damage is not suspected, the pile tip
may have penetrated a subsurface void. The penetration resistance should suddenly increase when the
bottom of the void is reached. Driving of piles at nearby locations will aid the determination of the lateral
extent of this void. A possible complication with subsurface voids is that they may become larger with time
and could eventually compromise the integrity of the foundation. Subsurface voids should be filled with a
cement-bentonite grout when they are found. Pile installation should continue at another location of the site
until the grout has had a chance to set. If the void in question is an underground cavern (as can occur in
limestone), grouting may not be practical because of the size of the void.
b. Problems with High Resistance. Encountered resistance that is too high may be caused by
inadequate driving equipment. If the driving equipment, driving energy, and piles are found adequate, then
soil relaxation may be a cause of the problem. Relaxation may occur in dense, fine, submerged sand,
inorganic silt, or a stiff, fissured clay or by driving a point bearing pile into friable shale or a clay stone. High
penetration resistances may also be caused by obstructions such as cobbles or boulders, an unexpected
dense or stiff soil, large bodies in landfills, interference with adjacent piles, or bedrock.
(1) Driving Equipment and Piles. Driving equipment that does not have an adequate hammer and
driving energy to install the piles will cause high penetration resistances with little or no set of the pile.
(a) The size and type of pile should be checked to be sure that the proper pile is being
installed to support the structure.
(b) Contract specifications will provide the minimum driving energy permitted for the work. The
driving energy should exceed the minimum permitted energy.
(c) If the PDA indicates that driving stresses are low, efficiency of the hammer is adequate,
and pile quality is adequate, energy of the hammer should be increased and driving continued. If the
efficiency of the hammer is low, then driving equipment requires maintenance.
(d) If the PDA indicates that driving stresses are high and near the allowable limits and
hammer efficiency and energy is adequate, then soil relaxation or obstructions may exist. If soil relaxation
or obstructions such as boulders or large bodies are not found, then the soil through which the pile is being
driven may be of high strength and assisted installation described in paragraph 3-4 may be required to
reach the design embedment depth. Soil data recorded during exploration should be checked to determine
if the designers had anticipated high-strength soil at depths above the design embedment depth. Adequate
bearing capacity may be obtainable at a lower embedment depth reducing the required length of the pile
and save on installation costs.
(2) Relaxation. Penetration resistances that continue to increase as the pile is driven may be
caused by soil that increases in strength with increasing depth. Driving may also reduce the pore water
pressures and cause the soil strength to increase for a time.
(a) Small displacement piles such as H-piles or open end pipe piles are recommended if
relaxation effects are significant. Open-end pipe piles can be cleaned periodically during driving to
minimize soil displacement. The pipe pile, after it had been cleaned, can also be filled with an internal core
of reinforcement steel and concrete to achieve adequate pile capacity. Consideration may be given to
withdrawing the pipe pile as the concrete is being placed if the pipe is not needed to support the concrete or
to provide adequate pile capacity.
(b) Driving of the pile should continue until driving stresses reach the allowable limit or until the