Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 238

thickness should be adequate to resist axial and lateral loads as well as the
stresses during pile driving.
According to Smith (1962) , it is possible to predict approximately the stresses
during pile driving using the principles of one-dimensional elastic stress wave
transmission by carefully selecting the parameters that govern the behavior of
soil, pile, cushions, capblock and hammer.
The above approach may also be used to optimize the pile hammer cushion
and capblock using the computer analysis commonly known as wave equation
analyses. The design penetration of driven piles should be determined, rather
than correlation of pile capacity with the number of blows required to drive
the pile a certain distance into the seabed.
If a pile stops before it reaches design penetration, one or more of the fol-
lowing actions can be taken:
1. Review of all aspects of hammer performance, possibly with the aid of ham-
mer and pile-head instrumentation, which may identify problems that can be
solved by improving hammer operation and maintenance or by the use of a
more powerful hammer.
2. Reevaluation of design penetration by reconsideration of loads, deforma-
tions and required capacities of both individual piles and the foundation
as a whole, which may identify reserve capacity available. An interpretation
of driving records in conjunction with the instrumentation mentioned above
may allow design soil parameters or stratification to be revised and pile
capacity to be increased.
3. Modifications to piling procedures (usually the last course of action), which
may include one of the following: The soil plug inside the pile is removed
by jetting and air lifting or by drilling to reduce pile-driving resistance. If
plug removal results in inadequate pile capacities, the removed soil plug
should be replaced by a gravel grout or concrete plug having sufficient
load-carrying capacity to replace that of the removed soil plug. Attention
should be paid to plug/pile load-transfer characteristics. Note that plug
removal may not be effective in some circumstances, particularly in cohe-
sive soils.
Soil below the pile tip is removed by drilling an undersized hole or by low-
ering jetting equipment through the pile, which acts as the casing pipe for the
operation. The effect on pile capacity of drilling an undersized hole is unpre-
dictable unless there has been previous experience under similar conditions.
According to API RP2A (2007), jetting below the pile tip should be avoided
because of the unpredictability of the results.
A first stage or outer pile is driven to a predetermined depth, the soil plug
is removed, and a second stage or inner pile is driven inside the first-stage pile.
In this case, grouting will be inserted in the annulus between the two piles
to provide load transfer between the two piles and to develop composite
action.