Pile testing - Piling Engineering - page 340

Measurement of stress wave data is achieved by the use of light-weight strain cells
and accelerometers that are attached to the pile a few diameters below the hammer.
A description of the instrumentation, and one of the most widely used monitoring
systems (the Pile Driving Analyzer, or PDA) has been given by Likins (1984).
The first stage of interpretation of stress wave data is performed in real-time in the
field by equipment such as the Pile Driving Analyzer. Strain and acceleration data are
processed, generally through electronic hardware, to obtain force and velocity data.
From these data, various parameters may be derived. Thus, integration with time of the
product of force and velocity up to the time at which the product becomes negative
leads to a figure for the maximum energy transmitted to the pile. This allows the
overall operating efficiency of the hammer to be assessed in terms of its rated energy.
If additional information is available on the ram velocity at impact, then the energy
losses may be subdivided into mechanical losses in the hammer, and losses in the
impact process due to inelasticity and bounce of the components.
In traditional pile driving formulae, one of the largest sources of error in esti-
mating the overall pile capacity is uncertainty in the energy transmitted to the pile.
Measurement of the actually transmitted energy allows use of simple pile driving for-
mulae with increased confidence. Such formulae provide a means whereby information
obtained on instrumented piles may be extrapolated in order to assess the quality
of uninstrumented piles driven on the same site. Of course, for piles where stress
wave data are obtained, more sophisticated techniques may be used to assess the pile
capacity.
The relationships presented in section 9.3.1 allow the dynamic and static soil resis-
tance to be estimated from the stress wave data. Equation (9.8) implies that, as the
stress wave travels down the pile, the magnitude of the force will decrease by half
the total (dynamic plus static) shaft resistance, T . Thus, at the bottom of the pile, the
downward travelling force is

F d = F d 0 − T / 2 (9.22)

where F d 0 is the original value at the pile head. Similarly, equation (9.13) may be used
to obtain the upward travelling force, after reflection at the pile tip as

F u = Q b − F d = Q b + T / 2 − F d 0 (9.23)

On the way back up the pile, provided the particle velocity at each position is still
downwards, implying upward forces from the soil on the pile, the upward travelling
wave will be augmented by half the shaft resistance, to give a final return wave of

F ur = Q b + T − F d 0 (9.24)

where the subscript r refers to the return (upward travelling) wave at a time 2 l c later

than the time at which the value of F d 0 was obtained ( l being the length of pile below
the instrumentation point). The total dynamic pile capacity is then

/

R = Q b + T = F d 0 + F ur (9.25)