Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
positioned at least two diameters or twice the length of the longest side of the pile section below
the pile head to ensure a reasonably uniform stress field at the measuring elevation.
In the test, the strain and acceleration measured at the pile head for each blow are recorded.
The signals from the instruments are transmitted to a data recording, filtering and displaying
device to determine the variation of force and velocity with time.
9.2.6.3 Methods of Interpretation
a) General
Two general types of analysis based on wave propagation theory, namely direct and indirect
methods, are available. Direct methods of analysis apply to measurements obtained directly from
a (single) blow, whilst indirect methods of analysis are based on signal matching carried out on
results obtained from one or several blows.
Examples of direct methods of analysis include CASE, IMPEDANCE and TNO method, and indirect
methods include CAPWAP, TNOWAVE and SIMBAT, CASE and CAPWAP analyses are used
mainly for displacement piles, although in principle they can also be applied to cast-in-place
piles. SIMBAT has been developed primarily for cast-in- place piles, but it is equally applicable to
displacement piles.
In a typical analysis of dynamic loading test, the penetration resistance is assumed to be
comprised of two parts, namely a static component, Rs, and a dynamic component, Rd.
b) CAPWAP method
CAPWAP (CAse Pile Wave Analysis Program) analysis is the common analysis adopted by the local
tester in Malaysia. In a CAPWAP analysis, the soil is represented by a series of elasto-plastic
springs in parallel with a linear dashpot similar to that used in the wave equation analysis
proposed by Smith (1912). The soil can also be modelled as a continuum when the pile is
relatively short. CAPWAP measures the acceleration-time data as the input boundary condition.
The program computes a force versus time curve which is compared with the recorded data. If
there is a mismatch, the soil model is adjusted. This iterative procedure is repeated until a
satisfactory match is achieved between the computed and measured force-time diagrams.
The dynamic component of penetration resistance is given by:
R d = j s v p R s (9.14)
Where:
j s = Smith damping coefficient
v p = velocity of pile at each segment
R s = static component of penetration resistance
Input parameters for the analysis include pile dimensions and properties, soil model parameters
including the static pile capacity, Smith damping coefficient, js and soil quake (i.e. the amount of
elastic deformation before yielding starts), and the signals measured in the field. The output will
be in the form of distribution of static unit shaft resistance against depth and base response,
together with the static load-settlement relationship up to about 1.5 times the working load. It
should be noted that the analysis does not model the onset of pile failure correctly and care should
be exercised when predicting deflections at loads close to the ultimate pile capacity.
9-28 March 2009