Civil Engineering Reference
In-Depth Information
straightforward to carry out the matching process manually. Experience soon enables
assessment of where values of soil resistance, damping or stiffness need to be adjusted
in order to achieve an improved fit. A satisfactory fit may generally be achieved after
5-10 iterations of adjusting the parameters and re-computing the response.
Limitations in the soil models used for pile driving analysis entail that the computer
simulation will not exactly match the real situation. A consequence is that the final
distribution of soil parameters should not be considered as unique, but rather as a best
fit obtained by one particular operator. Generally, the total static resistance computed
will show little variation provided a reasonable fit is obtained. However, the distribu-
tion of resistance down the pile, and the proportion of the resistance at the pile base,
may show considerable variation (Middendorp and van Weele, 1986).
An interesting investigation of operator dependence in the analysis of stress wave
measurements has been reported by Fellenius (1988). Eighteen operators were given
four sets of stress wave data to analyze, covering a range of pile types and soil con-
ditions. One of the sets of data was from a re-drive of a pile that was subjected to a
static load test the following day. All the operators were using the same computer pro-
gram, CAPWAP, which is one of the most widely utilized programs for such analyses,
originating from the work of Goble and his co-workers (Goble and Rausche, 1979).
The study by Fellenius shows a good measure of agreement among the participants
in predicting the static pile resistance, with the coefficient of variation being under
10% for three out of the four cases considered. For the one pile that was subjected to
a static load test, the predictions spanned the measured static capacity.
An example of matching stress wave data is shown in Figure 9.31. The measured
force has been used as input data, andmeasured and computed velocities are compared.
The agreement is reasonably good. The fit has been obtained for a static shaft capacity
of 1.78MN, and assumed base capacity of 2.24MN (giving 4.02MN total capacity).
However, the blow was not sufficient to mobilize the full base resistance. Analysis
of the data showed a residual force at the pile base of 1.17MN, and a maximum
base resistance during the blow of 1.50 kN. The minimum static pile capacity then lies
between 1
28MN. In this case, without
the use of a heavier hammer, it is not possible to be more specific concerning the
ultimate capacity of the pile. Note, however, that the minimum pile capacity exceeds
the simple Case estimate, due to partial rebound of the pile during the return passage
of the stress wave.
.
78
+
1
.
17
=
2
.
95MN, and 1
.
78
+
1
.
50
=
3
.
9.3.3 General comments
There are a number of different systems available commercially to undertake dynamic
pile monitoring and interpretation. Probably the two most widely used are (1) the Pile
Driving Analyzer and CAPWAP program from Pile Dynamics Inc., USA, and (2) the
TNO equipment (Figure 9.32) and TNO-WAVF program, from the Institute TNO for
Building Materials and Building Structures, Holland.
Regardless of which system is used, there are a number of intrinsic limitations in
assessing the static pile capacity from dynamic data:
1 The capacity of a driven pile generally increases with time following installation.
This phenomenon, referred to as 'set-up', is generally attributed to dissipation
