Civil Engineering Reference
In-Depth Information
viscous effects. Similarly, the dynamic base capacity may exceed the static capacity
significantly due to viscous effects. In both cases, differences between dynamic and
static capacity are greater in soft cohesive soil, than in stiffer or coarser material.
Allowance for damping is made by appropriate choice of damping parameters
in the dynamic analysis. However, the deduced static capacity is relatively sen-
sitive to the choice of damping constant, and it is clear that further research is
needed in order to provide better guidance on damping parameters for differ-
ent soil types. It is strongly recommended that whenever possible at least one
static load test be performed on a given site in order to calibrate the dynamic
analyses.
5 One area which has received insufficient attention is the different response of
H-section piles and open-ended pipe piles under dynamic and static conditions.
It may be shown that both types of pile tend to drive in an 'unplugged' condition,
with soil moving up the inside of the pipe, or filling the space between the flanges
of the H-pile. However, during a static load test the reverse is true. The frictional
resistance of the soil plug is such that both types of pile will tend to fail as a
solid body. Thus, during a dynamic test, these piles will show relatively high shaft
friction but low end bearing. By contrast, during a static test, the shaft capacity
will be just that on the outside of the pile, while the end-bearing resistance will act
over the gross area of the pile. It is essential that estimates of the static capacity
of such piles take account of differences in the failure modes during dynamic and
static penetration.
6 Residual stresses acting down the length of the pile can have a significant effect
on the calculated pile response under dynamic conditions. These stresses will gen-
erally build up along the pile, with some locked in end-bearing stress, balanced
by negative shear stresses acting along the pile shaft (particularly the lower part).
Accurate assessment of pile capacity and, in particular, the relative shaft and base
capacity, necessitates modelling of the residual forces locked into the pile between
each blow.
Dynamic methods for estimating the static resistance of piles have evolved consid-
erably over the last 20 years. However, it is still recommended that such methods are
not relied upon as the sole means of assessing the acceptability of piles, except in areas
where the geology is similar and a large body of experience has been accumulated. For
general applications, it is suggested that the dynamic methods should be calibrated
against static load tests, before extensive testing of contract piles is undertaken. Used
in this way, dynamic monitoring of piles during driving can enable economies to be
made in the static pile testing programme, and allow a larger proportion of contract
piles to be tested, giving increased confidence in the foundation performance.
9.4 Statnamic testing
The Statnamic test was was developed by the Berminghammer Foundation Equipment
in Canada in the late 1980s as a means of testing high capacity piles without the need
for an expensive reaction system, and avoiding some of the problems associated with
conventional dynamic pile testing (Bermingham and Janes, 1989). The test uses the
principle of equivalence of action and reaction, with the action in this case comprising
