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API REPORT 13 Document Information:
Title
Axially-Loaded Centrifuge Pile Tests
American Petroleum Institute
Publication Date:
Aug 10, 1981
Scope:
PREFACE
R. F. Scott
A previous report to API (1980) discussed an attempt at driving model
piles in the centrifuge during flight by means of a model pile-driver.
Both electrical solenoid and air-operated systems were tested, and
some of the results were presented in that report. A further
modification of the pneumatic pile-driver was subsequently tried out;
it was not successful. While pile-driving apparatus was being worked
on experiments on axially loaded piles continued. A typical pile was
located in the centrifuge container with its tip imbedded in a layer
of soil at the bottom of the container at one g. Soil was subsequently
placed and compacted by hand around the pile, still at one g. After
the installation of the usual instrumentation, the centrifuge was
activated to bring the pile, soil, and container up to the selected g
- level, for axial load tests. In this report, discussion of the
futile pile-driving tests is omitted, and attention is concentrated on
the pile-loading experiments.
A number of the latter were designed to be the model equivalent of
some full-scale pile tests, so that quantitative comparisons of
behavior could be made. It was intended that the analyses of the
performance of the model piles would be followed by a detailed
discussion of the pile-soil shear stress-displacement functions ("t-z"
curves) in relation to soil properties, with a view to constructive
guidelines for t-z curve development. However, it is apparent in the
material which follows that the axial behavior of the model piles is
so much softer than that of the prototype piles that there is a real
question as to the identification of these model tests with prototype
performance. It was therefore decided that, although t-z curves had
been developed for each model test, it was not appropriate to try to
relate them to idealized soil models for the construction of template
functions. In the description of the work which follows, this last
step, therefore, does not appear.
The bulk of this report is taken up with a Civil Engineer's thesis
devoted to the pile tests, and written by John E. Christenson, under
the guidance of Ronald F. Scott. In the thesis, the opinions expressed
in many instances are Christenson's alone, although here and there
they are modified by interaction between Christenson and Scott. In
particular, with reference to the lack of an apodictic correspondence
between the model and prototype compliances, Christenson ascribes the
difference principally to the variation of acceleration along the
model pile, and to interaction between model pile and the container
wall. He considers the effect of driving the model pile in flight to
be of secondary importance. The interaction between pile and wall is
considered by Christenson to have an effect through wall friction;
that is to say, as the centrifuge is brought up to speed, the tendency
of the soil to compress is resisted by pile and container wall
friction, and thus lateral pressure on the pile does not fully
develop. This contention could most easily be evaluated by performing
tests on the same pile in the same soil in a larger container. Such a
vessel is not available for the Caltech centrifuge. The writer assigns
a greater importance to the model pile-driving requirement from two
points of view. First, if the soil could be centrifuged without the
pile, then the lateral pressures might be more realistic even in the
limited size of the existing container. Second, subsequent driving of
the pile, at scale g, into the soil would tend to break down any
arching action that did develop, as well as, and more importantly, to
develop the proper pressure distribution in the immediate pile
vicinity, and lateral pressure on the pile. Soil volume changes next
to the pile which presumably play a significant part in the pile's
subsequent response would be generated by the driving. For the
present, this must remain an open question. There remains, of course,
the usual soil mechanics problem, that the prototype soil, although
essentially of the same type and at the same unit weight or void ratio
as the model material, may have entirely different deformational
properties, because of its different structure, or through the
development of interparticle bonds or cementation in time. If this
accounts for the differences in the present study, then the centrifuge
pile tests do have validity, and the t-z curves produced would be
worth examination. The only way to come to a conclusion on this matter
would seem to lie in either carrying out model pile-driving studies,
to see if driving changes the pile behavior significantly, or in
performing both full-scale and centrifuge pile studies concomitantly,
so that close control can be exercised over the soil characterization
tests.
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