Geotechnical Bearing Capacity of Timber Piles in the City of Amsterdam

Abstract

In the city of Amsterdam, many structures,
such as houses,bridges and quay walls, are founded on wooden piles. In order to
gain insightinto the safety of such structures, assessment of the foundations
is required.As part of an experimental framework assessing the safety of
bridges and quaywalls in the city of Amsterdam, a number of piles are
instrumented with fiberoptic sensors and load tested in compression. These
tests aim to providedetailed information on the behavior of timber piles
subjected to loading. Thisinformation can be used to determine the geotechnical
bearing capacity of suchpiles. An in-depth analysis is conducted on 8 timber
piles tested within thisframework.  The
conducted analysis has resulted in a variation ofoutcomes. Without including
the effects of residual loads, an average baseresistance of 130 kN is observed.
Upon inclusion of residual loads the averagetrue base resistance increases to a
value of 188 kN. The average shaftcapacities in the bearing sand layer for
scenarios excluding and includingresidual effects are 48 and 60 kN
respectively.  The correlation factor αp
is derived usingthree cone resistance averaging techniques. The Koppejan method
hasconsistently resulted in the highest derived αp factors
withvalues of 1.09 and 1.61 for scenarios excluding and including residual
loadsrespectively.  The scenario
excluding residual effects has resulted in anaverage αsof 0.009 for
the bearing sand layer. Upon inclusion of residual loads, thederived αsfactor
for the first sand layer amounts to 0.012. This increase in shear forcesis also
observed in the Pleistocene peat layer. Therefore, residual loadsredistribute
the capacity of the piles by increasing the shaft resistance inthe bearing sand
and Pleistocene peat, while simultaneously decreasing theshear stresses in the
Holocene layers above.  Conclusively,
timber pile characteristics such as variationin geometry and mechanical
properties have significant effects on the capacityof the piles. Additionally, these
variations result in fluctuations in thecalculated load distribution along the
pile. Local smoothing of thesefluctuations results in higher apparent loads,
most specifically at the pilebase. Therefore, smoothing algorithms are not
implemented in this analysis. Thevariation in diameter along the entire length
of each pile directly affects theload distribution. Despite this influence, no
trend is observed for thevariation of αp factors with respect to
pile tip diameter. Anapparent relationship between the tapering of the pile in
the bearing sandlayer and the derived αs factors suggests that
tapering effectivelyincreases the shear forces along the shaft in that layer.  Furthermore, the usage of fiber optic sensors
on woodenpiles has proven to be effective. The variation in local behavior of
wood isclearly illustrated through the conducted analysis. As a consequence of
thebiological nature of wood, the local behavior of wooden piles is best
capturedby sensing technologies measuring strains at high spatial frequencies.