Six cyclic settlement models for sand are evaluated to
analyse the settlement of automatic stacking crane (ASC) rail tracks at the
Rotterdam World Gateway (RWG) container terminal. During Phase 1 of the RWG
container terminal settlement of the rail tracks occurred at multiple locations
after the ASCs became operational. This has repeatedly led to (unplanned)
downtime of parts of the RWG container terminal due to rail track maintenance.
Settlements are caused by densification of the sand fill, which is a result of
the cyclic load applied by ASCs moving continuously over their rail tracks.
The aim of this research is to contribute to prevent
unplanned downtime in Phase 2 of the RWG container terminal due to rail track
settlements. Also, reliable settlement predictions can be used to determine the
intensity and extent of the ground compaction that are needed to meet the
settlement requirement of 20 mm for ASC rail tracks.
The cyclic settlement models, which have been validated
to predict the cyclic settlement of rail tracks and shallow foundations, are
obtained from literature. The available soil data include CPT’s, boreholes and
standard laboratory soil testing. In addition, settlements of the ASC rail
tracks in Phase 1 had been measured for a period of almost one year. The cyclic
settlement models are evaluated at six different locations, where the sand is
medium to very dense and settlements up to 32 mm have been measured. The
load is modelled as a quasistatic load equivalent to a vertical stress of 60
to 90 kPa applied to the ballast-sand interface. The model parameters
of the cyclic settlement models are determined by correlation, (FE) modelling
of the first load cycle, extrapolation and estimation.
The zone of influence was found to reach around 6 m
below the shallow foundation. Densification of the sand fill is substantial
within the entire zone of influence. The maximum densification was found not to
coincide with the minimum void ratio, it is a variable that depends on the
initial state of the sand and the loading and soil conditions. After order 104
load cycles densification of the sand was found to become negligible. To meet
the settlement requirement for ASC rail tracks the sand fill must consist of
sand layers with a minimum and average relative density of at least 65% and
85%, respectively.
Cyclic settlement increases with the number of load
cycles, amplitude of the load and extent of the zone of influence and decreases
with relative density, stiffness of the sand and volumetric threshold strain.
However, correlations used to calibrate the model parameters lead to model
predictions that are over- or insensitive to parameters that affect the cyclic
settlement. The cyclic settlement predictions of the terminal density model are
most reliable and match best with the settlement measurements, for loose and
medium dense sand the model predictions underestimate the settlement.
Instead of using correlations to obtain the model
parameter values and decrease their uncertainty it is recommended to measure
the:
·
disturbance of the sand fill underneath the ASC
rail tracks due to construction;
·
maximum densification of the sand underneath ASC
rail tracks in Phase 1 at locations where rail track settlement has stopped,
i.e. where the sand reached its maximum densification;
·
model parameters that characterise the cyclic
densification behaviour of sand in cyclic soil tests.
This will improve the reliability of the cyclic
settlement predictions of ASC rail tracks constructed on a sand fill. To
validate the cyclic settlement models for ASC rail tracks on sand, measurements
of the settlement with depth as function of the number of load cycles are
needed.
