Title:
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Centrifuge modelling of low energy dynamic compaction
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Low Energy Dynamic Compaction (LEDC) is a recently adopted approach for the rapid
improvement of the mechanical behaviour of soil to a relatively shallow depth over a limited
area. This method of compaction is termed `low energy' because the energy input per blow
is low compared with that imparted by traditional dynamic compaction techniques.
The field apparatus for this method was designed originally by BSP/DRA for the rapid
repair of bomb damaged airfield runways, but later adopted as a method of ground
improvement.
This thesis describes the design and operation of a unique model compactor simulating the
action of a low energy compactor in the centrifuge. The centrifuge study has been
undertaken at 20g using a tamper mass of 0.875 kg falling through 100mm onto a stiff
aluminium target, having a mass 0.268 kg and a diameter of 100mm. This simulated a field
scale tamper mass of approximately seven tonnes falling through two meters onto a target
having a base area of 3.14m2.
This research required the development, for use in the centrifuge, of a process monitoring
system proposed by Allen (1996), where the improvement in the ground characteristics
may be evaluated with the execution of the improvement process. The procedure was based
on the principles of the WAK test ( Briaud and Lepert, 1990 ) which was devised to
provide, by means of a very simple test, an estimate of the static stiffness of a soil/footing
system, thus removing the necessity for expensive pre- and post- test investigations. The
monitoring system required the innovative deployment of miniature instrumentation on the
pounder and the target to measure both input force and output accelerations
In order to analyse the soil response to the impact it is necessary to obtain both the
signature of the energy input and soil response in terms of acceleration and transient earth
pressures for each impact. This was achieved by the use of a dynamic load cell mounted on
the drop weight, an accelerometer on the target and accelerometers and dynamic earth
pressure cells embedded within the soil mass.
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