38 Ground improvement by deep vibratory methods
When the vibrators are in operation, the ground response causes—as well
as the dampening of acceleration and oscillation width—a certain reduction
in the operational speed of the vibrator motor. This reduction is generally
less with electric motors where it reaches up to 5%, corresponding with the
slip in asynchronous motors. More recently, electronic control technology
by frequency converters has made it possible to operate electrically driven
depth vibrators at variable speeds. In order to avoid the otherwise loss of
lateral impact force when reducing rotational speed, designers have developed special divisible eccentric weights that increase their eccentricity substantially when reversing the rotational direction to continue compaction at
reduced speed.
One of the key objectives of machine design, besides keeping wear and
tear low and repair costs within economically acceptable limits, is to
maintain substantial amplitude in the ground. This depends on the interaction of vibrator characteristics and soil response—a complex problem especially with soil constraints differing not only from site to site,
and with depth on the same site, but also with compaction time at any
given depth. It is, therefore, desirable to know as precisely as possible the
operational condition of the vibrator at any time and depth during compaction. The specialists working in this field have developed registration
and control devices that record a multitude of parameters as a function
of time:
• Depth of the vibrator
• Amperage or oil pressure as a measure of the power output
• Operational frequency
• Air or water flow pressure
Although these working parameters are recorded generally and continuously as a standard procedure on every project, temperature and acceleration measurements at selected critical locations inside the depth vibrator are
possible and help in protecting and improving the machine characteristics.
Visual display and recording of these working parameters is today essential and allows the operator to perform the compaction work in an orderly,
repeatable, and verifiable manner. Temperature measurements at selected
locations within the vibrator motor can also be displayed at the control
device serving as motor protection in extreme working conditions.
The outer surface of the depth vibrator is subjected to strong abrasive
forces from the surrounding soil. They are particularly severe for broken
quartz particles but less so when rounded calcareous material is encountered. Aggressive groundwater, and indeed seawater, accelerate corrosion
and may require special attention. For these reasons, the vibrator is generally
protected by special wearing plates or other protective means on its outside.
The efficiency of the vibratory systems, when working on-site, is very
important and all potential problems leading to unwanted interruptions
When the vibrators are in operation, the ground response causes—as well
as the dampening of acceleration and oscillation width—a certain reduction
in the operational speed of the vibrator motor. This reduction is generally
less with electric motors where it reaches up to 5%, corresponding with the
slip in asynchronous motors. More recently, electronic control technology
by frequency converters has made it possible to operate electrically driven
depth vibrators at variable speeds. In order to avoid the otherwise loss of
lateral impact force when reducing rotational speed, designers have developed special divisible eccentric weights that increase their eccentricity substantially when reversing the rotational direction to continue compaction at
reduced speed.
One of the key objectives of machine design, besides keeping wear and
tear low and repair costs within economically acceptable limits, is to
maintain substantial amplitude in the ground. This depends on the interaction of vibrator characteristics and soil response—a complex problem especially with soil constraints differing not only from site to site,
and with depth on the same site, but also with compaction time at any
given depth. It is, therefore, desirable to know as precisely as possible the
operational condition of the vibrator at any time and depth during compaction. The specialists working in this field have developed registration
and control devices that record a multitude of parameters as a function
of time:
• Depth of the vibrator
• Amperage or oil pressure as a measure of the power output
• Operational frequency
• Air or water flow pressure
Although these working parameters are recorded generally and continuously as a standard procedure on every project, temperature and acceleration measurements at selected critical locations inside the depth vibrator are
possible and help in protecting and improving the machine characteristics.
Visual display and recording of these working parameters is today essential and allows the operator to perform the compaction work in an orderly,
repeatable, and verifiable manner. Temperature measurements at selected
locations within the vibrator motor can also be displayed at the control
device serving as motor protection in extreme working conditions.
The outer surface of the depth vibrator is subjected to strong abrasive
forces from the surrounding soil. They are particularly severe for broken
quartz particles but less so when rounded calcareous material is encountered. Aggressive groundwater, and indeed seawater, accelerate corrosion
and may require special attention. For these reasons, the vibrator is generally
protected by special wearing plates or other protective means on its outside.
The efficiency of the vibratory systems, when working on-site, is very
important and all potential problems leading to unwanted interruptions