Improvement of fine-grained and cohesive soils 113
Modern data acquisition systems controlling both column building and
operational parameters of the equipment employed will eventually, together
with GPS systems, allow the contractor to construct large number of stone
columns per working hour of uniform quality and in a fully automated
way. Telemetric systems are already used to transmit not only service and
machine performance indicators (fuel consumption, oil pressure, etc.) but
also machine location, and all relevant process data as compiled by the
control and data acquisition unit employed.
To limit or even avoid operational errors and mistakes when constructing these stone columns a specialist contractor recently proposed an operator
guidance system (Betterground, 2014) enabling also less experienced operators
to install the stone columns according to specifications in difficult soils. Based
on the data acquired during the installation process, the onboard computer
helps the operator to decide when the stone filling process for a predetermined
column diameter in specific soils is completed and the vibrator can be lifted
into the next position where the filling process will start again. The system
allows programming the up and down movements of the vibrator, for example, in the following way (Betterground, 2014):
• Start column at a certain depth
• Build at least an 80 cm column, unless a specific amperage (say
240 A) of the depth vibrator motor is reached in which case also
smaller column diameters are allowed
• If the amperage stays below a certain value (say 150 A) soft soil is
encountered and a larger column diameter (say 140 cm) is to be built
or the threshold amperage is reached, whatever comes first
Needless to say that the interpretation of the real-time recordings of trial
vibro stone columns carried out at the beginning of a foundation project
requires considerable experience.
Modern vibrocats are designed to install stone columns to depths of up to
20 m. To expedite setting up of the vibro replacement equipment on-site special rigs were developed for shorter, more frequently used columns of up to
10 m, which represents the depth range of the majority of sites, allowing the
complete bottom feed depth vibrator to remain on the rig when being transported. To make use of standard excavators (which are plentifully available
on the construction equipment market) as base machines for carrying depth
vibrators to construct vibro stone columns specialist contractors have developed purposely built attachments. These provide essentially the same characteristics necessary for safe and controllable execution of stone columns;
they do however, develop only a moderate pull-down force. Figure 4.4 shows
such equipment capable of building 11 m deep stone columns.
For stone columns deeper than 20 m, and for columns to be constructed
over open waters, the crane-hung bottom feed method can be used. Base
machines for this purpose are generally heavy crawler cranes that carry the
Modern data acquisition systems controlling both column building and
operational parameters of the equipment employed will eventually, together
with GPS systems, allow the contractor to construct large number of stone
columns per working hour of uniform quality and in a fully automated
way. Telemetric systems are already used to transmit not only service and
machine performance indicators (fuel consumption, oil pressure, etc.) but
also machine location, and all relevant process data as compiled by the
control and data acquisition unit employed.
To limit or even avoid operational errors and mistakes when constructing these stone columns a specialist contractor recently proposed an operator
guidance system (Betterground, 2014) enabling also less experienced operators
to install the stone columns according to specifications in difficult soils. Based
on the data acquired during the installation process, the onboard computer
helps the operator to decide when the stone filling process for a predetermined
column diameter in specific soils is completed and the vibrator can be lifted
into the next position where the filling process will start again. The system
allows programming the up and down movements of the vibrator, for example, in the following way (Betterground, 2014):
• Start column at a certain depth
• Build at least an 80 cm column, unless a specific amperage (say
240 A) of the depth vibrator motor is reached in which case also
smaller column diameters are allowed
• If the amperage stays below a certain value (say 150 A) soft soil is
encountered and a larger column diameter (say 140 cm) is to be built
or the threshold amperage is reached, whatever comes first
Needless to say that the interpretation of the real-time recordings of trial
vibro stone columns carried out at the beginning of a foundation project
requires considerable experience.
Modern vibrocats are designed to install stone columns to depths of up to
20 m. To expedite setting up of the vibro replacement equipment on-site special rigs were developed for shorter, more frequently used columns of up to
10 m, which represents the depth range of the majority of sites, allowing the
complete bottom feed depth vibrator to remain on the rig when being transported. To make use of standard excavators (which are plentifully available
on the construction equipment market) as base machines for carrying depth
vibrators to construct vibro stone columns specialist contractors have developed purposely built attachments. These provide essentially the same characteristics necessary for safe and controllable execution of stone columns;
they do however, develop only a moderate pull-down force. Figure 4.4 shows
such equipment capable of building 11 m deep stone columns.
For stone columns deeper than 20 m, and for columns to be constructed
over open waters, the crane-hung bottom feed method can be used. Base
machines for this purpose are generally heavy crawler cranes that carry the