216 Ground improvement by deep vibratory methods
Where vibro replacement stone columns are concerned, the specifications
should not give too many details unless trial stone columns have been constructed beforehand, allowing the specialist contractor sufficient latitude
in choosing the appropriate equipment and in the detailed construction
procedure (Barksdale and Bachus, 1983). This philosophy is particularly
valid when looking at the range of depth vibrators in use today. End result
specifications therefore appear to be preferable restricting the key requirements to a certain design bearing pressure and providing limits for the total
and differential settlement.
The construction process will, in the majority of projects, involve the dry
bottom feed method since the use of jetting water necessary for wet stone
columns requires special considerations in their construction (sludge handling and disposal of process water), and this is only rarely possible due to
environmental restrictions. In the case of soft ground extending to ground
surface, the owner and the engineer need to address the requirement of a
granular working platform which is necessary for safe operation of the
heavy equipment and the efficient execution of the work.
An effective data acquisition system recording all relevant process parameters, in particular the actual amount of stone backfill used, is indispensable
for all vibro stone column projects. A realistic measurement of the stone
volume over depth, currently state of the art with the bottom feed method,
enables determination of the stone column diameter. It must be remembered
that the column diameter and the friction angle of the stone column material
are the key parameters determining their behavior under load. Stone consumption is therefore an essential element of quality control procedure and
a key assumption made in the design. Measurement and control of the stone
quantity is particularly important in very soft soils in which an overtreatment is to be avoided which otherwise may lead to an unwanted reduction
of local strength of the surrounding soil by remolding. Similarly important
are measures to avoid excessive heave, particularly in the final stage of column construction. Pre-boring, which can also become necessary to penetrate obstructive layers, is an effective measure in these instances.
Load testing will not be required when the performance of stone columns can be assessed with sufficient confidence from similar projects with
comparable ground and loading conditions. In all other cases, and particularly for large contracts, load test on single columns, or on groups of
columns—known as zone tests—is recommended and will help to verify
bearing capacity and settlement performance requirements. Procedures for
testing and instrumentation need to be specified together with the observation of the pore water pressure development in the ground to allow time to
elapse before commencing the load test procedures.
Measurement of vibro compaction is generally done by the volume in cubic
meter of sand to be compacted to a specified density. Pre- and posttesting at
specified frequencies can also be included in this item. The compaction work
includes rough site leveling. It may or may not include surface compaction
Where vibro replacement stone columns are concerned, the specifications
should not give too many details unless trial stone columns have been constructed beforehand, allowing the specialist contractor sufficient latitude
in choosing the appropriate equipment and in the detailed construction
procedure (Barksdale and Bachus, 1983). This philosophy is particularly
valid when looking at the range of depth vibrators in use today. End result
specifications therefore appear to be preferable restricting the key requirements to a certain design bearing pressure and providing limits for the total
and differential settlement.
The construction process will, in the majority of projects, involve the dry
bottom feed method since the use of jetting water necessary for wet stone
columns requires special considerations in their construction (sludge handling and disposal of process water), and this is only rarely possible due to
environmental restrictions. In the case of soft ground extending to ground
surface, the owner and the engineer need to address the requirement of a
granular working platform which is necessary for safe operation of the
heavy equipment and the efficient execution of the work.
An effective data acquisition system recording all relevant process parameters, in particular the actual amount of stone backfill used, is indispensable
for all vibro stone column projects. A realistic measurement of the stone
volume over depth, currently state of the art with the bottom feed method,
enables determination of the stone column diameter. It must be remembered
that the column diameter and the friction angle of the stone column material
are the key parameters determining their behavior under load. Stone consumption is therefore an essential element of quality control procedure and
a key assumption made in the design. Measurement and control of the stone
quantity is particularly important in very soft soils in which an overtreatment is to be avoided which otherwise may lead to an unwanted reduction
of local strength of the surrounding soil by remolding. Similarly important
are measures to avoid excessive heave, particularly in the final stage of column construction. Pre-boring, which can also become necessary to penetrate obstructive layers, is an effective measure in these instances.
Load testing will not be required when the performance of stone columns can be assessed with sufficient confidence from similar projects with
comparable ground and loading conditions. In all other cases, and particularly for large contracts, load test on single columns, or on groups of
columns—known as zone tests—is recommended and will help to verify
bearing capacity and settlement performance requirements. Procedures for
testing and instrumentation need to be specified together with the observation of the pore water pressure development in the ground to allow time to
elapse before commencing the load test procedures.
Measurement of vibro compaction is generally done by the volume in cubic
meter of sand to be compacted to a specified density. Pre- and posttesting at
specified frequencies can also be included in this item. The compaction work
includes rough site leveling. It may or may not include surface compaction