Vibro compaction of granular soils 57
of the vibro compaction probe spacing is either by experience from similar
cases or by field trials. In view of today’s diversity of available depth vibrators, design charts, as have been proposed and widely used in the past
(D’Appolonia, 1954; Thorburn, 1975), are not to be recommended anymore. With modern machines, probe spacing can be increased to more
than 3.0 m for clean sand, allowing safe bearing pressures in excess of
500 kN/m 2 with settlements less than 25 mm (see also Greenwood and
Kirsch, 1984). One should always remember though that the main objective of settlement control is in most cases to reduce differential settlement.
This aim normally does not require compaction to maximum density. Very
often, equalization by a moderate widely spaced compaction may be sufficient whereby the sand deposit receives an equally distributed density and
stress history for its loaded areas.
Considerably greater depths may become necessary for large raft foundations as well as for dams, embankments or tanks, or where compaction is required to prevent liquefaction or settlement of deeper deposits due
to earthquakes or other artificial vibrations. In these cases it is necessary
to ensure that the stress levels induced into the soil by vibro compaction
exceed those which might occur under design conditions.
We have seen earlier that the radius of influence of vibro compaction
depends, for given sand, primarily on the characteristics of the depth vibrator
and the chosen mode of operation. It generally varies between 2 and 4 m and
it is therefore impracticable and not advisable to specify the necessary spacing of the probes together with the required density criterion. Since the latter
is determined by the requirements of the overall design of the project, the
necessary spacing follows the choice of machine, and is generally established
Figure 3.16 Typical arrangements of compaction probes below isolated and strip
footings.
of the vibro compaction probe spacing is either by experience from similar
cases or by field trials. In view of today’s diversity of available depth vibrators, design charts, as have been proposed and widely used in the past
(D’Appolonia, 1954; Thorburn, 1975), are not to be recommended anymore. With modern machines, probe spacing can be increased to more
than 3.0 m for clean sand, allowing safe bearing pressures in excess of
500 kN/m 2 with settlements less than 25 mm (see also Greenwood and
Kirsch, 1984). One should always remember though that the main objective of settlement control is in most cases to reduce differential settlement.
This aim normally does not require compaction to maximum density. Very
often, equalization by a moderate widely spaced compaction may be sufficient whereby the sand deposit receives an equally distributed density and
stress history for its loaded areas.
Considerably greater depths may become necessary for large raft foundations as well as for dams, embankments or tanks, or where compaction is required to prevent liquefaction or settlement of deeper deposits due
to earthquakes or other artificial vibrations. In these cases it is necessary
to ensure that the stress levels induced into the soil by vibro compaction
exceed those which might occur under design conditions.
We have seen earlier that the radius of influence of vibro compaction
depends, for given sand, primarily on the characteristics of the depth vibrator
and the chosen mode of operation. It generally varies between 2 and 4 m and
it is therefore impracticable and not advisable to specify the necessary spacing of the probes together with the required density criterion. Since the latter
is determined by the requirements of the overall design of the project, the
necessary spacing follows the choice of machine, and is generally established
Figure 3.16 Typical arrangements of compaction probes below isolated and strip
footings.