180 Ground improvement by deep vibratory methods
4.7.6 Ground improvement for the foundation of
a petroleum tank farm in the Middle East
The tank farm area which serves as a case history is located in the Hamriya
Free Zone in Sharjah/UAE and is typical for many other industrial developments in the Middle East. The subsoil below the relatively level ground
surface at 2 m above sea level consists of about 2–3 m of artificial sand fill
followed by slightly silty fine to medium-grained sand, which is medium
dense to about 4 m; it is very loose from 4 to 6 m, and very dense below
7 m. The sand layer above this level has typically in excess of about 15%
fines, which renders its suitability for compaction rather questionable. The
groundwater table is at 2 m below ground surface.
The tank farm consists of 16 nos. steel tanks of the fixed roof type with
diameters of 10, 15, and 20 m. The height of the tank is 18 and 24 m. The bottom of the tank rests on a 40 cm thick concrete slab with a reinforced concrete
ring beam placed directly below the tank shell. In view of the deformation
criteria of the tanks, the engineer proposed to improve the existing ground
conditions which he found to be inadequate for the purpose accordingly. Cone
penetration tests (CPTs) carried out for the project revealed the presence of
loose sand between 3.50 and 6 m requiring compaction. In view of its high silt
content and a friction ratio of above 0.5, its suitability for vibro compaction
was only marginal. It was therefore decided to perform a field trial to define if
the soils could be improved sufficiently by vibro compaction, alone or if ground
improvement by vibro replacement with coarse backfill stone was necessary.
For vibro compaction, three different triangular probe grids with 2, 2.25,
and 2.5 m distances were carried out, each consisting of 18 nos. vibro compaction probes of 7 m depth. A Keller S340 depth vibrator (see Table 3.1)
was used employing a standard compaction procedure with 50 cm lifts and a
holding time of 40 s. Backfill sand material was taken from a nearby borrow
area to compensate the subsidence occurring during compaction keeping the
working platform at its original level. In total 23 nos. of pre- and post compaction CPTs were carried out together with a zone load test on a 3 m square
concrete foundation loaded to design load of 280 kPa in the 2.25-m grid area.
For the vibro replacement trial area, a 1.6 m triangular stone column grid
was chosen consisting of 19 nos. Stone columns of 7 m length each. A Keller
M1455 depth vibrator (see Table 3.1) was used employing the wet method
with stone supply from ground surface. According to the consumption of the
25–100 mm crushed stone backfill measured during compaction, a column
diameter of about 85 cm was achieved. Pre- and post compaction CPTs were
performed in the midpoints of the column grid, and 5 days after the stone
columns were completed a zone load test was carried out on a 3 m square
concrete test foundation which was also loaded to 280 kPa design load.
It was not surprising to see that vibro compaction failed to achieve, even
with the closest spacing, the specified minimum cone resistance of 8.0 MPa in
the silty sand layer at about 4 m depth. The zone load test carried out in the
4.7.6 Ground improvement for the foundation of
a petroleum tank farm in the Middle East
The tank farm area which serves as a case history is located in the Hamriya
Free Zone in Sharjah/UAE and is typical for many other industrial developments in the Middle East. The subsoil below the relatively level ground
surface at 2 m above sea level consists of about 2–3 m of artificial sand fill
followed by slightly silty fine to medium-grained sand, which is medium
dense to about 4 m; it is very loose from 4 to 6 m, and very dense below
7 m. The sand layer above this level has typically in excess of about 15%
fines, which renders its suitability for compaction rather questionable. The
groundwater table is at 2 m below ground surface.
The tank farm consists of 16 nos. steel tanks of the fixed roof type with
diameters of 10, 15, and 20 m. The height of the tank is 18 and 24 m. The bottom of the tank rests on a 40 cm thick concrete slab with a reinforced concrete
ring beam placed directly below the tank shell. In view of the deformation
criteria of the tanks, the engineer proposed to improve the existing ground
conditions which he found to be inadequate for the purpose accordingly. Cone
penetration tests (CPTs) carried out for the project revealed the presence of
loose sand between 3.50 and 6 m requiring compaction. In view of its high silt
content and a friction ratio of above 0.5, its suitability for vibro compaction
was only marginal. It was therefore decided to perform a field trial to define if
the soils could be improved sufficiently by vibro compaction, alone or if ground
improvement by vibro replacement with coarse backfill stone was necessary.
For vibro compaction, three different triangular probe grids with 2, 2.25,
and 2.5 m distances were carried out, each consisting of 18 nos. vibro compaction probes of 7 m depth. A Keller S340 depth vibrator (see Table 3.1)
was used employing a standard compaction procedure with 50 cm lifts and a
holding time of 40 s. Backfill sand material was taken from a nearby borrow
area to compensate the subsidence occurring during compaction keeping the
working platform at its original level. In total 23 nos. of pre- and post compaction CPTs were carried out together with a zone load test on a 3 m square
concrete foundation loaded to design load of 280 kPa in the 2.25-m grid area.
For the vibro replacement trial area, a 1.6 m triangular stone column grid
was chosen consisting of 19 nos. Stone columns of 7 m length each. A Keller
M1455 depth vibrator (see Table 3.1) was used employing the wet method
with stone supply from ground surface. According to the consumption of the
25–100 mm crushed stone backfill measured during compaction, a column
diameter of about 85 cm was achieved. Pre- and post compaction CPTs were
performed in the midpoints of the column grid, and 5 days after the stone
columns were completed a zone load test was carried out on a 3 m square
concrete test foundation which was also loaded to 280 kPa design load.
It was not surprising to see that vibro compaction failed to achieve, even
with the closest spacing, the specified minimum cone resistance of 8.0 MPa in
the silty sand layer at about 4 m depth. The zone load test carried out in the