Vibro compaction of granular soils 103
In this period, an area of 126,000 m 2 was treated by vibro compaction to
depths between 11 and 26 m. In areas where preloading was necessary (ca.
30,000 m 2 ), the vibro compaction treatment depth increased by 5.6 m, the
height of the sand surcharge. PVDs were carried out beforehand to depths
of up to 40 m (ca. 20 m sand fill, 10 m sandy clay fill, and 10 m marine clay)
to reach the bottom of the marine clay layer. Drain distances were designed
conservatively to 1 m, in square grid, in order to achieve 80% consolidation
within three months’ time under the 5.6 m sand surcharge. Consequently,
some 1,300,000 lin. m of prefabricated vertical drains were executed.
To accommodate the deformation criteria for the various parts of the
large hull structure, the postvibro compaction soil stiffness was specified
by the soil consultant in terms of carefully selected, standardized CPT
acceptance criteria. Several different criteria were proposed according to
the design loading and allowable settlements, but generally varied between
8 and 16 MPa, in a stepwise manner increasing with depth.
The sand fill material was hydraulically deposited by a standard dredging operation. As in many instances in Singapore, it is taken from the seabed and imported to the reclamation area. It consists primarily of quartz
sand, with less than 10% carbonate content, and can be described by the
USCS as SW material (well graded clean sand with less than 5% fines) and
reaches after dredging generally just medium density.
The specialist contractor chose to perform precontract compaction trials
to establish the necessary compaction parameters (probe distance, compaction time) to meet these specifications. Figure 3.53 shows the locations of
the five CPTs performed within a 3.6 m triangular compaction pattern.
With the five postcompaction CPT results, an area weighted average was
calculated which then served as reference curve for a treated representative
Vibro probe
4
3.6 m
1.8 m
1.0 m
0.5 m
5
3
2
1
CPT
Figure 3.53 Post-CPT locations to establish acceptance criteria for vibro compaction.
In this period, an area of 126,000 m 2 was treated by vibro compaction to
depths between 11 and 26 m. In areas where preloading was necessary (ca.
30,000 m 2 ), the vibro compaction treatment depth increased by 5.6 m, the
height of the sand surcharge. PVDs were carried out beforehand to depths
of up to 40 m (ca. 20 m sand fill, 10 m sandy clay fill, and 10 m marine clay)
to reach the bottom of the marine clay layer. Drain distances were designed
conservatively to 1 m, in square grid, in order to achieve 80% consolidation
within three months’ time under the 5.6 m sand surcharge. Consequently,
some 1,300,000 lin. m of prefabricated vertical drains were executed.
To accommodate the deformation criteria for the various parts of the
large hull structure, the postvibro compaction soil stiffness was specified
by the soil consultant in terms of carefully selected, standardized CPT
acceptance criteria. Several different criteria were proposed according to
the design loading and allowable settlements, but generally varied between
8 and 16 MPa, in a stepwise manner increasing with depth.
The sand fill material was hydraulically deposited by a standard dredging operation. As in many instances in Singapore, it is taken from the seabed and imported to the reclamation area. It consists primarily of quartz
sand, with less than 10% carbonate content, and can be described by the
USCS as SW material (well graded clean sand with less than 5% fines) and
reaches after dredging generally just medium density.
The specialist contractor chose to perform precontract compaction trials
to establish the necessary compaction parameters (probe distance, compaction time) to meet these specifications. Figure 3.53 shows the locations of
the five CPTs performed within a 3.6 m triangular compaction pattern.
With the five postcompaction CPT results, an area weighted average was
calculated which then served as reference curve for a treated representative
Vibro probe
4
3.6 m
1.8 m
1.0 m
0.5 m
5
3
2
1
CPT
Figure 3.53 Post-CPT locations to establish acceptance criteria for vibro compaction.