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Vibroflotation

Case Study: Compaction of Reclaimed Soil through Vibroflotation

On- and Offshore Vibro-Compaction for an Oil Pipeline in Singapore (Wehr & Raju, 2002)

Background

Jurong Island is a man made island that was formed from the combination of seven islands southwest of the main island of Singapore.  The installation of a crude oil pipeline on the island mandated the densification of the existing sandfill in order to reduce future settlements beneath the pipeline as well as to stabilize the seaside slope of the sand embankment adjacent to the pipeline jetty.

Site Characteristics

Jurong Island is an artificial island made of reclaimed soil.  The reclaimed soil consisted of medium to coarse sand with fines contents less than 5 percent, making it ideal for vibroflotation.  The groundwater table was found approximately 16 feet below the ground level surface.

Cone penetration tests were performed both in the area where the pipeline was to be installed as well as at the sand embankment at the pipeline jetty. Cone penetration resistances ranged from 725 pounds per square inch to 1160 pounds per square inch with friction ratios of approximately 0.5 percent.

Statement of Problem

Based on the results from the cone penetration tests, it was evident that densification of the reclaimed soil would be necessary in two areas.  The area beneath where the pipeline was to be installed required densification in order to reduce potential settlements.  The area of the sand bund where the pipeline jetty was to be located also required densification in order to reduce settlements and stabilize the adjacent seaside slopes.  The project posed a unique challenge in selecting the ground improvement method to be used.  The method selected had to be of minimal disturbance to nearby areas because of an existing gas pipeline situated approximately 6 feet to 10 feet below the ground surface.  This was especially crucial in the area where the crude oil pipeline and the gas pipeline were to cross.

Figure 15: Jurong Island site layout (Wehr & Raju, 2002)

Figure 18: Jurong Island site layout (Wehr & Raju, 2002)

Solution and Design

Beneath the proposed pipeline, an area of nearly 2.8 miles in length and 65 feet in width was selected for compaction to 70 percent relative density.  Based on field trials, an equilateral triangular scheme with 11.5 foot spacing was selected.  The area of the sand bund where the VLCC (very large crude carrier) jetty was to be located required offshore compaction spanning a distance of over 3200 feet in length and 65 feet in width.  This area was also compacted to 70 percent relative density using the same 11.5 foot equilateral triangular spacing scheme.  The slope adjacent to this area had slopes of 1:3, vertical to horizontal.  A 13 foot equilateral triangular spacing scheme was utilized at the inclined slopes in order to achieve ISO/IEC JTC specifications for compaction of inclined slopes.  A crane situated on a barge was used for offshore compaction.

Results

Post compaction cone penetration tests were performed throughout the improved areas. The criteria requiring 70 percent relative density in all areas of the project was achieved without any major issues.  Additionally, vibration measurements were carried out to assure there wasn’t any damage to the existing gas pipeline. Measurements showed maximum particle velocities well within permissible limits.

Conclusions

The densification of reclaimed soil both on and off shore on Jurong Island was achieved through the use vibroflotation.  Vibroflotation was an appropriate method for this project because of its relatively low cost, on and off shore flexibility as well as its low disturbance level during construction when compared with other techniques.

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