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Vibroflotation

Case Study: Vibroflotation for Densification of Hydraulic Fill

Application of Vibro Techniques for Infrastructure Projects in India (Sharma, 2004)

Background & Site Characteristics

The Seabird Naval Base at Karwar in the Indian state of Karnataka required the construction of a breakwater structure over a distance of more than 3 miles.  The existing seabed was composed of clay and soft silt.  In order to create favorable soil conditions for construction of the breakwater, the existing seabed was dredged to a depth of nearly 20 feet.  Hydraulic sand fill obtained from nearby borrow pits was used to the backfill the dredged area.

Figure 12: Project seabird (Sharma, 2004)

Figure 15: Project Seabird (Sharma, 2004)

Statement of Problem

After the hydraulic fill was placed, cone penetration tests were performed to assess the conditions of the sand fill.  The tests revealed the need for compaction of the top 13 feet of the fill in order to reduce potential settlements and mitigate liquefaction potential.

Figure 13: Project seabird soil profile (Sharma, 2004)

Figure 16: Project Seabird soil profile (Sharma, 2004)

Solution and Design

Vibro compaction is a technique commonly used to densify hydraulic fills in offshore projects.  For the construction of the breakwater structure at the Seabird Naval Base an area of about 35 acres was selected to be compacted to a depth of about 13 feet.

 Keller Grundbau performed the work over a period of 10 working months.  The equipment setup included the use of four 49 foot long vibrators suspended from a crane situated on a barge in order to allow for the compaction of the soil beneath 33 feet of seawater.  The spacing scheme selected was a 9.8 foot by 9.8 foot center to center grid.

Figure 14: Project seabird set up (Raju et al., 2003)

Figure 17: Project Seabird setup (Raju et al., 2003)

Results

Seven days after compaction was completed, cone penetration tests were performed every 164 feet along the breakwater structure.  The results demonstrated that the 13 feet of compacted fill achieved a twofold to threefold increase in penetration resistance compared to the uncompacted values. Settlement was monitored after the construction of the breakwater and range between 0.65 feet and 0.98 feet.

Conclusions

Offshore densification of hydraulic fills can be a challenging task.  Vibro flotation proves to be an easily implemented and economical technique for performing the necessary ground improvement. The results from the use of vibro flotation at the Seabird Naval Base in Karwar support this.  Vibro flotation densified the hydraulic fill beneath the breakwater structure which increased the penetration resistance of the fill and reduced future settlements.

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