Keller Singapore was awarded the offshore gravel fill compaction works for the Changi Outfall Construction project which is part of the Water Reclamation Plant in Singapore. The soft seabed material was excavated along the route of an offshore pipeline section with a length of approx. 500 meter and replaced with gravel fill. This gravel fill required compaction to achieve a suitable foundation platform for the pipelines to be laid on.
Keller with its subsidiaries worldwide was involved in the technical proposals for the compaction and quality testing works before the contract was finally awarded. The proposed offshore gravel fill compaction was an alternative to the original tender proposal of underwater soil cement stabilization.

The Project
Prior to commencement of the main works, a compaction trial was carried out on land to model and set the execution parameters. From the trial a spacing of 2.6 sq.m. per compaction point was chosen for the production compaction works. The quality control from the client was carried out by Pile Driving Analysis for each 3.000 cum compacted material.

Site works
The sandy gravel material was placed on the seabed and compacted by powerful S-300 vibrators in twin configuration in day and night operation. The usual Keller crane barge set up, with a 200 ton crane mounted on the deck, including workshop, office and accommodation facilities, was used for the project. The gravel fill thickness ranged from 2 m to 13 m and compacted in a maximum water depth of up to 38 m. The unusual depth of offshore compaction challenged the site team to set up water proof connections at the top of the vibrator strings and to extend the GPS antennas.

Quality control
Approx. 40 Pile Driving Analysis (PDA) tests were undertaken for the post compaction control. All tests were carried out and evaluated by the Consultant. The results were consistently above the required criteria. The settlements after compaction were continuously monitored by the Client using a hydrographic survey. Total settlement of 14% was achieved by the Gravel Compaction works.