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Extension of an existing Quay Wall using Marine Soil Improvement

Foundations

Berth 306
South Africa

Client : National Ports Authority
Main Contractor : Basil Read
Consulting Engineer : Transnet
Works Carried Out: : Vibro Stone Columns
Date Of Works : Sept 2005
 

Main Quantities

  • 33 No. Cone penetration tests were carried out (ahead of the main works)
  • 25,000 tonnes of marine stone columns
  • 20,000 linear metres of marine sand vibro-compaction
  • The stone columns were approximately 2.7m to 3.3m, and extended up to almost 30m into the soft silty sand and clayey marine sediments.
  • The water depth at the treatment location reached approximately 20m.
  • The quay wall between caisson 1 and 7 was treated with stone columns, due to the presence of clay zones.
  • The quay wall, between caisson 7 and 11, was treated with pure sand vibro-compaction instead of forming stone columns, on the basis that no clay was present.
  • The exact scope of works and matrix of column/compactation points was determined following the cone penetration tests and trial columns between caissons 6 and 7.
  • Started: January 2006, Ended: May 2006


Extension of an existing Quay Wall using Marine Soil Improvement

General description of Works
Richards Bay Coal Terminal is the largest export coal terminal in the world. Currently exporting over 72 million tonnes per year., 364 days of the year (stopping for Christmas Day only). The new expansion works aim to increase the export to 92 million tonnes per year and to achieve this, a new berth has been constructed to take the increase in traffic of some of the largest ocean going vessels in the world.

The patented installation process
The patented marine double lock gravel pump has a snorkel hose (6), which is attached at the air exhaust lock (16b) to the receiver tank (8). Snorkel hose (6) and locks (16a, 16b, 9) are operated in such a way that during the gravel transport through hoses (5), there us always atmospheric pressure in the receiver tank (8), independent of the actual water depth. In this way standard 7.5 bar compressors can accomplish the pneumatic gravel transport from the blow tank (4) to the receiver tank (8). A high pressure compressor, feeds directly into the pressure tank (10), and thus (since either lock 9 or locks 16 are closed at all times) supplies sufficient pressure to surmount the water and soil pressures in the gravel tube (11) at the tip. The new Berth consists of 11 No 22m high concrete caissons that had to be backfilled with sand. Deep soil improvement was carried out below the caissons, to a depth of up to 45m below sea level. Dura Soletanche Bachy made use of their patented Marine Double Lock gravel pump to install the required stone columns through the deep silty clay and sand, all under an approximate water depth of 20m. The installation works started in January 2006, with the trial section indicating that vibro-compaction would not be possible due to the variability in stiffness and clay content of the soils. A total of 1,120 columns are required on a triangular grid layout with depths ranging from 15 to 20 metres below sea level.

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