For many years there has been a need for the installation of high quality stone columns in an offshore environment. Previous attempts to install offshore stone columns often relied on the assumption that a mattress of gravel dumped on the seabed could be worked into the soil by moving a vibroprobe up and down.
No adequate means of quality control were available to demonstrate the integrity of columns installed in such a way. Proper documentation involving monitoring of column diameter variation with depth derived from measured batches of gravel placed at defined depth intervals was totally out of reach. With the new Marine Gravel Pump technology, the problem of installing high quality offshore stone columns has been solved. The Marine Double Lock Gravel Pump ®guarantees integral columns by continuously pressurized stone discharge. Offshore platforms or dams under cyclic loading or earthquake loading can now be founded cost efficiently and reliably on stone columns.
Port of Patras, Greece, construction of a seawall on loose liquefiable sandy and silty sediments.
The stone columns for the foundation of a breakwater and quay wall in Patras serve as drainage for excess pore pressures that build up during construction of the seawall and also provide additional strength under earthquake loading. The 1.0 m diameter stone columns in a 2.7 m to 3.3 m grid extend up to 20 m into the soft silty and clayey marine sediments. The water depth at the treatment location reaches up to 32 m. Both the breakwater and the quay wall are treated with stone columns, as detailed below:
Breakwater : 4830 No. stone columns, 16 m average length, 77280 linm, 60665 m3 (1 m diameter), average square spacing 2.7 m.
Quaywall : 4500 No. stone columns, 10 m average length, 45000 linm, 35300 m3 (1 m diameter), average square spacing 2.85 m.
Quality Control
While in a land based operation a stone column can be assessed with load tests, or a borehole can be drilled into the column to check for continuity, such controls are not readily available under water.
When recording only Ampere and depth information there is no control to ensure that gravel is placed at the required location. Problems such as blockage of the gravel transport tube or loss of gravel on the sea bed may not be detected.This danger does not exist in a land based operation, where excessive spillage of gravel on the surface can be visually controlled.
