Cofra - Offshore Vertical Drains
Since the development of the synthetic prefabricated vertical drain (PVD) in the late 1970ties, Cofra is worldwide one of the leading contractors in this segment. Cofra has installation rigs available for each soil type, varying from very soft soils to very stiff intermediate layers that can be deployed on pontoons. Our installation rigs are designed for containerized transport and thus easy to ship to any project location in the world. Being part of maritime service provider Boskalis gives us a unique access to resources and floating equipment to help execute your project
This techniques involves the installation of artificial drainage paths or vertical drains from a barge into soft compressible soils that are located off shore. The prefabricated vertical drain (PVD) used by Cofra are specially designed to work effectively under high settlements (i.e. under buckled or kinked conditions).
Highly permeable soils like sand can adjust rapidly to loads and the corresponding compression / reduction in voids due to the ability to easily drain the excess porewater. Low permeable soils like clay and peat do not have this ability and show a delayed compression with the dissipation of the excess pore water. This process of dissipation of excess pore pressures, also known as consolidation, can take up to years depending on the thickness of the deposit. Vertical drains or also known as wick drains are used to shorten the dissipation path of the excess pore water and to accelerate the consolidation process to a few months or even weeks when the PVD is placed at close center to center distances.
Vertical drains in an offshore application are mainly used to govern the stability of the embankment to be constructed. When off shore vertical drain installation is used the following conditions most of time apply:
- The strength of the subsoil is too low to support the thickness of the embankment under the site specific design slopes.
- PVD is required underneath the slopes of the embankment
- The fill material is too course to allow for installation of PVD (rock fill)
The drains themselves are used to accelerate the consolidation and reduce the consolidation period such that it fits within regular construction schedules and a faster lifting schedule of an embankment can be used. The strength of the subsoil has a relation with the degree of consolidation (adjustment of the subsoil to the placed load).