Cofra MebraDrain - Prefabricated Vertical Drains System
Since the development of the synthetic prefabricated vertical drain (PVD) also known as wick drain in the late 1970ties, Cofra is worldwide one of the leading contractors. Cofra has installation rigs available for each soil type, varying from very soft soils to very stiff intermediate layers. Installing worldwide, we encounter depths of 3 to over 60 meter! Our heavy installation sets are the strongest in the world and able to penetrate 25 to 30MPa sands. For working offshore, we have the necessary experience and equipment. Our installation rigs are designed for containerized transport and thus easy to ship to any project location in the world.
This techniques involves the installation of artificial drainage paths or vertical drains into soft compressible soils. 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.
The application areas for vertical drainage are:
- Soil improvement of residential or construction areas.
- The construction of infrastructure (roads, railways, harbours, airports).
- Construction of dykes.
- Land reclamation projects.
The advantages of applying vertical drainage are:
- The slight disturbance of the different soil layers.
- Guaranteed water drainage, even at high soil pressures and settlements.
- The possible adjustment of the core and filter to the local soil conditions.
- Shortening the consolidation period by applying a close drain distance.
- Rapid installation: on average 8000m per day, per machine.
- No water required for installation.
- Installation to a maximum depth of 65m.
- Monitoring of installation with a data logger, if required equiped with GPS.
- Clean execution without releasing soil
The required drain distance and installation depth of vertical drains are dependent on the project and are determined using design calculations. The drain distance is highly dependent on the available time for the consolidation process and on the requirements with regards to the residual settlements after the construction period. The depth of installation is in its turn dependent on the local geology and geohydrological conditions.
There are, next to simple formulas for a 1-layer system, several calculation methods available for the design of vertical drains, like the Dutch Koppejan method and several isotach methods. These methods are programmed into commercial software to make calculations easier. A well known software package in Holland is Msettle (www.delftgeosystems.nl), able to calculate, next to a drain distance, the settlements and residual settlement also the application of BeauDrain.
Cofra uses, next to Msettle, an internally developed program based on the isotach theory. Cofra is able to provide you with the complete design package for small and large projects, by using the expertise of Hydronamic, the engineering firm of Boskalis Westminster, with over 10 geotechnical engineers. Two of them are resident engineers based in Amsterdam. Next to design work, second opinions can be given on completed designs.
The calculations, using the internally developed calculation program, are performed for a multi layered 1 dimensional profile using the isotach theory of Yin and Graham 1989 with Terzaghi consolidation. The drains are modeled using the derivation of Barron-Carillo. It is possible to model, within the calculation model, a staged construction with and without vertical drains and calculate the settlement, residual settlement and degree of consolidation in time.
- existing surface level
- future surface level
- sketch indicating the area and location where the ground improvement is required
- soil profiles ( boreholes, SPT data, CPT data)
- Geotechnical parameters ( unit weights, compressibility characteristics of the soil)
- maximum extend of the consolidation period
- wishes with regards to surcharge levels or drain distances
Settlement, residual settlement or minimal degree of consolidation demands.