MebraDrain - Prefabricated Vertical Drains System
From Vertical Drainage
Prefabricated vertical drains, also known as wick drains, are one of the most commonly used techniques to make soft compressible subsoil with a low bearing capacity constructible. Vertical drainage together with a surcharge reduces the time of settlement substantially. Area’s that are sensitive to settlement are characterized by a high water content as well as an open structure between the grains.
When loads are placed, excess pore pressures are formed because of the low permeability of the soil layers. Without taking measures, this excess pore water will flow out very slowly. This leads to a long period with high settlements and possible stability problems when loads are being placed too soon. Cofra’s vertical drainage system consists of a synthetic prefabricated vertical drains that are being installed vertically from the surface level up to the bottom of the weak layer. This creates vertical drainage paths in the subsoil that shorten the drainage path of the excess pore water considerably. Vertical drainage results in a reduction of the consolidation process from tens of years to half a year or even less. It will also increase the stability which makes it possible to place embankments faster.
Since the development of the prefabricated vertical drain, Cofra has been one of the largests and most advanced installers in the world. The experience, gained in the Netherlands as well as in other countries, has made it possible to make a sound estimation of which equipment is most suitable for the execution of a specific project. This is done with the support of our geotechnical department, and an extensive database of previously completed projects.
Cofra has a solution for every size, depth and difficulty of a vertical drain project. With the use of several excavator sizes, ranging from small (CAT 330) to very large (larger than CAT 385), and stitcher types available, various projects can be executed ranging from small in very soft soils, to large in difficult soil requiring equipment for deep or difficult installation. Even for working 'off shore' Cofra has the necessary experience and equipment. Furthermore, Cofra has at its immediate disposal, drilling equipment for extreme hard layers encountered during installation (CPT values above 25-30 Mpa over thick sections), specialized equipment constructed to work near or below high voltage power lines and well-equipped workshops for long and large projects.
The vertical drains are installed using a wide range of exclusively built patented stitchers, developed in-house by Cofra. The principal of installation of the drainage is based on the insertion of a steel mandrel with a drain inside. This mandrel is moved up and down through a system of cylinders and winches, which in turn are propelled by the excavators’ hydraulic system. The drain extension at the bottom of the mandrel is connected to an anchor plate which closes off the opening so no soil can enter. The mandrel then takes the drain to the desired depth. When the mandrel is at this depth, it is withdrawn and the resistance created by the anchor plate upon retraction ensures that the drain remains in place at the right depth. After the mandrel has reached the surface, the drain is cut and a new anchor plate is connected to the bottom of the next drain.
During the installation period, a wide range of parameters can be registered. These guarantee a high quality standard and a proven end product. Through specially developed computer programmes; depth, time, penetration force, speed, position and straightness can be recorded. Subject to the wishes of the customer these results can be reported after installation. Furthermore, machines are always operated by qualified operators with extensive experience, supported by a professional technical service team.
Even if Cofra has not previously encountered a specific problem, a solution will be guaranteed, using the company’s innovative qualities. This is, of-course, done in close co-operation with the customer.
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.