A.5 Drainage blanket and working platform

For the efficiency of the vertical drainage system an appropriate drainage blanket (a layer of granular material of appropriate thickness and/or a geotextile or geotextile-related products) should be installed. The consolidation settlement causes a depression of the central part of the drainage blanket. Temporary wells for removing drained water from the drainage blanket may therefore be required, especially in cases where the width of the drainage blanket is large. Protection of the drainage blanket against frost effects should be considered when relevant.

The permeability of the drainage blanket shall be high enough not to cause backpressure in the drains in the way shown in Figure A.12.

The execution of a vertical drainage project requires the presence of a working platform with an upper surface suitable to facilitate the vertical installation of the drains. The working platform needs to be capable of carrying the installation equipment. The presence of pockets and lenses of soft soil in the working platform can significantly reduce the local bearing capacity and result in overturning of the installation rig. The placement of a geotextile separation layer underneath the working platform may be a way of avoiding the risk of heterogeneities in the working platform.

A.6 Loading

The loading operation usually consists of placing a surface load on top of the drainage blanket. This is a critical phase of vertical drainage projects. Loading needs to be carried out in such a way that the stability of the ground is not endangered. Therefore, the unit weight of the fill used for loading has to be defined and controlled. The un-drained shear strength of the soil may be detrimentally affected, not only by the drain installation in itself, but also by the loading operation if carried out with heavy equipment. In most cases, it is important that the filling operation is monitored by settlement and pore pressure observations.

If the shear strength of the soil is too low to permit placement of the fill to full height, loading berms are required. Alternatively, loading has to be carried out stepwise, followed by investigation of the gain in shear strength and dissipation of excess pore water pressure during the consolidation process, required to permit the placement of the next load-step, and so on. In the case of stepwise loading the specified thickness of each embankment layer need to be checked in order to avoid excess loading and consequential failure.

Groundwater lowering in permeable strata in connection with the drains can also be used as an alternative to, or in combination with, external loading.

At sites of drain installation where the stability conditions are unsatisfactory, the surface load can be replaced or augmented by the vacuum method, Figure A.13. In this case the drainage blanket is overlain by an airtight cover and sealed hermetically along its outer borders. The drainage blanket is connected to a vacuum pump, which produces under-pressure in the drains in relation to the pore water pressure in the soil and results in consolidation [9] [10]. The under-pressure achieved by the vacuum method in this case is maximum 70 kPa to 80 kPa.

Key

• u_d = pore water pressure in the drains
• u_vac = under-pressure (assumed equal to a vacuum of 70 % of atmospheric pressure)
• a) pore pressure dissipation caused by the drains
• b) pore pressure dissipation without drains
• 1 airtight cover
• 2 to vacuum pump

Another method to achieve vacuum [40] is shown in Figure A.14. In this system, the band drains are cut at the bottom of ditches, excavated to a depth of 1 m below the bottom of the working platform along each row of vertical drains. Each row of band drains is then connected to a horizontal circular drain, which is covered with a strip of liner. The cylindrical drains are connected to a vacuum pump and the under-pressure thus achieved in the cylindrical drains is transferred to the vertical drains.

An advantage of this system is that an airtight cover over the total area is not needed as in the conventional system. A disadvantage is that no under-pressure is achieved in the upper 1 m layer. In this case the maximum under-pressure achieved is about 50 kPa.

A.14a	A.14b	A.14c

Key

• 1 sand
• 2 liner strip
• 3 to vacuum pump
• 4 clay
• 5 vertical drains