A.4 Drain types

A.4.1 Band drains

A.4.1.1 General

Prefabricated band drains consist typically of a central core surrounded by a filter sleeve, Figure A.3. The width of the band drains is typically 100 mm.

A.4.1.2 Types of drains

a) Channel-shaped core with glued filter

b) Channel-shaped core with wrapped filter

c) Geo-mat with edge-sealed filter

d) Cusp-shaped core with wrapped filter

A.4.1.3 Methods of installation

Band drains are installed inside a hollow mandrel with rectangular, rhomboid or circular cross-section. The size of the mandrel is normally adapted to leave a free inside space for the band drain during installation. Moreover, the bending rigidity of the mandrel needs to be high enough to ensure verticality of the drain installed.

An anchor, which is fixed to the drain tip before installation, prevents the drain from being dragged up when the mandrel is withdrawn, Figure A.4. During installation the soil should be prevented from intruding between the inside surface of the mandrel and the drain. Otherwise, the drain will be subjected to high tensile forces upon withdrawal. The shape of the mandrel and the anchor needs to be fitted to prevent soil intrusion into the mandrel.

The penetration of the mandrel is either performed by means of a static load or by dynamic action, using a vibratory or impact hammer. Static installation is preferable in soils sensitive to disturbance.

After withdrawal of the mandrel, the drains should be cut in a way to ascertain good contact with the drainage blanket, preferably about 25 cm above the working platform.

A.4.1.4 Precautions for the drain installation

The tensile strength of the band drain needs to be high enough to prevent breakage of the drains during and after installation. The required tensile strength depends upon the type of execution equipment, installation technique, soil conditions and depth of the drain.

If possible, the mandrel should be filled with water during installation to avoid the band drain from becoming surrounded by air when the mandrel is withdrawn. The presence of air will reduce the filter permeability and the horizontal permeability of the soil surrounding the drain as well as the discharge capacity. A hydrophilic finish on the filter surface improves the affinity for water.

Static installation is preferable to dynamic installation in soils sensitive to disturbance.

The drain installation produces a zone of smear around the mandrel in which the permeability in the horizontal direction for certain types of soil, particularly fine-grained soils with coarser layers, may be considerably reduced.

Nevertheless, in some cases the un-drained shear strength of the soil may be high enough to resist a collapse of the hole created by the mandrel and thus leave an open space between the drain and the soil when the mandrel is withdrawn. This makes it difficult to estimate the effect of smear as well as the nominal drain diameter to be used in the design.

A.4.1.5 Factors influencing the band drain efficiency

Discharge capacity

It is important that the discharge capacity of the drains installed (the amount of water flow per time unit in the vertical direction through the drain under a hydraulic gradient equal to one) is sufficient to achieve the required degree of consolidation in accordance with the design.

The required discharge capacity (see Annex B) depends on the depth of drain installation, the drain spacing (higher with increasing depth of installation and decreasing drain spacing) and the consolidation characteristics of the soil (higher with increasing permeability and compressibility).

The actual discharge capacity of the drains installed in the soil is influenced by the band drain properties, by the drain installation method (including the effects of smear zone, the hole created by the mandrel and the presence of air in the drain) and by the interaction between the soil and the drain (lateral earth pressure against the drain, possible clogging of the filter and/or the core and effect of buckling).

In highly compressible soil (e.g. peat and gyttja) the relative compression, taking place during the consolidation process, may cause buckling or kinking of the drains, which may seriously reduce their discharge capacity, see Figure A.5. Buckling usually takes place in the upper part of the soil. However, the extreme buckling conditions shown in Figure A.5 can be expected only in very deformable soils with vertical strains of the order of 50%. This is not the case in ordinary soil and loading conditions, where the vertical strains are typically 10 % to 15 % and buckling phenomena have no influence on the discharge capacity.

A.4.1.6 Drainage blanket

For the efficiency of the vertical drainage system, an appropriate drainage blanket (a layer of granular material of appropriate thickness and/or an appropriate drainage system of geotextile or geotextile-related products) needs to be installed to eliminate the risk of a build-up of backpressure in the drains by the water squeezed out through the drains (see A.4.1.5). Backpressure in the drains reduces the hydraulic gradient created between the soil and the drains and prolongs the consolidation process.

The drainage blanket should be protected from frost effects when used in cold regions.