6.3.5 Discharge capacity The discharge capacity and the filtration characteristics are the most important properties. The following factors influence the discharge capacity:

  • a) due to increasing lateral effective pressure during the consolidation process the filter is squeezed into the channel system of the core, which reduces the channel area;
  • b) the vertical compression of the soil taking place during the consolidation process may lead to buckling of the relatively incompressible band drains, which may reduce the channel area;
  • c) fines may intrude through the filter into the core and cause blocking of the channel system;
  • d) soil temperature has an influence on the compression resistance and creep of the drains and thus on the discharge capacity. The required discharge capacity of the band drain is largely dependent upon the purpose of ground improvement, the consolidation parameters of the soil, the drain spacing and the depth of drain installation (see Annex B). The discharge capacity shall be high enough to satisfy the design requirements. The recommended value of the discharge capacity is given in Annex B. The discharge capacity test should be carried out in accordance with EN 12958 with the modifications given in A.4.1.2 in Annex A. For usual applications, the discharge capacity test should be performed at the laboratory temperature and then the test report should be referred to a temperature of 20 °C. For applications in tropical environment, the discharge capacity test should be performed at a temperature corresponding to the soil temperature at the place of drain installation and then the test report should be referred to that specific temperature. The test period should be long enough to yield a constant value of discharge capacity, preferably at least two days at the maximum static pressure stipulated by the designer.

6.3.6 Filter of band drains The filter sleeve should be composed of a non-woven material consisting of fibres that are mechanically, chemically or thermally bonded. The filter sleeve should have a regular structure. The occurrence of creases, tears, holes and/or other defects shall not be allowed. The seams of the filter sleeve shall be constructed in such a way that fines cannot intrude into the core of the band drain. Visual inspections for damage shall be made regularly during production in accordance with the factory production control plan.

6.3.7 Tensile strength per unit width of filter The tensile strength of the filter shall be sufficient to prevent breakage during and after installation. Testing should be carried out in accordance with EN ISO 10319. The average of the individually measured values for the tensile strength should not be lower than 3 kN/m in the longitudinal direction. For installations deeper than 25 m or in difficult soil conditions, a minimum tensile strength of 6 kN/m in the longitudinal direction is recommended.

6.3.8 Velocity index of filter

Testing should be carried out in accordance with EN ISO 11058. The average of the individually measured values of the velocity index (vh50) should be higher than 1 mm/s. In case of drain installation for liquefaction problems, the filter pore size should be adapted to ensure adequate permeability of the filter for this application, see 6.4.

6.3.9 Pore size of filter The pore size of filter shall be selected to ensure sufficient discharge capacity and avoid serious loss of discharge capacity due to clogging of the filter and/or the core by soil particles. The seams of the filter shall not have an opening size bigger than that of the geotextile filter. Primarily, the requirements for the filter sleeve characteristics shall be given in the project, considering the soil properties at the site and the installation conditions (dry or wetland, offshore). The value of the characteristic opening size O90, measured in accordance with EN ISO 12956 should not be higher than 80 µm.

NOTE The value of O90 may be influenced by project-specific requirements and higher values may be acceptable. In silty soils and silt the characteristic opening size O90 of the filter should be adapted to the soil conditions according to the following criteria:

  • a) < d85,soil in silty soils, which are problematic from the point of view of filtering technique;
  • b) < 1,5 d50,soil to 2,8 d50,soil in soils, which are difficult from the point of view of filtering technique, mainly medium and coarse silt.

6.3.10 Quality control The band drain shall comply with all European requirements and conformity assessment procedures that apply to it. The properties shall be within the limits announced in the accompanying document accredited by a Notifying Body. The filter and drain characteristics and corresponding testing methods, as well as the proposed testing frequency, are given in Table 1, adapted from EN 13252. The sampling procedure for the different testing methods shall comply with EN ISO 9862.

NOTE The on-site testing frequency should be decided between the parties involved.

Table 1 — Proposed testing frequency for fabrication control
Property Proposed test frequency Required standard
Thickness 25 000 m2 EN 9863-1
Mass per unit area 25 000 m2 EN 9864
Pore size 200 000 m2 EN 12956
Velocity index 200 000 m2 EN 11058
Tensile strength in the longitudinal direction 200 000 m2 EN 10319
Tensile strength in the cross direction 200 000 m2 EN 10319
Drain composite:    
Width and thickness 25 000 m EN 9863-1
Mass per unit length 25 000 m EN 9864
Tensile strength in the longitudinal direction 100 000 m EN 10319
Elongation at maximum tensile force 100 000 m EN 10319
Discharge capacity straight 500 000 m Annex A
Discharge capacity buckled 500 000 m Annex A
Tensile strength of filter seam 100 000 m EN 10321
Durability 500 000 m EN 13252

EN 15237:2007 Execution of special geotechnical works — Vertical drainage