6.4 Prefabricated cylindrical drain

6.4.1 Shape and structure of cylindrical drains A cylindrical drain consists of an annular-corrugated and perforated open core, surrounded by a filter sock. The drain diameter is typically 50 mm in outer diameter and 45 mm in inner diameter. Tears and/or other defects shall not be allowed to occur. Visual inspections for damage shall be made regularly as part of the production quality control.

6.4.2 Measurements

The diameter and thickness of the core should comply with the dimensions given by the manufacturer (within allowable deviations given by the manufacturer).

6.4.3 Durability The durability of the drain shall comply with the durability aspects given in EN 13252 (Annex B, B.1 weathering, and B.2, required service lives up to five years or more when the drains are installed to mitigate soil liquefaction). The drains should be protected against weathering during storage on the site. The product shall not be exposed longer than the time announced by the producer for CE Marking (EN 13252) unless the product is protected by a wrapping material or stored in-house. The recommendations of the supplier shall be followed.

6.4.4 Tensile strength and elongation The required tensile strength of the cylindrical drain is very dependent upon the type of installation machine, installation technique and depth of the drain. Tensile strength of the drains in the longitudinal direction shall be high enough to prevent breakage during and after installation. Testing of tensile strength and elongation of the cylindrical drain should be made in accordance with the standard tensile test with modified clamps. The strength of the seam, measured according to EN ISO 10321 in a range of temperatures which apply to the project site, shall be at least 1 kN/m.

6.4.5 Discharge capacity and filtration characteristics The discharge capacity of the cylindrical drains is usually much larger than is required for soil consolidation. It may decrease if the cylindrical core is crushed due to an increase of lateral effective pressure during the consolidation process and/or buckling. The perforation of the pipe (core) shall not be sealed due to compression of the filter sleeve. The pipe and filter of drains used for liquefaction mitigation shall resist the effect of ageing during the design lifetime of the structure.

6.4.6 Filter of cylindrical drains The filter sleeve should be composed of an arrangement of fibres that is 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.

6.4.7 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 shall be compliant with the recommended value.

6.4.8 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. The average of the individually measured values shall comply with the recommended value. 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.

6.4.9 Pore size of filter

The requirements are identical to 6.3.9.

6.4.10 Quality control

The prefabricated drain product 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.

6.5 Sand drains

6.5.1 The sand drain has a circular cross-section, built up of granular material with high permeability. The diameter of the sand drain can vary typically from 150 mm to 500 mm.

6.5.2 The grain size distribution of the material used for sand drains should preferably fall within the limits given in Annex A.

6.5.3 The permeability of the sand should be high enough to avoid significant well resistance. The permeability requirements depend on the permeability of the surrounding soil and the depth of drain installation (see Annex B) and should primarily be defined by the designer.

6.5.4 Material used for sand drains shall not cause pollution of soil or groundwater when installed.

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