33 of 49 Construction of Giresun Wastewater Treatment Plant  Tender Dossier Volume 3 Employer’s Requirements  Section 05: Earth Works, Pipe Works and Ground Improvement  5.4.5 Prefabricated vertical drains (Wick drains - strip drains)  5_Vol3_d4.3_Section 5 Earth and pipe works_en_GIR_rev6.docx  The drain capacity could be the limiting factor in cases where PV drains are designed for sites where  there are deep compressible layers with surcharge loading. When PV drains are designed to function  near their maximum capacity, the installations shall be monitored carefully. Before using wick drains  below a depth of 45m, a specialist shall be consulted. A working platform shall be formed to support drain installation equipments. Drainage layer may be  used for this purpose; however, thickness of the drainage layer may need to be increased to support the  equipment as required.  Surface settlement measuring plates will be buried in natural ground at the preloading area and these  will be protected and extended as described.  Fill material will be laid uniformly in 200-300mm thick layers and spread uniformly. No compaction  tests are required and compaction will be achieved by the truck traffic over the fill construction.  Therefore it is mandatory to place the fill homogeneously and uniformly.  Preloading fill materials shall be laid around the perimeter of the large structures and the base of the  fill shall be extended at least 4 meter outside the structure’s borders as shown on the drawings. Then  the fill will be placed with 1H:1V side slope, to a maximum height of 4 meters.  Finally, another separation geotextile shall be laid over the granular drainage layer over the wick drain  field. Before any preloading fill material is placed, settlement columns and magnetic extensometer  shall be installed according to the Method Statement, in order to find out the future settlements. The  settlements shall be measured and recorded until the displacements have reached the desired level. A drainage ditch shall be excavated around the perimeter of the wick drain field. This drainage trench  will collect the discharge coming from the granular drainage layer over the wick drain field.  Wick drains shall be placed at constant load and high speed in order to avoid disturbance of soil. Ver- tical misalignments shall be avoided by checking with a water balance.  Later, the vertical drain locations shall be marked. Prefabricated wick drains shall be installed down to  the depth stated in the Method Statement using a special steel mandrel mounted on the service crane.  The mandrel shall be pushed into the soil using a vibratory hammer.  First stage in the application of the prefabricated drain, shall be removing the topsoil, installing surface  settlement plates, laying the separation geotextile and placing the drainage layer according to the  thickness stated in the Method Statement and extend this drainage layer to the drainage trenches  around.  The Contractor is required to prepare a method statement for preloading with wick drains, based on  the geotechnical report. This method statement shall include the following data:   materials   monitoring equipments   spacing and depth of wick drains (drain layout drawings)   spacing and depth of settlement gauges (monitoring system layout drawings)   height of preloading fill   time required for achieving design settlements   record keeping   expected vertical displacements (swell/settlements) during trial loading, preloading, excava- tion for foundations and in long term (for centres and edges of foundations). 