1. Material delivery
Ensure the correct gabion materials as specified are delivered to site.
Check the rockfill is to the correct grading (normally 100-200mm) and that it is of a hard durable stone. It is normally acceptable to have 6% by weight of rockfill less than the 100mm minimum dimension. To improve filling of gabion units the maximum size grading of 200mm can be reduced to 150mm.
* Gabions can be assembled by 1 of 3 methods.
* Lacing: units should be continually laced through every mesh opening of joints to form the box shape. The start and end of each length of lacing should be secured by 3 turns of wire with the cut end being turned into the unit. Lacing wire is to have the same corrosion protection as the mesh wire and to be a minimum of 2.0mm wire diameter.
* Helicals: - The helical should extend to the full length of the joint with the free ends turned in to lock the spirals into place. Helicals are to be formed from wire having the same or better corrosion protection as the mesh wire and to be a minimum of 3.0mm wire diameter.
* Ring System: - C Rings to be 3.0mm diameter to the same or better specification than the mesh wire. The rings are to be closed with a pneumatic closing tool such that the free ends overlap by a minimum of 10mm. Rings must be placed at 1 every other mesh opening, otherwise the joint strength is insufficient. Gabions that have had mesh damaged in transportation or during storage on site should not be used.
The foundation to the gabion structure should be prepared to the correct grades and levels as required by design (poor foundation preparation will result in settlements and poor alignment both vertically and horizontally). Any soft areas of the foundation must be excavated and replaced with granular fill or rockfill on a geotextile separator. Positioning of geotextile if required should take place with a minimum of 300mm overlaps on adjacent pieces.
Unless it is proposed to prefill and lift units into place, the empty units should be positioned to line and level as required by design and jointed together on all verticals and where the gabion is placed on an existing gabion or mattress it should be laced to the units below at the front and rear prior to fillng. Where units are placed prefilled, the units must be jointed to the adjacent units as above.
5. Filling of units
The units must be filled in maximum 330mm lifts. Once placed into the gabion, the rockfill should be hand picked over to ensure that no large voids or arching of rock pieces occurs. Where required the facing should be hand placed to give a dry stone wall appearance. At each lift a windlass tie should be placed at mid width of each cell to maintain the face alignment. The windlass should be tightened to the correct width of the unit, over tightening will cause bulging and under tightening will cause sagging of the units. Windlassing is required to all external faces of the gabion structure unless the units are to be prefilled and lifted, in which case they are required to all cells and in both directions. For each lift the above procedures should be observed.
At each stage of filling, a visual inspection of the face should be made to ensure no large voids are present and that rockfill of size less than the mesh opening are not present on the face. When filled to the full height of the unit, the lid is closed such that it bears down on the rockfill. The lid must then be fixed down by one of the above techniques along all sides and across each diaphragm. In no circumstances during the filling operations should there be a difference greater than 1 lift of rockfill between adjacent cells, unless the cells are braced in both directions.
Backfill to the gabions should be carried out in layers and compacted. Positioning of geotextile, if required, should take place with a minimum of 300mm overlaps on adjacent pieces.