ITEM 511 – GABIONS AND MATTRESSES

511.1 Description

This Item shall consist of furnishing, forming wire mesh baskets, and placing rocks installed at the locations designated, in accordance with this Specification and in conformity with the lines, grades, dimensions, and arrangements shown on the Plans or as directed by the Engineer.

511.2 Material Requirements

511.2.1 General

Gabions shall be constructed of wire mesh and shall be supplied in various lengths and heights. A double twisted wire mesh container of variable sizes, uniformly partitioned into internal cells, interconnected with other similar units, and filled with stones at the project site to form flexible, permeable, monolithic structures such as retaining walls, sea walls, channel linings, revetments and weirs for erosion control. The lengths shall be multiples of 2, 3 or 4 times the width of the gabion and heights shall be0.50 m to 1.00 m or as shown on the plans. The horizontal width shall not be less than one meter. Gabion furnished shall be of uniform width.

The width, height and length of the gabion as manufactured shall not differ more than ±5% from the ordered size prior to filling.

Mattresses are double twisted wire mesh container uniformly partitioned into internal cells with relatively small height in relation to other dimensions, having smaller mesh openings than the mesh used for gabions. Mattresses are generally used for riverbank protection and channel linings. The length shall be 3.00 m to 6.00 m, the width shall be 2.00 m and the height shall be 0.17m, 0.23 m or 0.30 m or as shown on the Plans.

The width and length of the revet mattress as manufactured shall not differ more than ±5%, and the height shall not differ more than ±10% from the ordered size prior to filling.

511.2.2 Wire

The wire used in the manufactured of double-twisted mesh for use in gabions and mattresses shall conform to the specifications as shown below as appropriate for the style ordered.

511.2.2.1 Style 1 double-twisted mesh shall be manufactured from zinc-coated steel wire conforming to Specification ASTM A641, Class 3 coating, soft temper.

511.2.2.2 Style 2 double-twisted mesh shall be manufactured from Zn-5A1-MM-coated steel wire conforming to Specification ASTM A 856/A 856 M, Class 3 coating, soft temper.

511.2.2.3 Style 3 double-twisted mesh shall be manufactured from the same type of metallic-coated steel wire as style 1 with an additional PVC coating extruded into the metallic-coated steel wire. The PVC coating shall conform to the following requirements:

                                                                                       Test Method

1. Specific Gravity                          1.30 to 1.35                      D 792

2. Tensile Strength, min                  20.6 MPa                       D 412

3. Modulus of Elasticity, min          18.6 MPa                       D 412

4. Hardness, shore “D”                  between 50 & 60            D 2240

5. Brittleness Temp, max              -9⁰C (15⁰F) or lower      D 746

6. Resistance to Abrasion, %w       temperature                D 1242

    weight loss,                                   max. 12%

The PVC coating shall not show cracks or breaks after the wires are twisted in the fabrication of the mesh.

511.2.2.4 Style 4 double-twisted mesh shall be manufactured from aluminum-coated steel wire conforming to Specification ASTM A 809, soft temper.

511.2.3 Lacing Wire and Stiffener

Lacing wire and stiffeners shall be made of wire having the same coating material as the double-twisted wire mesh conforming to Specification ASTM A 641, A 856/A 856 M or A 809 with a tensile strength in accordance with subsection 509.2.7.

511.2.4 Fasteners made from zinc-coated steel wire, zinc - 5% aluminum mischmetal alloy-coated steel wire and aluminum-coated steel shall conform to specification A 764, Type A, B, or C, Table 2 or Table 3.

511.2.5 Gabion and mattresses shall be manufactured with all components mechanically connected at the production facility with the exception of the mattresses lid which is produced separately from the base. All gabions and mattresses shall be supplied in the collapsed form, either folded and bundled or rolled, for shipping

.511.2.6 Dimensions

The minimum size of the galvanized and PVC coated wire to be used in the fabrication of the gabion and mattresses shall be as follows:

Gabions and Mattresses Wires	Diameter, mm
	Gabion		Mattresses
	Metallic Coated	PVC Coated	Metallic Coated	PVC Coated
Body Wire Selvedge or Perimeter Wire Tying and Connecting Wire	3.05 3.80 2.20	2.70 3.40 2.20	2.20 2.70 2.20	2.20 2.70 2.20

Diameter Tolerances for Galvanized Wire to be used in the fabrication of gabion and mattress shall be ±0.10mm.

The nominal and the minimum thickness of PVC coating shall be 0.50 mm and 0.38 mm, respectively.

511.2.7 Mechanical Properties

Tensile Strength – The tensile strength of Zinc-coated wire used in the fabrication of gabion and mattresses when tested in accordance with Test Methods ASTM A 370, shall be as follows:

Gabions and Mattresses Wires	Strength, Mpa
	Gabions	Mattresses
Body Wire Selvedge or Perimeter Wire Tying and Connecting Wire	350-485 350-485 350-515	350-515 350-485 350-515

511.2.8 Weight of Coating

The minimum weight of zinc per unit area of uncoated wire surface shall be in accordance with ASTM A 975 or as follows:

Wire Diameter, mm	Class 3 or A Coating, g/m2, ASTM A 641
Over 1.90 to 2.30 Over 2.30 to 2.70 Over 2.70 to 3.10 Over 3.10 to 3.50 Over 3.50 to 3.90	220 230 240 260 270

511.2. 9 Rock Fill

Rock used in the gabions and mattresses shall consist of hard, durable rock pieces that will not deteriorate when submerged in water or exposed to severe weather conditions. Rock pieces shall be generally uniformly graded in sizes ranging from 100 mm to200 mm. Filled gabions shall have a minimum density of 1,400 kg/m³. Voids shall be evenly distributed.

No rock size shall exceed 2/3 the mattress depth and at least 85% by weight of the stone shall have a size greater than 80mm. No stones shall be able to pass through the mesh.

The rock shall meet the requirements of AASHTO M 63 except that the sodium sulphate soundness loss shall not exceed 9% after 5 cycles.

511.2.10 Filter Fabric

Filter cloth shall consist of 70% polypropylene and 30% polyethylene.

511.3 Construction Requirements

511.3.1 Fabrication

1. Gabions and mattresses shall be in the form of rectangular baskets of the required dimensions and shall be manufactured from wire as specified in Subsection 509.2.2. Gabions shall be made of steel wire double twisted forming a uniform hexagonal mesh type 8 x 10 having a nominal mesh openings of 83 by 114 mm. Mattresses shall be made of steel wire double twisted forming a uniform hexagonal mesh type 6 x 8 having a nominal mesh openings of 64 by 83 mm. Tolerances on the hexagonal, double-twisted wire mesh opening shall not exceed ±10% on the nominal dimension D values, 64 mm for mattresses and 83 mm for gabions. The edges shall be formed into a securely connected selvedge adequate to prevent raveling.

Individual basket ties and connections shall be made by using a quantity of wire not less than 8% of the weight of each basket.

2. When the gabion length exceeds its width, it shall have securely tied diaphragms connected at all edges to formindividual cells of equal length and width.

Gabions shall be fabricated in such a manner that the sides, ends, lids and diaphragms can be assembled at the construction site into rectangular baskets of the specified sizes. Gabions shall be of single unit construction, base, lids, ends and sides shall be either woven into a single unit or one edge of these members connected to the base section of the gabion in such a manner that the strength and flexibility at the point of connection is at least equal to that of the mesh.

The gabion shall be equally divided by diaphragms, placed at not more than 1.0 m intervals, and of the same mesh and gauge as the body of the gabions, into cells the length of which does not exceed the horizontal width. The gabion shall be furnished with the necessary diaphragms secured in proper position on the base in such a manner that no additional tying at this junction will be necessary.

3. Four cross-connecting wires shall be provided in each cell having a height of one half the width or less, and eight cross-connecting wires shall be provided in each cell having a height greater than one half the width.

All perimeter edge of the mesh forming the gabion shall be securely selvedged so that the joints, by tying the selvedges, have at least the same strength as the body of the mesh.

Selvedge wire used through all the edges (perimeter wire) shall not be less than 3.80 mm diameter and shall meet the same specifications as the wire mesh.

511.3.2 Assembly and Construction:

1. Gabions shall be installed in a workmanlike manner. The gabions shall be placed on a smooth foundation. Final line and grade shall be approved by the Engineer.

Each gabion unit shall be assembled by binding together all vertical edges with wire ties on approximately 152 mm spacing or by a continuous piece of connecting wire stitched around the vertical edges with a coil every 102 mm. Empty gabion units shall be set to line and grade as shown on the Plans or as described by the Engineer. Wire ties or connecting wires shall be used to join the units together in the same manner as described above for assembling. Internal tie wires shall be uniformly spaced and securely fastened in each cell of the structure.

A standard fence stretcher, chain fall, or iron rod may be used to stretch the wire baskets and hold alignment.

2. When possible the subgrade of the mattress and gabion shall be properly compacted to a depth of 150 mm. The Contractor shall consider the cost of subgrade preparation in the unit prices. Filter fabric as beds of gabions and mattresses forming the structure shall be suitably leveled and shall be securely connected along the complete length of all contact edges by means of the above specified tying and connecting wire.

3. Before the filling material is placed, the gabions and mattresses shall be carefully selected for uniformity of size, and the pieces shall be hand placed to provide a neat appearance as approved by the Engineer.

The gabions shall be filled with stone carefully placed by hand or machine to assure alignment and avoid bulges with a minimum voids. Alternate placing of rock and connection wires shall be performed until the gabion is filled. After a gabion has been filled, the lid shall be bent over until it meets the sides and edges. The lid shall then be secured to the sides, ends and diaphragms with the wire ties or connecting wire in the manner described for assembling.

The vertical joints of gabions and mattress baskets shall be staggered as in running bond in brickwork.

           

4. The cells in any row shall be filled in stage so that local deformation may be avoided. That is at no time shall the cell be filled to a depth exceeding 300 mm more than the adjoining cell.

5. Filter fabric shall be placed between earth surface and gabion or mattress structures. Filter fabric shall be rolled out into a flat non-rutted surface free from sharp objects, weighing down the edges. Construction equipment shall not be allowed into unprotected fabric. Jointing is normally affected by overlapping not less than 300 mm, but it is preferable to joint by sewing or industrial stapling. Joint edges should be facing downwards to avoid protruding through the surface material.

511.4 Method of Measurement

The quantities to be paid for shall be the number of cubic meter of gabions and mattresses and the area of filter cloth completed and accepted.

511.5 Basis of Payment

Quantities determined as provided above shall be paid for at the appropriate contract unit price per unit of measurement for the Pay Item shown in the Bid Schedule, which price and payment shall constitute full compensation for all necessary excavation, subgrade preparation, for furnishing, placing wire baskets and fill materials and for all labor, equipment accessories, tools, and incidentals necessary to complete the Item.

Payment will be made under:

Pay Item Number	Description	Unit of Measurement
511 (1) 511 (2) 511 (3)	Gabions Mattresses Filter Cloth	Cubic Meter Cubic Meter Square Meter