Gabions are double twisted hexagonal woven wire mesh compartmented baskets of various sizes and dimensions. Gabions baskets are filled with natural stone to create flexible monolithic gravity mass structures.
Gabion Baskets have been used world wide for retaining wall, slope stabilization, channel lining and other Civil Engineering applications for over 100 years. Due to the low cost, environmentally sensitive, long term solution, double twisted mesh gabions provide for these applications their usage has become common with many Governmental Agencies, and Private Land Developments.
As gabion usage grew domestically, the requirement for an industry standard of product quality became vital to ensure material consistency. The American Society for Testing and Materials is an organization that has long been recognized as requiring high quality testing standards, assisting manufacturing industries in establishing an industry standard of quality for specific materials and products. The American Society for Testing and Materials (ASTM) publishes a specification book which documents each specification in its entire format. Each individual product specification within the ASTM book is designated a specification number for reference. The ASTM specification number for Double Twisted Hexagonal Mesh Gabions is ASTM A975-97. In recent years manufacturers of welded wire mesh products, such as concrete reinforcement mesh, lobster cages, fence material etc have began marketing similar welded wire mesh panels as an equivalent to traditional double twisted hexagonal mesh gabions for the gabion applications referenced above. Due to the limited and poor history of welded wire panel gabion performance in the field for gabion applications, Terra Aqua strongly believes that Project Owners, Engineers, Contractors, Designers, and Development planners should understand the two ASTM Gabion Specifications and the gabion material each represents thoroughly and completely prior to accepting or utilizing any gabion material for construction.
Side By Side Comparison
Listed below are the two ASTM gabion specifications minimum strength and performance requirements. ASTM A975-97 is the specification for double twisted hexagonal mesh (woven wire) gabions. ASTM A974-97 is the specification for (welded wire panel) gabions. The two types of gabion materials have been assigned separate ASTM specifications by the American Society for Testing and Materials due to the significant differences in the two materials.
ASTM GABION SPECIFICATIONS
The two ASTM gabion strength test requirements specifications, in which all gabion material is manufactured to meet, are listed below side by side. The tests and their requirements are numbered for a direct comparison of the two specifications. All requirements listed are based on a nominal mesh wire diameter of 0.120 inches or U.S. 11 gauge.
ASTM A975-97 (Woven Wire)
ASTM A974-97 (Welded Wire)
1. Tensile of mesh strength test:
1. Weld shear strength test:
2. Pull apart resistance test:
2. Panel to panel resistance test:
3. Salt spray test of mesh:
3. Salt spray test of mesh: NO TEST!!
4. Mesh panel punch test:
4. Mesh panel punch test: NO TEST!!!
Explanation of Tests:
1. The Tensile of Mesh Test directly represents the strength of the mesh to withstand loading and shear.
2. The Pull Apart Resistance Test represents the strength of two separate gabion panels being connected together with approved fastening systems and then pulled apart.
3. The Salt Spray Test assist designers in determining the probable life span of gabion wire mesh panels in corrosive environments.
4. The Mesh Panel Punch Test represents a direct correlation to the puncture strength of the gabion wire mesh panel.
Comparison of Astm Test:
ASTM A974-97(welded wire panel gabion) specification does not include a Tensile Strength of Mesh Test, Mesh Panel Punch Test or a Salt Spray Test. These tests are excluded from ASTM A974-97 because the welded wire panel gabion material represented cannot and does not meet the minimum strength or time span requirements listed for ASTM A975-97(woven wire gabions). By excluding the Tensile of Mesh Test and Mesh Panel Punch Test ASTM A974-97 (welded wire panel gabion) specification fails to represent the overall strength of the welded wire mesh in any way. This leaves designers without any specific data to determine if the welded wire panel gabion material offers sufficient strength to perform for a specific application. The exclusion of the Salt Spray Test leaves designers without any specific data to base probable life span of the welded wire gabion mesh in corrosive or hydraulic applications. The Weld Shear Strength represented in ASTM A974-97 (welded wire panel gabion) specification of 450 lbs/ft does not correlate or give any indication as to the overall strength of the welded wire mesh.
A direct side by side comparison of the two ASTM gabion specifications shows that the Minimum Strength Test Requirements of ASTM A975-97(Woven Wire Gabions) are far superior to the Minimum Strength Test Requirements of ASTM A974-97(welded wire panel gabions) and include much higher minimum strength requirement values. It is logical to conclude that if the minimum strength test requirements of each gabion material specification are not equal then the gabion product each represents cannot be considered equal. Further more, the two gabion products cannot be expected to and do not perform equally. The ASTM A975-97(woven wire gabion) minimum strength test requirements and the product this specification represents are what the empirical design and performance data accumulated over the last 100 years utilized for gabion structure design are based on. Most all gabion design and performance data available to the market place and utilized by Governmental Agencies, Designers, and Engineers is based on woven wire gabion design criteria and the strength requirements of ASTM A975-97(woven). The design and performance data does not apply to material manufactured to meet strength requirements of ASTM A974-97(welded wire panel gabion). Material and projects designed to meet ASTM A974-97(welded wire gabion) requirements should utilize different more conservative design values than material and projects designed to meet ASTM A975-97(woven). Structural failures have occurred when designers have utilized the design criteria for woven wire gabions and specified or accepted welded wire panel gabions as an accepted equal.
Performance Problems With Material Manufactured
To Meet STM A974-97(Welded Wire Panel Gabions)
Used For Typical Gabion Applications
Welded wire panel gabions are Rigid, not flexible like Woven wire gabions
Flexibility is one of the most important performance characteristics and benefits of traditional woven wire gabions as well as woven wire gabion structures. Gabions structures are commonly utilized in poor soil conditions and drainage applications and require minimal surface preparation prior to placement. These conditions frequently result in substantial sub surface ground settlement resulting in structural settlement. Woven wire gabions are manufactured and designed specifically to elongate and be flexible without any loss of structural integrity. Welded wire panel gabions have a rigid square grid mesh pattern and are not flexible, often breaking (450 lbs/ft weld shear strength) welds and shearing the welded wire mesh when structural settlement occurs, resulting in loss of stone fill material and ultimate structural failure (as shown in picture B).
Woven wire gabions allow for mesh Elongation Welded wire panel gabions do not
Woven wire gabions forming a double twisted hexagonal mesh pattern has deformability sufficient to permit a minimum of mesh elongation equivalent to 10% of an unstretched piece of mesh. The ability of the woven wire gabion mesh to elongate, without loss of strength, is vital to its flexibility and allows the gabion unit to conform and maintain contact to foundation soils upon structural settlement. Welded wire gabion panels manufactured to ASTM A974-97 having a rigid square grid mesh pattern do not allow for mesh elongation. If the Welded wire mesh panels are required to elongate, due to structural settlement, the welds break and the entire welded wire mesh panel fails. This is one reason ASTM A974-97 (welded wire panel gabion) does not have a specification requirement for mesh elongation or more importantly Tensile Strength of Mesh Test such as ASTM A975-97 (woven wire gabion) does.
Reduction of Stone Fill and deformation of welded wire panel gabions
Welded wire panel gabions have a rigid square grid mesh lid panel as well as rigid square grid mesh bottom and side panels. The rigid square grid mesh lid requires that the welded wire panel gabion be under filled with stone fill so that the lid will lay shut and close without stretching or bending the rigid lid panel, which often results in broken welds and, or permanent panel deformation. Reducing the stone fill of the welded wire panel gabions can be detrimental to the overall stability of a gabion structure. When each layer of a gabion retaining wall structure is under filled, to allow for lid closure, it allows for localized movement and increased settlement of each layer resulting in instability of the overall structure. Since gabion retaining wall structures are gravity walls the reduction in stone fill resulting from under filling the welded wire panel gabions on each vertical layer has a negative effect on the structure meeting the intended design requirements for specific gravity. When a gabion channel lining or slope stabilization revetment is under filled the result is generally that the structure does not meet the design requirements for velocity, shear stress or deformation. When welded wire panel gabions are deformed by flow velocity, debris impact or structural settlement they do not allow for a new state of equilibrium of the stone fill to occur because the rigid square grid welded wire panels do not elongate, flex or completely conform to the foundation soils allowing for bridging between the welded wire panel gabion and the soil.
Welded wire panel gabions do not have a larger 9 gauge selvedge or perimeter wire like woven wire gabions to anchor the mesh or to make basket to basket fastening connections. All wires making up a welded wire panel are of the same wire diameter and oriented at 90 degree angles forming straight lines, one vertical and one horizontal. Comparatively, all wires making up a woven wire gabion panel form a hexagonal mesh pattern with the wire double twisted between each hexagonal opening of the mesh which prevents unraveling of the mesh. The woven wire hexagonal mesh does not unravel in the event of a broken wire within the panel. Instead the stress of the broken wire is diverted to the entire panel due the double twisted hexagonal mesh pattern. The welded wire square grid mesh pattern does not offer any resistance to unraveling or unzipping. Once one wire is broken within the welded wire square grid mesh all the stress is diverted to the weld point once that (450lbs/ft) weld point fails or is broken, all the stress is then diverted to the next weld point and so on down the panel vertically or horizontally until the panel has an unzipping effect resulting in a loss of stone fill and structural failure (as shown in pictures A & B).
Rolled stock welded wire mesh panels
Welded wire panel gabion manufacturers consistently provide materials to project sites that do not meet the specification requirements of ASTM A974-97. The product fails to meet the conditions listed within the specification that denote the material shall be provided to the project site made of single unit construction from the manufacturing facility. The most common form of this product type is called rolled stock material in which the manufacturer simply provides a series of welded wire mesh panel rolls and requires the contractor to determine the layout and placement intervals of internal diaphragms, end panels etc. The detriment is that this results in an exclusion of mesh panels within the structure reducing the steel reinforcing the structure and resisting the lateral and hydraulic forces being placed on it. The reduction in the steel wire mesh contained within the gabion structure has a negative effect on the overall performance of the structure and is magnified when structural movement occurs. Rolled Stock type material reduces the mesh as a result of the construction process. When standard gabion units are placed in a linear row and connected to one another there are two mesh panels being connected together an end panel from each gabion unit resulting in a double layer of steel mesh between each linear or horizontal gabion to gabion connection. This double layer of mesh reinforcement is also required when additional standard gabion units are stacked vertically on layers below. Because the lid of the gabion units on the layer below has been closed prior to placement of the second layer of gabion units and the base of the gabion unit on the second layer is placed directly on the lid of the gabion unit on the layer below the result is a double layer of mesh for each vertical layer of the gabion structure. Gabion structures built with standard gabion units contain this double layer of mesh for each vertical and horizontal connection of gabion units contained within it. Gabion structures that are built from welded wire rolled stock material typically reduce one panel from each vertical and horizontal connection contained within the structure in some cases this may reduce the amount of steel mesh reinforcement within the structure by 1/3 to 1/2.
Woven wire gabions being flexible allow the downward pressure of the stone fill material contained within the structure, normal to the foundation of the structure, to be transported through the stone fill material to the ground surface.
Welded wire panel gabions having rigid square grid mesh lids, bottom and side panels do not allow all the downward pressure of the stone fill material contained within the structure to be transported through the stone fill material to the ground surface. Instead some loading of the stone fill material is transported through the rigid vertical side panels of the welded wire panel gabions applying pressure on the side panels resulting in adverse loading effects. The loading of the vertical side panels is magnified when welded wire panel gabions are under filled to allow for lid closing as previously discussed (see Reduction of stone fill).
Fastening and connections
Welded wire panel gabions are most commonly used in conjunction with a spiral binder fastening system. This spiral binder fastening system is used for assembly, basket to basket connections and to close the lids of the welded wire panel gabions. During the lid closing operation it is very difficult to manipulate a prefabricated spiral binder horizontally through the gabion as to wrap every mesh opening due to the basket being full of stone. The only way to alleviate the problem is to either move the stone by hand along all horizontal edges or under fill the welded wire gabion so that the stone fill does not interfere with the spiral binding process. As discussed previously, under filling the welded wire gabion contributes to instability of the structure.
Shipping, Handling and Delivery
Welded wire gabions being rigid and non flexible are easily damaged during shipping and handling on and to the job site. More importantly the rigid welded wire panels can easily be deformed when dumping stone fill into placed gabion units. Once a welded wire panel has been bent or deformed it generally cannot be reshaped to its original form without stressing or breaking the welds, if at all.
Woven wire gabions can be deformed during shipping, handling, or stone filling and easily stretched and or straightened back into original position without any loss of strength, quality or structural integrity.
American Society of Materials and Testing (ASTM A975-97)
American Society of Materials and Testing (ASTM A974-97)