What You Need to Know
The application of mesh/net material to slopes is a common method of mitigating small-scale rockfall that occurs over large initiation areas. The typical solution involves hanging netting from a support rope that runs along the crest of the slope and draping the material downwards. Material from the slope is thereby controlled as it moves under the influence of gravity. In most cases, a catchment ditch is constructed at the toe of the slope to intercept debris as it moves through the system. These are typically called "drape mesh systems" or something similar.
There are many application guidelines found throughout the world that describe how the drape mesh systems are to be constructed, with some detailing preferred materials. The guidelines are normally published by government agencies responsible for the care and maintenance of roadways (see references below).
Another method of mitigation that uses geohazard netting are slope retention systems or anchored mesh systems. These systems use a dense pattern of soil or rock anchors to pin the mesh to the slope. The combination of mesh and anchors help increase the global stability of the slope with regards to shallow landsliding and slumping.
Currently there are no widely recognized guidelines for the design and application of anchored mesh systems. Some manufacturers have propriety recommendations, but no standards have been developed.
In both types of systems, it is up to the design engineer to select the materials and configuration that best suit the site. For this, there are four primary netting characteristics that can be specified that affect the design: tensile strength, puncture strength, unit mass, and mesh opening.
The tensile strength of a netting is reported as a force per unit length (typically kN/m or lbs/ft) both parallel and perpendicular to the weave direction. The value is obtained by applying a tensile force to a sample until the point of failure. The maximum load applied is recorded. The tensile strength is the most commonly cited strength parameter used for design. This is partially because the testing procedures are well documented and accepted. Furthermore, tensile strength test results are much easier to compare between products of varying mesh sizes and can be considered an index test.
There are a few standards available that describe accepted testing methodology. Some examples are EN 10223-3, ISO 17746 and ASTM A975.
One common mistake is to confuse the tensile strength of an individual wire with the tensile strength of the netting. For example, a lower tensile wire used in a mesh with a higher wire count can be many times stronger than a high-tensile wire mesh. Furthermore, the shape and weave pattern of the netting plays a major role in the tensile strength of the netting, in particular if it has asymmetric strength or not. As such it can be deceiving when a product is specified as being high-tensile though it may have a low strength and even have a lower strength in a second direction.
Tensile test according to ASTM A975
Tensile test according to EN10223-3
Puncture strength of a netting is reported as a force (typically kN or lbf) and is measured in a direction perpendicular to the plane of the sample. The value is obtained by applying a force to the netting via a plate or punch device until point of failure. The maximum measured force is recorded. For this type of testing, the size, form and placement of the plate has a major impact on the results. Furthermore, if the testing is being carried out with regards to an anchored mesh system, it is vital that the same anchor plate for the project is used while carrying out the test.
To further complicate matters, there are large differences between the various testing procedures used between manufacturers. For example, some guidelines use punch devices that are on the order of 30 cm (e.g. ASTM A975 or EN 10223-3) while others are 1 m (e.g. ISO 17746); some use beveled edges on the plates and others not and still others use a plate where the entire surface is curved (e.g. ISO 17746); and some tests place a deformable medium (such as sand) beneath the mesh prior to carrying out the test while others are conducted in open air. As such, there is currently no accepted punch test that can act as an index test to compare products.
Puncture strength as a design parameter for drape mesh systems can generally be neglected in favor of the tensile strength, unless high velocity impacts or point loading of the system is a primary concern. In contrast, the puncture strength should be the primary consideration when designing an anchored mesh system. In this case, as stated above, the relevant tests should be based on proprietary plates used for the system and an open-air testing methodology.
"Open-air" puncture test according to ASTM A975
"Open-air" proprietary puncture test
"Supported" proprietary puncture test
There are several other parameters that can be considered when specifying a geohazard netting. Some of the more common are the mesh opening size, unit weight, a minimum strand/wire diameter and the type/level of corrosion protection. Though there are currently no specific geohazard specifications regarding these, it is important that they are specified in an unbiased manner . Make sure the specified attribute is relevant to the design and does not create an technically incorrect bias such as by specifying the tensile strength of an individual wire which does not equate to the tensile strength or puncture strength of the mesh itself. As an example a mesh with a wire tensile strength of 1770 N/mm2 and a mesh opening of 83 mm x 143 mm will have a lower mesh tensile strength than a product with a wire tensile strength of 800 N/mm2 and which has a mesh opening of 50 mm x 50 mm. For certain parameters such as wire parameters or corrosion protection, appropriate specifications can be found in well established national and international material standards.
ASTM (2011) A975 Standard specification for double-twist hexagonal mesh gabions and revet mattresses (metallic-coated steel wire or metallic-coated steel wire with poly(vinyl chloride) (PVC) coating). ASTM International, West Conshohocken, USA.
ISO (2014) ISO/DIS 17746 Steel wire rope net panels and rolls - Definitions and specifications. International Organization for Standards, Geneva, Switzerland.
CEN (2013) EN 10223-3 Steel wire and wire products for fencing and netting - Part 3: Hexagonal steel wire mesh products for civil engineering purposes. European Committee for Standardization, Brussels, Belgium.