Geotechnical - Mass Gravity Retaining
Structures
Gabions, fabricated from reeds, and filled with rocks, were
first employed by Egyptians over 2000 years ago for fortifications
and for erosion control along the Nile river.
These days, gabions are box shaped cages of steel wire mesh.
The units are filled with rocks and connected to one another
to form monolithic mass gravity retaining structures.
The gabions can be easily configured in an infinite number
of ways to suit the uniqueness of construction projects!
Gabion retaining walls and structures offer advantages over
traditional retaining structures;
- Economic to design due to the modular nature of the units
(Maccaferri GawacWin™ 2003 design software)
- Economic to install
- Flexible
- Permeable
- Can include soil bioengineering techniques to promote
vegetation and soften aesthetics
- Easy to customize units on site to meet unforeseen conditions
or demands
Extensive testing of Maccaferri double twisted wire
mesh products prove that stresses are transferred
in two dimensions. This phenomenon even exists when
wires within the structure are cut! The double twist
mesh can therefore flex, transfer stresses, provide high connection
strength and accept puncture loads without breaking,
unzipping, unraveling or rupturing. Copies of these
reports are available on request.
Important gabion considerations;
- Wire Coating & Durability
- Connection Strength
- Differential Settlement
- Pre-assembled Units
- Puncture Forces
- Assembly
- Soil Bioengineering
If you are specifying wire strength, shouldn't you also consider
the points above concerning the mesh and gabion units?
The integrity of your structure could depend upon
it! Accept no compromise!
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Wire Coating & Durability
Maccaferri manufactures gabions from woven double twisted
galvanized steel wire mesh, in accordance with ASTM 975-97.
When the gabion structure is to be used in a demanding application;
in a river, under high loads, or where a long design life
is required, the galvanized mesh can be additionally PVC coated.
The wire is coated before it is woven into the mesh to provide
a fully protected connection.
If we coated the mesh with PVC after it had been woven, it
would be difficult to maintain the integrity of the PVC coating
as the connections inevitably flex whilst the structure settles
and functions on the job-site. A PVC coated product is likely
to last 2-3 times longer than a galvanized product.
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Connection strength
Double twisted wire gabions offer a substantial benefit over
their welded mesh counterparts - the joint strength between
wires in woven mesh is the same strength as the parent wire.
This is crucial where the mesh meets the outer 'selvedge'
or seam wires around the edges of Maccaferri gabions. Maccaferri
double twist mesh uses a thick selvedge seam wire around the
perimeter of the mesh panels. Units are connected to each
other via this selvedge wire and loads are transferred between
adjacent gabion units through it. If the connection is weaker
than the parent wire, consider the following;
- Will the unit-to-unit strength be compromised?
- What happens when the construction experiences differential
settlement and loads are transferred through the mesh?
- Can the stresses and strains generated by differential
settlement be dissipated throughout the structure or will
they be focused at weak points?
- Where are those loads concentrated?
There can be no compromise!
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Differential Settlement
Differential settlement is prevalent in many construction
projects, particularly those where water is present. Differential
settlement causes some portions of the structure to settle,
whilst others remain static. Loads are induced in the gabion
mesh as it flexes to accommodate the changing ground conditions.
These stresses need to be transferred throughout the gabion
units and not concentrated at mesh connections, particularly
the joints between wires. Good connection strength within
the mesh, and between adjacent units is therefore fundamental.
Double twisted mesh gabions are more able to accommodate
differential settlements than welded gabion units. Woven units
will not rupture and 'unzip' under loads, as loads are focused
at the joints between wires.
A gabion structure is only successful when one amorphous,
contiguous mass is formed, because weaknesses in one part
of the system can compromise the entire solution.
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Pre-assembled Units
Gabion products in accordance with ASTM have to be pre-assembled
by the manufacturer under factory controlled conditions, not
supplied in separate panels to the work site, and assembled
by the contractor. A pre-assembled unit has significant advantages;
- The connection on every unit will not have to be inspected
on site
- The connection between panels will be of a consistent
quality
- The connection will be the same throughout the entire
installation
Therefore, by minimizing the amount of connections performed
in the field, a better quality solution is provided.
The integrity of your structure depends upon it -
Follow the ASTM!
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Puncture forces
A tall gabion structure will be exposed to high internal
compressive loads. This overburden tends to try and squeeze
the rock fill outwards, loading the vertical faces of the
gabions. It is therefore fundamental that gabions are capable
of accommodating these punching forces. Double twisted woven
mesh can accommodate high puncture loads due to the capabilities
of the mesh and joint strength as outlined above.
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Assembly
Gabions and Reno mattresses are filled with rock. Rock is
a granular material, and all granular materials settle slightly,
as the rocks shift and ‘lock’ together. See ASTM
D6711 for details on rock fill for gabions and mattresses
To allow for the settlement of gabion fill material, all
gabions and mattresses must be overfilled by 1-2 inches (25-50mm).
The lid of the unit is then stretched over the rock fill,
and laced. Often a crow-bar or a Maccaferri ‘lid closing
tool’ is used to stretch the lid properly over the fill.
If no allowance for settlement is made, then the rock fill
will settle leaving a void in the top of the basket. This
can cause problems for the installation; for example a 1"
settlement in a 3ft high gabion unit translates to a 5"
settlement over a 15ft high structure.
Overfilling is equally important in mattresses and hydraulic
works. The shear resistance of the system is dependent upon
the units confining the rock fill. If the rock is not confined
by the mesh lid, it can move around under hydraulic loads.
This reduces the shear resistance of the system, compromising
the design.
All gabions and mattresses have to be overfilled.
Rock is a granular material! It will settle!
Assembly of gabions and mattresses can be speeded up dramatically
by the use of our Spenax Pneumatic Fastening Tool. This tool
uses ‘hog rings’ to clamp the edges of the wire
mesh together. It can speed up the assembly of units by 3
times. Please visit the download area for more details.
These tools are available for purchase or hire, and the fixing
rings are all available from Maccaferri Inc.
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Soil Bioengineering
When designed appropriately, soil bioengineering can be used
on gabion retaining structures to establish vegetation, and
soften the aesthetics of the solution.
The structural integrity of the retaining wall is of paramount
importance, and vegetation establishment should be designed
accordingly. Please see the Soil Bioengineering Manual
and Brochure in the download area for details.
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