Design GuidelinesGeneral Design Guidelines

Basic Principles

The effectiveness of a gabion structure relies on the integrity of the wire baskets throughout their design life. High

quality wire with appropriate corrosion protection is therefore essential- use of low grade mesh, selvedge wire or tie

wire is a false economy.

Cheap hand twisted mesh soon unravels

and a uniform mesh size is difficult to

achieve.

The first step in the successful use of

gabionsis thereforeselectionof an appro-

priate supplier:

The basic design of gabion structures is relatively straightforward,but it is essential to take due account of the

followingprinciples:

Stability: gabion structuresmust be designedas gravity retaining structures with no reliance on tension in the

wire baskets to resist overturning.

Drainage: all gabion structures are permeable and thus free drainage is assured, but a filter (sand, gravel,

geotextiIe)may be requiredbehind or beneath the structureto prevent washingout of fine material

fromthe subgradeor backfill.

Erosion: structures exposed to flowing water or waves must have adequate protection against undermining by

erosion of the foundation (gabion mattresses often provide an ideal means of achieving this).

Flexibility: where appropriatethe design should take advantage of the gabion's inherent flexibility to ensure

stabilityin achangingenvironment.

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Advantages

Each principle highlights advantages of a gabion structure over other forms of construction:

Stability: their use as earth retaining structures provides a low cost alternative to concrete or masonry retaining

walls.

Drainage: the permeability of gabion structures has the benefit of reducing pore water pressures behind the struc-

ture and allowing free drainage.

Erosion: the use of gabions allows erosion protection to be achieved using smaller stones and reduced thickness

than would be required for conventional rip-rap. Gabions can also provide a cost effective, more aes-

thetically pleasing and environmentally friendly alternative to concrete systems.

Flexibility: the ability of gabions to deform while still retaining their integrity has advantages over other types of

construction. When exposed to differential settlement or erosion comparable concrete structures may

crack and fail.

Drainage

Erosion Flexibility

In addition, the simple construction techniques required to erect a gabion structure can be advantageous when

compared to other forms of construction:

. there is no requirement for heavy plant or machinery, which reduces costs and facilitates construction in

remote areas or where plant is difficult to obtain;

. local labour can rapidly be trained to construct high quality structures;

. construction makes use oflow cost locally available stone;

. structures may be added to at a later date;

. rapid construction makes them ideal for emergency works.

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Underlayer or Filter

In most applications. gabions are provided with an underIayer (or filter). This layer can be formed from gravel or

geotextile (or both) and serves one or more of the following functions:

. filtration to prevent loss of subsoil material as a result of drainage flow through the gabion;

. separation to prevent movement of subsoil material into the gabion;

. provision of an even formation surface for laying gabions;

. secondary protection between the subsoil and flowing water.

Geotextile underIayers should be designed using manufacturers' guidelines for retention, permeability and strength.

Gravel filters are designed using formulae based on the gradings of the filter material, the retained soil and the velocity

of any water flow.The design of filters is a complex area and should be carned out by an engineer who is competent and

experienced in this field.

Hydrostaticpressurescan build up behind retaining walls if geotextilesbecome clogged with fine material. The

pressuremaybe relievedby providingdrainagepipesthroughthe walland drainagealongthe wall.

During design the construction sequence should be carefully considered so the filter can be easily laid. Care is essential

when lapping the geotextile and laying the gabions to prevent damage.

Layout and Detailing

As with all engineeringstructures,getting the layout and detailing

right is important.Layoutdrawingsshouldbe preparedto enablethe

proposeddesigntobeconstructedwiththeminimumcuttingandjoin-

ing of gabions.Careshouldbe takento ensurethat, where possible,

layers of gabionsare staggered(in a similar manner to brickwork

courses)in all threedimensions,to improvethe stabilityandintegrity

of the structure.

Keep lransiUonssimple to reduce

culUne

Detailing at the ends of the structure is also important to ensure a neat

finish that blends well with its surroundings and, in the case of river

works, prevents outflanking of the structure.

Coatings and Finishes

Galvanising provides a good protection to the mesh for normal applica-

tions. In more aggressive environments PVC coated galvanised wire is

recommended. Such environments include coastal areas, rivers where the bed load is high or includes gravel or boul-

ders, and near roads where de-icing salts are used. In areas where vandalism or theft is a problem, the mesh may be

sprayed with a light coating of cement grout or bitumen mastic to discourage deliberate damage.

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