maccaferri. the use of gabions

8/12/2019 Maccaferri. the Use of Gabions

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Copyright Maccaferri

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Stormwater Industry Association 2005 Regional Conference, Port Macquarie, NSW

SUSTAINABLE STORMWATER: You Are Responsible Justify Your Decisions.20-21 April 2005

THE USE OF GABIONS AND RENO MATTRESSES IN RIVER

AND STREAM REHABILITATIONBy Dale Chaychuk (Maccaferri Pty LTD)

1 HISTORYThe word Gabion comes from the Italian word Gabbione, which means Big cage. First

records of the use of the system go back more than 2000 years, when the Egyptians usedcylindrical willow baskets filled with small stones to protect the banks of the Nile River

from erosion. This system was unchanged until as late as the nineteenth century, when

willow was replaced by Gabions made of wire netting.

The first industrial manufacture of Gabions began in 1894 when a blacksmith from

Bologna, Italy named Maccaferri came up with the idea of using mesh to contain rocks.

The first project on record was the protection of the river banks on the River Reno where24000m of Gabions were used. (Figure 1). Since then, improvements in mesh

configurations and coatings have been adopted to better suit todays demands and to make

use of modern technology.

Figure 1. The first Gabion protections on the River Reno.


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2 UNDERSTANDING THE SYSTEM

There are many advantages of the Gabion type system over conventional protections that

make it an appealing solution for the rehabilitation of riverine environments.

Woven mesh Gabions and Reno Mattresses are a flexible system, which unlike blockwalls, reinforced concrete or concrete mattresses, are able to deform to differentialsettlements while maintaining their functionality.

The fastening (lacing) procedure ensures the units act as one monolithic homogeneousstructure. The same cannot be said for loosely placed rock armour where each rock acts

individually and therefore needs to be excessively large.

Due to the fact that Gabions and Reno Mattresses are approximately 30% voided, porewater pressures are easily dissipated through the structure without build up of

hydrostatic pressures. This inherent benefit ensures that the natural banks are not

isolated from the river as would occur with impermeable or semi-impermeableprotections.

The double twisted hexagonally woven mesh configuration ensures that if one wire wasto accidentally break, the mesh would not unravel. The heavily coated zinc galvanisedor Galmac (inclusion of Aluminium into the coating process) coated wire with an

additional PVC coating, disproves the fallacy that Gabion type structures are temporary

solutions.

Gabion type structures are widely known to be an environmentally friendly solutionwhere vegetation may easily establish, eventually re-creating the pre-existing

environment. The added use of bio-degradable coir fibre mats (Biomac), Jute blanketsor Ecologs to create planter boxes within the units will accelerate the vegetative

process. (Figures 2.1 and 2.2)

Figures. 2.1 and 2.2.The creation of planting areas within the Gabions.


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When considering Gabion or Reno Mattress protections, one must look at the system

holistically. The system comprises of the double twisted hexagonally woven Gabions orReno Mattresses, rock fill, geotextile separation layer, foundation preparation and backfill

procedure. If one of these components is not as per the correct specification, the structural

integrity of the entire system may be jeopardized.

2.1.1 Woven Mesh Gabions

Gabions are flexible cages made of hexagonally woven double twisted wire mesh. The wire

used to manufacture the baskets is heavily zinc coated or Galmac (95% Zinc + 5%

Aluminium + Mischmetals) coated mild steel with or without an additional PolyvinylChloride (PVC) coating. (Figure 2.3) These units are laced together to create a monolithic

structure, packed with selected stone and act as building blocks. They are commonly used

in the construction of retaining walls, weirs, culvert inlets/outlets, bridge abutments andother civil structures. (Figure 2.4)

Figure 2.3. The wire used inwoven mesh manufacture.

Figure 2.4. Woven mesh Gabions.


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Steel wire used for the manufacture of Gabions and Reno Mattresses is protected against

corrosion by the wire coating. The degree of protection afforded depends upon the type ofcoating and the thickness of the coating; the thinner the coating, the more susceptible it is to

abrasion and mechanical damage.

Zinc Galvanised or Galmac coated wire with additional 0.5mm radial thickness PVCcoating should be used in the following instances:

River or fluvial conditions where abrasive sediment loads may exist Marine conditions and salt spray areas Industrial waste Mining Applications Effluent Polluted water (pH less than 7 and above 12) Where the soil is highly acidic or alkalineZinc Galvanised or Galmac coated wire (without PVC coating) can generally be used in allother neutral conditions. Zinc Galvanised coated wire has a life expectancy of

approximately 40 years, Galmac coated approximately 60 years and PVC coated wire up to120 years. Some manufactures have certification from international specification bodies

showing their specific products have been tested and reported to provide a life expectancy

of up to 120 years (in specific conditions).

Unfortunately, there have been instances where inferior products have been used under the

assumption that they have the same technical characteristics as double twisted hexagonally

woven mesh products. When considering Gabions, of vital importance is that the meshmust fulfil the following requirements:

Have high mechanical resistance Have a high resistance to corrosion Good deformability Will not unravel easilyIn recent years there has been much debate over the equality of the different types of mesh

materials and mesh configurations available for the manufacture of Gabion products, in

particular woven wire mesh Gabions and welded mesh gabions. The use of chain link or

diamond mesh Gabions are beyond the scope of this paper as they are of such inferiorquality, performance and durability that any of the four requirements above are seldom

attained.

Experience has shown that when failures occur, they are generally viewed as a Gabion

failure rather than a woven or weld mesh failure. The perception that woven wire mesh and

welded wire mesh as applied to Gabion structures perform equally well when subjected tothe same loads, is untrue. These are two different types of mesh, generally made from the

same material, but in terms of the end product and field performance are very different.

Welded mesh does not offer the strength and flexibility that woven mesh does. (Figure 2.5)


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Figure 2.5. Welded mesh Gabions demonstrating their inability to accommodate settlement.

2.1.2 The Reno Mattress System

Reno Mattresses are thin flexible cages (less than 500mm thick), made of hexagonal double

twisted wire mesh with the same coating options as that of Gabions. The Mattress isdivided into cells by internal diaphragms positioned at 1 m centers and due to their flexible

nature, are used mainly as channel linings, revetments and scour protections (Figure 2.6)

Figure 2.6. The Reno Mattress System.


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It is generally recommended to use anchorage when installing Reno Mattresses on slopes

steeper than 1v:1.5h. This anchorage is either in the form of star pickets installed at centresand depths dependant on the underlying soil or with the inclusion of a Gabion anchor at the

top of the slope.

Some manufactures are able to supply Reno Mattresses with a double diaphragm (i.e. thebase of the unit and the diaphragms are manufactured from one continuous mesh panel) to

offer maximum confinement and minimize internal rock movement during high flow

conditions. This double diaphragm also ensures that when the units are cut into smallersections, each segment will have side panels so additional cutting and fastening is not

required. These Mattresses units are known as Castoro Type Reno Mattresses.

2.2 The Rock Fill

The rock fill used in Gabion and Reno Mattress construction must be clean, sufficiently

durable, non friable and not show any signs of weathering. It should be evenly graded, be

angular to provide interlock and have a specific gravity (SG) of at least 2.4t/m. Typically,basalt, granite and hard sandstones fulfil these criteria. Rock with an SG less than 2.4t/m

can be used, provided further stability analyses and weathering tests are carried out.

For Gabions, the distance between the twists is 80mm nominal and 60mm nominal forReno Mattresses (Figure 2.7). A rock fill size between say 1.5D to 3D is recommended to

limit the void ratio and maximize productivity. Where Gabions are used, rock on the

smaller side of the spectrum can be packed in the centre of the unit without compromisingthe structural integrity. Reno Mattresses on the other hand are a lot thinner than Gabions so

the rock grading is a lot more critical. Under no circumstances should a rock size be used in

Reno Mattress that can be removed by hand through the mesh.

Figure 2.7. Woven mesh configuration.


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2.3 The Geotextile Separation Layer

The use of geotextiles is essential in Gabion construction. The geotextile must be installed

at all mesh and soil interfaces with the primary function being that of separation and

secondly one of filtration. Generally, woven geotextiles are not suited to Gabion

construction as the filaments tend to slip on each other ultimately leading to piping failures.They are also prone to installation damage from wires and sharp rocks which may tear the

geotextile. Non-wovens have high energy absorption, robustness and are more resistant to

installation damage making them ideal for Gabion construction.

The geotextile will not perform its function if it is damaged and it is proven that they incurthe biggest damage during installation. Supervisors on site must ensure that the contractor

takes due care to minimize damage by not driving machines over the geotextile or by

dropping rocks unnecessarily onto the material. If they can withstand installation damage,they generally withstand the in service stresses that they are exposed to. The geotextile

must be robust, be sufficiently permeable and be abrasive resistant in hydraulic applications

to ensure that the filaments do not detangle when sediments are carried in suspension in thewatercourse. (Figure 2.8)

Figure 2.8. A geotextile with poor abrasion resistance behind a Gabion wall.

Some engineers are of the opinion that geotextiles will clog over time and that this will leadto excessive pore water pressures imposed on the structures. With the advancements made

in the field of Geosynthetics in recent years, it is rarely seen that, provided the correct grade

and type of geotextile is being used, any clogging will occur. You are more likely to see thebackfill material washed through the Gabion or Reno Mattress when a separation layer is

not present. (Figure 2.9 and 2.10)


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Figure 2.9 and 2.10. The omission of geotextile ultimately leads to piping through the

structure

2.4 The Foundation Preparation

Gabions and Reno Mattresses are an engineered system. The foundation needs to be level,

compacted as per specification, must provide uniform pressure distribution and there should

be minimal disturbance to the soil around the structure (Figure 2.11). The contractors mustshow the same amount of effort as would be shown for a rigid type structure i.e. straight

lines, dumpy levels and compaction testing methods must be adopted. Generally,

specialized foundation techniques like concrete footings are not required. Compacted dump

rock is typically accepted where underlying material does not provide the necessary bearing

capacity. Where Gabions are installed on bedrock, concrete levelling pads with shear keys(dowels) are recommended to maintain horizontal levels and minimize the potential for

sliding.

Figure 2.11. Level foundation with minimal disturbance to surrounding in-situ soil


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2.5 The Backfill Procedure

When backfilling behind Gabions it is recommended to use a free draining granular type

material. Care must be taken during the backfilling procedure to ensure that the geotextile

separation layer is not damaged. Heavy compaction machinery must keep at least 1.0m

away from the back of the units to minimize the chance of damaging the mesh with walkbehind compaction machinery used directly behind the wall. Backfilling must be completed

after every lift of Gabion units.

3 CHOOSING THE RIGHT SOLUTION

3.1The Minimum Energy Level Concept

In choosing the most appropriate solution the following approach should be adopted:

Analysis of the problem (causes and effects). Evaluation of the stability factors (numerical). Use and knowledge of the performance limits, of the materials used. Understanding of the performance of the combined use of these materials over time.The most appropriate solution will be that defined by the Minimum Energy Level. This

is commonly defined as the minimum amount of intervention on the environment, which isrequired to solve the problem. It is illustrated in Figure 3.1 and ranges from the lowest

level of no intervention through to the highest energy level, which may necessitate the

construction of a massive retaining structure, or a similar type of intervention.

Figure 3.1. Types of intervention for various energy levels.


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The right solution for the problem is dependent on:

The ability of the solution to be effective immediately after installation, as well as in thelong term.

Acceptable safety factors. The degree and size of the risk associated with the failure of the structure used.There are three categories of solution that could be appropriate:

Heavy systems. This category includes systems like articulated concrete blocks(ACBs), Gabions or Reno Mattresses, Terramesh, rip rap, etc.

Light systems. This category includes Green Terramesh, erosion control blankets(ECBs), turf reinforcement mats (TRMs), Ecologs, etc.

Soil Bioengineering techniques. This category may include many sorts of vegetativespecies and treatments (without the use of combined inert materials) ranging from

herbaceous or woody plants, to a wide variety of treatments and systems.

4 DESIGNING THE SOLUTIONA proper design approach involves a complete analysis of the river and its catch basin in

order to identify the causes of land failure and possible restoration works. We should not

condemn extensive intervention if this is required to amend a compromised situation. Inplanning river training, the designer may use three fundamental classes of training and

hydraulic protection structures:

Longitudinal works, defined as structures parallel to the river flow. They are used to:a. Provide bank protection due to excessive erosion resulting in slope

instability;b. Provide adequate flow conveyance during the flood event;c. Provide sediment control and stabilisation of the streambed.

Transverse works (weirs, spillways, dams), to provide longitudinal stabilisation to thewatercourse.

Groynes, to convey the flow direction towards the central part of the channel, when thebanks may be subject to excessive erosion.

It is generally advised to approach a river training design by allowing streams to maintainthe most natural geometry, and combining experience and best management practices to

establish the most environmentally friendly protective system.

The stability against erosion of a river section is made by calculating the water velocity andshear stress values generated by the hydraulic flow regime and checking whether the river

bed material is suitable to withstand/resist these effects without permanent damage.

The most general and widespread method used is referred to as the tractive force method.

It is applicable to any kind of solution and requires that the river bed material being adopted

has a defined shear resistance, limit velocity and critical velocity.


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When considering rock filled Gabions and Reno Mattresses, Critical velocity is the velocity at

which the revetment will remain stable without movement of the rock fill, while Limitvelocity is that which is still acceptable although there is some deformation of the protections

due to movement of the stones within the compartments. (Table 4.1)FILLING STONES CRITICAL

VELOCITYLIMITVELOCITYTYPE

THICKNESS

mStone size mm d50 m/s m/s

70 100 0.085 3.5 4.20.17

70 150 0.110 4.2 4.5

70 100 0.085 3.6 5.50.23

70 150 0.120 4.5 6.1

70 120 0.100 4.2 5.5

Castoro

Reno

Mattresses

0.30100 150 0.125 5.0 6.4

100 200 0.150 5.8 7.6Gabions 0.50 and 1.0

120 250 0.190 6.4 8.0

Table 4.1. Indicative Castoro Reno Mattress and Gabion thicknesses in relation to water

velocities.

Designing river training works using an environmentally friendly approach will oftenrequire you to take account of the vegetation establishment over time. This will most likely

have an effect on the flow conveyance and on the materials allowable shear resistance. It isrecommended in this case to verify the channel section under two scenarios:

End of installation, where the river section will provide the maximum flow conveyance(due to the low roughness), and the protective system the lowest allowable shear

resistance. This condition will normally be critical to the protection used and isdependent on the inert materials only.

Vegetation completely grown, where the resistance to erosion will be higher due to theconsolidating effect of the roots. Vegetation will most likely reduce the river

conveyance section due to the increased roughness and reduced cross sectional flowarea. Design flow capacity with the vegetation completely grown (minimum 3 years

after end of installation) needs to be verified.

Table 4.2 gives the fundamental parameters (c, l, n) taken into account in the bankprotection calculation where:

c critical shear stressl limit shear stressn Mannings n roughness coefficient


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END OF INSTALLATION VEGETATION

COMPLETELY GROWN

SYSTEM ADOPTED

Roughness n

(s/m1/3

)

Allowable

tractive force

l(Kg/m)

Roughness n

(s/m1/3

)

Allowable

shear stress c

(Kg/m)

Rip-rap 0.03-0.07

(b) (c)

0.07-0.4

(d)

35Gabions 1.0m thick 0.0301 50 0.07-0.4

(d) 50

Gabions 0.5m thick 0.0301 47 0.07-0.4(d)

50

Castoro Reno Mattress

0.17m thick

0.0277 22.4 0.07-0.4(d)

40

Castoro Reno Mattress

0.23m thick

0.0277 26.9 0.07-0.4(d)

45

Castoro Reno Mattress0.30m thick

0.0277 33.6 0.07-0.4(d)

45

MacMat-R (TRM) 0.0303 15-18(a)

0.07-0.4(d)

35(a) = Function of the flood duration(b) = The coefficient shall be computed on the basis of the real typology of the work, taking intoaccount shape and dimensions of the stones

(c) = The actual resistant shear stress depends on the stone dimensions and may be computed(d) = Depends on the vegetation growth

Table 4.2. Allowable tractive force and roughness values

Vegetation can improve many of the factors and conditions causing earth slope and

riverbank instability. But we cannot ask the plant, or even their roots, to provide ussomething they will never be able to give us in the causes effects solution chronological

scale.

If the problem is a stability issue, we cannot take the soil shear strength increase offeredfrom the vegetation roots into account, because at the moment of the intervention, they do

not exist, or demonstrate to be sufficient: the solution is the use of a retaining structure(mass gravity or reinforced soil structure) that can solve the problem immediately.

If the problem is erosion control, we can use the widest range of solutions, from simple

seeding through the widest range of geosynthetics up to the heaviest stone revetment.Today the global infrastructural solution must create (or re-create) new habitats suitable for

the life of animals and plant communities, whose aim is the improvement of the global

local environmental quality.

Combining these two concepts gives way to the soil bioengineering concept, where themost appropriate inert material to provide an immediate solution, can be combined withplants to ultimately create a complex, unique building block which is living as it is

functioning in its restoration of a natural ecosystem.


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5 THE FINER DETAILS OF DESIGN

There are unfortunately a number of important details that are commonly omitted or

overlooked during either the design or installation phase of Gabion and Reno Mattress

works. While failures do happen, the majority of them are avoidable. They are normally as

a result of poor designs and inadequate design information, inferior or incorrect productsbeing used or sub-standard installation / construction techniques adopted on site.

The most important details that must be considered during design and installation are:

5.1 Scour Protection to Retaining Structures

When constructing a Gabion wall along a watercourse, one must consider the effect ofpossible scour at the toe of the structure especially in the case of highly mobile river beds.

When the potential scour is ignored or underestimated, the results can be catastrophic.

(Figure 5.1)

Figure 5.1. Inadequate scour protection leads to undermining of the structure.

The two most common scour protection options adopted are:

Structures With Deep FoundationsThe wall must be founded at a level that is not affected by potential water scour. This type

of foundation is suitable where the bed material is virtually in-erodible or where it consists

of solid rock. It is also appropriate in mountainous regions where a heavy bed load coulddamage Reno Mattress scour aprons protruding into the stream in front of the walls.


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Structures Built on Anti-Scour ApronsA Gabion or Rock Mattress apron should extend a minimum of 1.5 to 2 times the maximumexpected depth of scour horizontally beyond the vertical front face of the protection. To

ensure a monolithic structure, the anti scour apron should extend beneath the Gabion wall

to provide added structural integrity to the system. (Figure 5.2)

Figure 5.2. Gabion structure constructed on a Reno Mattress apron.

5.2 Keying In of structures into the banks

Outflanking due to inadequate key-ins is the most common mode of failure in river bankprotection works. The results can lead to costly repairs and what has been referred to as

monuments in the watercourse. (Figure 5.3)

Figure 5.3. Total outflanking due to non-existent key-ins.


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When designing Gabion type protections, it is advisable to key the Gabions into the

existing banks at between 30 and 90 to minimize the chance of scour around thestructure. (Figures 5.4 and 5.5)

Figures 5.4 and 5.5. The inclusion of Gabion keys prohibit scour behind the protection.

5.3 Scour protection around crest walls for Gabion weirs

Weir crest walls exist to direct the flow in a specific direction to minimize the scour around

the structure i.e. towards the centre of the weir (Figure 5.6). Their exclusion can lead to

excessive erosion around the structure (Figure 5.7).

Figure 5.6. Crest walls directing the flow to the centre of the weir.


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Figure 5.7. No crest walls can lead to scour around the weir structure.

5.4 Laying direction and thickness of Reno Mattresses

It is very important to ensure that Reno Mattresses are installed in the correct orientation tominimize rock fill movement which could ultimately abrade the woven mesh. Most Reno

Mattresses are manufactured with internal diaphragms at 1.0m centres to assist in confining

the rock fill and limiting rock movement during high flow conditions. This would implythat a 6 x 2 x 0.3 Castoro Reno Mattress with 5 double mesh internal diaphragms must be

installed such that they are perpendicular to the flow to provide confinement every 1.0m

(Figure 5.8). Mattresses installed with diaphragms parallel to the flow would only offerconfinement from the edge panels every 2.0m (Figure 5.9). Note: The allowable velocities

and tractive forces indicated in section 4 Designing the solution, are based on CastoroReno Mattresses installed in the correct orientation.

Figure 5.8. Diaphragms perpendicular to flow at the base.


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Figure 5.9. Reno Mattresses installed in the incorrect orientation. Note the excessive rock

fill movement.

6 CONCLUSION

The Gabion system has in the past been considered to be somewhat simple; a characteristic

which might be considered a drawback in an age of technological innovations. On the

contrary, the system is a highly engineered solution where specific parameters are utilizedto conduct complex designs. The use of materials with lower grade coatings, thinner wirediameters and inferior mesh configurations will put structures at risk.

The terms or similar approved and or equivalent are sometimes used very loosely and

designers must take a more proactive stance to obtain the relevant product technical data

from suppliers and contractors to ensure what is assumed during design stage is actuallywhat is being used on site.

On-going product knowledge training for both designers and installers by suppliers needsto be a priority, as there have been many developments in the system in recent years.

Supervision of installers needs to closely monitored by the designers (and suppliers who

offer this service), to ensure that they are suitably qualified in the installation of thesecomplex structures.

Suppliers must provide the necessary product information, design assistance andinstallation training to all parties involved. This will ensure that the Gabion solution does

not receive the negative criticism that it has in the past, due to inexperienced designers and

installers neglecting or overlooking details, which more than often compromise thestructural integrity of the systems. Figure 6.1 and Figure 6.2 show what can be achieved

when designers, suppliers and installers work closely together.


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Figure 6.1 Gabion weir structure with defined crest walls, key-ins and good installation

technique. The author provided design assistance and intensive on-site installation trainingfor this structure.

Figure 6.2 Large 250m/s Gabion weir structure utilizing 5000m of Gabions and CastoroReno Mattresses. Note the concrete capped horizontal steps to protect the mesh during high

flow conditions. The author provided design assistance and intensive on-site installation

training for this structure.


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7 REFERENCES

Agostini, R and Ciarla, M (1987) Flexible Reno Mattresses and Gabion structures in

Coastal Protection works, Technical report: The Fifth Symposium on coastal and OceanManagement, Seattle, Washington

Agostini, R, Conte, A, G, Malaguti and Pipettii, A (1985) Flexible Linings in Reno

Mattresses and Gabions for Canals and Canalized Water Courses

Pauselli, M (2003) Woven Mesh Gabions vs Welded Mesh Gabions The Facts!

Maccaferri Design Document Double Twist Wire Mesh Products for the Restoration of

Fluvial Environments

Brunet, G (1999)Bank Stabilisation with Ecological Engineering

US Army Corps of Engineers (1997)Bioengineering for Streambank erosion

maccaferri. the use of gabions

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