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GABIONS Subject:Technology-Geotecnical Engineering Asha.Kanaller(Think ID-2748, [email protected], ID-2748, [email protected], ph.no-8722069122) , Bhavana.Basavaraj(Think ID-2485,[email protected], ID-2485 ,[email protected], ph.  ph. no-9686085893), BVBCET, Hubli

Abstract  —  Gabions   Gabions

are boxes and mattresses made of hexagonal mesh fabric, which is a valid and effective technical solution in the design, construction and maintenance of a variety of protective flexible structures. The gabions are steel wire cages that vary in size and are designed to abate the destructive forces of water currents and erosion. Gabions are uniquely woven by twisting each pair of w ires twice or thrice continuously providing the inherent strength and flexibility. Gabion cages are designed to contain quarry run or river run stones available at the site of erection or from a nearby quarry. Cages are stacked to construct structures of great durability and flexibility. Gabions form flexible structures that can deflect and deform in any direction without fracture. It can withstand the movement of earth without inordinate structure deformation. This attribute enables the gabion structure to be built with a minimum foundation preparation. Gabion structures behave as perforated barriers, allowing water to gradually pass through them. This is a valuable characteristic, characteristic, as t he hydrostatic pressure never builds up behind or under the structure and cause failure to the gabion design. Gabion structures are regarded as permanent. In the early stages after installation, siltation takes place between the stone fill promoting vegetation and adding to the permanency of the structure. Gabions provides a cost effective, environmentally sensitive solution for numerous civil engineering applications such as retaining walls, vegetated slope reinforcement, slope stabilization, slope reinforcement, channel lining, channel relocation, slope and bank protection, shore protection, weirs,  jetties, drop structures, culverts, headwalls, bridge abutments, rock fall confinement and erosion control.

I. I NTRODUCTION Ground modification techniques have become a major part of civil engineering practice over the last 40 years, and their use is growing rapidly as worldwide development poses an increasing demand, and for land reclamation and the utilization of soft or unstable soils. Soil, one of our most abundant natural resources, deserves to be treated as wisely for engineering purposes as for agricultural uses. In its  broadest sense, any interference in the natural state of soil and rock could be termed as ground modification. Ground modification techniques are widely classified into 4 groups as mechanical modification, hydraulic modification, physical and chemical modification and modification by inclusions and confinement. Confinement may be produced by internal “inclusions” or external formwork or supports. The idea of confinement is very obvious in the construction of crib walls, bin walls and

Gabions. These are essentially gravity structures created by interlocking forms containing granular material.

Gabions are rectangular units made of hexagonal mesh fabric, which are usually 1m high and 2m to 4m long. The term mattress is used for units, which are thinner but larger in area; say 6m by 4m and 0.25m to 0.5m high. The meshes are made of 2mm to 3mm diameter wire, with openings from 60mm to 100mm.They may be coated with poly vinyl chloride (PVC) for protection against against corrosion. Individual units are wired together to form larger structures. They offer valid and effective technical solutions in the design, construction, and maintenance of a variety of protective flexible structures. In the early stages after installation, siltation takes place between the stone fill promoting vegetation and adding to the  permanency of the structure. Gabions and mattresses will revegetate over time as the structure collects soil and the airborne and the waterborne seeds of the locally occurring  plant life. The spontaneous growth will create a very natural environment and will increase the efficiency of the gabion structures as the plants enhance the soil stability. Though the concept of adopting flexible crate like structures has been used since ancient times, it is only in the last few decades that their widespread use has led them to become an accepted construction material in civil engineering. II. PHYSICAL STRUCTURE: A typical unit of a gabion is as shown in the fig 1. Each gabion unit is rectangular and is fabricated from double twisted hexagonal mesh (fig 2) of soft annealed heavily zinc coated steel. The double twist joints prevent the mesh from unraveling. The mesh panels are reinforced at all edges with wires of larger diameter than that used for manufacturing the mesh, to strengthen them and to facilitate construction. When the gabions are to be employed in marine or polluted environments, the wire prior to manufacturing the mesh, is  provided with a 0.4 to 0.6mm thick special PVC coating. A complete protection against possible corrosion is thus obtained. The mattress (fig.3) is a special form of gabion with a large  plan area / thickness ratio. It is fabricated from a similar but smaller double twist hexagonal mesh to that used to manufacture the gabions. The wire characteristics are the same. Diaphragms are spaced usually at 1m centers, and a continuous panel of mesh forms the base, the side and the end walls of the unit to obtain an open-topped multicell container. The same mesh is used for the base, diaphragms, and the separate lid. All panel edges are selvedged with a wire of larger diameter than that used for the mesh, so as to strengthen the structure.

to sites economical and easy. Careful handling of gabions is necessary to ensure that there is no damage to the PVC coating. IV. GABION ASSEMBLY:

Fig.1. Double Twisted Hexagonal Mesh

The gabion box is opened and unfolded on a hard surface and the creases are pressed out. The front and rear sides, ends and diaphragms are lifted into position to form a box shape as shown in fig.4. The edges are laced together starting from the top corner in a continuous operation. Empty gabion units shall  be assembled individually and placed on the approved surface to the lines and grades as directed. Finished gabion structures shall have no gaps along the  perimeter of the contact surfaces between the adjoining units. All adjoining empty gabion units shall be connected along the  perimeter of the contact surface in order to obtain a monolithic structure. All lacing wire terminals shall be securely fastened. The initial line of gabion basket units shall be placed on the  prepared surface and adjoining empty baskets are set to line and grade, and common sides with adjacent units are thoroughly laced or fastened. They shall be placed in a manner to remove any kinks or bends in the mesh and to uniform alignment. The basket units shall be partially filled to provide anchorage against deformation and displacement during the filling operation.

Fig.2. Structure of a gabion

Fig.3. Mattress

By virtue of their characteristic structure, gabions and mattresses have inherent strength and flexibility. The compartments or cells are of equal size and dimension and are formed by internal diaphragms being placed within the basket. The diaphragms have the function of reinforcing the structure and making the assembly and erection easier. The compartments or cells are filled with natural stone. Even distribution of stone fill ensures that the mattress maintains intimate contact with the foundation soil. III. CONSTRUCTION: Gabions are delivered to the site in collapsible box form as  bundles, in order to occupy less space and make transport ation

Fig.3. Gabion Assembly

The end gabion is partially filled with suitable stone to form an end anchor and bracing wires are fixed at 500 mm spacing to prevent the front side from bulging. The gabions are tensioned by applying a load to the end, remote from the anchor gabion ensuring that the load is distributed over whole area of the last gabion. Following procedure is adopted for tensioning of gabions: a) The empty gabion or group of gabions is placed in position. The ends are secured from which work has to be started by rods driven through the two corners into the ground and tied firmly to a ground anchorage. The anchorage should be at least the height of the gabions to prevent it collapsing.  b) The remaining empty gabions are wired one to another as work proceeds. Inserting bars into the bottom corners and

levering them forward stretch the opposite ends of the gabions. The top and bottom are then kept stretched until the gabions have been filled, by tying them to the fixed point. c) While the gabion is being stretched, it should be checked that the wiring together has been has been properly carried out and is not pulling apart. V. GABION STONE P LACEMENT: The stone fill shall have a gradation as specified within the specifications. The stone fill shall be placed into the gabion units in 0.3m (1’) lifts. Cells shall be filled to a depth not exceeding 0.3m (1’) a time. The fill layer should never be more than 0.3m (1’) higher than any adjoining cell. Connecting wires shall be installed from the front to back and side to side of the individual cells at each 0.3m (1’) vertical interval for gabions with a depth of 1m (3’). The voids s hall  be minimized by using well-graded stone fill .All stone on the exposed face shall be hand placed to ensure a neat compact appearance. Deformation and bulging of gabion units, especially on the wall face shall be corrected prior to additional stone filling.

Fig.5. Construction Process

All corners shall be securely connected to the adjoining basket of the same layer before filling the units. When more than one layer of gabions is required, in order for the individual units to  become incorporated into one continuous structure, the next layer of gabions shall be connected to the layer underneath after this layer has been securely closed. Gabions shall be uniformly overfilled by about 1-3 inches to account for future structural settlements and for additional layers. Gabions can be filled by any kind of earth filling equipment (fig.5). The maximum height from which the stones may be dropped into the baskets shall be 1m (3’).

VII. ADVANTAGES: The various advantages of gabions are described below: Flexibility:  The double twist hexagonal mesh  construction permits it to tolerate differential settlement without fracture. This property is especially important when a structure is on unstable ground or in an area where scour from waves or currents can undermine it. Permeability:  Gabions are naturally free draining.  Drainage is accomplished by gravity and by evaporation as the porous structure permits an active air circulation through it. Moreover as soil is deposited and plant growth invades the structure, transpiration further assist in removing moisture from the soil being protected. Hydrostatic heads do not develop behind a gabion structure because of their  permeable nature. They are free draining structures ideal for slope stabilization and retaining walls. Durability: Its efficiency increases instead of  decreasing with age since further consolidation takes  place as silt and soil collects in the voids and vegetation establishes itself. It is capable of undergoing structural movement without any loss of structural integrity, creating a highly durable structure. Strength: Gabions can withstand and absorb the  forces generated by retained earth or flowing water. The most important and relevant strength characteristics pertaining to gabion and mattress design standards are the mesh punch strength, the mesh tensile strength, the pull apart resistance strength, and the corrosion resistance of the mesh. Ecology:  They offer an environmentally sensitive  solution to soil retention and slope stabilization. They use natural stone for their stability. Soil will be gradually deposited in the small voids of the stone fill material during drainage and will promote vegetated growth inundating the structure. Cost Effectiveness: Gabion structures are easy to  construct. They require limited equipment, unskilled labour, minimum foundation preparation, no costly drainage provisions, low cost stone fill provided by local stone or rock supplier. They require very little if any maintenance and if properly designed can be a  permanent structure. Aesthetics: It can create visually pleasing structures.  Unlike other type of materials such as modular block walls, gabion stones do not discolor due to drainage. In fact the larger a gabion or a mattress structure, is in place and becomes inundated with vegetation, the more attractive it becomes.

VI. LID CLOSING: The lids of the gabion units shall be tightly secured along all edges, ends and diaphragms in the same manner as described for assembling. Adjacent lids may be securely attached at the same time. The panel edges shall be pulled to be connected using the appropriate closing tools where necessary. Single  point leverage tools, such as crowbars will not be acceptable. All end wire shall then be turned in.

VIII. PRECAUTIONARY MEASURES: Although gabion construction is simple, good supervision of the construction is essential. The most common construction deficiency is inadequate filling of the  baskets. Proper tensioning of the baskets is essential. Regular inspections, particularly after floods or heavy rain

have to be carried out. The following details have to be checked for: 

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Fig.6.e

Inadequate peripheral construction: If cutoffs, returns, and aprons appear inadequate, some form of additional work will be required. Loss of rock:  If loss of rock is excessive, replacement of rock, if possible or some overlay such as slurry concrete may be required. Fig.6.g

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Fig.6.f

Loss of shape:   If gabions subjected to high water impact lose their shape, their effectiveness and stability should be assessed. Torn Mesh:  Apart from negligence, debris and large angular rocks are the main offenders. Tears should be mended and rocks replaced where necessary. Abraded mesh:  Areas, which are subject to high abrasion, are crests, aprons, and energy dissipaters. If abrasion becomes significant, some form of protection or reinforcement should be applied. Inadequate anchoring: Early performance of a structure often indicates the need for additional staking or stronger anchoring.

Good design, quality materials and a high standard of work quality and supervision can make gabion construction a versatile construction technique for a civil engineer. IX. APPLICATIONS - EXAMPLES:

Fig .6.a

Fig .6.c

Fig .6.b

Fig.6.d

Fig.6.h

Fig.6. a) Switzerland-Retaining wall for protection of railway near Alpnach  b) Brasil-Bridge abutments on Turvo river, near S Jose dos Campos c) Canada-Ontario, Culvert protection near Cornwall d) Australia-Protection of abutments of the Sheridan Bridge ,State Highway e) Brasil- Seashore protection of the Praia Mansa at Caioba f) Italy- Canalization and Lining of the Fella Stream. g) Republic of Cape Verde-Series of soil conservation in Santiago Island h) Canada –  Cubec-Marina walls at Mantane X. CASE STUDIES: Gabion construction has been adopted world wide due to the numerous advantages it offers. Some case studies have been  presented here.  Eco-Compatible Gabion Retaining Walls On The Hilly Section Of The Mumbai –  Pune Expressway The government of Maharashtra assigned the construction of the access-controlled expressway between Mumbai and Pune to the MSRDC. While the stretches between Kon and Adoshi (Sections A and B) and Kusagaon and Ozarde (Sections C and D) obtained environmental clearance from the GOI, the lengths between Adoshi and Kusagaon, considered as ecologically sensitive areas, failed to do so. MSRDC therein, decided to take up the improvements in the existing Ghat  portion of NH-4. Being a hilly terrain, widening of the road could either mean cutting of hill or filling on the valley side. Keeping in mind the environmental considerations, a balancing method of partly cutting from the hill and filling the same muck and stones on the valley side by adopting gabion walls turned out to be an ideal solution. The typical sizes of boxes used for Gabion wall construction along the ghat portion were 4 x 1 x 1m, 2 x 1 x 1m, and 1.5 x 1x1m, and mesh type 10 x 12 with the horizontal  perpendicular distance between the axes of double twists  being D=99mm. At the present site, corrosion is caused due to the emission of the carbon monoxide and carbon di oxide by vehicles and by heavy rainfall. Hence additional PVC coating is provided to the galvanized wire. Gabion boxes were hand  packed with stones conforming to BS : 5390. Stepped rear face wall configuration was found to be more suitable under the existing conditions as they required

minimum rock cutting. The gabion walls were placed on the PCC leveled rocky strata or 95% compacted embankment as  per the situation. At some other locations, gabion walls were founded on M-20 grade PCC founding blocks with 25 mm dia  bars for anchoring the blocks to the rock bed. To prevent the build up of hydrostatic pressure behind the  backfill and to provide free drainage of water t hrough the wall, drainage arrangements were made. This comprised of larger size material at the lower level of the fill and drains for collecting the water. At regular intervals along the walls, transverse drains made of heavy duty PVC were laid at a slope of 1:15 to carry water away from the foundation. At the back of the wall, to prevent the ingress of fine soil from the backfill into the gabion wall structure Terram 1000 geotextile layer was provided. Gabion walls of height ranging from 1m to 6m were provided. The total quantity of work involved was of the order of about 35000 cum. The gabion retaining walls along the ghat portion of NH-4 have successfully withstood some reasonably heavy showers following the first monsoon after the completion of works As expected some degree of the sub grade settlement was observed at locations where the gabion walls were constructed on large thick rock-fill deposits. Gabion walls lived to the expectations and withstood the settlement without any structural damage. They appear to have functioned as designed with little or no lateral movement taking up even the settlements that occurred in the lower layers of the dumped rock fill. Practically there was 30 to 40% saving in the cost by adopting gabion retaining walls instead of the conventional RCC wall. Crotch Lake Dam Rehabilitation Project (Eastern Ontario) The earth dam was located on Crotch Lake on the Mississippi River in eastern Ontario. Crotch Lake Dam provides water storage for the power production at High Falls Generating Station. It also regulates water levels for recreational activities and residential uses. The existing dam was built in 1926 as a rock filled timber crib structure. Over the past few decades the timber crib core had deteriorated, the waterproofing membrane had failed and the rock covering the structure had settled. A Dam Safety Assessment of the existing dam was conducted  by Ontario Hydro, concluding that severe damage and  potential failure could result, if the dam was overtopped. The reconstruction of the dam was not to alter the existing cross section geometry or crest elevation. It was proposed to use gabions and gabion mats to replace the existing dam. The design had to comply with the Canadian Dam Safety Guidelines. Ontario Hydro requested that an independent engineering consultant review the design XI. CONCLUSIONS: 

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Confinement is one of the ground modification technique and gabions which come under this category are easy to construct. They offer valid and effective technical solutions in the construction of a variety of protective flexible structures. They are cost effective and eco-friendly.

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They result in visually pleasing structures Because of all these plus points, they are widely used in foreign countries and are slowly gaining popularity in India. XII. REFERENCES:

[1] Manfred R.Hausman, (1990).”Engineering Principles of Ground Modification”, Singapore: McGraw Hill International edition [2] Momin S.S, Khatu.P.L, Gharpure A.D (June 2003). “EcoFriendly gabion retaining walls on the hilly section of Mumbai-Pune Expressway”, Indian Highways [3] “ For River training works, Earth control and soil conservation, retaining structures landscaping, Lining of Canals and dams, Marine works” (1996). Officine Maccaferri S.p.A.Publication Bologna-Italy [4] www .terraaqua.com [5] www.ieca.org/Photogallery/GabionIllustrations.asp [6] www.meccaferri-canada.com