1.0 HYBRID SYSTEM Any wall which uses facing units (of any type) tied to rods or strips (of any material) which have their ends anchored into the ground is a Hybrid Wall. These are walls which combine elements of both externally stabilized walls (e.g. gravity walls) and internally stabilized walls (e.g. reinforced soil): a) Tailed Gabion b) Tieback Anchor Wall / Anchored Earth c) Steel Sheet Pile Wall d) Tailed Concrete Block 1.1 Tailed Gabion 1.1.1 Definition Gabion walls are compartmented units filled with stone that are 4” to 8” in size. Each unit is a rectangular basket made of galvanized steel, geosynthetic grid, or polyvinylchloride (PVC) coated wire. Each gabion unit is laced together on-site and filled with select stone. Gabion walls can be designed with wire mesh or geosynthetic reinforcement that extends back into the retained soil from between the gabion unit. These wall systems are termed tailed gabions.
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1.0 HYBRID SYSTEM
Any wall which uses facing units (of any type) tied to rods or strips (of any material) which have
their ends anchored into the ground is a Hybrid Wall.
These are walls which combine elements of both externally stabilized walls (e.g. gravity walls)
and internally stabilized walls (e.g. reinforced soil):
a) Tailed Gabion
b) Tieback Anchor Wall / Anchored Earth
c) Steel Sheet Pile Wall
d) Tailed Concrete Block
1.1 Tailed Gabion
1.1.1 Definition
Gabion walls are compartmented units filled with stone that are 4” to 8” in size. Each unit is a
rectangular basket made of galvanized steel, geosynthetic grid, or polyvinylchloride (PVC)
coated wire. Each gabion unit is laced together on-site and filled with select stone. Gabion walls
can be designed with wire mesh or geosynthetic reinforcement that extends back into the
retained soil from between the gabion unit. These wall systems are termed tailed gabions.
Figure: Typical section of tailed gabions
Main components of tailed gabions are:
i) gabion structure
-consists of gabion box and stone filling
ii) mesh reinforcement
iii) compacted soil
- for back-filling behind the gabion.
Mesh reinforcementGabion box
1.1.2 Functions
This tailed gabion system is similarly functioned as the normal gabion. It can be used in many
situations including the stabilization of earth movement and erosion, river control, reservoirs,
canal refurbishment, landscaping, channel linings, gabion wires and also as retaining walls.
a) Retain at the river banks
It is useful to avoid erosion in streams where the mattress goes into the river and
underneath a gabion wall to stop the river undercutting the wall at the edge where the water
speed is fastest. It is also offer free drainage providing a higher bank stability when used for
river bank protection.
b) As landscaping
Gabions are customizable and can fabricate curves for home garden use. The gabion can
be planted in to soften edges. When it is used for landscaping purpose, the versatility is
limited by imagination.
c) As retaining walls
Gabion walls are suitable for retaining walls near to roads, dams, military context and also
domestic project. Gabions are easy enough to install that no experience is necessary to get
a good result.
d) As gabion cladding
Gabion used as cladding can provide an attractive finish to the buildings, walls and many
other situations. Gabions of 0.3m to 0.5m generally used and need to be attached to the
existing building using clamp bars. At 4m to 5m heights, 4mm diameter wire basket will be
used. Meanwhile, for more than 5m heights, 5mm diameter wire basket will be used.
e) As gabion fencing
Gabion now becomes an important feature in many situations and the construction thin walls
from narrow depth gabions. As required vertical faces and narrow depth, extra support to
provide stability and also can use metal or concrete posts at regular intervals inside the
gabion basket.
1.1.3 Design considerations
i) Must be flexible and free draining system
ii) The design must follow the Design Criteria BS8002- The Code of Conduct for Earth
Retaining Structure
iii) It is normal to incline the wall from vertical by 6/10° for aesthetic and stability reason.
iv) It can be build with speed and economy in all circumstances and particularly suitable for
liable to subside.
v) The capacity of gabion to deform makes them preferable to concrete wall which would
crack and collapse.
vi) It is fabricated by wire coated with polyvinyl chloride to prevent corrosion.
vii) It is usually filled with stones which require qualities of weather resistance, non-
dissolution and high stiffness.
1.1.4 Method of construction
i) Assembly and Installation
For easy handling and shipping, gabions are supplied folded into a flat position. They are readily
assembled by unfolding and by simply wiring the edges together and the diaphragms to the
sides. Adjoining gabions are wired together at their edges. Empty gabions, stacked on filled
gabions, are connected with lacing wire to the filled gabions at front and back. After the gabion
is filled, the top is folded shut and wired to the ends, sides, and diaphragms. Where practical,
several gabions should be wired together and stretched before filling to reveal weak areas of
lacing. Gabions may be filled by almost any type of earth handling equipment such as front-end
loader, crane, or modified concrete bucket. Some hand stone adjustment during the filling
operation is required to prevent undue voids. Exposed faces should be hand placed using
selected stone. This will add to the appearance of the structure and prevent the gabions from
bulging.
ii) Mechanically Applied Fasteners
Mechanically applied fasteners (rings or clips) can be used to connect panels, diaphragms or
adjacent cells in lieu of lacing wire. Only Class 3 coated galvanized clips or rings should be
used with galvanized gabions. Only stainless steel rings or clips should be used with PVC
coated gabions. Hog rings must have a minimum overlap of 1in (25mm) when closed.
iii) Erection of the Gabion Structure
This section applies to 3ft (1.0m), 1.5ft (0.5m), and 1ft (0.3m) thick gabions used in the
construction of retaining walls, weir walls, stepped revetments, and flared zones of transition
between retaining walls and sloped revetments. When gabion units, or modules, are placed into
the structure the inspector must be concerned with the alignment, grade, stretching, and
attachment of the individual units to each other. The alignment and grade are established by the
field engineer or surveyor but should be checked. The stretching and attachment of the gabions
to each other are of primary interest to the inspector. When several units, up to 100 ft. (33.0m),
are in place and aligned, they should be stretched to tighten the wire fabric. This operation
makes it much easier to control bulging during the filling operation. An anchor for the stretching
operation can best be obtained by completely filling the end cell of the end unit. This may be
accomplished either by machine or by hand filling. The inner tie wires (or stiffeners) shall be
properly installed in both directions in this anchor cell (tie wires will be discussed later). The
pulling force is then applied to the opposite end of the string. The attachment method for
applying the pulling force should be such that the force is applied to at least 4 places on the
heavy vertical selvedge wires forming the corners. The force should NOT be applied to the
fabric of the end panel.
While the stretching force is applied, the next to the last cell of the string is filled in the same
manner as before. This will leave the last cell empty to provide for easier attachment of
additional units. Subsequent gabions or gabion modules will be placed in the same manner with
only the next to the last cell being filled during the stretching operation. There will be times,
depending upon the type of structure or the physical limitations of the project site, when it will be
impossible to stretch the units. This will not affect the structural integrity of the structure.
However, additional care will be needed during the filling operation to maintain alignment and
prevent bulging. Typical situations in which this can occur include: (a) counter fortes behind
retaining walls, (b) the main wall of weirs or drop structures, and (c) curved or serpentine walls.
Gabion counter fortes and weir walls extend into narrow notches excavated into hill sides or
stream banks which allow for very little working room. The shorter the radius
of curvature of serpentine or curved walls, the more difficult it will be to apply a stretching force
to a long string of gabions.
iv) Filling Gabions Cells
The rock used to fill gabions must be of sound rip-rap quality, preferably graded from 4in to 8in
(100mm to 200mm) in size, and roughly cubical in shape. This will allow for the maximum
amount of machine filling. There will be some minus 4in (100mm) material present, due to
breakage in transit, but this should be limited to a maximum of about 5%. No minus 4in
(100mm) material should be permitted on any exposed face of the structure since it can fall out,
be washed out, or pulled out, thus creating a void in the structure. Soil (picked up from the
bottom of the rock pile) shall not be permitted to remain in the structure as a seam or “block”.
Some dirt or fine material is inevitable, but the amount must be minimal. Soil should be brushed
or manipulated in a manner so as to provide for the required point to point contact of the large
rock fill.
Excessive amounts of sand/soil will wash out and create large voids in the gabion structure.
Rock should be placed in 1ft lifts in the gabion structure, moving from cell to adjacent cell. A row
of 5in to 8in (125 to 200mm) size rock should be hand placed against the exposed face(s)
during the filling to present a pleasing appearance and minimize the size of exposed voids. The
first fill layer is then leveled to permit the installation of the inner tie wires. When at all possible,
the maximum difference of the rock level in adjacent cells should not exceed 1ft (0.3m). When
1.5ft (0.5m) deep gabions are used in a retaining wall or weir wall configuration they shall have
the inner tie wires, placed only at mid-height. 1ft (300mm) deep gabions, in the same situation,
do not need the inner tie wires, but should be filled in half height lifts. Inner tie wires or stiffeners
are needed to help brace exposed, or temporarily unsupported, faces of the gabions.
In a long retaining wall configuration they are placed transverse to the long axis of the wall.
Only the end cells, or temporary end cells, will need inner tie wires in both directions. The inner
cells of walls which have thick layers, say 9ft (225mm) or 12ft (300mm) or more, do not need
the inner tie wires if the adjacent cells are filled in 1ft (0.3m) lifts uniformly. After the first 1ft
(0.3m) lift of rock is laced and the inner tie wires installed, the second layer and inner ties and
then the third layer are placed using the same sequence of operations. The last layer should be
filled approximately 2in (50mm) above the top of the gabion to allow for subsequent settling of
the rock fill. The top should be roughly leveled (no large humps or voids), and the lid closed, and
then attached to the tops of the diaphragms, the ends, and the fronts.
v) Backfilling behind the Gabion Structure
The inspector should be very familiar with the entire project specifications involving the type of
backfill material and degree of compaction required. Backfilling any gabion structure is
performed in the same manner as in any other type of construction. Most care is needed to
ensure the gabion mesh wires are not damaged or broken by contact with the compaction
equipment. Caution should be exercised when backfilling a single long “string” of gabions (as in
a retaining wall). To achieve the normally specified degree of compaction, the compactive effort
may be sufficient to push the gabions out of alignment. If the gabions are not embedded below
grade, or laced to gabions which are embedded, alignment control may become a problem.
One possible solution would be to place a temporary fill in front of the gabion string to help hold
alignment. Sheets of plywood can be used to keep the temporary fill from infiltrating the voids of
the front face of the gabions. When compaction is completed, the temporary fill may be
removed. This method may cost a little in time and effort, but it is far cheaper, and much more
satisfactory, then trying to push the filled gabion string back into alignment. The degree of
compaction of backfill around gabions placed in notches in stream banks (weirs), or around
gabion counter fortes for retaining walls, is critical to satisfactory performance of the structure. If
doubts arise the Resident Engineer and the Design Engineer should be contacted immediately.
1.1.5 Advantages
a) Flexibility
It is subjected to alternating forces of tension and compression, the inherent flexibility of a
gabion structure enables it to deform rather than break. This is to prevent loss of efficiency.
Deforming in response to subsidence of foundation or internal stress is a functional feature.
b) Strength
Since gabions are bound together as a monolithic unit, the wire mesh is extremely strong
under tension. The wire mesh shell is not simply a container for the stone filling, but a
reinforcement of the entire structure. Additional strength is achieved by the use of the
vertical diaphragms. These diaphragms are affixed to the base of the gabions to restrict
internal movement of the stone filling and provide further reinforcement. Gabion efficiency ia
actually increasing despite of increasing in ages. During early periods of use, silt and
vegetation will collect within the rock filling to form a naturally permanent structure,
enhancing the environment.
c) Permeability
Interstitial spaces in the stone filling within the baskets provide a great degree of
permeability throughout the structure eliminating the need for a drainage system and
preventing build up of hydrostatic pressure which will displace and crack concrete
structures. In the river works, pressure and counter pressure on the banks due to the
variations in water depth between flood and low water are therefore also eliminated.
d) Economy
Wire mesh gabions are less expensive than most construction materials. Graded stone fill is
usually available and waste materials such as crushed concrete may be specified in place of
stone. Construction costs are reduced, unskilled labors can easily learn to erect modular
gabion system, fill them and close them properly. Many gabion structures may be built
without any mechanical equipment. Pilings, underwater drainage systems and excavations
are unnecessary. There is no need to drain the site or to construct a cofferdam for
underwater installation. The first layer gabions can be laid in water or in mud. Upon
completion, a gabion structure will take its full load immediately, without the waiting periods
of up to one month normally associated with concrete structures. Gabion structure are
virtually maintenance free.
e) Extendable
Extensions are simple. Additional units are simply attached to the existing ones. The gabion
wire diameter can range from 2mm to 8mm and the gabion basket can either be welded or
woven, galvanised or plastic coated, depending on application. Baskets come in flat pack
form and are assembled on site. They can be used in a range of applications from small
domestic projects to large industrial schemes. The infill to the baskets is usually locally
sourced stone ranging from 200mm - 100mm. Hand facing ensures that the wall has good
cosmetic appearance. Gabion advantages include flexibility, durability, strength,
permeability, economy, and landscaping.
f) Very porous, durable, reliable, easily constructed and ecological.
1.1.6 Disadvantages
May be considered unsightly by some people, they may take up quite a lot of space up on the
beach as lots of gabions are needed to provide plenty of protection. This makes the beach less
desirable to people as the beach may seem more crowded.
If the rocks inside the cage become worn down the whole cage will need to be taken out and to
be replaced with new ones. If the gabions are stacked up in a wall and a lower one needs
replacing which means to take out all the above ones to replace it as well as to rebuild the wall.
Hence, this would take lots of time and cost.
1.2 Tieback Anchor Wall/ Anchored Earth
1.2.1 Definition
An anchored wall is any non-gravity cantilevered wall (i.e. sheet-pile wall, soldier pile and