DETAIL ESTIMATING
GABIONS
B512 - GABIONS - OPSS 512
512.1
GENERAL
Gabions are contained rectangular units made of steel wire mesh and filled with
stone.
The double twisted wire mesh is galvanized and PVC (polyvinyl chloride) coated.
Gabions are flexible and permeable baskets or mats that when combined, may be used
for applications ranging from riverbank erosion protection to retaining systems.
Gabion baskets are supplied in collapsed form that require site assembly and hand
placement of stone (Figure 1). They are manufactured in standard sizes with the
following nominal dimensions:
Width
Length
Height
0.5m or 1.0m
1.5, 2.0, 3.0 or 4.0m
0.3, 0.5 or 1.0m
Figure 1 - Gabion Basket
Gabion baskets stacked vertically are referred to as gabion structures.
Gabion mats, generally used as channel lining, are supplied in 30 metre rolls with or
without filter cloth. They are manufactured in nominal widths or 2.0 or 3.0 m and
standard heights (depths) of 0.23 and 0.30 m.
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512.2
GABIONS
REFERENCES
Foundation Investigation and Design Report – project specific
MTO Drainage Management Manual
MTO Retained Soil Systems Guidelines
MTO Structural Manual
Maccaferri Gabions - Design and Installation Guides
512.3
TENDER ITEMS
The tender items associated with the placing of gabions and erection of gabion
structures are:
Gabions
Gabion Structures Not Exceeding 2m Height
512.4
SPECIFICATIONS
The installation of gabions is detailed in OPSS 512.
The installation of gabion structures is detailed in OPSS 512 amended by special
provision.
Materials for gabion baskets and mats are specified in OPSS 1430.
512.5
SPECIAL PROVISIONS
The designer should refer to Chapter “E” of this manual to review the special
provisions applicable to these tender items.
512.6
STANDARD DRAWINGS
There are no Ontario Provincial Standard Drawings for gabions.
512.7
DESIGN
The main sources of information are the Regional Geotechnical Section, Regional
Environmental Section, Pavements and Foundations Section and the Regional
Structural Section. For design application details, see manufacturers' publications.
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GABIONS
Gabion baskets may be used for the following installations:
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Retaining walls
Wingwalls
Weirs
Headwalls
Toe walls
Channel lining
Slope paving
Culvert aprons
Where gabion installation is of a simple nature, i.e channel lining, slope paving,
aprons or weirs, the work under the item "Gabions" is all-inclusive, covering any
excavation for gabions and bedding, placement of granular for bedding, use of
geotextiles and the installation of the gabion.
Geotextile may be used to separate the gabion baskets from the adjacent and
underlying earth and/or backfill material, depending on the existing soil conditions
and proposed backfill properties. The need for geotextiles should be assessed on a
project-specific basis. Where the OPSS 512 requirements for geotextile are not
appropriate, the geotextile requirements shall be specified in the Contract Drawings
or by Non-Standard Special Provision.
When a gabion installation has a retaining function and holds back an embankment,
i.e. headwall, wingwall, retaining wall or toe wall, it is described as a gabion structure
and the item "Gabion Structures not Exceeding 2m Height" should be used. Height is
measured from the base of the gabion structure and includes embedment depth, when
applicable. The designer may consult with the Regional Structural Section when
designing "Gabion Structures not Exceeding 2m Height". Design checks for bearing,
sliding and overturning are required.
For work under the item “Gabion Structures not Exceeding 2m Height”, excavation
for structure, granular bedding and backfill, and protection system (if required) are
covered by separate tender items. In the case of granular backfill, the tender item
would be the appropriate general granular item in the contract.
If the Contractor chooses or requires a protection system and the ‘Protection System’
tender item is not used to cover this in the contract, such protection system shall be
according to OPSS 539 and the costs would be included in the Gabion Structures Not
Exceeding 2 m Height tender item.
At MTO, retaining structures less than 2 m in height are considered landscape walls.
For gabion structures exceeding 2 m in height, the MTO Retained Soils Systems
process shall be followed.
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512.7.1
GABIONS
Information to be Provided to Bidders
The Designer should identify whether protection systems are required. The designer
shall consult with the Regional Geotechnical Section to determine whether protection
systems are required. If so, a Foundation Investigation Report describing the
subsurface conditions in the vicinity of the protection systems should be provided and
a separate OPSS 539 ‘Protection System’ tender item should be included in the
contract.
512.8
COMPUTATION
The items Gabions and Gabion Structures not Exceeding 2m Height are both Plan
Quantity Payment items.
The unit of measurement for Gabions and Gabion Structures not Exceeding 2m
Height is the cubic metre.
The volumes of Gabions and Gabion Structures not Exceeding 2m Height are
calculated from the number and dimensions of baskets shown on the design drawings.
Excavation for the Gabions item is carried out according to OPSS 206.
Excavation for the Gabion Structures not Exceeding 2m Height item is carried out
according to OPSS 902. When required, it should include excavation for bedding and
frost tapers. The designer is referred to Section B902 of the CDED.
512.9
DOCUMENTATION
512.9.1
Contract Drawings
Both new and existing gabions, or gabion structures requiring extension are shown on
the plans and profiles of the contract drawings.
Typical cross-sections should be included in the contract drawings that show
dimensions, any bedding and backfill requirements and excavation limits.
Where there is a requirement for a protection system, the line of protection or a
protection system shall be shown on the contract drawings depending on the
magnitude of the protection required. The actual length and location of the protection
system required is the contractor’s responsibility. Performance levels should be
included on the contract drawings. A separate tender item is provided to cover the
protection work.
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GABIONS
Material for bedding and backfill must be identified on the drawings even when it is
to be provided under the Earth Excavation (Grading) or Earth Borrow item.
The OPSS 1004 gradation of the gabion stone, G-3 or G-10, shall be identified on the
drawings.
512.9.2
Quantity Sheets
The quantity for each Gabion or Gabion Structure location is shown separately on
"Quantities - Miscellaneous 1" sheet. The column total provides the tender quantity.
512.9.3
Documentation Accuracy
Stations are recorded to the nearest whole metre.
Offsets are recorded to 0.1 of a metre.
Quantity entries are recorded to the nearest cubic metre.
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