B512 - - 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 0.5m or 1.0m Length 1.5, 2.0, 3.0 or 4.0m Height 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. January 2014 Page 1 of 5 B512
512.2 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. January 2014 Page 2 of 5 B512
Gabion baskets may be used for the following installations: 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. January 2014 Page 3 of 5 B512
512.7.1 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. January 2014 Page 4 of 5 B512
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. January 2014 Page 5 of 5 B512