Reinforced Earth Structures – Materials and Design Criteria

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Reinforced Earth or Mechanically stabilized earth (MSE) is soil constructed with artificial reinforcing. It can be used for retaining walls, bridge abutments, dams, seawalls, and dikes. Although the basic principles of MSE has been used throughout history, MSE was developed in its current form in the 1960s. The reinforcing elements used can vary but include steel and geosynthetics. The reinforcement materials of MSE can vary. Originally, long steel strips 50 to 120 mm (2 to 5 in) wide were used as reinforcement. These strips are sometimes ribbed, although not always, to provide added resistance. Sometimes steel grids or meshes are also used as reinforcement. Several types of geosynthetics can be used including geogrids and geotextiles. The reinforcing geosynthetics can be made out of high density polyethylene, polyester, and polypropylene. These materials may also be ribbed and come in varying sizes and strengths.

Design of Reinforced Earth Structures

Reinforced Soil Structure consists of facing panels and compacted fill incorporating reinforcing elements (e.g. strips or grids). The reinforced soil block consists of the compact fill and reinforcing elements.

Design Requirements

Design Criteria

• The structures shall be designed for a minimum life of 100 years.

• The design loadings transferred from bridge abutments to the structures shall be specified on the Drawings. The live loading from the bridge approaches and the vertical and lateral loads from the earthwork shall be taken into account in the design.

• The structure shall satisfy the stability requirements of the AS 5100 for earth retaining structures.

• Allowable bearing pressures and settlements on the founding material under the reinforced soil block shall be determined by the designer from the geotechnical investigations.

• Drainage provisions shall be provided where necessary to intercept or divert groundwater and surface water to prevent scour or the development of hydrostatic pressure behind facing panels or saturation of any fill (approach and reinforced earth), and shall incorporate provisions to prevent saturation of the backfill in the event of a leaking water main. The drawings shall clearly show all drainage provisions.

• For straight walls the design shall provide for a 1 in 40 slope on the vertical wall face.

• If the design is to cater for future extensions (refer to the Project Design Brief or the Drawings), the design shall allow for both initial and future configurations. For walls which are to be extended in height, initially placed reinforcement and facing panels shall be designed to resist the future height wall and any surcharge without adjustment (except for removal of non standard capping panels). For walls that are to be extended laterally, the extension shall be able to be constructed without disturbing reinforcing straps that are to remain in place.

Materials for Reinforced Earth Structures

1. Wall Facings

Wall facing panels shall be of incremental height, precast reinforced concrete, manufactured in accordance with Division 3 "Concrete". The minimum concrete grade shall be S32. Steel reinforcement shall be a minimum of 450 mm2 per meter in each of two directions at right angles to each other and located at mid depth of the panel thickness. Wall facing panels shall be designed to prevent relative displacement. Wall facing panels shall incorporate anti graffiti measures and aesthetics in the design. A footing shall be designed to accommodate the wall facing panels.

2. Joint Fillers

Joint fillers between wall facing panels shall be composed of durable inert material resistant to attack from the soil material and the atmosphere. Joint fillers shall be provided to allow for joint rotation without spalling of concrete edges and to prevent loss of fines from the backfill material and staining of the panel faces.

3. Soil Reinforcing

Reinforcing strips or grids and their connections shall be fabricated from approved reinforcing products. Such products shall be sufficiently strong, stiff, stable and durable to satisfy the performance and design requirements of major reinforced soil structures and this Specification with a minimum of 10 years data from laboratory and site applications in representative conditions. Steel reinforcing shall comply with AS 3679, with a minimum base metal thickness of 5mm and hot dip galvanised after fabrication in accordance with AS 4680 with a minimum average coating thickness equivalent to 600 grams per square metre. Steel mesh shall comply with AS 4671 and hot dip galvanised after fabrication with zinc to AS4680 with a minimum average coating thickness equivalent to 600 grams per square metre. Synthetic material shall comply with a British Board of Agreement Certificate and demonstrated by testing in a NATA accredited laboratory to satisfy the performance and design requirements of this Specification.

4. Backfill

Select backfill shall comply with the requirements of the designer and have a particle size distribution, shear strength and coefficient of friction value to ensure the design parameters are achieved. If the backfill is in contact with galvanized steel components, it shall comply with the properties specified in Clause 420.8 "Verification Requirements". Pulverised fuel ash (PFA) shall not be used as select backfill.

5. Connections

Materials connecting the wall facing panels with the reinforcing elements shall be electrolytically compatible to ensure that corrosion will not be promoted through the use of dissimilar metals. All materials forming connections shall be adequately protected for the in situ conditions, consistent with the protection provided for adjacent components and for the defined structure life. Read More: Concept of Reinforced Earth Structure Design