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Retaining structures are a common feature of road construction in hilly and mountainous areas and can account for up to 20% of the total construction cost. Retaining structures comprise:

  • gravity walls, where the weight of the wall and its backfill provide most of the stabilizing force (masonry, gabion and reinforced concrete cantilever walls are typical examples);

  • embedded walls, where the soil in front of and behind the structure and anchors (if any) provide the stabilizing force (sheet pile or bored pile walls are typical examples);

  • reinforced soil, where the in situ soil mass is reinforced with nails or dowels (usually behind a protective face);

  • reinforced fill, where steel or geosynthetic geogrids or straps are embedded into the fill during its emplacement.

Due to cost considerations on most low-cost roads, retaining walls are usually designed as gravity structures. Consequently, this chapter focuses on gravity walls constructed from masonry, gabion, mass concrete and reinforced concrete. However, consideration is given to the use of soil nails to strengthen cut slopes (Section C5.2.5) and to the use of reinforced fill structures (Section C5.2.6), as these can provide useful alternatives under certain circumstances.

Walls are constructed in above-road and below-road locations; see Figure C5.1 for illustrations of these terms.

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