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Chapter 2 — Systems and Project Evaluation

2.1 Applications

Reinforced soil is used wherever a soil mass must be made steeper, stronger, or more stable than it would be unreinforced, but where a conventional concrete structure would be too costly, too heavy, or too slow to build. The manual groups applications into three families:

  • MSE walls — highway embankment widening, grade separations, bridge abutments and wing walls, marine and waterfront walls, and urban retaining walls where the right-of-way is tight.
  • Reinforced soil slopes — new cut or fill slopes built steeper than the soil would naturally stand; repair of failed or creeping slopes; widening of an existing roadway where the toe cannot move outward.
  • Hybrid and specialty uses — reinforced soil over soft ground, reinforced embankments over piles, and structures combining MSE with conventional elements.

2.2 Advantages and disadvantages

Advantages of MSE walls

  • Cost and speed. Precast facing and granular fill are cheap and fast compared with cast-in-place concrete walls.
  • Flexibility. The facing tolerates differential settlement far better than a rigid wall, and can be built to accommodate curves, steps, and complex geometry.
  • Constructibility. The system is built from the bottom up with small equipment and no large formwork or curing time.
  • Aesthetics. Facing textures and finishes are easy to vary.

Advantages of reinforced soil slopes

  • Uses site fill. Marginal on-site soil can often be used in the reinforced zone, reducing import and disposal.
  • Vegetation. The sloping face can be vegetated, so the structure can blend into the landscape — a decisive advantage in environmentally sensitive areas.
  • Repairs. A slope can be stabilized without a large structural wall.

Disadvantages

  • Reinforced fill quality matters. The fill in the reinforced zone must meet gradation, plasticity, and electrochemical limits — it is not "any soil".
  • Corrosion and durability. Steel reinforcement must be protected against aggressive backfill and water; geosynthetics must resist creep, installation damage, and chemical ageing.
  • Drainage is mandatory. The design assumes free-draining fill; a blocked or omitted drainage system can produce hydrostatic pressures the design never contemplated.
  • Deformation. MSE walls move — the facing tilts and the wall bulges outward slightly — and the design must accommodate this, especially where utilities or structures sit behind the wall.
  • Not for every site. Soft or compressible foundations, aggressive groundwater, or tight tolerances may rule reinforced soil out.

2.3 Relative costs

Reinforced soil is normally the low-cost option within a band of wall heights — the manual presents cost comparisons showing MSE walls becoming competitive with conventional retaining walls above roughly a few metres, and reinforced soil slopes becoming competitive where the required slope angle is modest and site fill is available. Cost is driven by fill availability, facing type, reinforcement quantity, and foundation treatment rather than by height alone. The manual's cost figures (Appendix C, and Chapter 7's project cost estimates) are dated and must be re-based to local prices.

2.4 Description of MSE/RSS systems

Systems differentiation. The first question is whether the reinforcement is inextensible or extensible:

  • Inextensible (steel strips, bar mats, welded wire grids) strains very little relative to the soil, so the soil's own deformation mobilizes the reinforcement's tension. Design uses a coherent-gravity model with a Rankine-type active earth pressure and a maximum tensile force line.
  • Extensible (geotextiles, geogrids) strains comparably to the soil, so internal strain compatibility governs the distribution of load. Design uses a tie-back wedge model with a coherent-gravity stress distribution that is more uniform with depth.

The manual treats both within one framework but flags the different stress distributions and the different pullout and connection models.

Types of systems. MSE wall systems are differentiated by facing (precast concrete panels, modular concrete blocks, wrapped geosynthetic, timber, or welded-wire) and by reinforcement (strips, grids, bar mats, geotextiles, geogrids). Reinforced soil slope systems are differentiated mainly by how the face is treated (bare wrap, vegetated wrap, armored wrap).

Facing systems. Facing types include:

  • Segmental precast concrete panels — the classic MSE wall face, attached to the reinforcement by mechanical connections or friction.
  • Concrete modular block (MBW) — dry-stacked block units with pins, cores, or tails that connect to the reinforcement.
  • Flexible (wrapped) facings — geosynthetic wrapped around the face fill and held by a return wrap; sometimes with a protective mesh or shotcrete skin.
  • Temporary facings — used during construction, later covered.

Reinforcement types. Steel: smooth or ribbed strips, bar mats, welded wire grids, and (in some systems) steel anchors. Geosynthetic: woven and non-woven geotextiles, and uniaxial geogrids. Selection depends on strength, stiffness, durability, connection to the facing, and cost.

Reinforced backfill materials. The reinforced fill must be frictional and free-draining: typically a granular soil meeting limits on fines content, plasticity index, gradation, and electrochemical aggressivity. Where a suitable granular fill is unavailable, a lower-quality fill may be acceptable only with additional design measures (increased reinforcement, extra corrosion protection, or drainage layers).

Miscellaneous materials. Facing joint materials, bearing pads, drainage aggregate and pipes, filter geotextiles, and erosion-control materials.

2.5 Site evaluation

Site exploration. A reinforced-soil project needs the same subsurface information as any retaining structure: stratigraphy, groundwater regime, and the engineering properties of the foundation soil.

Field reconnaissance. Walk the site: identify slope geometry, seepage, existing instability, drainage paths, utilities, right-of-way, and access for construction equipment.

Subsurface exploration. Borings, test pits, and in-situ tests establish the foundation profile and the depth to competent bearing strata — decisive for settlement and global stability.

Laboratory testing. Strength and consolidation testing of foundation soils, and gradation, plasticity, compaction, and electrochemical testing of candidate reinforced fills.

2.6 Project evaluation

The choice among a conventional wall, an MSE wall, and a reinforced slope turns on several factors:

  • Structure selection factors — required height and face batter, foundation conditions, fill availability, and construction access.
  • Geologic and topographic conditions — the presence of soft, collapsible, or expansive strata; slope stability of the surrounding ground; seismic setting.
  • Environmental conditions — aggressive groundwater or soil chemistry (chlorides, sulphates, acidity), which governs steel corrosion and geosynthetic durability; and ecological constraints, which may favour a vegetated RSS.
  • Size and nature of the structure — the load the structure must carry (including bridge abutment loads) and the required service life.
  • Aesthetics — the visible face and its context.
  • Questionable applications — the manual flags situations where reinforced soil is a poor fit, including very soft foundations, aggressive environments without adequate protection, and cases where tolerances are unrealistically tight.

2.7 Establishment of project criteria

Alternates. Compare at least one conventional solution against the reinforced-soil option on cost, constructibility, and risk.

Facing considerations. Facing type, texture, and jointing are chosen for performance, durability, and appearance.

Performance criteria. Set the limits on facing deformation, differential settlement, and joint opening that the structure must satisfy — the criteria the design must meet and the monitoring must verify.

Design life. Fix the required service life (commonly 75–100 years for highway structures), because it directly sets the corrosion allowance for steel reinforcement and the reduction factors applied to geosynthetic strength.

2.8 Construction sequence

  • MSE walls with precast facings — place the levelling pad, erect the first panel course, connect and place reinforcement, place and compact fill in lifts, then repeat course by course.
  • MSE walls with flexible facings — build in lifts, wrapping the geosynthetic face around the fill and holding it with a return wrap, then compact.
  • RSS construction — place reinforcement at the design elevations as the fill is raised, benching the foundation, then treat the face (vegetation or armor).

2.9 Proprietary aspects

Many MSE systems are proprietary, protected by patents, and supplied with their own design methods, facing details, and connection capacities. The manual therefore distinguishes materials (which must meet specification regardless of supplier) from special applications (where the system supplier's own design and test data are needed). This is the origin of the end-result and supplier-design contracting approaches in Chapter 8.

2.10 Key takeaways

  • Choose the structure family first: MSE wall (vertical face) or RSS (sloping face) — they fail differently and are designed differently.
  • Classify the reinforcement as extensible or inextensible; it changes the internal-stability model.
  • Fill quality, corrosion/durability, and drainage are the three recurring failure causes — evaluate them at the project stage.
  • Design life is a design input that propagates into every durability factor.