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Chapter 6 — Reinforced (Steepened) Soil Slopes: Project Evaluation

6.1 Introduction

A reinforced soil slope (RSS) is a soil slope, steeper than the soil could stand unreinforced, whose stability is provided by horizontal layers of reinforcement extending back from the face into the stable ground behind the critical slip surface. Unlike an MSE wall, the face is sloped, and the structure's stability is a global slope-stability problem, not a rigid-body problem.

RSS is the natural solution when the required slope angle is moderate, when site fill is available, when vegetation is wanted, and when the foundation is too weak for a wall. It is also the standard repair technique for a failed or creeping slope.

6.2 Reinforced soil slope systems

Types of systems. RSS systems are differentiated by:

  • Reinforcement type — geosynthetic (geotextile or geogrid), which is the most common, or metallic (grids, bar mats) for specific applications.
  • Face treatment — bare wrap, vegetated wrap (the dominant form, often called a vegetated reinforced soil slope, VRSS), or armored wrap.
  • Construction staging — built up as a new fill, or used to repair an existing slope by excavating in benches.

Construction materials. The reinforced fill must again be frictional and free-draining; the reinforcement must meet the same creep, installation-damage, and durability requirements as for walls; the face treatment materials (erosion control mats, vegetation, or armor) are chosen for the environment and the design life.

6.3 Design approach

Use considerations. RSS is preferred where the required face angle is relatively flat (so the reinforcement is efficient), where the foundation is weak, where vegetation is required, and where the failure mode is a shallow or intermediate slide rather than a deep-seated one.

Design of reinforcement for compaction aid. A lightly reinforced slope may be used simply to allow compaction of a steep face fill — the reinforcement's role is to hold the face during construction, not to provide long-term stability.

Design of reinforcement for steepening slopes and slope repair. The reinforcement is designed to raise the factor of safety of the critical slip surface to the required value. The mechanics are the modified limit-equilibrium analysis with reinforcement described in Chapter 7.

Computer-assisted design. Because the critical surface is unknown, RSS design is normally done with a slope-stability program that searches for the critical surface and accounts for reinforcement forces.

Evaluation of external stability. The reinforced slope can still fail externally — by sliding along a weak foundation layer, by bearing failure, or by a deep-seated slide passing below the reinforced zone — so external stability must be checked independently of the internal reinforcement design.

6.4 Construction sequence

Build the reinforced slope in lifts: prepare and bench the foundation, place the reinforcement at the design elevations, place and compact fill, wrap the face, then repeat. The face is protected during construction and vegetated or armored afterward.

6.5 Treatment of the outward face

Grass-type vegetation. Seed or hydromulch the wrapped face; simple and cheap, but limited to gentle face angles and mild climates, and vulnerable before the vegetation is established.

Soil bioengineering (woody vegetation). Live cuttings, brush layers, and other bioengineering techniques are used on steeper faces and in wetter climates; the roots reinforce the face soil while the plants provide surface protection.

Armored. Where vegetation will not survive, or where the face is exposed to flow, the face is armored — with rock, articulated concrete blocks, or a protective mesh or shotcrete skin over the wrap.

6.6 Design details

  • Guardrail and traffic barriers — carry the impact load to a structural element, not into the slope face.
  • Drainage — intercept surface water above the slope and convey it away; control subsurface water so it does not raise pore pressures in the reinforced zone.
  • Obstructions — utilities and drainage structures interrupt reinforcement; detail around them.

6.7 Case histories

The manual documents five built projects that illustrate the range of RSS applications:

  • The Dickey Lake Roadway Grade Improvement Project — a reinforced slope used to widen and steepen a roadway grade.
  • Salmon–Lost Trail Roadway Widening Project — roadway widening with a reinforced slope where the toe could not move.
  • Cannon Creek Alternate Embankment Construction Project — an alternative embankment built with reinforcement.
  • Pennsylvania SR 54 Roadway Repair Project — repair of a failing roadway slope.
  • Massachusetts Turnpike — Use of Soil Bioengineering — a bioengineered face treatment on a reinforced slope.

6.8 Standard RSS designs

As for walls, the manual provides standard RSS designs for common geometries and conditions, so routine slopes need not be redesigned from scratch.

6.9 Key takeaways

  • An RSS is a slope-stability problem, not a rigid-body problem — the critical slip surface governs.
  • The same reinforcement as a wall is used, but the face is sloped and usually vegetated.
  • Check internal (reinforcement) and external (foundation) stability separately.
  • The face treatment (vegetation or armor) is a design decision driven by climate, angle, and exposure.
  • Drainage is the most common cause of RSS distress.