Reinforced Soil Structures

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Description

Reinforced soil structures (Figure 1) are formed by compacted layers of soil 50 to 150 cm thick in which reinforcing elements of appropriate length are interposed to improve overall resistance; the external face of the structure is protected by a facing which may consist of shotcrete and wire mesh, geogrid/geotextile sheets, modular facing blocks, cast-in-situ or prefabricated panels or similar (Figure 2). The facing may incorporate biotechnical elements, typically for aesthetic purposes only.

Reinforced soil structures are generally applicable to situations where the reinforcement elements and the fill are placed as the wall is constructed. The concept of reinforcing the backfill behind retaining walls was developed by H. Vidal in France in the mid 1960s).

These structures offer several advantages As highlighted for example by Mitchell and Villet (1987), reinforced soil structures:

• are coherent and flexible to tolerate relatively large displacements;

• can use a wide range of backfill materials;

• are easy to construct;

• are relatively resistant to seismic loading; however their use in areas of high seismicity is still somewhat restricted because of the lack of definitive research on this issue; in particular the connection between the reinforcing elements and the facing elements may be critical (Allen and Holtz, 1991).

• can form aesthetically attractive retaining walls and slopes because of available facing types

• are often less costly than conventional retaining structures, especially for high steep slopes and high walls.

 

General principles

In reinforced soil structures the reinforcing elements provide the structure with a component of tensile strength. As the height of the wall increases, the overburden pressure increases and the shear stresses within the soil mass build up. There is a tendency for the face of the wall to displace outwards which increases as the height of the wall increases. The outward movement of the soil is resisted by the reinforcing elements which go into tension as frictional forces develop along them. Because of the thin nature of the reinforcing elements used in this type of structure, they can only provide tensile resistance. The tensile forces acting in the reinforcements also contribute to the normal stress acting along potential slip-surfaces within the reinforced soil mass, thus increasing the frictional resisting force along them. In the case of reinforcements consisting of grid mesh, with orthogonal strips running parallel to the face of the wall, there is also a component of resistance generated from their edge bearing against the soil infilling the gaps between the strips.

The maximum tensile forces in the elements occur within the reinforced soil mass rather than at the facing. The locus of the point of maximum tensile force in each row of reinforcing elements separates the reinforced soil mass in two distinct zones, an “active” zone immediately behind the facing and a “passive” zone. Contrary to soil-nailing structures, the position of the line of maximum tension can be reasonably estimated in cases of reinforced soil structures due to their uniform geometry and the “known” characteristics of materials.

Reinforcing elements

The reinforcing elements may consist of:

• Metallic strips (Reinforced Earth or Terre Armée);

• Polymeric strips;

• Geotextile sheets;

• Geogrids;

• Metallic grids.

Strip reinforcing elements

The mechanism of stress transfer between the reinforcement and the soil is essentially friction developed at the surface of the reinforcing strip (Mitchell and Villet, 1987; Christopher and Holtz, 1989; Christopher et al., 1990).

Early experiments with fibreglass-reinforced polymers, stainless steel and aluminium strips were not successful so all Reinforced Earth (Terre Armée) walls are currently constructed using galvanized steel strips (Schlosser, 1990).

As corrosion rates of metals in soil are very difficult to predict, also in presence of galvanized steel strips free-draining sand and gravel fills are specified to reduce corrosion potential. Epoxy-coated steel strips have been developed and may offer higher resistance to corrosion (Elias, 1990). In theory, steel reinforcement could be designed with a sacrificial thickness, but this is seldom economic considering the small initial thickness of the reinforcement elements and the need to provide sacrificial steel all round.

Since the mid 1970s, non-metallic strips have been also developed (Holtz, 1978; Jones, 1978), consisting of continuous glass fibres embedded in a protective coating of epoxy resin or of geosynthetic strips.

The reinforcement elements are connected to vertical prefabricated reinforced concrete panels or inclined steel mesh facing panels progressively assembled as the structure is constructed.

In an attempt to improve the stiffness and pull out resistance of the reinforcement, bar-and-mesh systems or bar-mats formed by cross-linking steel reinforcing bars were developed by California Department of Transportation, Caltrans (Forsyth, 1978); laboratory tests showed that the bar-and-mesh reinforcement could produce significantly higher pull-out resistances compared to longitudinal bars only (Chang et al., 1977). Evolving from the Caltrans project other bar mats systems has been developed and used (see for example Anderson et al., 1987; Hausmann, 1990; Mitchell and Christopher, 1990). The main problems with bar mats systems are given by the corrosion of the steel bars.

Getextile sheets

The use of geotextiles in reinforced soil structures followed shortly after the introduction of Reinforced Earth (Terre Arméè), (Bell and Steward, 1977; Yako and Christopher, 1988; Allen at al., 1992).

The mechanism of stress transfer between the reinforcement and the soil is essentially friction developed at the surface of the reinforcing sheets (Mitchell and Villet, 1987; Christopher and Holtz, 1989; Christopher et al., 1990).

A large variety of nonwoven or woven polyester and polypropylene geotextiles, with a wide range of mechanical properties, is available (Christopher and Holtz, 1989; Koerner, 1990).

Coarse grained soils ranging from silty sands to gravels are commonly used as fill.

The most common facings are formed by wrapping the geotextiles around the exposed soil. Since the geotextiles are subjected to vandalism, mechanical damage and deterioration, the exposed materials must be covered with shotcrete or asphalt emulsion, modular facing elements, gabions or soil and vegetation. In the latter case, the facing typically includes additional layers specifically designed to control erosion, consisting of variable combinations of geogrids, geomats and/or biodegradable mats, to hold the soil in place until the vegetation has taken hold.

The use of geosynthetics sheets instead of steel strips has been introduced and it has become progressively more popular mainly on account of their lower cost and greater corrosion resistance. However, doubts persist on the durability and longevity of geosynthetic materials because of chemical and biological attack (Elias, 1990; Allen, 1991; Brand and Pang, 1991). The mechanical characteristics of geosynthetics also give rise to issues related to their lower stiffness and their susceptibility to significant creep (Rimoldi and Ricciuti, 1992).

Geogrids and Metallic grids

In grid reinforcement, polymeric or metallic elements are arranged in rectangular grid shape, with the long side oriented parallel to the direction of the movement between the reinforcement and the soil; therefore, the grid-soil interaction involves both friction acting on the long side grid elements and passive bearing resistance on the short side grid elements. Due to the contribution of the passive bearing resistance grid reinforcements provide higher resistances to pull-out than flat strips; it should be considered, however, that passive bearing resistance develops after relatively large displacements (5 to 10 cm), see for example Schlosser (1990).

Polymeric geogrids represent the most commonly used element for soil reinforcement; they are made by polypropylene, polyethylene or PVC coated polyester. Since the 1970s, advances in the formulation of polymers led to significant improvement in their strength and stiffness and in their use for several applications, including repair of slope failures (O’Rourke and Jones, 1990; Murray and Irwin, 1981; Murray, 1982; Jones, 1985; Szymoniak et al., 1984; Forsyth and Bieber, 1984; Mitchell and Christopher, 1990). As with the geotextile sheets, polymeric geogrids are susceptible to environmental deterioration, large deformations and creep.

Coarse grained soils ranging from silty sands to gravels are commonly used as fill.

Requirements and details of facings are similar to those described above for structures constructed with geotextile sheets.

Whatever the reinforced soil structures, provision of drainage behind the facing and the reinforced soil mass is important, to maximize effective stresses within the fill and available shear strength at the soil reinforcement interfaces. Allowance for drainage from the facing should also be made.

For a more comprehensive description and discussion on reinforced soil structures reference can be made, for example, to Lee et al. (1973), Jones (1985), Mitchell and Villet (1987), Christopher et al. (1990), Mitchell and Christopher (1990), O’Rourke and Jones (1990), DoT Advice note HA/68/94 (1994), BS 8006 (1995), Love and Milligan (1995), Jewell (1996), Jones (1996), Berg et al. (2009).

 

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Design methods

The thickness of the layers used depends on the nature of the fill and reinforcement and on the geometry of the structure.

The filling material shall be suitable for compaction; granular fill is typically compacted to 95% of the maximum dry density determined in Modified Proctor Test.

The type of reinforcement, facing and connections depend on soil type, wall height, slope, etc. Usually polymeric geosynthetic are considered extensible, while steel strips are considered inextensible; using extensible or inextensible reinforcements may determine differences in the method of analysis (see for example BS 8006, 1995).

The toe of the facing should be embedded below the ground surface to prevent against local punching failure at the base of the facing and to prevent flow of the soil under the wall from water flow due to a head building up behind the facing (piping).

The basic design of the reinforced soil structures under the thrust exerted by the (unstable or potentially unstable) sloping ground behind it includes both external and internal stability evaluations (see for example Ghionna, 1995).

 

External stability evaluation will include consideration of:

• Overturning;

• Sliding at or below the base;

• Bearing failure of the foundation.

• Overall stability including the unstable or potentially unstable ground behind the reinforced soil system.

They are carried out in static and seismic conditions according to simplified methods normally adopted for conventional earth retaining structures.

Sliding should be checked using the weakest relevant frictional properties considering that sliding might occur through the foundation soil, the fill or along the interface of the reinforcing element used at the base of the structure.

Although not a stability criterion, the settlement of the ground induced by the reinforced soil system should be considered; excessive settlements can cause problems for example with drains and services; care should be taken when earth retaining structures are built adjacent to other existing structures.

Internal settlements of reinforced soil structures are governed by the nature and compaction of the fill and the vertical stresses within it (which depend on the height of the structure and surcharges). Differential settlements generally cause the most severe effects on a completed structure; the facing is the most critical part of the structure.

The internal stability is checked for each stage of construction to ensure that failure does not occur in/or around the reinforcements and the facing.

The internal stability is checked by limit equilibrium methods in which the additional forces provided by the reinforcing elements are added. Only ultimate limit states are examined with these methods; the magnitude of deformations, which govern the serviceability limit states, is usually controlled by applying adequate factors of safety to account for variations in material properties, loads, methods of analysis, etc. However, where deformations are critical it is necessary to resort to numerical analysis to estimate displacements.

The internal stability evaluations will include:

• Structural checks on the reinforcement, to verify that their tensile strength is sufficient to withstand with adequate factors of safety the tensile forces generated by the interaction with the soil.

• Geotechnical checks on the reinforcement, to verify that their length is sufficient to provide adequate pull-out resistance to withstand with adequate factors of safety the tension generated by the interaction with the soil.

• Structural checks on the facing, with particular reference to the connections with the reinforcement and local bending and shear in the facing

Various methods of analysis have been developed to evaluate these three aspects; the various methods have been validated to different extent with full scale experiments and instrumentation. A comprehensive descriptions of the various methods can be found, for example in DoT Advice note HA/68/94 (1994), BS 8006 (1995), Love and Milligan (1995), Jewell (1996), Jones (1996), Berg et al. (2009).

To account for creep and temperature effects, the rupture strength of polymeric reinforcements is governed by the characteristic strength corresponding to the required design life and temperature conditions; partial factor of safety are applied to this strength to account for both variations in material strength and environmental conditions, possible damage during construction, the need to limit creep deformations and the extent to which extrapolation of experimental data is required where the design life of the structure exceeds available long term tests. The rupture strength of metallic strip reinforcements is usually the quoted yield strength of the material:

The bond resistance is usually determined from considerations of the frictional properties at the interface between the soil and the reinforcing elements, estimating normal stress acting at the interface. This approach is well suited to reinforced soil structures where the reinforcing material and fill are reasonably well controlled. The angle of friction is obtained from direct shear box tests with shearing carried out on the reinforcing materials at an appropriate range of normal stresses or from large scale pull out tests under controlled conditions.

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• Carter P. (1992) “Dewatering”. In: Drainage Design, P. Smart and J.G. Herbertson editors,  blackie & Son Ltd, Glasgow

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• Allen T.A., Holtz R.D. (1991). ”Design of Retaining Walls Reinforced wuth Geosynthetics”. Proc. of Geotechnical Engineering Congress 1991, Boulder, Colorado, Geotechnical Special Publication 27, ASCE, F.G. Mc Lean, D.A. Campbell and D.W. Harris editors, 970-987.

• Allen T.A., Christopher B.R., Holtz R.D. (1992). ”Performance of a 12.6 m high geotextile wall in Seattle, Washington”. Proc. of the International Symposium on Geosynthetic Reinforced Soil Retaining Walls, Denver, J.T.H. Wu  editor,Balkema, 81-100.

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• Berg. R.R., Christopher B.R., Samtani N.C. (2009). ”Design Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, vol. 1 and vol. 2”. Reports n° FHWA-NHI-10-024  and FHWA-NHI-025, National Highway Institute, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C..

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