Reinforced Retaining Wall

Concept

Retaining walls are required where a soil slope is uneconomical or not technically
feasible. When compared with conventional retaining structures, walls with reinforced
backfills offer significant advantages. They are very cost effective, especially for higher walls. Furthermore, these systems are more flexible than conventional earth retaining walls such as reinforced concrete cantilever or gravity walls. Therefore, they are very suitable for sites with poor foundations and for seismically active areas.

Modern reinforced soil technology was developed in France by H. Vidal in the mid
1960s. His system is called Reinforced Earth and is shown in Fig. 4. Steel strips are used to reduce the earth pressure against the wall face. The design and construction of Vidaltype reinforced earth walls are now well established, and many thousands have been successfully built throughout the world in the last 25 years. Other similar proprietary reinforcing systems have also been deve loped using steel bar mats, grids, and gabions.
The use of geotextiles as reinforcing elements started in the early 1970’s because of
concern over possible corrosion of metallic reinforcement. Systems using sheets of
geosynthetics rather than steel strips are shown in Figure

The maximum heights of geosynthetic reinforced walls constructed to date are less
than 20 m, whereas steel reinforced walls over 40 m high have been built. A significant benefit of using geosynthetics is the wide variety of wall facings available, resulting in greater aesthetic and economic options. Metallic reinforcement is typically used with articulated precast concrete panels or gabion-type facing systems.


Design Considerations



Reinforced wall design is very similar to conventional retaining wall design, but
with the added consideration of internal stability of the reinforced section. External stability is calculated in the conventional way--the bearing capacity must be adequate, the reinforced section may not slide or overturn, and overall slope stability must be adequate. Surcharges (live and dead loads; distributed and point loads) are considered in the conventional manner. Settlement of the reinforced section also should be checked if the foundation is compressible.

A number of different approaches to internal design of geotextile reinforced
retaining walls have been proposed (Christopher et al., 1990; Allen and Holtz; 1991; Holtz,1995), but the oldest and most common--and most conservative--method is the tieback wedge analysis. It utilizes classical earth pressure theory combined with tensile resisting “tiebacks” that extend back of the assumed failure plane (Fig. 6). The KA (or Ko) is assumed, depending on the stiffness of the fa cing and the amount of yielding likely to occur during construction, and the earth pressure at each vertical section of the wall is calculated. This earth pressure must be resisted by the geosynthetic reinforcement at that section.

To design against failure of the reinforcement, there are two possible limiting or failure cond itions: rupture of the geosynthetic and pullout of the geosynthetic. The corresponding reinforcement properties are the tensile strength of the geosynthetic and its pullout resistance. In the latter case, the geosynthetic reinforcement must extend some distance behind the assumed failure wedge so that it will not pull out of the backfill. Typically, sliding of the entire reinforced mass controls the length of the reinforcing elements. For a detailed description of the tieback wedge method, see Christopher and Holtz (1985), Bonaparte et al. (1987), Allen and Holtz (1991), and Holtz et al. (1997). Recent research (e.g., Lee et al., 1999; Lee, 2000; Bathurst et al.,2000) has indicated that the tieback wedge approach is overly conservative and uneconomical, and modifications and deformation-based designs are rapidly being deve loped. Other important design considerations include drainage and potential seismic loading.


Material Properties

Geosynthetic properties required for reinforced walls are similar to those listed in Table 1, Section 8.3 and discussed in Section 9.3 for reinforced slopes. Properties are required for design (stability), constructability, and durability. Allowable tensile strength and soil- geosynthetic friction are required for stability design, and similar to reinforced slopes, a partial factor or reduction factor approach is common. The ultimate wide width strength is reduced to account for uncertainties in creep strength, chemical and biological degradation effects, installation damage, and joints and connections. Berg (1993), Holtz et al.(1997), and Koerner and Hsuan (2001) give details about the determination of the allowable geosynthetic tensile strength. They also describe how soil- geosynthetic friction is measured or estimated.

Backfill for geosynthetic reinforced walls should be free draining if at all possible. If not, then adequate drainage of infiltrating surface or groundwater must be provided. This is important for stability considerations because drainage outward through the wall face may not be adequate. Soil properties required include gradation, percent fines,chemical composition, compaction, unit weight, and shear strength. To insure stability,appropriate consideration of the foundation and overall slope stability at the site is also important (Holtz et al., 2001b).


Wall Facing Considerations

A significant advantage of geosynthetic reinforced walls over conventional retaining structures is the variety of facings that can be used and the resulting aesthetic options that can be provided. Aesthetic requirements often determine the type of facing systems. Anticipated deflection of the wall face, both laterally and downward, may place further limitations on the type of facing system selected. Tight construction specifications and quality inspection are necessary to insure that the wall face is constructed properly; otherwise an unattractive wall face, or a wall face failure, could result.

Facing systems can be installed (1) as the wall is constructed or (2) after the wall is built. Facings installed as the wall is constructed include segmental and full height precast concrete panels, interlocking precast concrete blocks, welded wire panels, gabion baskets, treated timber facings, and geosynthetic face wraps. In these cases, the geosynthetic reinforcement is attached directly to the facing element. Systems installed after construction include shotcrete, cast-in-place concrete facia, and precast concrete or timber panels; the panels are attached to brackets placed between the layers of the geosynthetic wrapped wall face at the end of wall construction or after wall movements are complete. Facings constructed as the wall is constructed must either allow the geosynthetic to deform freely during construction without any buildup of stress on the face, or the facing connection must be designed to take the stress. Although most wall design methods assume that the stress at the face is equal to the maximum horizontal stress in the reinforced backfill, measurements show that considerable stress reduction occurs near the face, depending on the flexibility of the face. See Allen and Holtz (1991) and Holtz et al. (1997) for a detailed discussion of wall facing systems.


Constuction

Construction procedures for geosynthetic reinforced walls and abutments are given
by Christopher and Holtz (1985) and Holtz et al. (1997). Procedures are relatively simple and straightforward, but failures are surprisingly common, especially with proprietary precast segmental concrete block-faced wall systems. It appears that most of these failures are due to (1) inadequate design, particularly of the foundation and back slope of the wall, and/or (2) problems in construction. The latter include poor inspection and quality control, poor compaction, use of inappropriate backfill materials, lack of attention to facing connections, and lack of clear lines of responsibility between designers, material suppliers, and contractors.

Reinforced Steep Slope (with geosynthetics)

Concept

The first use of geosynthetics for the stabilization of steep slopes was for the reinstatement of failed slopes. Cost savings resulted because the slide debris could be reused in the repaired slope (together
with geosynthetic reinforcement), rather than importing select materials to reconstruct the slope. Even if foundation conditions are satisfactory, costs of fill and right-of-way plus other considerations may require a steeper slope than is stable in compacted embankment soils without reinforcement. As shown in Fig.3, multiple layers of geogrids or geotextiles may be placed in a fill slope during construction or reconstruction to reinforce the soil and provide increased slope stability. Most steep slope reinforcement projects are for the construction of new embankments, alternatives to retaining walls, widening of existing embankments, and repair of failed slopes. Another use of geosynthetics in slopes is for compaction aids (Fig. 3). In this application, narrow geosynthetic strips, 1 to 2 m wide, are placed at the edge of the fill slope to provide increased lateral confinement at the slope face, and therefore increased compacted density over that normally achieved. Even modest amounts of reinforcement in compacted slopes have been found to prevent sloughing and reduce slope erosion. In some cases, thick nonwoven geotextiles with in-plane drainage capabilities allow for rapid pore pressure dissipation in compacted cohesive fill soils.


Design Considerations

The overall design requirements for reinforced slopes are similar to those for unreinforced slopes--the factor of safety must be adequate for both the short- and long-term conditions and for all possible modes of failure. These include: (1) internal--where the failure plane passes through the reinforcing elements; (2) external--where the failure surface passes behind and underneath the reinforced mass; and (3) compound--where the failure surface passes behind and through the reinforced soil mass. Reinforced slopes are analyzed using modified versions of classical limit equilibrium slope stability methods (e.g., Terzaghi et al., 1996). Potential circular or wedge-type failure surfaces are assumed, and the relationship between driving and resisting forces or moments determines the factor of safety. Based on their tensile capacity and orientation, reinforcement layers intersecting the potential failure surface increase the resisting moment or force. The tensile capacity of a reinforcement layer is the minimum of its allowable pullout resistance behind, or in front of, the potential failure surface and/or its long-term design tensile strength, whichever is smaller. A variety of potential failure surfaces must be considered, including deep-seated surfaces through or behind the reinforced zone, and the critical surface requiring the maximum amount reinforcement determines the slope factor of safety. The reinforcement layout and spacing may be varied to achieve an optimum design. Computer programs are available for reinforced slope design which include searching routines to help locate critical surfaces and appropriate consideration of reinforcement strength and pullout capacity. Additional information on reinforced slope design is available in Christopher et al. (1990), Christopher and Leshchinsky (1991), Berg (1993), Holtz et al.(1997), and Bathurst and Jones (2001).
For slide repair applications, it is very important that the cause of original failure is addressed in order to insure that the new reinforced soil slope will not have the same problems. Particular attention must be paid to drainage. In natural soil slopes, it is also necessary to identify any weak seams that could affect stability.


Material Properties

Geosynthetic properties required for reinforced slopes are similar to those listed in Table 1. in the previous post Properties are required for design (stability), constructability, and durability. Allowable tensile strength and soil-geosynthetic friction are most important for stability design. Because of uncertainties in creep strength, chemical and biological degradation effects, installation damage, and joints and connections, a partial factor or reduction factor concept is recommended. The ultimate wide width strength is reduced for these various factors, and the reduction depends on how much information is available about the geosynthetics at the time of design and selection. Berg (1993), Holtz et al. (1997), and Koerner and Hsuan (2001) give details about the determination of the allowable geosynthetic tensile strength. They also describe how soil-geosynthetic friction is measured or estimated. An inherent advantage of geosynthetic reinforcement is their longevity, especially in normal soil environments. Recent studies have indicated that the anticipated half-life of reinforfcement geosynthetics in between 500 and 5000 years, although strength characteristics may have to be adjusted to account for potential degradation in the specific environmental conditions. Any soil suitable for embankment construction can be used in a reinforced slope system. From a reinforcement point of view alone, even lower-quality soil than conventionally used in unreinforced slope construction may be used. However, higher-quality materials offer less durability concerns, are easier to place and compact, which tends to speed up construction, and they have fewer problems with drainage. See Berg (1993) and Holtz et al. (1997) for discussion of soil gradation, compaction, unit weight, shear strength, and chemical composition.


Construction

Similarly to reinforced embankments, proper construction is very important to insure adequate performance of a reinforced slope. Considerations of site preparation, reinforcement and fill placement, compaction control, face construction, and field inspection are given by Berg (1993) and Holtz et al. (1997).


Reference :
R.D. Holtz, Ph.D., P.E., Geosynthetics Soil Reinforcement, Department of Civil & Environmental Engineering, University of Washington