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

Reinforced Embankment on Soft Foundation (Using Geosynthetics)

Concept

The design and construction of embankments on soft foundation soils is a very challenging geotechnical problem. As noted by Leroueil and Rowe (2001), successful projects require a thorough subsurface investigation, properties determination, and settlement and stability analyses. If the settlements are too large or instability is likely, then some type of foundation soil improvement is warranted. Traditional soil improvement methods include preloading/surcharging with drains; lightweight fill; excavation and replacement; deep soil mixing, embankment piles, etc., as discussed by Holtz (1989) and Holtz et al. (2001a). Today, geosynthetic reinforcement must also be considered as a feasible treatment alternative. In some situations, the most economical final design may be some combination of a traditional foundation treatment alternative together with geosynthetic reinforcement. Figure 2a shows the basic concept for using geosynthetic reinforcement. Note that the reinforcement will not reduce the magnitude of long-term consolidation or secondary settlement of the embankment.


Design Considerations

As with ordinary embankments on soft soils, the basic design approach for reinforced embankments is to design against failure. The ways in which embankments constructed on soft foundations can fail have been described by Terzaghi et al. (1996), among others.



In figure above shows unsatisfactory behavior that can occur in reinforced embankments. The three possible modes of failure indicate the types of stability analyses that are required for design. Overall bearing capacity of the embankment must be adequate, and the reinforcement should be strong enough to prevent rotational failures at the edge of the embankment. Lateral spreading failures can be prevented by the development of adequate shearing resistance between the base of the embankment and the reinforcement. In addition, an analysis to limit geosynthetic deformations must be performed. Finally, the geosynthetic strength requirements in the longitudinal direction, typically the transverse seam strength, must be determined. Discussion of these design concepts as well as detailed design procedures are given by Christopher and Holtz (1985), Bonaparte et al. (1987), Holtz (1989 and 1990), Humphrey and Rowe (1991), Holtz et al. (1997), and Leroueil and Rowe (2001). The calculations required for stability and settlement utilize conventional geotechnical design procedures modified only for the presence of the reinforcement. Because the most critical condition for embankment stability is at the end of construction, the total stress method of analysis is usually performed, which is conservative since the analysis generally assumes that no strength gain occurs in the foundation soil. It is always possible of course to calculate stability in terms of effective stresses provided that effective stress shear strength parameters are available and an accurate estimate of the field pore pressures can be made during the project design phase. Because the prediction of in situ pore pressures in advance of construction is not easy, it is essential that the foundation be instrumented with high quality piezometers during construction to control the rate of embankment filling. Preloading and staged embankment construction are discussed in detail by Ladd (1991) and summarized by Leroueil and Rowe (2001).


Material Properties

Based on the stability calculations, the minimum geosynthetic strengths required for stability at an appropriate factor of safety can be determined. In addition to its tensile and frictional properties, drainage requirements, construction conditions, and environmental factors must also be considered. Geosynthetic properties required for reinforcement applications are given in Table 1.

Table 1. Geosynthetic properties required for reinforcement applications



When properly designed and selected, high-strength geotextiles or geogrids can provide adequate embankment reinforcement. Both materials can be used equally well, provided they have the requisite design properties. There are some differences in how they are installed, especially with respect to seaming and field workability. Also, at some very soft sites, especially where there is no root mat or vegetative layer, geogrids may require a lightweight geotextile separator to provide filtration and prevent contamination of the embankment fill. However, a geotextile separator is not required if the fill can adequately filter the foundation soil. A detailed discussion of geosynthetic properties and specifications is given by Holtz et al. (1997) and Koerner and Hsuan (2001), so only a few additional comments are given below. The selection of appropriate fill materials is also an important aspect of the design. When possible, granular fill is preferred, especially for the first few lifts above the geosynthetic.

Environmental Considerations
For most embankment reinforcement situations, geosynthetics have a high resistance to chemical and biological attack; therefore, chemical and biological compatibility is usually not a concern. However, in unusual situations such as very low (i.e., < 3) or very high (i.e., > 9) pH soils, or other unusual chemical environments (for example, in industrial areas or near mine or other waste dumps), chemical compatibility with the polymer(s) in the geosynthetic should be checked. It is important to assure it will retain the design strength at least until the underlying subsoil is strong enough to support the structure without reinforcement.

Constructability (Survivability) Requirements
In addition to the design strength requirements, the geotextile or geogrid must also have sufficient strength to survive construction. If the geosynthetic is ripped, punctured, torn or otherwise damaged during construction, its strength will be reduced and failure could result. Constructability property requirements are listed in Table 1. (These are also called survivability requirements.) See Christopher and Holtz (1985) and Holtz et al. (1997) for specific property requirements for reinforced embankment construction with varying subgrade conditions, construction equipment, and lift thicknesses. For all critical applications, high to very high survivability geotextiles and geogrids are recommended

Stiffness and Workability
For extremely soft soil conditions, geosynthetic stiffness or workability may be an important consideration. The workability of a geosynthetic is its ability to support workpersons during initial placement and seaming operations and to support construction equipment during the first lift placement. Workability is generally related to geosynthetic stiffness; however, stiffness evaluation techniques and correlations with field workability are very poor (Tan, 1990). See Holtz et al. (1997) for recommendations on stiffness.

Construction
The importance of proper construction procedures for geosynthetic reinforced embankments cannot be overemphasized. A specific construction sequence is usually required in order to avoid failures during construction. Appropriate site preparation, low ground pressure equipment, small initial lift thicknesses, and partially loaded hauling vehicles may be required. Clean granular fill is recommended especially for the first few construction lifts, and proper fill placement, spreading, and compaction procedures are very important. A detailed discussion of construction procedures for reinforced embankments on very soft foundations is given by Christopher and Holtz (1985) and Holtz et al. (1997). It should be noted that all geosynthetic seams must be positively joined. For geotextiles, this means sewing; for geogrids, some type of positive clamping arrangement must be used. Careful inspection is essential, as the seams are the “weak link” in the system, and seam failures are common in improperly constructed embankments. Finally, soft ground construction projects usually require geotechnical instrumentation for proper control of construction and fill placement; see Holtz (1989) and Holtz et al. (2001a) for recommendations.



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