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

Using Geogrid in Flexible Pavement

Geogrid technology has developed steadily since the products werefirst introduced in the early 1980’s. The initial geogrid products rapidlygained popularity within the civil engineering industry, principallydue to their ability to provide simple, cost-effective solutions in variousroadway and grade separation applications.They have gained widespread acceptance over the last 25 yearsas a solution to problems associated with roads constructed on softor problematic subgrades, but their use with roads on competentsubgrades has been less common. Clear, well-established, design methodology from the AmericanAssociation of State Highway and Transportation Officials (AASHTO)is now available that allows the design engineer to quantify the benefitsof using geogrids to extend pavement design life. This approach canbe applied for the design of major highways or light duty pavementsassociated with local housing or retail store developments.


Geogrid Composition



A geogrid is a regular grid structure of polymeric material used toreinforce soil or other geotechnical engineering related materials.Products are generally classified as either Uniaxial Geogrids orBiaxial Geogrids, depending on whether their strength is predomi-nantly in one or two directions. Uniaxial Geogrids are principally used in grade separation appli-cations for retaining walls and steep slopes. Biaxial Geogrids areused mainly in roadway applications to either stabilize a soft subgrade, or to provide reinforcement to the unbound base coursematerials (referred to as base reinforcement). Benefits of usingBiaxial Geogrids for base reinforcement are typically a reduction ofrequired base course material thickness and/or a significant extension in service life of the pavement structure. In base reinforcement applications, the existing subgrade is of afirm nature or has been rendered such through the use of a subgradeimprovement technique. One of the principal failure mechanisms ofa pavement under these firm subsoil conditions is rutting–resultingfrom progressive lateral movement of the aggregate base courseduring traffic loading. The amount of lateral movement can be reduced greatly byincluding a Biaxial Geogrid within or at the bottom of the basecourse layer. Partial penetration of coarse aggregate particlesthrough the geogrid apertures and subsequent compaction, resultsin “mechanical interlock” or “confinement” of the aggregate particles.


Application Benefits



The principal benefit of using a geogrid within the unbound aggregatecomponent of a flexible pavement is less rutting at the surface. Thisis due to reduced lateral spreading of the unbound aggregate. However, an additional feature of the reinforcement is that thegeogrid confined aggregate results in a much stiffer base courselayer and a lower dynamic deflection of the pavement structureduring traffic loading. Fatigue cracking of the asphalt is thereforealso reduced due to the presence of the geogrid reinforcement.In order for geogrids to work successfully in base reinforcementapplications, they must have the capacity to facilitate efficient loadtransfer between the aggregate and the geogrid. Webster (1992) reported a large-scale research program undertakenby the U.S. Army Corps of Engineers to investigate and determinethe key physical properties of a geogrid required to create optimalinteraction and load transfer. A summary of the key material propertiesdetermined in the study are presented in the table below.Key geogrid properties as determined by the U.S. Army Corps of Engineers.


Expanded Use



As the population of our towns and cities continues to expand rapidly,new or recently constructed housing in the form of sub-division devel-opments are becoming increasingly commonplace. One of the morefrequent problems associated with the roads in these developmentsis adirect result of their method of construction.Phased construction has become an extremely common practice,particularly in residential developments. In order to build a roadwayto gain site access, contractors will initially placethe aggregate component of the pavement and,usually, a thin asphalt layer on top. This techniqueis particularlyuseful when local trenches arerequired for installation of utilitypipes and cables. Pavement distress in the form of asphalt crackingat the surface is common on phased roads withinsub-divisions. In many cases, these “alligatorcracks” start to appear within a very short periodof time following construction – perhaps as little asone or two years. The simple solution to this problem is a layer ofBiaxial Geogrid installed at the bottom or withinthe base course during initial construction. Forrelatively little additional expense at the start of construction, thelifetime of their road is extended enormously, while expensive anddisruptive rehabilitation or reconstruction activities are avoided.Another use of geogrid technology can be found in the developmentof pavements around retail stores. Typically, thicker heavy-duty pavements are adopted in the loading areas around suchstores, while thinner lighter duty pavements are used for the carparking areas.
One of the main problems associated with this approach is thepotential for a “bath tub” effect – this is where the subgrade is at alower level in the areas of the heavy duty pavements. These areas areprone to water ingress and build up resulting in a reduction in thelong-term strength of the pavement. In colder regions, these areas are also more susceptible to theeffects of freeze-thaw activity. Both of these situations result in areduction in the design life of the pavement but there are additionalpractical problems for the contractor associated with this more complicated method of construction.In addition to offering protection against the “bath tub” problemsdescribed above, the reinforced sections offer significant material cost savings. Additional benefits result from increased speed of construction – fewer stake out procedures, less undercut/disposal offill, simpler construction, etc.


A Glimpse Into The Future

Currently, AASHTO provides guidelines for the design of flexible pave-ments in their current design guide (AASHTO,1993) and in InterimStandard PP46-01 (AASHTO, 2001). New pavement design approaches,based on advanced mechanistic-empirical (M-E) principles, are beingdeveloped and refined by AASHTO and other entities. A few DOT’s have already made the leap to improved M-E designmethods, but most are still awaiting official publication of AASHTO’snew design guide which will advocate adoption of this approach topavement design. Official publication of the new AASHTO designguide may still be several years out, but the availability of a M-E baseddesign method incorporatinggeogrids within the pavement structureis currently being finalized by The University of Illinois atChampaign-Urbana.