Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil

Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil Soil improvement and ground modification methods chapter 14 geosynthetic reinforced soil

Geosynthetic Reinforced Soil

This chapter provides an overview of earthwork construction where forcing materials (predominantly geosynthetics) are used to provide addedstrength and capacity to engineered fill, stability for embankments over softground, stability for steepened slopes, and for resistance to erosion and otherdeterioration This type of soil reinforcement has become very popular andhas continued to grow in use for a wide variety of applications For manyyears, retaining structures were typically made of reinforced concrete anddesigned as gravity or cantilevered walls, which are rigid and cannot accom-modate significant deformations Earth walls stabilized with geosyntheticinclusions provide a cost-effective and technically sound alternative.Reinforced soil slopes offer a solution to myriad slope stability issues thathave historically caused tremendous losses and/or expensive “fixes” and/

rein-or redesigns Other types of soil reinfrein-orcement include use of structuralinclusions placed in situ and soil confinement mechanisms These othermethodologies are described in later chapters

14.1 HISTORY, FUNDAMENTALS, AND MATERIALS FORSOIL REINFORCEMENT

14.1.1 History of Soil Reinforcement

In ancient times soil reinforcement consisted of mixing straw with mud,reinforcing with woven reeds, and using branches and other plant material

to improve strength and capacity to support greater loads Modern soil forcement uses stronger and more durable materials, but employs many ofthe same fundamental mechanisms that provided strength in these earlyapplications

rein-Early versions of “modern” soil reinforcement were developed in theearly 1960s with Henri Vidal’s patented Reinforced Earth®for construction

of self-supporting retaining walls These walls were constructed using nized steel strips with “ribs” to provide lateral resistance against earth pres-sures (Figures 14.1 and 14.2) These types of wall (and similarly slope)structures are generically referred to as mechanically stabilized earth (MSE).Construction of earth walls with geosynthetic reinforcing materials was

galva-343

Soil Improvement and Ground Modification © 2015 Elsevier Inc.

introduced in the 1980s (Federal Highway Administration, 2011) Since thattime there has been an explosion of the use of geosynthetic reinforcementfor soil structures as well as for many other geotechnical applications.

14.1.2 Soil Reinforcement Materials

A number of different geosynthetic materials have and continue to be usedfor soil reinforcement As described in a previous section, early versions ofMSE wall used steel strips as reinforcement Many walls were constructed inthis way and are still in service today with a generally good track record.Some issues and concern with the use of metallic reinforcement have arisendue to corrosion of these elements As a result, use of metallic reinforcingmembers has been replaced by polymeric, geosynthetic materials for someapplications Corrosion of metallic inclusions is dependent on a number

of factors, including salt and oxygen content in the ground, degree of

Select backfill Reinforcement strips

Figure 14.1 Representation of steel strip reinforced wall and detail of ribbed galvanized strip.

saturation, acidity, and sulfate content, among others Corrosion rates may

be predicted with some accuracy and some corrosion allowance is usuallypart of design In addition, modern measurement techniques allow for

in situ testing of metal reinforcement “condition,” allowing “health” itoring of structures built with these types of inclusions The use of steel rein-forcement is still widely practiced due to the high strength of thesereinforcing members A variety of steel reinforcement types include the dis-crete steel strips previously described, and welded wire bar and mat arrange-ments (Figure 14.3)

mon-Geotextiles have long been recognized for their ability to reinforce neered fill constructed as walls or slopes They are also used to distributeloads beneath embankments and roadways over soft subgrade soils to reducesettlements and lateral deformations Geotextiles have the additional advan-tage of providing a separation function keeping dissimilar material from mix-ing, such as when aggregate or base coarse is placed over a fine-grainedsubgrade They may also function as filters depending on the application,

engi-as discussed in Chapter 8 Geotextiles used for reinforcement are usuallywoven, and are available in a wide range of weights, thicknesses, modulus,and strengths For applications where small deformations (strains) are ofFigure 14.2 Construction of a metallic strip reinforced wall Courtesy of The Reinforced Earth Company.

concern or should be monitored, some fabrics, such as Tencate’s tect®, incorporate fiber optic inclusions that can measure strains as low as0.02% (www.tencate.com).

GeoDe-For applications where higher reinforcement strength is required, meric geogrids may be utilized The primary function of geogrids is clearlyreinforcement The open apertures of geogrids (relatively large openingsbetween ribs) allows interconnectivity of the soil above and below, andtherefore provides additional passive resistance along the sides of the trans-verse ribs A full explanation of the reinforcing mechanisms of geogrids will

poly-be provided in the next section Geogrids come in a variety of typesdescribed below There are three fundamentally distinct categories based

on the manufactured geometry of the geogrid:

• Uniaxial geogrids (Figure 14.4), are typically manufactured from a sheet ofhigh-density polyethylene (HDPE) that has been punched and drawn inone direction This unidirectional draw provides high tensile strengthwith minimum elongation in one direction Uniaxial geogrids are idealfor applications where the stresses (loads) are primarily oriented in onedirection, for example walls and embankment slopes

• Biaxial geogrids (Figure 14.5) are commonly punched HDPE sheets drawn

in two directions and so provide good reinforcement in orthogonal(or random) directions While they may have a somewhat lower ultimateFigure 14.3 Welded wire bars and mats used for MSE wall reinforcement Courtesy

of The Reinforced Earth Company.

tensile strength than uniaxial grids, depending on design they may havenearly equal strength in the transverse direction as in the longitudinal direc-tion This makes them more suitable for resisting two-dimensional stresses.Many geogrids are manufactured by bonding two sets of orthogonal ribstogether to form a grid matrix A version of biaxial geogrids manufacturedwith fiberglass or polyester is used primarily in roadway applications andwill be described later in this chapter Other high-strength geogrids are usedfor foundation reinforcement or within reinforced soil masses.

• Triaxial geogrids (Figure 14.6) are relatively new on the market and vide a multidirectional reinforcement With triangular apertures,increased rib thickness, and better junction efficiency, they provide ahigher-strength alternative to biaxial geogrids, with the improved aggre-gate interlock and confinement of a reinforced soil mass Researchhas shown that the use of triaxial geogrid beneath a roadway base coarse

pro-Figure 14.4 Uniaxial geogrid and stabilized wall construction Courtesy of Tensar International Corp.

Figure 14.5 Traditional HDPE biaxial geogrid.

has allowed for reduced base thickness on the order of 25-50%(www.tensar.com).

While the majority of geogrids have traditionally been made of HDPE, thereare now many other materials and designs with a wide range of strengths,geometries, and attributes for an equally wide variety of application condi-tions (Figure 14.7) Tensile strengths of up to 1300 kN/m (92,500 lb/ft) arenow readily available in grids constructed of tensioned multifilament poly-ester cores, coextruded and encased with polyethylene (HDPE) protection

to maintain geometric stability (www.maccaferri-usa.com;Koerner, 2005)(Figure 14.8) These grids provide high strength reinforcement withFigure 14.6 Triaxial geogrid Courtesy of Tensar International Corp.

Figure 14.7 Other (non-HDPE) bonded and “green” woven geogrids.

minimal deformation for high load and stress applications, such as basal forcement of embankments over soft ground.

rein-Strengths of geogrids are commonly measured by single rib strength andwide-width tensile strength (ASTM D6637), as well as junction strength(where longitudinal and transverse ribs intersect) Anchorage (pullout)strength is computed as a combination of interface shear strength for bothlongitudinal and transverse ribs, plus the passive resistance provided bythe bearing strength against the sides of transverse ribs Allowable strengthsused for design take into consideration a number of other potential issuesincluding endurance properties of installation and creep, as well as possibledegradation due to chemical and biological attack

14.1.3 Soil Reinforcement Fundamentals

Soil is inherently weak in tension and stronger in compression and shear.The shear resistance of reinforcing materials placed within soil can bedescribed as a combination of the interface friction between materials, adhe-sion between materials, and in some cases passive resistance of reinforcementinclusions Since the development and implementation of patented soil rein-forced walls in the 1960s, when metallic strips were used as reinforcing ele-ments, there has been continued interest and growth in geosyntheticallyreinforced slopes and walls

The general mechanics of geosynthetic soil reinforcement is based on anumber of criteria usually involving specified test parameters such as materialFigure 14.8 Very high strength polyester/polyethylene composite geogrid from Maccaferri.

interface resistance, tensile strength, tear strength, elongation, and so forth.The friction and/or adhesion resistance between a geosynthetic material and

a particular soil is commonly measured in a manner similar to a direct sheartest, and is referred to as the interface friction or adhesion The resistance is,therefore, the multiple of the unit interface resistance and the area of thematerial in contact with the soil When a geogrid is used, there is typically

a strikethrough and interlock of the soil material placed above and beneaththrough the open apertures of the grid There is also an added passive resis-tance against the leading edge of the transverse member of a geogrid (orwelded mat, or transverse ribbed surface of other geosynthetic material)Figure 14.9

14.2 MSE WALLS AND SLOPES

14.2.1 Geosynthetic Reinforced Wall and Slope Basics

MSE refers to the use of reinforcement constructed between compactedsoil layers to build earth structures such as retaining walls, bridge abut-ments, embankments, and steep, yet stable slopes Various reinforcingmaterials have been used including steel strips, welded wire mats, geo-textiles, and geogrids Use of geotextiles for reinforcement began inthe 1970s, while geogrids have been used since the early 1980s Asdescribed earlier, versions of these MSE walls were developed in the early1960s Since that time, many tens of thousands of MSE walls have beenand continue to be constructed due to a number of desirable attributesthat they possess It has been estimated that more than 850,000 m2(9 mil-lion ft2) of MSE wall is constructed each year in the United States and hasbeen used in every state (Federal Highway Administration, 2010) MSEstructures of this kind are used not only for retaining walls, but also

Pullout forces P

TL TT

Figure 14.9 Forces acting on a geogrid or mat to resist pullout TL, interface shear strength on the (top and bottom) surfaces of longitudinal ribs; TT, interface shear strength on the (top and bottom) surfaces of transverse ribs; P, passive bearing force

of the leading edge of transverse ribs.

for bridge abutments, approach ramps, cut-and-cover tunnels, andnoise walls.

MSE walls have several advantages over conventional gravity or forced structural walls:

rein-• Relatively lightweight wall facing provides much lower bearing loads

• System has high flexibility, providing the ability to undergo small tomoderate deformations

• Fundamentally simple construction

• Usually a very economical alternative to other earth-retaining structures

• Typically, significantly reduced construction time compared tostructural walls

MSE walls may be faced in a number of ways and with a variety of differentmaterials These may be precast segmental panels (with or without) artisticdesigns; cast-in-place panels; rock-filled gabion cages; and welded wiremesh, timber, or integrated modular blocks (Figure 14.10) The majority

of MSE walls in the United States are designed as permanent structures structed with segmental precast facings connected to galvanized steel stripswith heights up to 46 m (150 ft) (Federal Highway Administration, 2010).Details with respect to connections between facing units and reinforcementwill differ depending on facing and reinforcement type, and loading condi-tions (Figure 14.11) Facing may also be constructed by a “wraparound” ofgeosynthetic material used as the primary or secondary reinforcement of thestructure (Figure 14.12) The use of geosynthetic reinforcement in engi-neered earth slopes and embankments adds significant stability and strength,providing the ability to construct steep slopes that require a smaller footprint

con-to achieve the same height

14.2.2 Failure Design Modes

While the design of earth-retaining structures is based largely on lateralearth pressure theory, and slope designs are generally based upon slope sta-bility analyses, designs for retaining walls and slopes reinforced with geo-synthetic inclusions begin to have much greater similarities Bothapplications depend on internal soil-reinforcement interaction (pulloutresistance of the reinforcing members), and tensile rupture (tear strength)

of the geosynthetic material Designs must include calculated resistance tointernal stability, as well as external stability, or “global” stability of a MSEmass Designs for the reinforcement strength and placement within a sta-bilized soil mass will be described in the next section Global stability must

adequately satisfy the requirements of overturning (usually taken as tion about the toe), sliding (translation along the base of the reinforcedmass over foundation soils), slope failure (encompassing the entire rein-forced mass), and bearing (capacity of the underlying foundation soil tosupport the load of the MSE system) This last global stability modemay be enhanced or solved by reinforcing the subgrade/foundation soils,

rota-as will be addressed inSection 14.3

A significant design detail that should not be overlooked, especially forgeotextiles, is the survivability of the materials, both during installation andfor working loads This is generally related to strength and stiffness of mate-rials as defined by grab tensile strength tests (ASTM D1682), but may alsoconsider puncture or tear strength

Figure 14.10 Example of some wall facings: top —precast panels, wrapped Courtesy of The Reinforced Earth Company.

bottom—wire-Figure 14.11 Typical wall connections for precast facing panels Courtesy of The Reinforced Earth Company.

Figure 14.12 Geosynthetic wrapped-face wall Courtesy of Tencate-Mirafi.

14.2.3 Reinforcement Design for MSE Walls

The design of MSE walls will depend on size load requirements and nency of the structures For temporary applications, such as limited-timeaccess roads, construction staging areas, or surcharge fills, less expensive(and less capacity, durability, and strength) geotextiles may be employed

perma-In many of these shorter-term applications, the wall facing is simply awrapped tail of reinforcement held in place by the confining stress of mate-rial placed in above layers (wrapped geotextile wall) (Figure 14.13) Forshort-term protection, these types of walls may be covered with a thin shot-crete or other nonstructural facing material For longer-term and permanentapplications, MSE walls can be constructed with a design life of 75-100

years, and will typically have a semirigid facing of concrete panels, cribbing,

or confined rockfill (e.g., gabion construction; see Chapter 16) (www.reinforcedearth.com)

The basic premise for internal stability design of MSE walls stems fromlateral earth pressure theory For simple design of soil reinforcement, a Ran-kine active earth pressure is assumed Any additional loads (e.g., static sur-charge loads or “live” vehicle loads) must be included, and added to thelateral earth pressures Static surcharge loads are assumed to be “at-rest” con-ditions Live loads are distributed to the soil mass using Boussinesq elastictheory as described in classic soil mechanics texts or design manuals (i.e.,NAVFAC DM 7.2, Dept of the Navy, 1982)

Design for internal stability involves calculating vertical spacing of forcement layers (Sv), embedment length to resist pullout from behind theactive zone (Le), and for geotextile-wrapped walls, the overlap lengthsneeded to ensure integrity of the wall face.Figure 14.14shows the basic lay-out for a MSE wall Design details can be found in dedicated geosynthetictexts (i.e.,Holtz et al., 1997; Koerner, 2005), or from guidelines and spec-ifications provided by suppliers Maximum vertical spacing is inversely pro-portional to lateral stress, so in general, layer spacing must be closer lower in awall Embedment length is a function of the lateral stresses, vertical spacing,and interface shear strength between backfill soil and geosynthetic, but itshould always penetrate at least 1 m beyond the theoretical active slip surface(Koerner, 2005)

rein-Original foundation soil Leveling pad

panels

Sv

failure surface

Le Select fill

backfill

Figure 14.14 Fundamental MSE wall design components.