2.2.3.3 Epoxy-Coated Bar and Epoxy-Coated Filled StrandEpoxy-coated bar (AASHTO M284) and epoxy-coated filled strand (supplement to ASTM A882)while not used extensively for highway applications, are becoming more widely used for damtiedown projects.The epoxy coating provides an additional layer of corrosion protection in theunbonded and bond length as compared to bare prestressing steel.For epoxy-coated filled strand, in addition to the epoxy around the outside of the strand, the centerwire of the seven-wire strand is coated with epoxy.Unfilled epoxy-coated strand is notrecommended because water may enter the gaps around the center wire and lead to corrosionUnlike bare strand, creep deformations of epoxy-coated filled strands themselves are relativelysignificant during anchor testing.When evaluating anchor acceptance with respect to creep, thecreep of the epoxy-coated filled strands themselves mustbe deducted from the total creepmovements to obtain areliable measurement of themovementsinthe bond zone.Estimatesofintrinsiccreepmovementsofepoxy-coatedfilledstrandareprovidedinPTI(1996)CENTRALIZERSTRANDSHEATHGROUTTUBESPACERFigure6.Cutawayviewofstrandtendon.2.2.3.4 Other Anchor Types and Tendon MaterialsIn addition to cement grouted anchors incorporating high strength prestressing steels, alternativeanchor types and tendon materials are used in the U.S.Examples include Grade 60 and Grade75grouted steel bars, helical anchors, plate anchors, and mechanical rock anchors. The design andtesting methods described in this document are used for cement grouted anchors that use highstrength prestressing steels.These methods may not be appropriate for use with the alternativeanchortypesmentionedabove.10
10 2.2.3.3 Epoxy-Coated Bar and Epoxy-Coated Filled Strand Epoxy-coated bar (AASHTO M284) and epoxy-coated filled strand (supplement to ASTM A882), while not used extensively for highway applications, are becoming more widely used for dam tiedown projects. The epoxy coating provides an additional layer of corrosion protection in the unbonded and bond length as compared to bare prestressing steel. For epoxy-coated filled strand, in addition to the epoxy around the outside of the strand, the center wire of the seven-wire strand is coated with epoxy. Unfilled epoxy-coated strand is not recommended because water may enter the gaps around the center wire and lead to corrosion. Unlike bare strand, creep deformations of epoxy-coated filled strands themselves are relatively significant during anchor testing. When evaluating anchor acceptance with respect to creep, the creep of the epoxy-coated filled strands themselves must be deducted from the total creep movements to obtain a reliable measurement of the movements in the bond zone. Estimates of intrinsic creep movements of epoxy-coated filled strand are provided in PTI (1996). CENTRALIZER SHEATH SPACER GROUT TUBE STRAND Figure 6. Cut away view of strand tendon. 2.2.3.4 Other Anchor Types and Tendon Materials In addition to cement grouted anchors incorporating high strength prestressing steels, alternative anchor types and tendon materials are used in the U.S. Examples include Grade 60 and Grade 75 grouted steel bars, helical anchors, plate anchors, and mechanical rock anchors. The design and testing methods described in this document are used for cement grouted anchors that use high strength prestressing steels. These methods may not be appropriate for use with the alternative anchor types mentioned above
Research on the use of fiber reinforced plastic (FRP)prestressing tendons is currently beingperformed (e.g, Schmidt et al., 1994). FRP tendons have high tensile strength, are corosionresistant, and arelightweight.These products,however,are not used in current U.S.constructionpractice.Othermaterials such as fiberglass and stainless steel have been used experimentallybutcostand/orconstructionconcernshaverestrictedwidespreaduse,2.2.4CementGroutAnchor grout for soil and rock anchors is typically a neat cement grout (ie., grout containing noaggregate) conforming to ASTM C150 although sand-cement grout may also be used for largediameter drill holes. Pea gravel-sand-cement grout may be used for anchor grout outside the tendonencapsulation.High speed cementgrout mixers are commonly used which can reasonablyensureuniform mixing between grout and water. A water/cement (w/e) ratio of 0.4 to 0.55 by weight andType I cement will normally provide a minimum compressive strength of 21 MPa at the time ofanchor stressing.For some projects, special additives may be required to improve the fluid flowcharacteristics ofthegrout.Admixtures are not typically requiredfor most applications,butplasticizers may be beneficial for applications in high temperature and for longgrout pumpingdistances.2.3ANCHOREDWALLS2.3.1GeneralA common application of ground anchors for highway projects is for the construction of anchoredwalls used to stabilize excavations and slopes.These anchored walls consist of nongravitycantilevered walls with one or more levels of ground anchors.Nongravity cantilevered walls employeither discrete (e.g., soldier beam) or continuous (e.g, sheet-pile) vertical elements that are eitherdriven or drilled to depths below the finished excavation grade. For nongravity cantilevered walls,support is provided through the shear and bending stiffness of the vertical wall elements and passiveresistance from the soil below the finished excavation grade.Anchored wall support relies on thesecomponents as well as lateral resistance provided by the ground anchors to resist horizontal pressures(e.g.,earth, water, seismic,etc.)acting on the wall.Various construction materials and methods are used for the wall elements of an anchored wall.Discrete vertical wall elements often consist of steel piles or drilled shafts that are spanned by astructural facing.Permanent facings are usually cast-in-place (CIP) concrete although timber laggingor precast concrete panels have been used.Continuous wall elements do not require separatestructural facing and include steel sheet-piles,CiP or precast concrete wall panels constructed inslurry trenches (i.e., slurry (diaphragm) walls), tangent/secant piles, soil-cement columns, and jetgroutedcolumns.11
11 Research on the use of fiber reinforced plastic (FRP) prestressing tendons is currently being performed (e.g., Schmidt et al., 1994). FRP tendons have high tensile strength, are corrosion resistant, and are lightweight. These products, however, are not used in current U.S. construction practice. Other materials such as fiberglass and stainless steel have been used experimentally but cost and/or construction concerns have restricted widespread use. 2.2.4 Cement Grout Anchor grout for soil and rock anchors is typically a neat cement grout (i.e., grout containing no aggregate) conforming to ASTM C150 although sand-cement grout may also be used for large diameter drill holes. Pea gravel-sand-cement grout may be used for anchor grout outside the tendon encapsulation. High speed cement grout mixers are commonly used which can reasonably ensure uniform mixing between grout and water. A water/cement (w/c) ratio of 0.4 to 0.55 by weight and Type I cement will normally provide a minimum compressive strength of 21 MPa at the time of anchor stressing. For some projects, special additives may be required to improve the fluid flow characteristics of the grout. Admixtures are not typically required for most applications, but plasticizers may be beneficial for applications in high temperature and for long grout pumping distances. 2.3 ANCHORED WALLS 2.3.1 General A common application of ground anchors for highway projects is for the construction of anchored walls used to stabilize excavations and slopes. These anchored walls consist of nongravity cantilevered walls with one or more levels of ground anchors. Nongravity cantilevered walls employ either discrete (e.g., soldier beam) or continuous (e.g., sheet-pile) vertical elements that are either driven or drilled to depths below the finished excavation grade. For nongravity cantilevered walls, support is provided through the shear and bending stiffness of the vertical wall elements and passive resistance from the soil below the finished excavation grade. Anchored wall support relies on these components as well as lateral resistance provided by the ground anchors to resist horizontal pressures (e.g., earth, water, seismic, etc.) acting on the wall. Various construction materials and methods are used for the wall elements of an anchored wall. Discrete vertical wall elements often consist of steel piles or drilled shafts that are spanned by a structural facing. Permanent facings are usually cast-in-place (CIP) concrete although timber lagging or precast concrete panels have been used. Continuous wall elements do not require separate structural facing and include steel sheet-piles, CIP or precast concrete wall panels constructed in slurry trenches (i.e., slurry (diaphragm) walls), tangent/secant piles, soil-cement columns, and jet grouted columns
2.3.2Soldier Beam and Lagging Wall2.3.2.1 GeneralSoldier beam and lagging walls are the most commonly used type of anchored wall system in theU.S.This wall system uses discrete vertical wall elements spanned by lagging which is typicallytimber,butwhichmayalsobereinforced shotcrete.Thesewall systemscanbeconstructedinmostground types, however, care must be exercised in grounds such as cohesionless soils and soft claysthat may have limited “stand-up"time for lagging installation.These wall systems are also highlypervious. The construction sequence for a permanent soldier beam and lagging wall is illustrated infigure 7 and is described below.STEP1:InstallsoldierbeamSTEP4:CompleteexcavationSTEP2:Excavate and installSTEP 5:Install headed studsandlaggingprefabricateddrainageSTEP 3:Install and test ground anchorSTEP 6:Pour cast-in-place facingFigure7.Construction sequenceforpermanent soldierbeamand laggingwall12
12 2.3.2 Soldier Beam and Lagging Wall 2.3.2.1 General Soldier beam and lagging walls are the most commonly used type of anchored wall system in the U.S. This wall system uses discrete vertical wall elements spanned by lagging which is typically timber, but which may also be reinforced shotcrete. These wall systems can be constructed in most ground types, however, care must be exercised in grounds such as cohesionless soils and soft clays that may have limited “stand-up” time for lagging installation. These wall systems are also highly pervious. The construction sequence for a permanent soldier beam and lagging wall is illustrated in figure 7 and is described below. Figure 7. Construction sequence for permanent soldier beam and lagging wall
2.3.2.2Soldier BeamThe initial step of construction for a soldier beam and lagging wall consists of installing the soldierbeams from the ground surface to their final design elevation. Horizontal spacing of the soldierbeams typically varies from 1.5 to 3 m.The soldier beams may be steel beams or drilled shafts,althoughdrilled shafts are seldomused incombination withtimberlaggingDrilled-in SoldierBeamsSteel beams such as wide flange (WF) sections or double channel sections may be placed inexcavated holes that are subsequently backfilled with concrete.It is recommended that theexcavated hole be backfilled with either structural or lean-mix concrete from the bottom of the holeto the level of the excavation subgrade.The selection of lean-mix or structural concrete is based onlateral and vertical capacity requirements of the embedded portion of the wall and is discussed inchapter5.Fromtheexcavationsubgradetothegroundsurface,theholeshouldbebackfilledwithlean-mix concrete that is subsequently scraped off duringlagging and anchor installation.Structuralconcrete is not recommended to be placed in this zone because structural concrete is extremelydifficult to scrape off for lagging installation. Lean-mix concrete typically consists of one 94 Ib bagof Portland cement per cubic yard of concrete and has a compressive strength that does not typicallyexceed approximately1MPa.As an alternativetolean-mix concretebackfill,controlled lowstrengthmaterial (CLSM)or"flowablefillmaybeused.Thismaterial,in addition to cement,contains fine aggregate and fly ash.When allowing lean-mix concreteor CLSMfor backfillingsoldier beam holes, contract specifications should require a minimum compressive strength of 0.35MPa.Like lean-mix concrete, CLSM should be weak enough to enable it to be easily removed forlagginginstallation.Ground anchors are installed between the structural steel sections and the distance between thesections depends upon the type of ground anchor used.Drill hole diameters for the soldier beamsdepend uponthestructural shape and thediameter ofthe anchor.Replacement anchors canbeinstalled between the structural sections at any location along the soldier beam.Theground anchorto soldierbeamconnectionfordrilled-insoldierbeamscanbeinstalledonthefrontfaceofthestructural sectionsorbetweenthesections.For small diameterground anchors,the connection maybe prefabricated before the soldier beams are installed.The connections for large-diameter anchorsaremadeaftertheanchorshavebeeninstalled.DrivenSoldierBeamsSteel beams such as HP shapes or steel sheet piles are used for driven soldier beams. Driven soldierbeams must penetrate to the desired final embedment depth without significant damage. Drive shoesor points" may be used to improve the ability of the soldier beams to penetrate a hard stratum. Highstrength steels also improve the ability of the soldier beams to withstand hard driving.If the soldierbeams cannot penetrate to the desired depth, then the beams should be drilled-in.Thru-beamconnections or horizontal wales are used to connect ground anchors to driven soldier beams.13
13 2.3.2.2 Soldier Beam The initial step of construction for a soldier beam and lagging wall consists of installing the soldier beams from the ground surface to their final design elevation. Horizontal spacing of the soldier beams typically varies from 1.5 to 3 m. The soldier beams may be steel beams or drilled shafts, although drilled shafts are seldom used in combination with timber lagging. Drilled-in Soldier Beams Steel beams such as wide flange (WF) sections or double channel sections may be placed in excavated holes that are subsequently backfilled with concrete. It is recommended that the excavated hole be backfilled with either structural or lean-mix concrete from the bottom of the hole to the level of the excavation subgrade. The selection of lean-mix or structural concrete is based on lateral and vertical capacity requirements of the embedded portion of the wall and is discussed in chapter 5. From the excavation subgrade to the ground surface, the hole should be backfilled with lean-mix concrete that is subsequently scraped off during lagging and anchor installation. Structural concrete is not recommended to be placed in this zone because structural concrete is extremely difficult to scrape off for lagging installation. Lean-mix concrete typically consists of one 94 lb bag of Portland cement per cubic yard of concrete and has a compressive strength that does not typically exceed approximately 1 MPa. As an alternative to lean-mix concrete backfill, controlled low strength material (CLSM) or “flowable fill” may be used. This material, in addition to cement, contains fine aggregate and fly ash. When allowing lean-mix concrete or CLSM for backfilling soldier beam holes, contract specifications should require a minimum compressive strength of 0.35 MPa. Like lean-mix concrete, CLSM should be weak enough to enable it to be easily removed for lagging installation. Ground anchors are installed between the structural steel sections and the distance between the sections depends upon the type of ground anchor used. Drill hole diameters for the soldier beams depend upon the structural shape and the diameter of the anchor. Replacement anchors can be installed between the structural sections at any location along the soldier beam. The ground anchor to soldier beam connection for drilled-in soldier beams can be installed on the front face of the structural sections or between the sections. For small diameter ground anchors, the connection may be prefabricated before the soldier beams are installed. The connections for large-diameter anchors are made after the anchors have been installed. Driven Soldier Beams Steel beams such as HP shapes or steel sheet piles are used for driven soldier beams. Driven soldier beams must penetrate to the desired final embedment depth without significant damage. Drive shoes or “points” may be used to improve the ability of the soldier beams to penetrate a hard stratum. High strength steels also improve the ability of the soldier beams to withstand hard driving. If the soldier beams cannot penetrate to the desired depth, then the beams should be drilled-in. Thru-beam connections or horizontal wales are used to connect ground anchors to driven soldier beams
A thru-beam connection is a connection cut in the beam for a small diameter ground anchor. Thru-beam connections are usually fabricated before the beam is driven.This type of connection isdesigned sotheground anchorload is applied atthecenter of the soldierbeam in linewiththewebof the soldier beam.Large-diameter(ie.,greater than approximately150mm)ground anchorscannot be used with thru-beam connections.Thru-beam connections are used when few groundanchor failures are anticipated because when a ground anchor fails, the failed anchor has to beremovedfromtheconnection oranewconnectionhastobefabricated.A sidewinderconnectionmay be used with a replacement anchor for a temporary support of excavation wall, but it is notrecommended for a permanent wall.A sidewinder connection is offset from the center of the soldierbeam, and the ground anchor load is applied to the flange some distance from the web. Sidewinderconnections subject the soldierbeams to bending and torsion.Horizontal wales may be used to connect the ground anchors to the driven soldier beams. Horizontalwales can be installed on the face of the soldier beams, or they can be recessed behind the frontflange.Whenthewalesareplacedonthefrontflange,theycanbeexposedorembeddedintheconcrete facing.If the wales remain exposed, then the ground anchor tendon corrosion protectionmay be exposed to the atmosphere and it is therefore necessary that the corrosion protection for theanchorage be well designed and constructed. However, since exposed wales are unattractive andmustbeprotectedfromcorrosion,theyarenotrecommendedforpermanentanchoredwalls.Walesplaced on the front face of the soldier beams require a thick cast-in-place concrete facing.Wales canberecessed to allow a normal thickness concrete facing to be poured.Recessed wales must beindividually fabricated and the welding required to install them is difficult and expensive.If a waleis added during construction, the horizontal clear distance to the travel lanes should be checkedbeforeapproval ofthechange.2.3.2.3 LaggingAfter installation of thesoldier beams.the soil infront of thewall is excavated in lifts.followedbyinstallationoflagging.Excavationforlagginginstallationiscommonlyperformedin1.2to1.5mlifts, however, smaller lift thicknesses may be required in ground that has limited “stand-up"timeLagging should beplacedfromthetop-downas soonaspossible after excavation tominimizeerosion of materials into the excavation.Prior to lagging installation, the soil face should beexcavated to create a reasonably smooth contact surface for the lagging.Lagging may be placedeither behind the front flange of the soldier beam or on the soldier beam. Lagging placed behindsoldier beam flanges is cut to approximate length, placed in-between the flanges of adjacent soldierbeams, and secured against the soldier beam webs by driving wood wedges or shims. Lagging canalso be attached to the front flange of soldier beamswith clips or welded studs.In rarecircumstances, lagging can be placed behind the back flange of the soldier beam.With eitherlagging installation method, gaps between the lagging and the retained ground must be backpackedto ensure good contact. Prior to placing subsequent lagging a spacer, termed a “louver, is nailed tothe top of the lagging board at each end of the lagging.This louver creates a gap for drainagebetween vertically adjacent lagging boards.The size of the gap must be sufficiently wide to permitdrainage,whileatthesametimedisallowingtheretainedsoiltofalloutfrombehindtheboardsTypically, placing vertically adjacent lagging boards in close contact is considered unacceptablehowever, some waterproofing methods may require that the gap between the lagging boards beeliminated.In this case,the contractormust provide an alternate meansto provide drainage.14
14 A thru-beam connection is a connection cut in the beam for a small diameter ground anchor. Thrubeam connections are usually fabricated before the beam is driven. This type of connection is designed so the ground anchor load is applied at the center of the soldier beam in line with the web of the soldier beam. Large-diameter (i.e., greater than approximately 150 mm) ground anchors cannot be used with thru-beam connections. Thru-beam connections are used when few ground anchor failures are anticipated because when a ground anchor fails, the failed anchor has to be removed from the connection or a new connection has to be fabricated. A “sidewinder connection” may be used with a replacement anchor for a temporary support of excavation wall, but it is not recommended for a permanent wall. A sidewinder connection is offset from the center of the soldier beam, and the ground anchor load is applied to the flange some distance from the web. Sidewinder connections subject the soldier beams to bending and torsion. Horizontal wales may be used to connect the ground anchors to the driven soldier beams. Horizontal wales can be installed on the face of the soldier beams, or they can be recessed behind the front flange. When the wales are placed on the front flange, they can be exposed or embedded in the concrete facing. If the wales remain exposed, then the ground anchor tendon corrosion protection may be exposed to the atmosphere and it is therefore necessary that the corrosion protection for the anchorage be well designed and constructed. However, since exposed wales are unattractive and must be protected from corrosion, they are not recommended for permanent anchored walls. Wales placed on the front face of the soldier beams require a thick cast-in-place concrete facing. Wales can be recessed to allow a normal thickness concrete facing to be poured. Recessed wales must be individually fabricated and the welding required to install them is difficult and expensive. If a wale is added during construction, the horizontal clear distance to the travel lanes should be checked before approval of the change. 2.3.2.3 Lagging After installation of the soldier beams, the soil in front of the wall is excavated in lifts, followed by installation of lagging. Excavation for lagging installation is commonly performed in 1.2 to 1.5 m lifts, however, smaller lift thicknesses may be required in ground that has limited “stand-up” time. Lagging should be placed from the top-down as soon as possible after excavation to minimize erosion of materials into the excavation. Prior to lagging installation, the soil face should be excavated to create a reasonably smooth contact surface for the lagging. Lagging may be placed either behind the front flange of the soldier beam or on the soldier beam. Lagging placed behind soldier beam flanges is cut to approximate length, placed in-between the flanges of adjacent soldier beams, and secured against the soldier beam webs by driving wood wedges or shims. Lagging can also be attached to the front flange of soldier beams with clips or welded studs. In rare circumstances, lagging can be placed behind the back flange of the soldier beam. With either lagging installation method, gaps between the lagging and the retained ground must be backpacked to ensure good contact. Prior to placing subsequent lagging a spacer, termed a “louver”, is nailed to the top of the lagging board at each end of the lagging. This louver creates a gap for drainage between vertically adjacent lagging boards. The size of the gap must be sufficiently wide to permit drainage, while at the same time disallowing the retained soil to fall out from behind the boards. Typically, placing vertically adjacent lagging boards in close contact is considered unacceptable, however, some waterproofing methods may require that the gap between the lagging boards be eliminated. In this case, the contractor must provide an alternate means to provide drainage