steel (bar or strand) to the ground surface or the supported structure.Anchorage components for abar tendon and a strand tendon are shown in figure 2 and figure 3, respectively.The unbondedlength is that portion of the prestressing steel that is free to elongate elastically and transfer theresistingforce from the bond length to the structure,A bondbreaker is a smooth plastic sleeve that isplaced over the tendon in the unbonded length to prevent the prestressing steel from bonding to thesurrounding grout.It enables the prestressing steel in the unbonded length to elongate withoutobstruction during testing and stressing and leaves the prestressing steel unbonded after lock-offThe tendon bond length is that length of the prestressing steel that is bonded to the grout and iscapableof transmittingtheapplied tensileload into theground.Theanchorbond length should belocatedbehind thecriticalfailuresurface.A portion of the complete ground anchor assembly is referred to as the tendon.The tendon includesthe prestressing steel element (strands or bars),corrosion protection, sheaths(also referred to assheathings),centralizers, and spacers,but specifically excludes the grout.The definition of a tendon.as described inPTI(1996),also includes theanchorage;however,itis assumedhereinthatthetendon does not include the anchorage.The sheathisa smoothorcorrugatedpipeortubethatprotects the prestressing steel in the unbonded length from corrosion. Centralizers position thetendon in the drill hole such that the specified minimum grout cover is achieved around the tendon.For multiple element tendons, spacers are used to separate the strands or bars of the tendons so thateach element is adequatelybonded totheanchor grout.Thegrout is a Portland cementbasedmixture that provides load transferfrom the tendon to the ground and provides corrosion protectionforthetendon.BEARINGPLATEANCHORNUTBARFigure 2. Anchorage components for a bar tendon.5
5 steel (bar or strand) to the ground surface or the supported structure. Anchorage components for a bar tendon and a strand tendon are shown in figure 2 and figure 3, respectively. The unbonded length is that portion of the prestressing steel that is free to elongate elastically and transfer the resisting force from the bond length to the structure. A bondbreaker is a smooth plastic sleeve that is placed over the tendon in the unbonded length to prevent the prestressing steel from bonding to the surrounding grout. It enables the prestressing steel in the unbonded length to elongate without obstruction during testing and stressing and leaves the prestressing steel unbonded after lock-off. The tendon bond length is that length of the prestressing steel that is bonded to the grout and is capable of transmitting the applied tensile load into the ground. The anchor bond length should be located behind the critical failure surface. A portion of the complete ground anchor assembly is referred to as the tendon. The tendon includes the prestressing steel element (strands or bars), corrosion protection, sheaths (also referred to as sheathings), centralizers, and spacers, but specifically excludes the grout. The definition of a tendon, as described in PTI (1996), also includes the anchorage; however, it is assumed herein that the tendon does not include the anchorage. The sheath is a smooth or corrugated pipe or tube that protects the prestressing steel in the unbonded length from corrosion. Centralizers position the tendon in the drill hole such that the specified minimum grout cover is achieved around the tendon. For multiple element tendons, spacers are used to separate the strands or bars of the tendons so that each element is adequately bonded to the anchor grout. The grout is a Portland cement based mixture that provides load transfer from the tendon to the ground and provides corrosion protection for the tendon. BAR ANCHOR NUT B P E L ARING ATE Figure 2. Anchorage components for a bar tendon
WEDGEPLATEBEARINGPLATEWEDGESTRANDFigure3.Anchoragecomponentsforastrandtendon2.2.2Types ofGroundAnchors2.2.2.1 GeneralThere are three main ground anchor types that are currently used in U.S. practice: () straight shaftgravity-grouted ground anchors (Type A); (2) straight shaft pressure-grouted ground anchors (TypeB); and (3) post-grouted ground anchors (Type C).Although not commonly used today in U.S.practice, another type of anchor is the underreamed anchor (Type D). These ground anchor types areillustrated schematically in figure 4 and are briefly described in the following sections.Drilling methods for each of the three main soil and rock ground anchors include rotary,percussionrotary/percussive, or auger drilling. Detailed information on these drilling techniques may be foundin Bruce (1989). The procedures and methods used to drill holes for ground anchors are usuallyselected by the contractor.The choice of a particular drilling method must also consider the overallsite conditions and it is for this reason that the engineer may place limitations on the drilling method.The drilling method must not adversely affect the integrity of structures near the ground anchorlocations or on the ground surface. With respect to drilling, excessive ground loss into the drill holeand ground surface heave are the primary causes of damage to these structures.For example, the useof largediameter hollow stem augered anchors should be discouraged in sands and gravels since theauger will tend to remove larger quantities of soil from the drill hole as compared to the net volumeof the auger.This may result in loss of support of the drill hole.In unstable soil or rock, drill casingis used.Water or air is used to flush the drill cuttings out of the cased hole.Caution should beexercised when using air flushing to clean the hole.Excess air pressures may result in unwanted6
6 Figure 3. Anchorage components for a strand tendon. 2.2.2 Types of Ground Anchors 2.2.2.1 General There are three main ground anchor types that are currently used in U.S. practice: (1) straight shaft gravity-grouted ground anchors (Type A); (2) straight shaft pressure-grouted ground anchors (Type B); and (3) post-grouted ground anchors (Type C). Although not commonly used today in U.S. practice, another type of anchor is the underreamed anchor (Type D). These ground anchor types are illustrated schematically in figure 4 and are briefly described in the following sections. Drilling methods for each of the three main soil and rock ground anchors include rotary, percussion, rotary/percussive, or auger drilling. Detailed information on these drilling techniques may be found in Bruce (1989). The procedures and methods used to drill holes for ground anchors are usually selected by the contractor. The choice of a particular drilling method must also consider the overall site conditions and it is for this reason that the engineer may place limitations on the drilling method. The drilling method must not adversely affect the integrity of structures near the ground anchor locations or on the ground surface. With respect to drilling, excessive ground loss into the drill hole and ground surface heave are the primary causes of damage to these structures. For example, the use of large diameter hollow stem augered anchors should be discouraged in sands and gravels since the auger will tend to remove larger quantities of soil from the drill hole as compared to the net volume of the auger. This may result in loss of support of the drill hole. In unstable soil or rock, drill casing is used. Water or air is used to flush the drill cuttings out of the cased hole. Caution should be exercised when using air flushing to clean the hole. Excess air pressures may result in unwanted
removal of groundwater and fines from the drill hole leading to potential hole collapse or theseexcesspressuresmayresultingroundheaveTypeA:Straight shaft gravity-groutedType B:Straight shaft pressure-groutedType C: Post-groutedTypeD:UnderreamedFigure 4. Main types of grouted ground anchors (modified after Littlejohn, 1990, GroundAnchoragePractice",Design and Performance of Earth Retaining Structures, Geotechnical SpecialPublication No.25, Reprinted bypermission of ASCE)2.2.2.2StraightShaftGravity-GroutedGroundAnchorsStraight shaft gravity-grouted ground anchors are typically installed in rock and very stiff to hardcohesive soil deposits using either rotary drilling or hollow-stem auger methods.Tremie (gravitydisplacement) methods are used to grout the anchor in a straight shaft borehole. The borehole maybe cased or uncased depending on the stability of the borehole.Anchor resistance to pullout of thegrouted anchordepends on the shear resistance that ismobilized at thegrout/ground interface
7 removal of groundwater and fines from the drill hole leading to potential hole collapse or these excess pressures may result in ground heave. Figure 4. Main types of grouted ground anchors (modified after Littlejohn, 1990, “Ground Anchorage Practice”, Design and Performance of Earth Retaining Structures, Geotechnical Special Publication No. 25, Reprinted by permission of ASCE). 2.2.2.2 Straight Shaft Gravity-Grouted Ground Anchors Straight shaft gravity-grouted ground anchors are typically installed in rock and very stiff to hard cohesive soil deposits using either rotary drilling or hollow-stem auger methods. Tremie (gravity displacement) methods are used to grout the anchor in a straight shaft borehole. The borehole may be cased or uncased depending on the stability of the borehole. Anchor resistance to pullout of the grouted anchor depends on the shear resistance that is mobilized at the grout/ground interface
2.2.2.3 Straight Shaft Pressure-Grouted Ground AnchorsStraight shaft pressure-grouted ground anchors are most suitable for coarse granular soils and weakfissured rock.This anchor type is also used in fine grained cohesionless soils.With this type ofanchor, grout is injected into the bond zone under pressures greater than 0.35 MPa. The borehole istypically drilled using a hollow stem auger or using rotary techniques with drill casings. As theauger or casing is withdrawn, the grout is injected into the hole under pressure until the entire anchorbond length is grouted.This grouting procedure increases resistance to pullout relative to tremiegrouting methods by:(1) increasing the normal stress (i.e., confining pressure) on the grout bulbresulting from compaction of the surrounding material locally around the grout bulb; and (2)increasingtheeffectivediameterofthegroutbulb.2.2.2.4Post-groutedGroundAnchorsPost-grouted ground anchors use delayed multiple grout injections to enlarge the grout body ofstraight shafted gravity grouted ground anchors. Each injection is separated by one or two daysPostgrouting is accomplished through a sealed grout tube installed with the tendon. The tube isequipped with check valves in thebond zone.The check valves allow additional grout to be injectedunder high pressure into the initial grout which has set. The high pressure grout fractures the initialgrout and wedges it outward into the soil enlarging the grout body.Two fundamental types of post-grouted anchors areused.One system uses a packerto isolateeachvalve.The other system pumpsthe grout down the post-grout tube without controlling which valves are opened.2.2.2.5 Underreamed AnchorsUnderreamed anchors consist of tremie grouted boreholes that include a series of enlargement bellsorunderreams.Thistypeofanchormaybeusedinfirmtohardcohesivedeposits.Inadditiontoresistance through side shear, as is the principal load transfer mechanism for other anchors,resistance may also be mobilized through end bearing. Care must be taken to form and clean theunderreams.2.2.3Tendon Materials2.2.3.1Steel Barand Strand TendonsBothbar and strand tendons arecommonly used for soil and rock anchors forhighwayapplicationsin the U.S. Material specifications for bar and strand tendons are codified in American Society forTesting and Materials (ASTM) A722 and ASTM A416, respectively. Indented strand is codified inASTMA886.Bar tendons arecommonly available in26mm,32mm,36mm,45mm,and 64mmdiameters in uncoupled lengthsupto approximately18 m.Anchordesignloadsupto approximately2,077kN can be resisted by a single 64-mm diameter bar tendon.For lengths greater than 18 m andwhere space constraints limit bar tendon lengths, couplers may be used to extend the tendon lengthAs compared to strand tendons, bars are easier to stress and their load can be adjusted after lock-off8
8 2.2.2.3 Straight Shaft Pressure-Grouted Ground Anchors Straight shaft pressure-grouted ground anchors are most suitable for coarse granular soils and weak fissured rock. This anchor type is also used in fine grained cohesionless soils. With this type of anchor, grout is injected into the bond zone under pressures greater than 0.35 MPa. The borehole is typically drilled using a hollow stem auger or using rotary techniques with drill casings. As the auger or casing is withdrawn, the grout is injected into the hole under pressure until the entire anchor bond length is grouted. This grouting procedure increases resistance to pullout relative to tremie grouting methods by: (1) increasing the normal stress (i.e., confining pressure) on the grout bulb resulting from compaction of the surrounding material locally around the grout bulb; and (2) increasing the effective diameter of the grout bulb. 2.2.2.4 Post-grouted Ground Anchors Post-grouted ground anchors use delayed multiple grout injections to enlarge the grout body of straight shafted gravity grouted ground anchors. Each injection is separated by one or two days. Postgrouting is accomplished through a sealed grout tube installed with the tendon. The tube is equipped with check valves in the bond zone. The check valves allow additional grout to be injected under high pressure into the initial grout which has set. The high pressure grout fractures the initial grout and wedges it outward into the soil enlarging the grout body. Two fundamental types of postgrouted anchors are used. One system uses a packer to isolate each valve. The other system pumps the grout down the post-grout tube without controlling which valves are opened. 2.2.2.5 Underreamed Anchors Underreamed anchors consist of tremie grouted boreholes that include a series of enlargement bells or underreams. This type of anchor may be used in firm to hard cohesive deposits. In addition to resistance through side shear, as is the principal load transfer mechanism for other anchors, resistance may also be mobilized through end bearing. Care must be taken to form and clean the underreams. 2.2.3 Tendon Materials 2.2.3.1 Steel Bar and Strand Tendons Both bar and strand tendons are commonly used for soil and rock anchors for highway applications in the U.S. Material specifications for bar and strand tendons are codified in American Society for Testing and Materials (ASTM) A722 and ASTM A416, respectively. Indented strand is codified in ASTM A886. Bar tendons are commonly available in 26 mm, 32 mm, 36 mm, 45 mm, and 64 mm diameters in uncoupled lengths up to approximately 18 m. Anchor design loads up to approximately 2,077 kN can be resisted by a single 64-mm diameter bar tendon. For lengths greater than 18 m and where space constraints limit bar tendon lengths, couplers may be used to extend the tendon length. As compared to strand tendons, bars are easier to stress and their load can be adjusted after lock-off
Strandtendonscomprisemultipleseven-wirestrands.ThecommonstrandinU.S.practiceis15mmin diameter. Anchors using multiple strands have no practical load or anchor length limitations.Tendon steels have sufficientlylow relaxation properties to minimize long-term anchorload losses.Couplersareavailablefor individual seven-wirestrandsbutarerarelyusedsincestrandtendons canbemanufactured inanylength.Strand couplersarenotrecommendedforroutineanchorprojectsasthediameterofthecouplerismuchlargerthanthestranddiameter,butstrandcouplersmaybeusedto repair damaged tendons.Where couplers are used, corrosion protection of the tendon at thelocationofthecouplermustbeverified.2.2.3.2 Spacers and CentralizersSpacer/centralizer units are placed at regular intervals (e.g., typically 3 m) along the anchor bondzone. For strand tendons, spacers usually provide a minimum interstrand spacing of6 to 13 mm anda minimum outer grout cover of 13 mm.Both spacers and centralizers should be made of non-corrosive materials and bedesigned to permit free flow of grout.Figure5 and figure 6 show a cutawaysectionofabarandastrandtendon,respectivelySMOOTHPLASTICSHEATHINGCORRUGATEDPLASTICCEMENTSHEATHINGGROUTCENTRALIZERBARFigure5.Cutawayviewofbartendon9
9 Strand tendons comprise multiple seven-wire strands. The common strand in U.S. practice is 15 mm in diameter. Anchors using multiple strands have no practical load or anchor length limitations. Tendon steels have sufficiently low relaxation properties to minimize long-term anchor load losses. Couplers are available for individual seven-wire strands but are rarely used since strand tendons can be manufactured in any length. Strand couplers are not recommended for routine anchor projects as the diameter of the coupler is much larger than the strand diameter, but strand couplers may be used to repair damaged tendons. Where couplers are used, corrosion protection of the tendon at the location of the coupler must be verified. 2.2.3.2 Spacers and Centralizers Spacer/centralizer units are placed at regular intervals (e.g., typically 3 m) along the anchor bond zone. For strand tendons, spacers usually provide a minimum interstrand spacing of 6 to 13 mm and a minimum outer grout cover of 13 mm. Both spacers and centralizers should be made of noncorrosive materials and be designed to permit free flow of grout. Figure 5 and figure 6 show a cut away section of a bar and a strand tendon, respectively. Figure 5. Cut away view of bar tendon