Concrete lagging has been used, but its use may be problematic due to difficulties in handling andvery tight tolerances on the horizontal and vertical positioning of the soldier beam to ensure easyinstallation of standard length concrete lagging.Trimming of concretelagging is very difficult andfield splicing isnotpossible.Also,theconcretelagging neartheanchorlocationmay crackduringanchor testing or stressing.2.3.2.4ConstructionSequenceTop-down installation of lagging continues until the excavation reaches a level of approximately 0.6m below the design elevation of a ground anchor.At this point, the excavation is halted and theground anchor is installed. Deeper excavation (i.e., greater than 0.6 m) below the level of a groundanchor may be required to allow the anchor connection to be fabricated or to provide equipmentaccess.The wall mustbe designed towithstand stresses associated with a deeperexcavation.Theanchor is installed using appropriate drilling and grouting procedures, as previously described.When the grout has reached an appropriate minimum strength, the anchor is load tested and thenlocked-off at an appropriate load.Excavation and lagging installation then continues until theelevation of the next anchor is reached and the next anchor is installed. This cycle of excavation,lagging installation, and ground anchor installation is continued until the final excavation depth isreached.When the excavation and lagging reach the final depth, prefabricated drainage elements may beplaced at designed spacings and connected to a collector at the base of the wall. The use of shotcretein lieu of timber lagging can be effective in certain situations.However, since the shotcrete is of lowpermeability,drainage must be installed behind the shotcrete.Drainage systems for anchored wallsarediscussed further in chapter 5.For permanent walls, a concrete facing is typically installed.Thefacing is eitherprecast or CIPconcrete.2.3.3Continuous WallsGround anchors are also used in continuous wall systems such as sheet-pile walls, tangent or secantpile walls, slurry walls, or soil mixed walls.Continuous walls are commonly used for temporaryexcavation support systems.Sheet-pile walls are constructed in one phase in which interlockingsheet-pilesaredriven to the final design elevation.Wheredifficult drivingconditions areencountered, a template is often utilized to achieve proper alignment of the sheet-piles, however, itshould be recognized that these wall systems may not be feasible for construction in hard groundconditions or where obstructions exist. Interlocking sheet-piles may be either steel or precastconcrete, however,steel sheet-piles are normally used dueto availability andhigher strength thanprecast concrete sheet-piles.Additional information on wall construction procedures,materials,andequipment for other continuous wall systems is presented in FHWA-HI-99-007 (1999).Unlike soldier beam and lagging walls, continuous walls act as both vertical and horizontal wallelements.Cycles of excavation and anchor installation proceed from thetop of the excavation andthen between the level of each anchor.Because of the relative continuity of these wall systems,waterpressurebehindcontinuouswallsmustbeconsideredindesign.Incaseswherethecontinuouswall must resist permanent hydrostatic forces, a watertight connection must be provided at thegroundanchor/wall connection.15
15 Concrete lagging has been used, but its use may be problematic due to difficulties in handling and very tight tolerances on the horizontal and vertical positioning of the soldier beam to ensure easy installation of standard length concrete lagging. Trimming of concrete lagging is very difficult and field splicing is not possible. Also, the concrete lagging near the anchor location may crack during anchor testing or stressing. 2.3.2.4 Construction Sequence Top-down installation of lagging continues until the excavation reaches a level of approximately 0.6 m below the design elevation of a ground anchor. At this point, the excavation is halted and the ground anchor is installed. Deeper excavation (i.e., greater than 0.6 m) below the level of a ground anchor may be required to allow the anchor connection to be fabricated or to provide equipment access. The wall must be designed to withstand stresses associated with a deeper excavation. The anchor is installed using appropriate drilling and grouting procedures, as previously described. When the grout has reached an appropriate minimum strength, the anchor is load tested and then locked-off at an appropriate load. Excavation and lagging installation then continues until the elevation of the next anchor is reached and the next anchor is installed. This cycle of excavation, lagging installation, and ground anchor installation is continued until the final excavation depth is reached. When the excavation and lagging reach the final depth, prefabricated drainage elements may be placed at designed spacings and connected to a collector at the base of the wall. The use of shotcrete in lieu of timber lagging can be effective in certain situations. However, since the shotcrete is of low permeability, drainage must be installed behind the shotcrete. Drainage systems for anchored walls are discussed further in chapter 5. For permanent walls, a concrete facing is typically installed. The facing is either precast or CIP concrete. 2.3.3 Continuous Walls Ground anchors are also used in continuous wall systems such as sheet-pile walls, tangent or secant pile walls, slurry walls, or soil mixed walls. Continuous walls are commonly used for temporary excavation support systems. Sheet-pile walls are constructed in one phase in which interlocking sheet-piles are driven to the final design elevation. Where difficult driving conditions are encountered, a template is often utilized to achieve proper alignment of the sheet-piles, however, it should be recognized that these wall systems may not be feasible for construction in hard ground conditions or where obstructions exist. Interlocking sheet-piles may be either steel or precast concrete, however, steel sheet-piles are normally used due to availability and higher strength than precast concrete sheet-piles. Additional information on wall construction procedures, materials, and equipment for other continuous wall systems is presented in FHWA-HI-99-007 (1999). Unlike soldier beam and lagging walls, continuous walls act as both vertical and horizontal wall elements. Cycles of excavation and anchor installation proceed from the top of the excavation and then between the level of each anchor. Because of the relative continuity of these wall systems, water pressure behind continuous walls must be considered in design. In cases where the continuous wall must resist permanent hydrostatic forces, a watertight connection must be provided at the ground anchor/wall connection
2.4APPLICATIONSOFGROUNDANCHORS2.4.1HighwayRetainingWallsAnchored walls arecommonlyused forgrade separations to constructdepressed roadways,roadwaywidenings,androadwayrealignments.Theadvantagesofanchored walls over conventional concretegravity walls have been described in section 1.2. Figure 8 provides a comparative illustration of aconventional concrete gravity wall and a permanent anchored wall for the construction of adepressed roadway.The conventional gravity wall is more expensive than a permanent anchoredwall because it requirestemporaryexcavation support, select backfill, and possibly deepfoundationsupport.Anchored walls may also be used for new bridge abutment construction and end sloperemovalfor existing bridgeabutments (seeFHWA-RD-97-130, 1998)ConventionaTemporary groundconcretegravitywallanchorBackfillNewroadindationpTemporary exsupport(a)Conventional ConcreteGravityWallWallfPermanentgroundaoSacinNenbeamSoldierbeam(b) Permanent Anchored Soldier Beam and Lagging WallFigure 8.Comparison of concrete gravitywall and anchored wall fora depressed roadway16
16 2.4 APPLICATIONS OF GROUND ANCHORS 2.4.1 Highway Retaining Walls Anchored walls are commonly used for grade separations to construct depressed roadways, roadway widenings, and roadway realignments. The advantages of anchored walls over conventional concrete gravity walls have been described in section 1.2. Figure 8 provides a comparative illustration of a conventional concrete gravity wall and a permanent anchored wall for the construction of a depressed roadway. The conventional gravity wall is more expensive than a permanent anchored wall because it requires temporary excavation support, select backfill, and possibly deep foundation support. Anchored walls may also be used for new bridge abutment construction and end slope removal for existing bridge abutments (see FHWA-RD-97-130, 1998). Figure 8. Comparison of concrete gravity wall and anchored wall for a depressed roadway
2.4.2SlopeandLandslideStabilizationGround anchors are often used in combination with walls, horizontal beams, or concrete blocks tostabilize slopes and landslides.Soil and rock anchors permit relativelydeepcuts to be madefor theconstruction of new highways (figure 9a). Ground anchors can be used to provide a sufficientlylarge force to stabilize the mass of ground above the landslide or slip surface (figure 9b). This forcemay be considerably greater than that required to stabilize a vertical excavation for a typical highwayretaining wall. Horizontal beams or concrete blocks may be used to transfer the ground anchor loadsto the ground at the slope surface provided the ground does not "run" or compress and is able toresist the anchor reaction forces at the excavated face.Cost, aesthetics, and long-term maintenanceof the exposed face will affect the selectionof horizontal beams orblocks.2.4.3Tiedown StructuresPermanent ground anchors may be used to provide resistance to vertical uplift forces.Vertical upliftforces may be generated by hydrostatic or overturning forces.The method is used in underwaterapplications where the structure has insufficient dead weight to counteract the hydrostatic upliftforces. An example application of ground anchors to resist uplift forces is shown in figure 9c.Theadvantage of ground anchors for tiedown structures include: (1) the volume of concrete in the slab isreduced compared to a dead weight slab, and (2) excavation and/or dewatering is reducedDisadvantages of ground anchors for tiedowns include:(l)potentially large variations in groundanchor load resulting from settlement and heave of the structure; and (2)difficulty in constructingwatertight connections at the anchor-structural slab interface, which is particularly important forhydrostatic applications; and (3) variations in stresses in the slab.A major uplift slab thatincorporated tiedowns was constructed for the Central Artery Project in Boston, Massachusetts (seeDruss, 1994).Although not a highway application, permanent rock anchor tiedowns may be used to stabilizeconcrete dams (figure 9d). Existing dams may require additional stabilization to meet current safetystandards with respect to maximum flood and earthquake requirements. Anchors provide additionalresistancetooverturning,sliding,andearthquakeloadings.17
17 2.4.2 Slope and Landslide Stabilization Ground anchors are often used in combination with walls, horizontal beams, or concrete blocks to stabilize slopes and landslides. Soil and rock anchors permit relatively deep cuts to be made for the construction of new highways (figure 9a). Ground anchors can be used to provide a sufficiently large force to stabilize the mass of ground above the landslide or slip surface (figure 9b). This force may be considerably greater than that required to stabilize a vertical excavation for a typical highway retaining wall. Horizontal beams or concrete blocks may be used to transfer the ground anchor loads to the ground at the slope surface provided the ground does not “run” or compress and is able to resist the anchor reaction forces at the excavated face. Cost, aesthetics, and long-term maintenance of the exposed face will affect the selection of horizontal beams or blocks. 2.4.3 Tiedown Structures Permanent ground anchors may be used to provide resistance to vertical uplift forces. Vertical uplift forces may be generated by hydrostatic or overturning forces. The method is used in underwater applications where the structure has insufficient dead weight to counteract the hydrostatic uplift forces. An example application of ground anchors to resist uplift forces is shown in figure 9c. The advantage of ground anchors for tiedown structures include: (1) the volume of concrete in the slab is reduced compared to a dead weight slab; and (2) excavation and/or dewatering is reduced. Disadvantages of ground anchors for tiedowns include: (1) potentially large variations in ground anchor load resulting from settlement and heave of the structure; and (2) difficulty in constructing watertight connections at the anchor-structural slab interface, which is particularly important for hydrostatic applications; and (3) variations in stresses in the slab. A major uplift slab that incorporated tiedowns was constructed for the Central Artery Project in Boston, Massachusetts (see Druss, 1994). Although not a highway application, permanent rock anchor tiedowns may be used to stabilize concrete dams (figure 9d). Existing dams may require additional stabilization to meet current safety standards with respect to maximum flood and earthquake requirements. Anchors provide additional resistance to overturning, sliding, and earthquake loadings
嘉碧雪晶(b)SlopeStabilization(a) HighwayRetainingWall家易心智嘉智智晶雪(c) Uplift Slab(d)ConcreteDamStabilizationFigure9.Applications of ground anchors and anchored systems.18
18 Figure 9. Applications of ground anchors and anchored systems
CHAPTER 3SITEINVESTIGATIONANDTESTING3.1INTRODUCTIONThe purpose of this chapter is to describe basic site characterization and soil and rock propertyevaluation for ground anchor and anchored system design. These activities generally include fieldreconnaissance, subsurface investigation, in situ testing, and laboratory testing.The engineeringproperties and behavior of soil and rock material must be evaluated because these materials provideboth loading and support for an anchored system.Site investigation and testing programs are necessary to evaluate the technical and economicalfeasibility of an anchored system for a project application.The extent of the site investigation andtesting components for a project should be consistent with the project scope (ie., location, size,critical nature of the structure, and budget), the project objectives (i.e., temporary or permanentstructures),and the project constraints(i.e.,geometry,constructability,performance,andenvironmental impact).Typical elements of a site investigation and testing program are describedherein.3.2FIELDRECONNAISSANCEField reconnaissance involves visual inspection of the site and examination of available documentsregarding site conditions.Information collected during field reconnaissance should include thefollowing:surface topography and adjacent land use,surface drainage patterns, and surface geologic patterns including rock outcrops, landforms,existingexcavations,and evidenceof surfacesettlement,site access conditions and traffic control requirements for both investigation andconstructionactivities,areas of potential instability such as deposits of organic or weak soils, steep terrain slidedebris, unfavorably jointed or dipping rock, and areas with a high ground-water table,extent and condition (e.g., visible damage, corrosion) of existing above and below groundutilitiesand structures,andavailable right-of-way (ROW) and easements required for the installation of ground anchorsandanchoredsystems19
19 CHAPTER 3 SITE INVESTIGATION AND TESTING 3.1 INTRODUCTION The purpose of this chapter is to describe basic site characterization and soil and rock property evaluation for ground anchor and anchored system design. These activities generally include field reconnaissance, subsurface investigation, in situ testing, and laboratory testing. The engineering properties and behavior of soil and rock material must be evaluated because these materials provide both loading and support for an anchored system. Site investigation and testing programs are necessary to evaluate the technical and economical feasibility of an anchored system for a project application. The extent of the site investigation and testing components for a project should be consistent with the project scope (i.e., location, size, critical nature of the structure, and budget), the project objectives (i.e., temporary or permanent structures), and the project constraints (i.e., geometry, constructability, performance, and environmental impact). Typical elements of a site investigation and testing program are described herein. 3.2 FIELD RECONNAISSANCE Field reconnaissance involves visual inspection of the site and examination of available documents regarding site conditions. Information collected during field reconnaissance should include the following: • surface topography and adjacent land use; • surface drainage patterns, and surface geologic patterns including rock outcrops, landforms, existing excavations, and evidence of surface settlement; • site access conditions and traffic control requirements for both investigation and construction activities; • areas of potential instability such as deposits of organic or weak soils, steep terrain slide debris, unfavorably jointed or dipping rock, and areas with a high ground-water table; • extent and condition (e.g., visible damage, corrosion) of existing above and below ground utilities and structures; and • available right-of-way (ROW) and easements required for the installation of ground anchors and anchored systems