GROUND ANCHORS AND ANCHOR SYSTEMS

GROUND ANCHORS AND ANCHOR SYSTEMSPresented by:

Adil Umer 2011-MS-CEG-05(adilumer2@gmail.com)

Atif Hafeez 2011-MS-CEG-01(engratifhafeez@gmail.com)

1CONTENTSGround AnchorAnchored WallsApplicationsDesign Considerations

2Ground AnchorsMeaning of AnchorGround Anchors (Our concern)

TypesStatistics for useAnchored WallsAnchored ElementBearing PlateSheathingUnbound TendonBound Tendon (in grouted region) AnchorageSite Visit AlbumAnchor CapacityA device which can be relied upon as a source of support and stability.

A ground anchor functions as load carrying element, consisting essentially of a steel tendon inserted into suitable ground formation in almost any direction. Its load carrying capacity is generated as resisting reaction stabilized by stressing the ground along a specially formed anchorage zone.

A prestressed grouted ground anchor is a structural element installed in soil or rock that is used to transmit an applied tensile load into the ground. How they are installed? Let me tell you, first of all , hole is drilled, then tendon with sheathing is inserted, grouting of the unsheathed tendon, bearing plate installation, . To minimize lateral deformation, we prestress the tendon like this and thus it provides additional restraint against the lateral earth pressure.

Now, there are several different types of anchor used in practice depending upon type of load transfer method used, prestressing or no prestressing, and life of anchors. (Explanation on slides)

The ground condition may vary from rocks to soils which may be wet. Statistics indicate that 75%..... (Explanation on slide)

Now the anchor capacity, the maximum load in the anchor which it can sustain without critical failure (after consideration of variety of potential failures) include

3Types of Ground Anchors (Load Transfer Method, Load Applied on Anchored Element, Life)

Active and Passive Anchors

According to method of load transfer2.2.2.2 Straight Shaft Gravity-Grouted Ground AnchorsStraight 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 anchor depends on the shear resistance that is mobilized at the grout/ground interface.82.2.2.3 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)increasing the effective diameter of the grout bulb.2.2.2.4 Post-grouted Ground AnchorsPost-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 days.Postgrouting is accomplished through a sealed grout tube installed with the tendon. The tube isequipped with check valves in the bond 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 are used. One system uses a packer to isolate each valve. 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 bellsor underreams. This type of anchor may be used in firm to hard cohesive deposits. In addition toresistance 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.6Statistics for the use of Anchors on different Ground material types

The clear predominance of soil anchors indicates the frequency of soil substratum in the majority of urban and industrial excavations, consisting mostly of alluvium, silty sand and clay.

7Anchorage components for a strand tendon

4Cut away view of Strand Tendon

Centeralizer helps in ensuring a uniform grouting throughout periphery and length of tendon5Anchored WallsSoldier Beam and Lagging Walls.(Most Commonly used type)

Continuous Walls.

A 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 jetgrouted columns.9Factors affecting Anchor Capacity and PerformanceGround Characteristics.Installation Technique.Workmanship attained in the field.In general anchor capacity and performance are influenced by three main factorsGround characteristics, especially shear strengthInstallation sequence, particularly the method of fixing the bonding zoneWorkmanship attained in the field8How to decide bound and unbound length??Decision for Unbound and bound length of the tendons depends on:

and soil grout resistance respectively

The magnitude of the total anchor force required to maintain the wall in equilibrium is based on theforces caused by soil, water, and external loads. Anchors can provide the required stabilizing forceswhich, in turn, are transmitted back into the soil at a suitable distance behind the active soil zoneloading the wall, as illustrated in figure 12a. This requirement that the anchor forces must betransmitted behind the active zone generally defines the minimum distance behind the wall at whichthe anchor bond length is formed.The anchor bond length must extend into the ground to intersect any potentially critical failuresurfaces which might pass behind the anchors and below the base of the wall as illustrated in figure12b. The required depth to which anchors must be installed in the soil should be determined basedon the location of the deepest potential failure surfaces that have an insufficient factor of safetywithout any anchor force.12Soldier beam and Lagging Walls (Construction Sequence)

13Application of Ground anchors and Anchored Systems

2.4.1 Highway Retaining WallsAnchored walls are commonly used for grade separations to construct depressed roadways, roadwaywidenings, and roadway realignments. The advantages of anchored 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 requires temporary excavation support, select backfill, and possibly deep foundationsupport. Anchored walls may also be used for new bridge abutment construction and end sloperemoval for existing bridge abutments (see FHWA-RD-97-130, 1998).Figure 8. Comparison of concrete gravity wall and anchored wall for a depressed roadway.172.4.2 Slope and Landslide StabilizationGround anchors are often used in combination with walls, horizontal beams, or concrete blocks tostabilize slopes and landslides. Soil and rock anchors permit relatively deep cuts to be made for 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 selection of horizontal beams or blocks.2.4.3 Tiedown 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 reduced.Disadvantages of ground anchors for tiedowns include: (1) 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 additionalresistance to overturning, sliding, and earthquake loadings.14Steps for the Design of Anchored Wall (Courtesy FHWA)

Preliminary Survey, Geotechnical InvestigationForce AnalysisGround Anchor Geometry, Design of Wall15Software for Analysis for Geotechnical Capacity of Ground AnchorsDEEPXCav. Slope stability analysis Soil nailing walls with French Clouterre standards incorporated. Eurocode 2, 3, 7, 8 implemented. ACI 318-08, ASD 9th, LRFD Multiple wall beams can be activated at any stage. Irregular ground layers Geotechnical capacity calculations for ground anchors. Extensive verification examples. Cofferdam walls and bin type walls with different assumptions. Detailed summary reports in word and pdf. Copy - Export to Excel + More

Case Study of Pile-Anchor Support System for Deep Excavations in LahoreThe Project of IT Tower Gulberg, Lahore17Project Overview28 Storey (5B + G + 22F).Owned by M/s Urban Developers.Excavation Support System Designed and Supervised by M/s NESPAK.Depth of Excavation Required for 5B = 64 Ft.

18Design Parameters Adopted4 Rows of Anchors have been considered.The depth from NSL to the center of the Grouted portion of the first level anchor is typically taken as greater than 5 m.1st Anchor Level = 11 ft from NSL2nd Anchor Level = 26 ft from NSL3rd Anchor Level = 41 ft from NSL4th Anchor Level = 56 ft from NSL19Schematic Arrangements of Ground AnchorsExcavation Line46NSL64ML 1 = 115 Pcf 1 = 23SM 2 = 122 Pcf 2 = 33SM 3 = 125 Pcf 3 = 35112641561520Design Approaches UsedThree Design Approaches were usedDesign manuals of a) FHWA (Federal HighWay Administration)b) NAVFAC (NAVal FAcilites Command Services)c) Canadian Geotechnical ManualNote:Only the FHWA design procedure is explained in presentation.21FHWAAssuming full mobilization of Soil Shear Strength, active earth pressure is evaluated using conventional geotechnical analysis.Total Load, imposed by active earth pressure, on the face of excavation is then Calculated.

22Spacing Requirements for Ground Anchors

FHWA (1999)23Apparent Earth Pressure Diagram

24Apparent Earth Pressure Diagram (contd.)

25Apparent Earth Pressure Diagram (contd.)

Lateral wall movements and earth pressures during anchor stressing26Apparent Earth Pressure Diagram (contd.)

Lateral wall movements and earth pressures with excavation at lower anchor level27

Apparent Earth Pressure Diagram (contd.)Lateral wall movements and earth pressures with excavation at design grade.2828Recommended Apparent Earth Pressure Diagram for SANDS

FHWA (1999)29Increase by 1.3Calculations of Forces

FHWA (1999)3030Estimated Capacity of Soil Anchors and Bonded LengthAnchor bond lengths for gravity grouted, pressure grouted, post grouted soil anchors are typically 4.5 m 12 m .Beyond 12 m there is not significant increase in the capacity.The capacity for this case study was reduced by 1.2

31Estimated Capacity of Soil Anchors and Bonded Length32

FHWA (1999)Check the Design Against Applied LoadsFor 1st and 2nd anchor, bonded length = 12 m gave a capacity of 225 kips > 223 kips (i.e calculated anchor load from the recommended earth pressure diagram).For 3rd anchor 8 m and for 4th anchor 6 m bonded length gave sufficient capacity.

33Final Design as per FWHA34

ReferencesPublication No. FHWA-IF-99-015

A Comparative Study Of Pile Anchor Support System For Deep Excavations In Lahore. by Dr Aziz Akbar et. al.

35THANK YOU FOR YOUR PATIENCEQuestions Are Welcome36