Strand anchors

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Description

Strand anchors are structural elements installed and grouted in predrilled holes in soil or rock to transmit an applied tensile load into the ground. They are typically manufactured from high strength low relaxation class 1860 MPa steel in strands 15.7 mm (0.6”) in diameter; the  number of strands in ground anchors typically varies from 3 to 8 (Figure 1). Strand is typically the most economical tendon and often the most versatile due to its flexibility. The maximum length is nominally unlimited, since the strand can be manufactured and assembled in any length and it can be transported coiled; in practice, however, the maximum length is limited by drilling. Typical overall lengths are up to 35 – 40 m.

The basic components of a grouted ground anchor include the: (1) anchorage; (2) free stressing (unbonded) length; and (3) foundation or bond length. (Sabatini et al., 1999)

The anchorage is the combined system of anchor head, bearing plate and trumpet which allows a correct housing of the strands and of the wedge system and the transmission of the prestressing force from the strands to the ground surface or the supported structure; in permanent application it is provided also with a protection cap.

The free length represents the part of the anchor between the foundation or bond length and the head, in which the strands are free to elongate elastically during tensioning operations and transfer the resisting force from the bond length to the structure, nominally without load transfer to the surrounding ground. A bondbreaker is a smooth plastic sleeve filled with greese 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, providing corrosion protection at the same time.

The foundation or bond length is the part of the anchor which transmits the tensile stresses to the ground; this is usually obtained by means of cement grout providing adherence between the tendon and the drillhole. To increase the grout-steel adherence, strands are suitably shaped by means of spacers and straps. A typical assembly is shown in Figure 2.

Appropriate drilling methods must be selected to suit ground and groundwater conditions, supporting the drillhole with casing, if required. Bentonite or other mud suspensions should not be used, as “smear” on the drillhole walls can significantly reduce the grout-to-ground bond. Air flush should be used in argillaceous soils and rocks susceptible to rempulding. Typical drillhole size range from 100 to 200 mm; a minimum inclination of 10° below horizontal is recommended to facilitate grouting. Typically, drillholes in rock are self supporting. However, critical drilling conditions with potential loss of borehole stability may be encountered when drilling through higly fractured or milonitic zones, especially if water is also encountered in the drillhole. In this case, it may be simpler to grout and redrill the hole, rather than using a casing.

Strand anchors contribute to the stabilization of ground slopes operating according to the scheme reported in Figure 1 of fact sheet 6.0 on the “General aspects of hazard reduction by transfer of loads to more competent strata” or in combination with other structures such as piles (fact-sheet 6.2), barrettes (fact-sheet 6.3) or caissons (fact-sheet 6.4).

Permanent anchors must satisfy three basic requirements:

1. There must be a suitable method of anchoring the distal end of the strands (foundation, bond lenth) in the drill hole;

2. A known tension must be applied to the strand anchor without creep and loss of load over time;

3. The complete strand anchor assembly must be protected from corrosion for the design life of the project.

The most common method of anchoring the distal end of a strand anchor in the drill hole is cement grout. There are several techniques to form the grouted bond length, as follows (Wymer et al., 2003):

1. Gravity grouted shaft borehole, which may be lined or unlined depending on hole stability;

2. Low pressure (< 1 MPa) grouted borehole  via  a lining  tube  or  insitu packer where the diameter of the  fixed anchor is increased with minimal disturbance as the grout permeates  through  the pores or natural fissures in the ground;

3. High pressure (> 2 MPa) grouted borehole via lining tube or insitu packer, where the grouted fixed anchor is enlarged via hydrofracturing or compaction of the ground;

4. Gravity grouted  borehole in which a series of enlargements (underreams) have previously been mechanically formed.

In soils it is important to increase friction between grouting and the surrounding soil; therefore grouting of the bond zone is usually Type C, performed by means of pipes with valves equipped with manchettes placed at variable distance (typically 30 to 100 cm) depending on the soil characteristics, to permit repeated localized high pressure grouting and to repeat grouting after tensioning, if necessary, in case of insufficient friction.

The most appropriate method to ensure that bolts are not susceptible to creep and loss of load over time is to set operating loads significantly lower than the pullout resistance and below the level at which significant creep or fluage is observed in load tests. Specific test procedures have been developed for example by the Post Tensioning Institute (1985) and by AICAP (1993), which can detect the essential aspects of the behaviour of the anchor and the surrounding ground, to determine also the long term pullout resistancet rather than the short term resistance only.

As experience with ground anchors in general and with strand anchors in particular accumulated, increasing attention has been focused on durability and corrosion protection. Ever since the publication of BS 8081:1989, all standards and guidelines place great attention on this issue.

The protection degree of strand anchors is defined with reference to their design life and to the environmental aggressiveness. Protection in every part of the strand anchor is usually assured by:

• Bond length: cement grouting and plastic, dielectric, waterproof and corrugated sheath (permanent anchors).

• Free length: each strand is coated with soft corrosion protection compounds (grease, wax, etc.) and contained in a polyethylene pipe; an external sheath covers the whole bundle of strands (temporary and permanent applications).

• Anchorage: it is the critical element of the system and is the part most susceptile to corrosion and to transmission of stray currents; it requires a perfect sealing above and below the bearing plate. Protection below the bearing plate is assured by an insulating system consisting of a cylindrical chamber sealed to the anchor head and to the plastic sheath in the free length; after tensioning the cylindrical chamber is filled with anticorrosion compound. Protection above the bearing plate is assured by a concrete sealing, if it does not require in service checking or re-tensioning or by the installation of a metallic cap filled with anticorrosion compound. Further protection from stray currents may be allowed by the interposition between bearing plate and anchor head of dielectric materials.

The current European standard (EN 1537:2000) classifies anchors depending on their design life, distinguishing ”temporary” and ”permanent” anchors, having a design life less than and more than 2 years respectively. Double corrosion protection and dielectric isolation are mandatory for permanent anchors and according to strict interpretation, in situ grout cannot be considered as providing corrosion protection.

Strand anchors are used in all types of soils and highly weathered and/or fractured rocks or where rock bolting is impractical due to the lengths required (overall length more than 15 m). Examples are shown in the schematic section of Figure 3 and in Pictures 1 to 3.

A load distribution structure is normally required to spread the very high concentrated loads available at the anchorage. Reinforced concrete spreader slabs or beams are normally used for this purpose in permanent applications. Where the soil or the weathered rock  may degrade and ravel from under and in between the reinforced concrete slabs or beams, a full containment facing must be included in the anchorage system. Different solutions may be foreseen for the facing, including for example:

• Reinforced concrete walls: the wall acts both as a protection against raveling of the rock and as a large reaction plate for the rock bolts; the rock bolt will be drilled through sleeves in the concrete; it is also important that there be drain holes through the concrete to prevent buildup of water behind the wall.

• Shotcrete, reinforced with reinforcing mesh (typically steel, but other materials may be equally suitable).

• Reinforced wire mesh, with a network of steel cables.

• Reinforced wire mesh associated with reinstatement of vegetation.

A typical facing that was popular in the 1970’s and 1980’s is a network of “vertical” and “horizontal” reinforced concrete beams forming  a grid pattern on the slope, with strand anchors at the intersections; the open spaces between the beams were typically filled with soil to encourage re-naturalization.

A description of the use of anchors to stabilizate landslide is provided by Millet et al. (1992); interesting case histories describing the use of tiebacks together with drilled shaft or driven H-piles walls are presented in Weatherby and Nicholson (1982), Hovland and Willoughby (1982), Tysinger (1982).

 

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Design methods

For cases where strand anchors are used in conjunction with other mitigation measures such as piles, barrettes and caissons, they are explicitly considered in the respective global stability and soil-structure interaction analysis appropriate for each type of mitigation measure(see fact sheets 6.2, 6.3 e 6.4).

For stand alone anchored plates, the geotechnical design is carried out using the methods and criteria set out in fact-sheet 2.0 on the “general aspects of mitigation through changes in slope geometry and/or load distribution), taking into account the stabilizing effect of the anchor loads. Besides global stability analysis, it is necessary to verify the bearing capacity of isolated spreader slabs or beams, the local stability between isolated spreader slabs or beams, the adequacy of any facing and the structural design of slabs, beams and walls.

With regard to the design of the bond length, it should be noted that the stress distribution along the bond length is higly non-uniform. In practice, the required length of the bond zone can be calculated with the simplifying assumption that the shear stresses at the ground-grout interface is uniformly distributed along the bond length. In this respect, it is recommended to restrict the length of the foundation to maximum 12 to 15 m.

Limit values of the shear stresses at the grout-ground interface can be estimated by applying, for example, the recommendations given by Bustamante and Doix (1985) which consider different values of limiting friction as a function of both ground characteristics and the method of grouting. Care should be excercised in applying these or similar guidelines to anchors placed at shallow depth where the limited cover may preclude or render ineffective high pressure grouting.

The working shear strength of the corrugated sheat-grout interface is usually greater than the working strength of the ground-grout interface; for this reason the length of the bond zone is typically determined from the stress level of the ground-grout interface.

In all cases, it is highly recommended to verify the actual limit resistance of the anchor by full scale preliminary load tests before starting commercial production. A suitable testing procedure to check that the full design load is applied at the required depth and that there will be no loss of load with time shall be drawn; reference can be maid to recommendations given, for example, by the Post Tensioning Institute (1985) and by the AICAP (1993).

As far as experiences on maintenance and monitoring of permanent anchors are concerned, reference can be made to the paper by Littlejohn and Mothersille (2008a; 2008b).

 

 

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• AICAP (1993). ”Ancoraggi nei terreni e nelle rocce – Raccomandazioni”. In Italian, 43pp..

• BS 8081:1989 British Standard code of practice for Ground Anchorages. BSI, London.

• Bustamente M., Doix B. (1985). ”Une méthode pour le calcul des tirants et des micropieux injectés”. Bull. Liaison labo P. Et Ch., 140, ref. 3047, 75-92.

• EN 1537:2000 European Standard for Execution of special geotechnical work . Ground Anchors.

• Hovland H.J., Willoughby D.F. (1982). ”Slide stabilization at the Geyser’s Power Plant”. In Application of Walls to Landslide Control Problems, Proc. of two sessions, ASCE National Convention, Las Vegas, Reeves R.B. editor, 77-92.

• Littlejohn, S. (1992). "Ground Anchorage Technology - A Forward Look." Proceedings of the Conference on Grouting, Soil Improvement and GeoSynthetics, Vol. 1, Geotechnical Special Publication No. 30, ASCE, New Orleans, Louisiana, pp. 39-62.

• Millet R.A., Lawton G.M., Repetto P.C., Varga V.K. (1992). ”Stabilization of Tablachaca Dam”. Proc. of a Specialty Conference on Stability and Performance of Slopes and Embankments, Berkeley, California, R.B. Seed and Boulanger R.W. editors, Geotechnical Special Publication 31, ASCE,.1365-1381.

• Post Tensioning Institute (1985) ”Recommendation for Prestressed Rock and Soil Anchors”. 2nd edition, Phoenix, Arizona, 57 pp..

• Sabatini P.J., Pass D.G., Bachus R.C. (1999). ”Geotechnical Engineering Circular No. 4. Ground Anchors and Anchored Systems”. US Department of Transportation, FHWA, FHWA-A0-IF-03-017, Washington D.C.  (http://www.fhwa.dot.gov/engineering/geotech/pubs/if99015.pdf  ).

• Tysinger G.L. (1982) ”Slide stabilization 4th rocky fill, Clinchfield railroad”. In Application of walls to Landslide Control Problems, Proc. of two sessions, ASCE National Convention, Las Vegas, Reeves R.B. editor, 93-107.

• Watherby D.E., Nicholson P.J. (1982). ”Tiebacks used for landslide stabilization”. In Application of walls to Landslide Control Problems, Proc. of two sessions, ASCE National Convention, Las Vegas, Reeves R.B. editor, 44-60.

• Wymer P, Robinson R, Sharp D. (2003) “Ground Anchor Practice in New Zealand – A Review of Applications, Design and Execution”. NZ Geomechanics Society Symposium, Geotechnics on the Volcanic Edge, Tauranga.

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