Durable prestressed anchors – electrically isolated tendons (EIT)

Based on [13]

1. Introduction

Permanently anchored structures are prevalent in Switzerland due to the country’s topography. Since there are very few genuinely flat regions, most of all constructions are in the foothills or the mountains. Roads, railway lines, hydroelectric power stations, and buildings are numerous in the Alps.

Figure 1: Retaining wall with electrically isolated permanent ground anchors (EIT) [13]

Several ruptures of permanent anchors brought the issues of durability and inspection of permanent anchors to the attention of the building authorities in the mid-1980s (Thur Bridge in Dietfurt, Canton St. Gallen, strand tendon, Motorway A1, Dietli creeping landslide, St. Gallen, bar tendons). Luckily enough, nobody was hurt. For more than ten years, many Swiss regional and national authorities decided not to use permanent anchors any longer for safety reasons – even if this implied considerable additional construction costs.

Figure 2: Collapse of the Dietfurt Bridge in 1981, corrosion of the permanent anchor (P. Matt, [14])

2. Historical development of anchors in Switzerland – technology and normalization

1950 – 1975  Completey bonded anchors
1973 – 1983Anchors with  a free anchor length, without « double corrosion protection »; first national code for anchors (SIA 191)  
1983 – 1994Anchors with « double corrosion protection » and anchors with complete electrical isolation
Since 1995Anchors with a « comprehensive corrosion protection »; additional national code for anchors (SIA V 191);
– technical assessment for all anchor systems
– all permanent anchors with complete electrical isolation  
1998Technical assessment completed for three anchor systems with – complete electrical isolation
– for permanent anchors, federal authorities (highways/railways) only accept systems with complete electrical isolation  
1999Electrical resistance measurement for anchors is integrated in European standard EN 1537:1999 « Execution of special geotechnical work – ground anchors » as Appendix A  
2003New national standards for geotechnics (SIA 267 and SIA 267/1). Regulations for anchors remain more or less the same as in SIA V 191  
2013New version of European standard EN 1537:2013 « Execution of special geo­technical work – ground anchors ». Electrical resistance measurement is removed from the code.  
2019Establishment of EAD 160071-00-0102 as the basis for European technical assessment of anchors using prestressed steel strands
Figure 3: Several types of ruptures of tendons on anchored structures [13]

3. State of the art today (2022)

3.1  Overview of the system

Since 1995:     Anchors with a « comprehensive corrosion protection »; Additional national code for anchors (SIA V 191);

  • technical assessment for all anchor systems
  • all permanent anchors with complete electrical isolation
Figure 4: Anchor system with « double corrosion protection » [13]

3.2  Description of the anchor system

As a rule, anchors with a « double corrosion protection » (approx. 1983 to 1994) are sufficiently protected against corrosion. With this anchor generation and reasonably designed structures, damage can be expected to remain occasional. Besides, anchors of this type are generally equipped with surveillance installations (measuring anchors, extensometers or inclino­meters). Measurements, inspections of anchor heads and verification of the anchor force in the case of lift-off can be used to assess the condition of anchors. The effectiveness of anchors with a « comprehensive corrosion protection » (since 1995) can be verified by measuring electrical resistance [3]. If the electrical resistance requirements are fulfilled, there will be no damage. Measurements and spot-checking in the anchor head area can serve as a basis for assessing the condition of anchors.

4. Example: Zurich Stadelhofen Railway Station

Figure 5: Extension of Stadelhofen Railway Station in Zurich – retaining wall with 940 permanent ground anchors (1985) [13]
Figure 6: Extension of Stadelhofen Railway Station in Zurich – cross-section and corrosion risks for anchors

5. Example: A2 motorway, northern portal of the Seelisberg Tunnel

6. Outlook

  • Electrical isolation for permanent prestressed anchors, developed from 1985 to 1998, is now standard for the works of public authorities in Switzerland.
  • A low electrical resistance value does not indicate a corrosion process but only a damaged second corrosion protection barrier (PE pipes) on the anchor. Inner corrosion protection barriers may still be intact (strands on free length, grout in bond length).
  • Putting the technology into practice under construction site conditions is complex, and the testing method is very rigorous. Nevertheless, 5% to 10% of anchors not meeting the required electrical resistance are accepted.
  • Measuring electrical resistance on every anchor is a very effective quality control method when installing anchors (consolidation in the market of anchor systems and drilling companies)
  • Electrical resistance measurements can only be used in combination with visual inspections and measurements of forces and deformations for monitoring anchors during their useful life.
  • Please note: electrical resistance measurements are sensitive to weather conditions (impossible under rainy conditions) and correct electrical grounding.

7. References

[1]   SIA Dokumentation D 031, Korrosion und Korrosionsschutz, Teil 4, Anker und Spannkabel, Studientagung vom 09. März 1989, Zürich
[https://www.e-periodica.ch/cntmng?pid=sbz-003%3A1989%3A107%3A%3A33]

[2]   von Matt, U., « Repair and reinforcement of an anchored rock cut », Prestressed Concrete in Switzerland, 1990-1994, Fédération internationale de la Précontrainte, 12ème Congrès, Washington D.C., USA, 29 mai – 2 juin 1994
[https://fib-ch.epfl.ch/Pubs/FIP/FIP_Washington1994.pdf]

[3]   von Matt, U., « Durable prestressed ground anchors », Structural concrete in Switzerland, fib-CH, Swiss national group of the international federation for structural concrete, The first fib-Congress, October 13 – 19, 2002, Osaka Japan

[4]   Hunkeler, F., Matt P., von Matt, U., « Prestressing tendons, stay cables and ground anchors – lessons learnt from corrosion damage », Structural concrete in Switzerland, fib-CH, Swiss national group of the international federation for structural concrete, The second fib-Congress, June 5 – 8, 2006 Naples, Italy and fib-CH Betontag 2006, August 18, 2006, Zurich, Switzerland
[https://www.drvollenweiderag.ch/pdf/35%20Erkenntnisse%20aus%20Korrosionssch%C3%A4den.pdf]

[5]   Schw. Eidgenossenschaft, Bundesamt für Strassen, « Richtlinie Boden- und Felsanker », Ausgabe 2007, V3.12, ASTRA 12005
[https://www.astra.admin.ch/dam/astra/de/dokumente/standards_fuer_nationalstrassen/astra_12005_boden-undfelsanker2007v311.pdf.download.pdf/astra_12005_boden-undfelsanker.pdf]

[6]   Guntlin, M., « Stützwand Täntenholz, ein permanent verankertes Bauwerk der neuen Generation », Nr. 140, Mitteilungen der Schw. Gesellschaft für Boden- und Felsmechanik, Frühjahrstagung, 24. April 2000, Zürich     
[http://geotechnikschweiz.ch.vtxhosting.ch/wp-content/uploads/2017/04/Heft140.pdf]

[7]   EMPA und Expertengruppe Anker (EGA), « Leitfaden für die technische Zulassung von Ankersystemen gemäss Norm SIA 267 », Fassung vom 29. Januar 2004
[https://docplayer.org/70227820-Leitfaden-fuer-die-technische-zulassung-von-ankersystemen-gemaess-norm-sia-267.html]

[8]   Mayoraz, F., « Retour d’expériences à partir d’exemples réalisés par l’OFROU », n° 175, Publication de la Géotechnique Suisse, Journée d’étude du 26 octobre 2017, Berne
[http://geotechnikschweiz.ch.vtxhosting.ch/wp-content/uploads/2019/10/Heft175.pdf]

[9]   Pavone, M., Richina, N., N09 – Montreux – Roche, assainissement de murs de soutène-ment », n° 175, Publication de la Géotechnique Suisse, Journée d’étude du 26 octobre 2017, Berne
[http://geotechnikschweiz.ch.vtxhosting.ch/wp-content/uploads/2019/10/Heft175.pdf]

[10] Ryser, M., « Beobachtungsmethode bei der Erhaltung geotechnischer Bauwerke », Nr. 176, Mitteilungen der Geotechnik Schweiz, Frühjahrstagung, 17. Mai 2018, Yverdon-les-Bains

[11] Naterop, D., « St. Moritz, Brattas Kriechhang, Chesa Corviglia, 23 Jahre geotechnisches Monitoring, Nr. 176, Mitteilungen der Geotechnik Schweiz, Frühjahrstagung, 17. Mai 2018, Yverdon-les-Bains

[12] European Technical Assessment, ETA 21/0476 of 03.09.2021, « Forma 97 system,

« 16 – Reinforcing and prestressing steel for concrete and ancillaries, post-tensioning kits », ITeC, Institut de Tecnologia de la Construcció de Catalunya, Wellington 19, ES08018, Barcelona

[13] Ryser, M., “Ground anchors: the origin of EIT in Switzerland,” presentation, FHWA EIT GBP workshop, ETH-Zurich, May 22, 2019

[14] Hunkeler F., Matt P., von Matt U., Werner R., Prestressing tendons, stay cables and ground anchors – Description of the systems and lessons learnt from corrosion damages, Eidgenössisches Departement für Umwelt, Verkehr, Energie und Kommunikation,Bundesamt für Strassen, Forschungsauftrag AGB2000/470 auf Antrag der Arbeitsgruppe Brückenforschung (AGB), 2005
[https://www.tfb.ch/Htdocs/Files/v/5827.pdf/SchaedenanSpannstaehlenAnkern/VSS588ZusammenfassungenAugust2005.pdf?download=1]