SBMA Systems | Removable anchors

Removable Anchors

- Section 4.5 of Volume 2, 'Procedures of Geotechnical Engineering Handbook - 2003'
H Ostermeyer and A D Barley

Anchors for deep excavations in urban areas very often have to be placed in adjoining private ground or under public roads. If these anchors are left in the ground, they might obstruct later excavation or foundation works. Therefore, the removal of these anchors is required in an increasing number of cases.

Many types of removable anchors have been developed and removability demonstrated with mixed success (43). With all removable anchor types, cement grout and plastic components are left in the ground. The types of removable anchors differ in the extent of the tendons and other steel parts remaining in the ground, and the success rate in removal:

a) to achieve removal of the entire length of a tendon, the bond between a multi-strand tendon and the grout body, may be destroyed by blasting (25) or by pulling out a cone from the rear end of the anchor (33);

b) to accommodate short steel parts remaining within the fixed anchor length: - a saddle and compression bar at the end of each totally debonded strand loop of a multiple anchor (SBMA (10, 86)), - a compression tube of a compression tube anchor, - a number of short strand sections of an anchor with a staggered bond length (51);

c) to accommodate the tendon remaining with its entire bond length. The free length being disconnected for example (51) by - unscrewing of a threadbar -yielding of the strands by induction heating at the top of the fixed length, - yielding of the strands by inducement of mechanical weakness at the top of the fixed length.

Even when removable anchor works are carefully executed, certain risks will remain on whether the tendons can be removed as intended by design. Difficulties may arise from frictional resistance along the free tendon length or the diameter of the grout body being too large and hence preventing crack development and the debonding of the tendon. Fully debonded loop anchors probably carry the least risk since they require no breakage of the bond with the grout and are contained in low friction grease. Recent developments can provide a steel free borehole after removal where the remaining saddles and compression tubes are made of plastic.

References

Barley, A.D., Payne, W.D., McBarron, P.L. (1999): Six rows of high capacity removable anchors support deep soil mix cofferdam. Proc.12. Eur. Conf. Soil Mech. Geot. Engg. Amsterdam 1999, Geotechnical Engineering for Transportation Infrastructure (Edp. Barends et al). Rotterdam, pp. 1465-1471.

Dietz, K., Gross, T. (1999): Neues Verfahren zum vollstandigen Ausbsau von Vorspannankern. Felsbau 17, Nr. 4, pp. 294-296.

Gipperich, C., Triantafyllidis, T. (1997): Entwicklung eines ruckbaubaren Verpressankers. Bauin-genieur 72, pp. 221-234.

Herbst T F (1997): Removable ground anchors - Answer for urban excavatons. Proc. Inern. Conf: round anchorages and anchored structures. London. pp. 197-205.

Klockner, R. (1995): Neuere Entwicklungen in der Dywidag Ankertechnik: Der elctrisch isolierte Daueranker und der ausbaubare Temporaranker. Anker in Theorie und Praxis, Proc. Int. Symposium Salzburg Okt. 1995, Balkema Rotterdam, pp. 345-351.

Stockhammer, P., Trummer, F. (1995): Der wiedergewinnbare Litzenanker System Keller. Anker in Theorie und Praxis, Proc. Int. Symposium Salzburg Okt. 1995, Balkema Rotterdam, pp. 373-376.