Civil Engineering April 2021 | Vol 29 No 3

Civil Engineering April 2021 43 installed in 2013 in Cape Town are provided in Figure 8. This illustrates the concern when not using DCP in coastal areas. For active systems, EN1537 – Execution of special geotechnical work – Ground an- chors, referenced in EN1997-1, states that all steel that is stressed shall be protected against corrosion for its design life. The use of permanent anchors shall be with the use of a duct and does not include methods such as limiting crack widths exclusively (see Table 3 in EN1537 and SAICE Lateral Support in Surface Excavation Code of Practice). In permanent stressed grouted anchors, the anchors are proof tested be- fore lock-off close to the design value of the bar. This induces very large stresses in the grout body which could induce cracking. In soil nails these high stresses are not experienced unless the system reaches an ultimate limit state. This and the debonding void that could typically be left to ensure free movement of the strands or bars over the free length during stressing, necessitates the need for more conservative requirements on stressed grouted anchors. Do you think the thread shape plays a role in permanency (crack width size)? Two participants indicated that they believe a sharper thread (T-thread) would be better in permanent applications while the rest believe that there should be no difference between a sharper or rounder thread (rope-thread). Figure 8 (Left) Corrosion just below shotcrete facing at temporary SDA nail head (installed in 2018, photo taken in 2020) and (right) Corrosion of SDA-bolt connection (installed in 2013, photo taken in 2021) Figure 7 Atmospheric corrosion category for South Africa (CSIR, from ckit.co.za ) Atmospheric corrosion of Zinc Area Description Legend Severity of corrosion Corrosion plate uncoated mild steel Zinc coating loss pm/annum A < 50 m from high water mark Severe marine +300 μm/year > 30 μm/year B 50–500 m from high water mark Medium to severe marine 200–300 μm/year 15–30 μm/year C 50 m – 3 km from sea Average marine 80–200 μm/year 6–15 μm/year D 3–40 km from sea Mildly marine 20–80 μm/year 2–6 μm/year E 40–100 km from sea Very mild marine 10–20 μm/year 1–2 μm/year F Inland Rural < 10 μm/year < 0.8 μm/year G Inland Desert < 5 μm/year < 0.5 μm/year H Urban inland Industrial 10–30 μm/year 1–3 μm/year J Urban coastal Marine industrial 50–150 μm/year 5–15 μm/year The above map provides a general indication of corrosion rates throughout the sub continent. Micro climatic conditions can vary substantially from one local site to another depending on factors such as wind direction, land contours, height above sea level and industrial pollution, etc. Should the corrosion rate of zinc be unacceptable in terms of the required service life, it is recommended that a duplex system (zinc plus paint) be specified. See Chapter 17. Walvis Bay Maputo G E G E G G G G F F E F A B E E E F D C F E F G Cape Town (J) Port Elizabeth (J) B A C East London (J) D E Durban A B C C D E F F LESOTHO Kimberley (H) Bloemfontein (H) Pretoria Witbank Vereeniging Urban Area (H) F F D D J N D D B S.T. C.T. Cape Town Area Durban Area E D D B A D BA J