FEATURE ARTICLE Dam Anchoring Fundamentals — Part 1 On-site tendon fabrication for 49 to 91 strand tendons up to 95 m (312 ft) long at Hazelmere Dam, South Africa Of the thousands of dams that exist worldwide, many were constructed over the five decades between the 1920s and 1970s, which, in many cases, means that the service life is nearing expiry or has indeed fully expired. Consequently, dam restoration and strengthening have become the focus of asset owners. Over the passage of time, factors such as the introduction of new design standards, improved knowledge of seismic loading and reclassification of Probable Maximum Flood (PMF) have served to render many existing structures unsatisfactory from an overall stability standpoint. It is under these cir- cumstances that ground anchor technology has seen increased applications. Since the first use in Cheurfas Dam in Algeria in 1934, hundreds of dams have benefitted from enhanced overall stability created by post- tensioned rock anchors installed to resist the overturning and sliding forces imposed by reservoir storage. The increased use this technology in dams is largely because anchoring can usually be implemented at significantly reduced costs compared with other stabilization methods (e.g., use of mass concrete) whilst still, and often most importantly, managing to preserve the original appearance of the existing structure and usually without interference with the routine functioning of the dam. An interesting example is the use of anchors for the strengthening of Lock Arklet Dam in Scotland (Forrester, 2015). This heritage structure was constructed from 1909-1914 and is 320 m (1,050 ft) long, 10.6 m (35 ft) high with a non-overspill crest of only 2.7 m (9 ft). The successful con- struction of 64 anchors with working loads varying from 2,000 to 2,700 kN (450 to 607 kip) in drill holes of 311 mm (12.2 in) in diameter provided adequate stabilizing forces without altering the appearance of this historical significant structure. Anchor Design Fundamentals for Dams It is beyond the scope of this article to consider the design process for assessing the external forces acting on the structure since these aspects have been adequately covered elsewhere (e.g., Hobst and Zajic, 1983; and Xanthakos, 1991). Instead, focus will be placed on the fundamental principles associated with the design of the anchor itself as a tensile element within the structure. In this regard, the design process essentially covers two aspects: (1) evalua- tion of minimum free tendon length (satis- fying the requirements for external stability) and (2) determination of fixed anchor dimensions and tendon strength (satisfying the requirements for internal stability). Anchors installed through the narrow crest at Locke Arklet Dam, Scotland Worldwide, the design, construction and testing of dam anchors are governed by pivotal documents: PTI DC35.1-14 (U.S. and Canada), BS8081:1989 (South Africa, Middle East and other regions), BS8081:2015 (U.K.) and EN 1997-1:2004 +A1:2013 (Europe). Execution of dam anchors is gov- erned by EN 1537:2013 and testing by the soon-to-be-published EN ISO 22477-5: 2018. AUTHORS Dr. Devon K.V. Mothersille, CEng, FICE, Geoserve Global, and Professor Stuart Littlejohn, University of Bradford DEEP FOUNDATIONS • SEPT/OCT 2018 • 81
