The raising of Hazelmere dam, South Africa FEATURE ARTICLE Dam Anchoring Fundamentals – Part 2 In the September/October issue, Part 1 covered the fundamentals of anchor design and con- struction with particular reference to drilling, tendon fabrication and installation. Part 2 highlights some important aspects of grouting and testing which are essential prerequisites for successful execution. Part 2 also includes comments on monitoring, maintenance and the evaluation of legacy anchors. Grouting In current best practice, cement grout is used as the material for transmitting tensile forces from the tendon to the surrounding rock in the fixed anchor. The grout is introduced into the drill hole in fluid form and, after hardening, gains mechanical properties essential to the strength of the bond at the ground/grout , grout / encapsulation and grout/tendon interfaces. The important features required for satisfactory quality control (QC) are summarized below. Grout Properties The proposed cement grout mix should be designed for a 28-day unconfined compressive strength (UCS) of at least 40 MPa (5,800 psi). The UCS may be increased to a minimum of 60 MPa (8,700 psi) for anchor tendons comprising more than 60 strands. Before grouting, the following details should be available: • Type of cement and age • Water/cement ratio (by weight) • Type and concentration of admixture and/or filler • Flow reading from viscometer or flow cone • UCS development (e.g., at 3, 7, 14 and 28 days) • Free expansion or shrinkage • Bleed at two hours and initial setting time Grout Tests During construction, emphasis should be placed on those tests that permit the grout to be assessed prior to and during injection. QC related to batching and mixing should include measurements of fluidity by flow cone (or flow trough for sanded mixes) and specific gravity by mud balance. During grouting, fluidity, bleed and density should be checked regularly (e.g., every two to three hours or as appropriate for the circumstances). For the cured stage, on specimens measuring 100 mm (4 in) cubed, at least three crushing tests should be performed at 7 and 28 days to measure strength, noting that cube strengths are indicative and do not necessarily represent the grout strength in-situ. Grout Injection The sheer scale of dam anchors often warrants special techniques, and the grouting operation is a case in point. The specialist contractor may elect to use high- strength grout mixes in the fixed length (e.g., water:cement [w/c] ratio = 0.38 with additives). For economic reasons, the contractor may switch to a void filling mix (e.g., w/c ratio = 1:1) in the free length. This operation calls for special methods and can involve the installation of show pipes on the tendon to provide an indication of grout levels (Mothersille, 2017). During grouting, the level of grout in the supply tank should not be drawn down below the crown of the exit pipe, otherwise air will be injected. The following AUTHORS Dr. Devon K.V. Mothersille, CEng FICE, Geoserve Global, and Professor Stuart Littlejohn, University of Bradford DEEP FOUNDATIONS • NOV/DEC 2018 • 75
