DC35.1-14 and EN1537:2013, respectively. Unless ground conditions, special requirements or spacing of anchors dictate otherwise, European practice recommends that a drillhole deviation of 1 in 30 can be accepted. It should be noted, developments in drill rig technology have enabled greater accuracy and alignments in drilling and tighter tolerances can be achieved routinely (e.g., 1:300 at Hazelmere Dam [Mothersille, 2017], and up to 1:5000 (averaging 1:300) at Seven Mile Dam [Sutton et al, 2004]). These levels of deviation are particularly important when drill holes are near galleries, upstream faces of the dam wall and services within the dam structure. Installation of trial anchor tendon downstream of Hazelmere Dam At Stewart Mountain Dam (Bianchi and Bruce, 1992), ‘bond length determination tests’ were performed to conf irm production anchor fixed lengths of 10.6 m (34.8 ft) for anchors installed in quartz diorite and granite and of 13.7 m (45 ft) for anchors installed in intensely fractured mylonitized granite with fractured sub- vertical dipping joints. Trial anchors used for the derivation of in-situ bond stresses should ideally be loaded to failure and can, therefore, incorporate shorter fixed lengths or larger tendons so that failure can be induced at the ground/grout interface without exceeding the permissible tendon load (i.e., 80% of the characteristic tendon tensile strength). Anchor Construction – Quality Control Drilling All ground data obtained during anchor drilling should be recorded daily and can act as an early warning indicator should variation in strata levels or ground type require changes in the design or instal- lation method. Notes should also be included on water levels encountered, stratum changes, drilling rates, flushing losses or gains, and stoppages. BS8081:1989/2015 recommends that, for a specified alignment at entry point, the hole should be drilled to an angular tolerance of +/- 2.5 degrees; this tolerance is recommended as +/- 3 degrees and +/- 2 degrees in PTI Borehole Testing One common reason for unsatisfactory performance during anchor testing is primary (fixed anchor) grout loss through fissures in the rock mass. In European practice, the likelihood of cement grout loss can be assessed from an analysis of a falling head water test applied to the drill hole or the fixed anchor length via a packer. Pre-grouting is not required if water loss is less than 5 liter/min (1.32 gal/min) at an excess head of 0.1 MPa (1 bar) measured over 10 minutes. This is based on field studies that investigated the size of rock fissures that permit the passage of cement grout particles. The established flow rate relates to a 160-micron (0.0064 in) fissure and uses this as the minimum size of fissure into which suspended cement particles cannot pass, for the range of typical grout mixes in rock grouting technology (Littlejohn, 1975). Similarly, in U.S. practice, a water pressure test is most simply performed by filling the entire hole in the rock with water and subjecting this water to a pressure that is 0.035 MPa (5 psi) greater than the hydrostatic head. If the free length is in fractured rock, a packer can be used to allow the bond length portion of the hole alone to be pressure tested. If the water loss from the drill hole, over a 10-min period exceeds 10.3 liter (2.75 gal), then the hole is pre-grouted, redrilled and retested. If the subsequent falling head water test fails, the entire process should be repeated until the acceptance criterion is satisfied. DEEP FOUNDATIONS • SEPT/OCT 2018 • 83
