Civil Engineering April 2021 | Vol 29 No 3

36 April 2021 Civil Engineering attributed to the fact that casings need to be welded together during drilling and grinded during extraction. To the authors’ knowledge, drill rigs available in South Africa cannot currently install off-the- shelf interconnecting casing systems. All respondents indicated that they see SDAs as a feasible alternative for stressed grouted strand anchors. One respondent indicated that they will how- ever only apply a very small prestress load (50 kN maximum). If you use SDAs, do you install them with a free length and apply prestress to them? All respondents indicated that they would apply prestress if required. One respondent indicated that the apparent free length might not correspond to the expected apparent free length. This can most likely be attributed to friction that could develop between the bar and the debonding pipe and grout and coupler placed over the free length. How do you achieve a free length using SDAs? Two respondents indicated that they use PVC/ HDPE pipes sealed onto the bar (Figure 2 and 3) over the free length with grease impregnated tape applied over the couplers and often over the surface of the bar inside the pipe. Another indicated that they prefer using coupled (spigot-socket with PVC cement grout) smooth sheating to protrude over the full free length of the bar including the couplers to create the unbonded length (hole diameter permits). Another respondent indicated that they believe that smooth sheeting and grease impregnated tape is not effective on its own and that one needs to bail the hole over the free length. One way of achieving this is using a modified lance pipe/ flushing tube. This flushing tube needs to be shaped so that it does not allow flow to the grout placed over the fixed length (refer to FHWA-CFL/TD-10-01). The bailing of the hole over the free length would typi- cally result in lower ultimate capacities compared to a hole that is fully grouted. The void created between the debonding sheathing and the bar or grease impreg- nated tape over the bar increases the corrosion risk and is difficult to seal. If you apply prestress load to an equivalent SDA bar do you think the displacement of the system will be higher than that of a comparably stressed strand anchor? Three respondents indicated that displacement should be similar. Two re- spondents stated that displacement should be lower, with one stating that this is due to increased bond achieved in clays and another due to the increased steel cross- sectional area. Another respondent stated that displacement should be higher due to decreased bond capacity and comparison between the steel properties. The displacement of a comparable stressed lateral support system will be dependent on: Q Q Steel properties Q Q Ultimate bond capacity Q Q Bond load-displacement behaviour. The example provided in Part 2 of this series was adjusted, and the three-strand Figure 2 Debonded free length installed using smooth sheating Figure 3 Debonded free length with hollow bar SDA Grout body Debonding PVC pipe with grease tape over bar-free length. Pipe’s inner diameter should be slightly larger than the coupler’s diameter to ensure coupler can slide into pipe when stressed. PVC debonding pipe fixed to coupler with duct tape. Grease impregnated tape over coupler protruding over debonding pipe, PVC pipe heated at coupler and round steel hose clamp applied to seal at proximal end. Fully bonded SDA Bar