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Introduction
Delhi Metro Rail Corporation (DMRC), in a plan to connect the Indira Gandhi International (IGI) Airport with the New Delhi Railway Station, is constructing an exclusive Airport Metro Express Line (AMEL). As part of these plans, an underground metro station and multi level car park are being constructed (see Figure 1 for plan layout), which requires 11m to 19m deep excavation. The designed retaining wall system for the proposed excavation includes soldier pile wall in combination with 60T U-turn retrievable ground anchors at different levels.
Site Stratigraphy
The site in general consists of silty clays with depth of bedrock varying from as low as 5m to as deep as 20m. The rock can be described as weathered to hard Quartzite.
U-Turn Retrievable Ground Anchors
U-turn retrievable ground anchors facilitate the removal of anchor strands after construction of the intended wall. The steel strands are covered with PVC jacket, turned over a U-loop (U-turn saddle) at the bottom and connected to a reinforcement rod (see Figure 2). In addition, each loop has grout pipe for secondary grouting. The ground anchor is fabricated with combination of three individual U-loops (i.e. 6-strands at the top) of different lengths, spaced at predetermined design spacing.
The steel strands used are 12.7mm diameter (7ply) LRPC conforming to IS: 14268 – 1995, which has an ultimate tensile strength (breaking load) of about 187kN. The anchors are designed to withstand a working load of 60T and a test load of 66T (i.e. 1.1 times the working load).
The installed anchors are then allowed for curing period of about 10 days. In the mean while, the waler beam is fixed to enable the stressing activity at the end of curing period.
After the curing period, the each and every anchor is tested / stressed using a multi strand pre-stressing jack of requisite capacity. The anchors are stressed to a test load of 66T.
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