Pre-Stressed Ground Anchors for Underground Stations and Earth Retaining Structures-Paper

Paper

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The paper describes the design and installation of temporary, retrievable prestressed rock anchors for Mumbai Metro Line 3. Pavai Infra Geotech and TeampT provided earth-retaining works at six underground stations, where anchors support excavations under varying ground conditions and tight space constraints.<br /><br />The anchors use prestressing strands arranged in U-shaped loops within protective HDPE pipes, allowing them to be removed after construction. Design follows relevant British Standards, particularly BS 8081, because Indian guidance offers limited detail for retrievable systems. Anchor capacity, strand quantity, fixed bond length and free length are determined using project geotechnical data, then refined through investigative anchor tests that establish actual rock–grout bond strength.<br /><br />Testing is central to the workflow. Trial anchors are loaded in cycles to assess bond performance, elongation and creep. Working anchors are then checked through suitability and creep tests before being stressed and locked off. The paper also clarifies the distinction between the design working load and the prestressing force.<br /><br />Practical challenges included differences between borehole records and actual strata, groundwater inflow, difficult drilling and variable bond conditions. The team responded by inspecting recovered ground samples, adapting anchor lengths and monitoring working anchors. Innovations included a loop-making device, specialized drilling machines, additional corrosion protection and staged stressing to compensate for unequal loop lengths and achieve uniform loading.<br /><br />The authors report that coordinated design and construction efforts enabled work to proceed across all six stations despite challenging geology, restricted sites and stringent performance requirements.

Keywords

Mumbai Metro Line 3

temporary rock anchors

prestressed anchors

retrievable anchors

earth-retaining works

BS 8081

anchor testing

rock-grout bond

HDPE pipes

groundwater inflow