The Use of Permanent Tiedown Anchors for Underground Structures

During the design of a recent transit project, rock anchors were designed to resist high buoyancy forces at multiple underground structures. After considering other options, permanent prestressed double corrosion protected rock anchors were selected. Although such anchors have been used in other infrastructure applications, there are few cases of such anchors for underground structures. To advance the anchor design, the design team reviewed previous precedence at underground and other infrastructure projects, and evaluated the performance of permanent anchors. Identified risks included potential geotechnical failure mechanisms, leaks through the anchored invert slab, corrosion of the tendon and head connection, the inability to access anchor heads after construction, constructability issues and verification of capacity. Mitigation measures included conservative safety factors, groundwater corrosion studies and corrosion protection measures, anchor testing, specialized waterproofing details, and detailed construction sequencing. The findings of the evaluation and the development of the design are presented herein.