Underpinning and Earth Retention for Temple Square Renovation in Salt Lake City, Utah

The Salt Lake Temple, completed in 1893, is currently undergoing renovation and seismic retrofitting. A base-isolated foundation is being installed alongside expansive underground structures adjacent to the historic building. To support these efforts, a conventional anchored shoring system was constructed around the site perimeter, with excavations reaching depths of 45 ft (13.7 m). Within this shored area, new footings were built directly adjacent to and extending 18 ft (5.5 m) below the unreinforced stone masonry walls of the 170-million-pound Temple. Interior and exterior shoring walls supported this structure by laterally restraining soil “columns” directly beneath the footings. Midway through construction, a third underground level was added to the areas north and east of the Temple, requiring an additional 25 ft (7.6 m) of excavation within the zone of influence of the Temple’s foundations, necessitating very stiff shoring walls to protect the historic structure. Stiffness and construction precision were critical in limiting settlement to less than 1½ in. (3.8 cm). Shoring and underpinning techniques included micropiles, secant piles, jack-and-bore casings, soldier piles with lagging, tieback anchors, tensioned tie-rods, hand-dug piers, preloaded internal bracing, soil nailing, buttressing, and consolidation grouting.