Deep Foundations for Top Down-Construction of High-Rise Project-Paper

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The New Century Plaza project in Century City, Los Angeles, includes two 46-story towers and a renovated hotel. A section of the site’s driveway had to remain open while a basement excavation of up to about 40 feet proceeded beneath it, requiring top-down construction. The temporary support system also had to avoid becoming a “hard spot” that would settle less than the permanent mat foundation and cause differential settlement.<br /><br />A temporary deck was supported by eight drilled shafts fitted with steel columns through the future basement levels. Osterberg load cells (O-cells) tested the shafts’ capacity and preloaded their bases. Three trial shafts of different lengths helped establish design capacities and determine production shaft lengths, which were kept as short as practicable. As excavation progressed under the deck, the sand-cement slurry surrounding the columns was removed; the permanent mat foundation was later constructed around them. This approach allowed the temporary supports to settle compatibly with the mat under the completed towers.<br /><br />Five additional, 7-foot-diameter drilled shafts supported a western portion of the podium and driveway. These were designed to carry load through end bearing, while bitumen-coated casing acted as a bond breaker to limit shaft friction and support settlement compatibility with the mat.<br /><br />Construction encountered abandoned tieback anchors near the property line and thin, highly cemented sandstone and conglomerate layers in the dense subsurface deposits. Both required slow drilling with core barrels; shaft alignment was closely monitored because of the tight O-cell clearance.<br /><br />The project demonstrated how load testing, analytical design, and carefully controlled shaft construction can enable top-down work while managing foundation capacity and differential settlement.

Keywords

New Century Plaza

Century City

Los Angeles

top-down construction

basement excavation

drilled shafts

Osterberg load cells

differential settlement

temporary support system

mat foundation