Parking in a Land Down Under: A Paradigm to Constructing Basements Next to a River with Shallow Rock-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes construction of a five-level underground parking garage for a seven-story office building in Austin, near a river and shallow limestone. The basement was supported by a perimeter diaphragm wall made up of 60 panels. During excavation, the wall was stabilized with five rows of tiebacks—364 anchors in total—and corner bracing designed for loads of up to 250,000 pounds. Three tieback rows were below the groundwater table, which was about 30 feet below grade.<br /><br />Subsurface conditions included low-blow-count clayey and silty sands, sand with silt and gravel, and Georgetown Limestone with a reported strength of 3,900 psi. Tieback layouts required adjustments to avoid nearby wastewater pipes and pipe-pile foundations, while still achieving the required bond length in the limestone.<br /><br />Construction involved installing guidewalls, excavating panels with a hydromill, checking excavation verticality, placing reinforcement cages, and pouring concrete by tremie. Quality control covered drilling-fluid properties, cage inspections, and verticality testing. Crews also drilled, grouted, and stressed the tiebacks, installed corner bracing, and prepared the completed wall surface.<br /><br />Key challenges included developing a suitable tremie concrete mix amid limited local fly ash supply, utility conflicts and bentonite loss, drilling through gravel, and managing tight site logistics. Early contractor involvement through the design-build process helped the team achieve the project’s stated successes: a watertight basement and a condensed schedule.

Keywords

underground parking garage

diaphragm wall

tiebacks

excavation support

Georgetown Limestone

hydromill

tremie concrete

groundwater table

design-build

Austin construction