Abstract:
The city of Austin, TX has been leading the country in population growth for the past few years. Subsequently, significant investment and development has attempted to keep pace with the recent boom. Due to its proximity to both the Colorado River and numerous public facilities, the Austin Riverfront area is prime real estate in this budding locale. While the presence of the river above ground is an attractant to this area, it adds significant challenges to construction underground. Additionally, the subsurface conditions along the riverfront include shallow depth to rock, and granular overburden Terrace Deposits that are hydraulically connected to the Colorado River. Conventional shoring systems like soil nail walls and soldier pile and lagging do not provide water cutoff. Active dewatering would not be viable due to the volumetric pumping requirements and because of the risk of impacts to adjacent structures. The Georgetown Formation Limestone bedrock is not exceedingly strong but is competent enough to invalidate soil mixing techniques that would allow for a dry excavation.
The subject site is located 550 feet south of the Colorado River at 218 S Lamar Boulevard. The planned edifice, Zilker Point, is a seven-story 192, 000 square foot office building with 635 parking spaces in a five-level subterranean garage. The design-build method of support of excavation was chosen to meet the myriad challenges at this site. The proposed exterior foundation wall will be a 900 linear foot reinforced concrete diaphragm wall installed with slurry wall techniques. The 32-inch-thick walls will be supported with five levels of tiebacks and corner braces in the temporary condition. In the permanent condition, the walls will be supported by the floor slabs and the tiebacks will be detensioned. The walls will extend 65 feet from ground surface and will toe between five and nine feet in limestone bedrock. Groundwater is found 27 feet below ground surface. The construction and engineering challenges, solutions, and lessons learned on this project will be explored and discussed in this presentation.