INNOVATIVE EARTH RETENTION, UNDERPINNING AND WATER CONTROL TECHNIQUES FOR A MEGA-DEVELOPMENT IN IN SALT LAKE CITY, UTAH

摘要

The City Creek Center Block 76 Project includes the demolition and redevelopment of an entire city hlock for mixed use retail, office and residential properties directly adjacent to Historic Temple Square in Salt Lake City, Utah. Prior to construction, the Block 76 Project footprint was completely occupied by an existing shopping mall, subsurface parking structure and a twenty two story office building. Demolition of these structures, including the implosion of the office tower, was required to make way for the new construction on the project. However an adjacent light rail system and four existing buildings between 9 and 23 stories remained at the corners of the project.Excavation was required to 65ft (19.8m) below street grade and by as much as 45ft (13.7m) directly below adjacent buildings supported by shallow mat and strip type footings. Movements of the earth retention system and adjacent buildings were monitored in real time using automated equipment which complimented a suite of traditional geotechnical instrumentation.Due to the high profile nature of this project and considering deep excavations through granular soils adjacent to high rise structures on shallow foundations, careful consideration was applied to evaluate the proposed support system. Anchored diaphragm walls were employed in the areas immediately adjacent to the existing buildings greater than 10 stories. This is believed to be the first anchored diaphragm wall for earth retention in Salt Lake City, Utah.Soil nail wall systems were utilized at areas of the site not supported by diaphragm walls employed. The location of soil nail walls was generally adjacent to streets where larger wall movements were acceptable to the project team. Soil nail walls were also employed directly below existing structures, but were enhanced by an underpinning system consisting of Nicholson Pin Piles with an integral cap beam to limit anticipated movements to 1" (25mm).Jet grouting was employed as an alternate for water cutoff in lieu of dewatering to mitigate water inflow along the North and East sides of project. Prior to commencing production jet grouting, a field test program was instituted to optimize injection parameters and to confirm jet grout element geometric and mechanical properties. The test program involved the construction of test elements followed by a 3 day cure time. Jet grout test elements were exposed by excavation and geometric properties obtained by direct measurement. The jet grout wall consisted of overlapping secant columns and wasconstructed along two alignments, providing a connection between diaphragm walls adjacent to buildings along three corners of the project.This paper will provide an overall description of the Block 76 project and discuss the approach for the selection of the earth retention systems employed on the site, hi addition, the location and reasoning behind the use of underpinning and jet grouting will be discussed. A detailed account of the installation of the earth retention, underpinning and water cutoff systems will be provided. Soil conditions encountered at the site will also be discussed. Predicted versus actual earth retention wall movements will be presented. In closing a brief summary will be provided with regards to the 24" (600mm) diameter, 450tn capacity augercast piles currently in place for the new structure that is taking shape.