Geotechnical Exploration Phase Drilled Shaft Load Testing-Article

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The article argues that full-scale drilled-shaft load tests conducted during design can reduce uncertainty, improve constructability planning, and lower project costs—particularly where local load-testing experience is limited. Under LRFD, load-test results can support higher resistance factors and smaller foundations than designs based solely on geotechnical exploration. Modern testing methods, including Osterberg cells (O-cells), have also reduced testing time and expense.<br /><br />The authors describe a test program for the Utah Transit Authority’s North Temple Street Viaduct in Salt Lake City. Initial designs called for large drilled shafts to support service loads of 2,000–3,500 kips. One 6.56-foot-diameter, 126.5-foot-deep test shaft was constructed using full-depth temporary casing and tested with an O-cell installed 105 feet below grade. The test confirmed high shaft resistance and showed that the selected construction method was suitable for site conditions. Excavation also revealed coarse gravel and cobbles that conventional borings had not adequately characterized.<br /><br />The test demonstrated an equivalent top-down resistance of at least 8,300 kips at approximately 1.2 inches of displacement. Its results enabled a revised design using smaller-diameter shafts. The original estimated foundation cost was about $3.17 million; the accepted construction bid was about $2.00 million. After accounting for the $362,245 test and engineering program, the owner’s net savings were approximately $822,000. The process took less than a month, including the holiday period.<br /><br />The authors conclude that design-phase testing can provide both cost savings and early confirmation of construction feasibility. Results apply only to shafts built using the tested construction method. By contrast, a load test performed after design completion may merely verify assumptions; an unfavorable result could lead to costly delays, redesign, and disputes.

Keywords

drilled shafts

load testing

O-cell

LRFD

foundation design

constructability

shaft resistance

geotechnical exploration

temporary casing

cost savings