Publication Abstract

Proceedings - Fourth International Conference - Piling and Deep Foundations, 1991, Stresa, Italy, (DFI)

Design and Construction of Drilled Shafts in Coarse Gravel and Cobble Deposits
G.H Beckwith & A. Hirany; Sergent, Hauskins & Beckwith Geotechnical Engineers Inc., Phoenix, Ariz.

Drilled shaft foundations up to 43m in depth and 4m in diameter excavated through coarse gravel, cobble and boulder river deposits have been widely used in Phoenix, Arizona, and vicinity during the past 15 years. About 25 bridges over the Salt River and major tributaries and several electrical power transmission lines are supported on this type of foundation. The river deposits consist of very hard quartzite gravel, cobbles and boulders up to 0.5m nominal diameter. The large depths of foundations for bridges are dictated by potential scour during extreme floods, which is influenced by past gravel mining in the river beds. Foundation design practice for these conditions is described. Local geotechnical engineering practice includes exploratory drilling with the Becker Hammer drill, estimation of shear strength based on published triaxial shear testing on coarse gravel done for embankment dam design, and estimation of the lateral earth pressure coefficient, K, based on geologic history. Construction methods and equipment including the use of bentonite slurry are summarized.

 article #448; publication #37 (IC-1991-IT)