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Geologic Aspects of Strut-Overloading in Braced Excavations on the Southern Flank of the Elysian Park Anticline, Los Angeles, California

W. Roth1, M. Oborne2, B. Su1, and C. Jensen1
1URS, Los Angeles, CA, [email protected], [email protected], [email protected]
2City of Los Angeles Bureau of Engineering, [email protected]

North-south shortening in northern metropolitan Los Angeles is documented both by geodetic observations (Argus et al, 1999) and by determination of the horizontal components of measured earthquakes, such as the 1987 M 5.8 Whittier Narrows earthquake (Shaw and Shearer, 1999). The Elysian Park anticline, trending along the southern boundary of this defined zone of shortening in eastern Los Angeles, is interpreted as an active contractile surface structure resulting from slip on the underlying blind Elysian Park reverse fault. Geotechnical investigations in 1995 and 1996 for the Metro Red Line East Extension, as well as a 2001 investigation for the Gold Line extension, confirmed the generally compressive environment in this region. Pressure meter measurements conducted in the alluvial deposits overlying bedrock revealed near-surface in-situ stresses 1.5 to 4 times higher than would normally be expected in such soils.

Further evidence of excess horizontal stress came in the form of severe strut overloading experienced in the braced excavations for the Gold Line’s two underground stations on 1st Street at Boyle Avenue and at Soto Street. The shoring for these 60-ft deep stations had been designed assuming that excess in-situ stress would readily dissipate during excavation. However, when measured strut loads increased to up to 3 times the design value, and strut-waler connections began to buckle, additional struts had to be installed. An investigation into the cause of strut overloading concluded that shoring deflections were too small for the locked-in horizontal stress to be sufficiently relieved (Roth et al, 2008). In essence, the requirement for minimizing shoring deflections, aimed at avoiding settlement damage of adjacent buildings, was found to be incompatible with the specified shoring-pressure diagram which did not account for the unusually high in-situ stresses. The basic premise of this paper is that, given the geologic setting of this site, these horizontal stresses most likely are resultant of regional tectonic movement.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009