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Deep Freeze

by David Berti, P.E., (M.ASCE), Principal; Berti-Lindquist Consulting Engrs., Moraga, CA,
Eric Lindquist, P.E., (M.ASCE), Principal; Berti-Lindquist Consulting Engrs., Moraga, CA,
Lee Roesner, P.E., Proj. Mgr., Engr.; Mladen Buntich Constr. Co., Sunland, CA,


Serial Information: Civil Engineering—ASCE, 2002, Vol. 72, Issue 2, Pg. 68-74


Document Type: Feature article

Abstract:

In engineering, overcoming a failure can sometimes be more instructive than completing a project with unqualified success. Such was the case with the collapse and repair of a force main shaft in Los Angeles. The project involved replacing a force main beneath the main shipping channel at the Port of Los Angeles with a new, deeper force main that would accommodate future channel dredging. The 30 in. (760 mm) diameter ductile iron force main was installed within a steel casing pipe 44.5 in. (1,130 mm) in diameter. The jacking shaft was approximately 86 ft (26 m) deep and 20 ft (6 m) in excavated diameter and resisted a maximum hydrostatic head of 81 ft (25 m) of seawater at high tide. The receiving shaft was similar to the jacking shaft but was 15 ft (4.6 m) in excavated diameter. Both shafts were supported by steel soil-freezing pipes. At about 6:00 P.M. on December 23, 1998, when excavation for the jacking shaft was within 3 ft (1 m) of subgrade, water began leaking into the shaft excavation. Infiltration ceased by 9 P.M., but at midnight, the shaft failed suddenly when it flooded with a torrent of seawater and soil. This inflow created a large sinkhole at the ground surface. No one was hurt. A subsequent investigation established that the primary cause of failure was the salinity of the soil pore water, which decreased the strength of the frozen soil to a level insufficient to serve as ground support at the design temperature. To avoid failure during reexcavation, the engineering team redesigned the support for the jacking shaft using a 10 in. (255 mm) thick shotcrete shaft lining for top-down structural support. The shotcrete was reinforced with welded wire fabric. A similar top-down design was used at the receiving shaft.

Subject Headings: Shafts | Steel pipes | Pressure pipes | Jacking | Excavation | Soil water | Soil strength

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