Behavior of a Braced Excavation in Soft Clay and the Undrained Shear Strength for Passive Earth Pressure

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ABSTRACT

This report presents the behavior of a braced excavation in soft ground by analyzing data obtained from a construction site of the Tokyo International Airport. The excavation was 35 m wide and 11m deep. These dimensions are not large in respect of present construction techniques in Japan. However, because the total thickness of soft clay layers at this site was over 50 m, the toe of sheet piles did not extend into a hard stratum, thus the braced-wall structure floated in the soft clay. The large lateral displacements of the sheet piles, as well as heaving of vertical supports, caused the excavation work at some sections to be stopped. At these sections, earth pressures mobilized on the excavated side were considerably smaller than those based on Rankine’s passive earth pressure theory, using the unconfined compressive strength as the undrained shear strength. The paper examines effects of adhesion acting between the sheet pile and the ground, strength reduction due to swelling, strain rate and progressive failure, and differences in the confining conditions. As a result it turned out that the mobilized shear strength in the excavated ground is given by the undrained shear strength obtained from the triaxial extension test.

Key words

active earth pressure
case history
excavation
passive earth pressure
shear strength
sheet pile wall (IGC: H2/E5)

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