Current Issue

FORMATION MECHANISMS FOR VARIOUS FAILURE IN THE REAR-LINE REGION OF THE TAICHUNG PORT DOCK CAISSON

Tse-Shan Hsu

Founding President, Institute of Mitigation for Earthquake Shear Banding Disasters

Professor, Department of Civil Engineering, Feng-Chia University, Taiwan R.O.C.

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Hung-Chia Chang

Manager, Civil Engineering Maintenance Section, Maintenance Management Division, 

Port of Taichung, Taiwan Internation Port Corp. LTD., Taiwan, R.O.C.

Abstract

The current seismic design standards for dock caissons primarily focus solely on fortification against secondary ground vibration effects, making dock caissons susceptible to failure under primary shear banding effects. In view of this, this paper uses the case study of the damage to the rear-line region of the Taichung Port dock caisson during the 1999 Jiji earthquake in Taiwan to analyze the failure mechanisms. The analysis reveals the following findings: (1) Liquefaction in the rear-line region occurred only locally, and it was confined to areas of densely compacted backfill that had passed the compaction test prior to the earthquake, rather than in the loose sand zones as traditionally addressed by scholars. (2) The densely compacted backfill experienced plastic strain softening during the tectonic earthquake, which induced shear bands. This in turn caused the localized development of excess pore water pressure, leading to soil liquefaction. As a result, the dock caisson rotated or shifted significantly toward the seaward, causing substantial settlement in the rear-line region. (3) The direct placement of the caisson on the seabed, and the continued loss of shear band soils, both contributed to piping failure in the rear-line region. Based on these findings, the authors recommend that the seismic design codes for dock caissons be revised to clearly define the mechanisms for causing potential failures. Furthermore, engineers should be required to conduct verification tests to ensure effective disaster prevention measures are incorporated into the design, thereby achieving the intended safety objectives for the rear-line region of dock caissons.

Keywords: rear-line region, dock caissons, shear band, soil liquefaction, piping failure.