This paper presents a remedy case of a large-scaled trench collapse during the construction of a deep cylindrical diaphragm wall located in the south of Taiwan. Through a well-organized field monitoring system and suitable countermeasures, the potential disaster was mitigated in time. Thirty-two panels in total were designed to form the 70 m-diametered diaphragm with 1.2 m thickness and 90 m depth. During the trench excavation, a deep and large collapse of soils, approximately 110 m3, occurred at the depth of GL.-43 m to GL.-52 m in the trench. The overlapped reinforcing bars (rebar) of previously installed panels were serious deformed and impeded the coming rebar cage insertion. A series of monitoring activities including ultrasonic sounding and piezometer measurement was performed to figure out the causes and damaged situation. Relevant remedial measures as well as the monitoring data during the post-treatment are addressed. Three main concerns are concluded: (1) a well-organized field monitoring system during construction would be helpful for judgment in case any accident occurred, (2) an improper construction activity using percussion drill inside the coming-close cylindrical diaphragm wall built up the excess pore water pressure of the confined aquifer at the collapse depth and resulted in the serious collapse, and (3) the long stand-by duration of the excavated trench would increase the possibility of collapse.