Post-Earthquake Failure of a Tailings Dam Due to Liquefaction of Pond Deposit

At the time of the Izu-Ohshima-Kinkai earthquake of January 15, 1978 in Japan, two darns retaining tailings from Mochikoshi gold mine failed, leading to a release of a large volume of tailings. One of the darns, No.1 dike, collapsed almost simultaneously with the shaking of the main shock, but another darn, No.2 dike, failed about 24 hours later at a time when there was no shaking. The failure of the No.1 dike is known to have been triggered by the liquefaction which developed in the tailings deposit in the impoundment pond. The cause of the failure in the No.2 dike has not, however, been studied and left open to question. To provide a basis to answer this question, an attempt was made to analyze outward movement of phreatic surface within the dike fills which is motivated by an increase in pore water pressure due to liquefaction developing in the pond deposit behind the retaining dike. It was shown that the phreatic surface could move up to a level near the downstream surface by the time the failure of the dike took place. At each stage of the phreatic surface movement slope stability analysis was performed using the conventional method and factors of safety were calculated. Results of the analyses showed that the factor of safety drops approximately to unity at the stage when the phreatic surface reaches a location in proximity to the surface of the slope. The period of time elapsed for the phreatic surface to reach the near-surface location was also computed and shown to be nearly coincident with the time period between when the earthquake occurred and when the failure actually took place in the No.2 dike