Liquefaction-Associated Ground Damage During the Vrancea Earthquake of March 4, 1977

Abstract

During the Vrancea earthquake of March 4, 1977, severe damage was incurred to the ground in the old riverbed deposit of the Dimbovitza river in the city area of Bucharest, Romania, due to the development of liquefaction. Following the earthquake extensive in-situ investigations were carried out to clarify subsurface soil conditions by means of the Dutch cone penetration test and the Swedish automatic ram sounding test. On the basis of the results of these tests, analyses were made for the liquefaction at two sites in this area ;one site where apparent signs of liquefaction were observed and the other site where no such sign was visible on the ground surface following the earthquake. The result ot the analyses indicated that liquefaction could occur in the sand deposit underlying the surface layer composed of clayey silt. However, because the surface crust was not thick enough at one site, it was broken up and the liquefied sand migrated through the fissures upwards on the ground surface. In contrast to this, the surface layer at the other site was sufficiently thick to prevent such breach and consequent sand ejection from occurring during the earthquake.One of the most alarming aspects of the collapse in the ground was several huge sinkholes which developed in the flood plain area of the Danube river near the town of Giurgiu in Romania. As a result of boring investigation, cavernous limestone formation was found to exist at a depth of about 20 m below a thick sand deposit of alluvial origin. It is then speculated that the cave-in must have taken place as a result of ravelling failure in the weathered upper portion of the limestone formation. In order to provide some quantitative interpretation to the mechanism of the sinkhole development, an instability analysis was made with respect to the seepage conditions in the weathered limestone layer which might have been altered due to the increase in pore water pressure in the upperlying alluvial sand deposit during the earthquake. The result of the analysis indicated that the seepage-associated instability caused by the increase in pore water pressure in the sand deposit is most likely to be an immediate cause of the catastrophic sinkhole development in the ground during the Vrancea earthquake.