Liquefaction potential of sands at the Krško-Brežice field, Slovenia

The Krsko-Brežice field is one of the most seismically active areas in Slovenia. The most damaging recorded earthquake with an intensity of VIII (EMS) occurred on 29th January 1917. It caused damage and claimed two lives. In the last 100 years, 9 earthquakes with intensity higher than VI (EMS) have been recorded. At the investigated area, a top layer up to 5 m thick, consisting of recent deposit of very loose silts and sands (ML, SM, SP), covers the medium dense to dense Quaternary gravel, beneath which there are over-consolidated, uncemented Miocene silts and marls. The top layer could be prone to liquefaction, as reported for the close surroundings of Brežice, where the liquefaction phenomenon was observed during the Zagreb earthquake in 1880 and during the Kupa Valley earthquake in 1909. The paper presents the results of laboratory index tests, cyclic simple shear tests and field investigations (SPT, CPT, (S)DMT, vs measurements), which were carried out to assess the liquefaction potential of the top layer at the location of the Brežice Hydroelectric Power Plant (HPP). All results show that the top layer is prone to liquefaction for an earthquake with a 475 year return period. Cyclic simple shear test results show that the liquefaction potential of horizontal ground for an earthquake with a 475 year return period can be reduced by the densification of the top layer to at least 95% of maximum Proctor density.