Liquefaction Hazard Assessment in Marine and Volcanic Flow Deposits, New Ireland Province, Papua New Guinea

Lihir is one of four volcanic island groups which lie in a seismically and hydrothermally active area, north-east of the New Ireland coastline, some 600km north-east of the Papua New Guinea mainland. The island comprises a series of 5 stratovolcanoes of less than 3 million years old. One of these, the Luise volcano, located on the eastern coast of Lihir, is host to the Ladolam epithermal gold deposit, presently mined by Lihir Gold Ltd. The offshore area is dominated by a major debris flow associated with collapse of the volcano some 350,000 to 150,000 years ago, overlain by loose to medium dense interlayered marine sediment and tephra, and volcanic-rich sand. The site is expected to experience accelerations of 0.3g and 0.4g (Richter magnitude 7) at return periods of 80 and 500 years respectively. At the mine site, the island comprises volcanic hillslopes rising steeply above a fringing terrace, leaving little land area onshore for stockpiling low grade ore (LGO) for later processing. For this reason, LGO is being stockpiled on a reclamation constructed into the Harbour. Previous studies based on cone penetration test (CPT) results have indicated that sediments underlying the reclamation are susceptible to liquefaction, indicating a risk to the reclamation and potential loss of the stockpiled LGO in a significant earthquake. Additional field investigations comprising machine-drilled boreholes with standard penetrometer testing (SPT's) and laboratory testing including dynamic triaxial tests and accelerator mass spectrometry dating have been used to assess liquefaction hazard based on recently developed correlations and analysis techniques. The recent investigations highlight the importance of 'Good Practise' for in-situ testing, and indicate that the sediments are likely to be less susceptible to liquefaction than indicated from previous analyses based on CPT's.