Time-history dynamic seismic analysis for pile group foundation design in Newcastle
This paper presents a summary of the time-history seismic analysis for a proposed deep foundation design consisting of a screw pile group installed into problematic deep soft ground. The ground conditions generally comprised of fill up to 4.5m deep followed by peat swamp deposits of a very soft to soft consistency followed by very soft to soft estuarine silts and loose to medium dense sands. Alluvial and estuarine soils of a firm or better consistency were encountered at depths between 16m and 20m. The design analysis confronted two pivotal questions: the potential seismic liquefaction of the soil mass and the dynamic seismic loading effects on the pile group.
To respond to the first question, faced with the soft silts and loose sand layers on the site, analysis was conducted to identify the likelihood of the liquefication potential. Based on the site piezocone penetrometer testing (CPTu) data obtained, liquefaction analysis was undertaken using the earthquake design method (Robertson, 2009). A probabilistic assessment of earthquake-induced liquefaction potential was also quantitatively conducted to demonstrate that the anticipated probability of liquefaction (PL) at the site was less than 5% corresponding to a classification of ‘Almost certain that it will not liquefy’ based on the definition of liquefaction likelihood classification (Juang, et al., 2001).
For the evaluation of dynamic actions on the pile group, a scaled time-history accelerogram of the 1989 Newcastle Earthquake, recorded at Lucus Heights was employed as the excitation source. A two-dimensional (2-D) soil-foundation-structure-interaction (SFSI) analysis was performed using finite element program Plaxis to assess the dynamic seismic performance of the pile group. Key aspects discussed include input motions from the seismic source, consideration of tectonic and seismic settings in Newcastle, and 2-D time-history finite element analyses to capture seismically induced deformations, bending moments and shear forces along the pile group.
Additionally, an independent one-dimensional (1-D) assessment including structural analysis by Spacegass, calibrated with Finite Element Method (FEM) output, was conducted. A general agreement between the 2-D geotechnical Plaxis and 1-D structural outputs confirm the validation of the analysis.
The results of the time history and SFSI analyses reveal variation ranges of bending moments, shear forces, induced settlement and horizontal movement along the pile group during seismic events. Notably, the maximum variation of bending moment and shear force remained within the proposed pile group capacity. This study contributes valuable insights and methodologies applicable to similar infrastructure designs in the Australian region, emphasising robust design and analysis practices in challenging geotechnical and seismic environments.