2022 – Finite Element Analysis of an Embankment Dam Raise with 60 Years Monitoring Data

Joshua Chan – GHD Pty Ltd, Hobart, Qian Gu – GHD Pty Ltd, Perth, and Ben Hanslow – GHD Pty Ltd, Hobart.

This paper presents key outcomes from a detailed deformation analysis of an existing 85 m high central core rockfill embankment dam, considering both historical performance and a potential raise of 15 m. The adequacy of the raised dam geometry, its upper sloping core arrangement, protective filters and crest camber allowance are evaluated through deformation analysis of an embankment cross section focusing on embankment settlement, potential crack initiation and continuity of filter / core zones.

The original embankment construction, first filling and historical reservoir fluctuations were simulated with the Finite Element Analysis (FEA) by Plaxis 2D, using non-linear soil models with parameters derived from available testing and construction records. The analysis model was verified against the valuable deformation monitoring data for almost 60 years, including piezometer, surface monitoring, central cross arms, and hydrostatic settlement gauges within the rockfill shoulder. Further analyses were undertaken to predict the performance of the dam during and after the proposed embankment raise.

Dynamic seismic deformation analyses were also undertaken with Plaxis 2D on the raised embankment using basal excitation time histories. Appropriate soil models that can capture realistic shear modulus / damping variation with strain and cyclic softening were adopted for the dynamic analyses. Hydrodynamic pressure was modelled with “fluid” material which is considered more appropriate than the conventional Westergarrd’s added mass method.

When validated with monitoring data, FEA proved to be an effective tool that will inform future development of the proposed raise design. Valuable insights were gained into the deformational effects to the existing and raised embankment, particularly the risk of tension cracking in the raised sloping core of the embankment. Historical deformations were able to be captured within the FEA and the model was able to simulate 100 years of continued operations and the effects of a design earthquake.

The paper demonstrates the modern capability of Finite Element Analysis (FEA) to support the design and assessment of dams. It also highlights the importance and challenges of thorough model calibration as well as the value of reliable construction, operational and surveillance data to support the model calibration and verification process.