Abstract
This paper presents the assessment of the static and nonlinear dynamic responses of a nearly 100-year-old concrete arch-gravity dam located in Australia. Concrete coring and laboratory tests revealed the presence of debonded lift joints with calcite formation on the interfaces, raising uncertainty about the dam’s performance under flood and seismic loads that exceed the levels it had previously experienced. The dam’s load resistance is governed by the complex interaction of friction at the base and along unbonded lift joints and vertical contraction joints, cantilever action, as well as arch compression resisted by the abutments. Numerical analyses using 3D finite element modelling were performed to evaluate the dam’s stress and deformation behaviour under both static and dynamic loading conditions, focussing on the geometric nonlinear behaviour of joint interfaces. Additionally, stability evaluations were performed and assessed against ANCOLD (2013) acceptance criteria.
A finite element model that comprised the dam, foundation and interface joints was established using the software Midas FEA NX. Staged finite element analyses were performed, with the static analysis being baseline conditions. The dam is serviceability stage, and considering the age of the concrete, the heat of hydration has fully dissipated. 3D geometric nonlinear dynamic analysis was conducted using time history ground motions comprising two horizontal and one vertical components. The dynamic analysis was carried out for the significant duration of the ground motion, selected time history with 90% of Arias intensity. Damping ratios of 5% and 7% were considered for OBE and SEE loading scenarios respectively.
The extreme flood loading scenarios resulted in the highest static tensile stresses, while the safety evaluation earthquake caused the maximum tensile stress for seismic loading scenarios. The displacement analysis revealed that the maximum downstream displacement occurs under extreme flood conditions. Seismic events induce moderate displacement. The sliding factor of safety at the dam-foundation contact and at the lift joints, as well as the compressive and tensile stresses in the dam body, will remain within acceptable limits under all assessed load conditions. The results were used in a comprehensive risk assessment to assess the safety of the dam in accordance with regulatory guidelines.
1. Introduction
This paper presents the assessment of the performance of a nearly 100-year-old, 18 m high concrete gravity arch dam under both static and dynamic loading scenarios. Considering the age of the dam and the presence of calcite at the lift joints on the downstream face of the dam, concrete coring investigations were conducted during the safety review to assess the concrete condition and its parameters, along with assessing the state of the lift joints. The cores revealed the presence of debonded lift joints with calcite formation on the interfaces within the dam body, raising uncertainty about the dam’s performance when subjected to flood and seismic loads that exceed the levels it had previously experienced.
The dam’s load resistance is governed by the complex interaction of friction at the base and along unbonded lift joints and vertical contraction joints, cantilever action, as well as arch compression resisted by the abutments. The assessment considered static loads for various magnitude flood events, and dynamic loads resulting from various magnitude seismic activities. The numerical analyses were performed using a finite element model, focussing on the geometric non-linear behaviour of the joint interfaces. Additionally, stability evaluations were performed and assessed against acceptance criteria as outlined in the Australian National Committee on Large Dams (ANCOLD), 2013.
The results were used to perform a comprehensive risk assessment in accordance with the ANCOLD Guidelines on Risk Assessment (ANCOLD, 2022), ensuring compliance with the Queensland regulatory Guidelines on Safety Assessments for Referable Dams (DRDMW, 2023).
2. Background and history of dam
The dam was constructed in 1929 for the purposes of town water supply. The dam is a concrete arch gravity type structure, which is curved in plan and relies on both arch action and gravity effects for stability. The dam has a central, uncontrolled spillway that discharges onto a roller bucket type dissipator. The upstream face of the dam is vertical whilst the downstream face has a slope of 1V:0.48H. The dam has an approximate height of 18.0 meters, measured from the lowest downstream toe level. It is designed to hold a full supply storage volume of 9,500 megalitres (ML), while its crest storage capacity reaches up to 13,000 ML.
3. Concrete investigation
Visual inspection of the downstream face found several seepages along some of lift joints and vertical contraction joints. As a result, concrete coring investigations were conducted during the safety review to assess the concrete condition and its parameters, along with assessing the state of the lift joints. Core samples were collected within the parent concrete and at the lift joints. The laboratory testing included compressive strength, tensile strength, modulus of elasticity, Poisson’s ratio, as well as total chloride and total sulphate content.
A key focus of the concrete investigation was the condition of joints. Based on ANCOLD Guidelines on Design Criteria for Concrete Gravity Dams (ANCOLD, 2013) “… unless there is strong evidence to support bonded lift joints all concrete lift joints should be considered as unbonded …”.
When the samples arrived at the laboratory, it was observed that the lift joint surfaces were smooth and had a shiny appearance with a thin leached calcite formation. All the samples were fractured at the lift joint locations. Upon visual inspection of the joint surface condition, and in the absence of finding any bonded lift joints, for the purpose of the assessment, it was conservatively assumed that the joints are all currently unbonded. Direct shear testing could not be carried out due to the fractured samples.
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