Dams leak! But only some of the leaks require investigation and remediation. When they do, finding the pathway of the leak becomes an expensive and slow process, often characterised by drilling “trial and error” boreholes that further impair the integrity of the structure. A much better alternative is to collect specialised data with highly sensitive instruments along all relevant points, map the data using the latest groundwater geophysics technology or hydrogeophysics technology, create 3D models of the subsurface including the flow path of the leak in question, and finally use software filters and algorithms to predict ongoing effects of the water problem. In this paper three case studies are presented including the Bartley Dam, King George Dam, and the Samanalawewa dam. All of the dams had leaks that concerned the dam owners. The method was applied to determine the location of the seepage paths passing through the dam. Remediation was completed at the Bartley Dam and King George Dam confirming the results from the method. And there are plans for remediation at the Samanalawewa dam. The method saved the clients a significant amount of money because they had a focused remediation. Knowing the dam has been repaired and there are no other leaks provides peace of mind to the dam owners.
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Multiple-arch dam technology enjoyed a certain popularity between the fifties and seventies, but was later discontinued for practical reasons. The multiple-arch dam that is the subject of this paper is especially peculiar since it was built using prefabricated elements and a combination of several pre-stressed steel systems.
This dam consists of 17 buttressed arches with a maximum height of 35 m on a limestone and dolostone foundation. It has a crest length of 531 m and a 15 hm3 reservoir. After 55 years in operation, several apparent degradations have surfaced and a study on the safety of the dam is currently being carried out.
The main concern is the dam’s structural safety, which is apparently linked to the integrity of the post-stressed steel elements and the precast elements in the arches. This paper describes the approach chosen for the remediation study, the visual inspection, and the tests developed on the post-stressed steel and concrete, in order to feed a 3D numerical model of the structure.
The assessment of the geological foundations of arch dams is required as part of the asset owner’s safety obligations (ANCOLD 2003). The task is often made difficult due to steep topography where arch dams are commonly constructed. Between 2013 and 2017, GHD was engaged by South Australia Water (SA Water) to examine the geological and geotechnical conditions of the Sturt River Flood Attenuation Dam (South Australia) abutment foundations. The dam was constructed between 1964 and 1966 within the Proterozoic “Sturt Tillite”. The foundations of the dam are characterised by a folded and fractured rock mass which creates complex spatial relationships between discontinuities and outcrop expression, difficult to assess in two-dimensional space. In collaboration with Monash University’s School of Earth, Atmosphere and Environment, a high resolution ortho-photogrammetric survey of the downstream dam abutments was undertaken using an Unmanned Aerial Vehicle (UAV) in areas where traditional mapping could only be obtained by rope access methods. Monash also undertook digital geological mapping of inferred discontinuities based on the UAV imagery. The data was then used to construct a three-dimensional (3D) model of the shape and position of high-persistence discontinuities, potentially critical to abutment stability. In addition to digital data, a low cost, high value field investigation to “ground-truth” the digital data and reviewed existing geological information (including rope access scanline data, foundation mapping and rotary cored boreholes) to develop a holistic understanding of the persistent discontinuities in their geological context.
There are a number of software packages that have been developed to conduct Probabilistic Seismic Hazard Assessments (PSHA’s). Each one has advantages and disadvantages. Two such programs are compared; the licenced subscription-based EZ-FRISK software package developed by Fugro USA Land, Inc. and the open-sourced OpenQuake-engine (OQ) software package by the Global Earthquake Model (GEM) Foundation. Both of these packages use the classical PSHA methodology as described by Cornell (1968) and modified by McGuire (1976). Each of these packages offers different advantages; OQ is freely distributed, code based and provides easy access to a number of tools. EZ-FRISK doesn’t rely on command-line tools and instead provides an easy user interface with quick access to plots to check results. EZ-FRISK is computationally faster than the OQ program.
A simple rectangular source model with four sites was used to investigate the degree of agreement between these two software packages. Results indicate that hazard estimates from the two packages agree to within 4% for the two closest sites. At long return periods for the two furthest sites, the difference is larger.
Trustpower’s Mahinerangi Dam in New Zealand’s South Island is a concrete arch and gravity abutment dam built in 1931, subsequently raised in 1946 and strengthened with tie-down anchors in 1961.
This paper discusses a 3D finite element analysis of the dam and the predicted performance of the arch section under Safety Evaluation Earthquake (SEE) loading against identified potential failure modes.
Current guidelines and recent seismic hazard assessments recommend earthquake loadings higher than what was originally accounted for in previous decades. A Comprehensive Safety Review identified stability under SEE loading as a potential deficiency, so a programme of works was commenced to evaluate and better understand the seismic risk by using modern day tools and technology to evaluate the dam against current performance standards.
The final model incorporated the results of extensive laboratory testing, high-resolution LiDAR survey data and dynamic calibration using ambient-vibration monitoring. Motion recordings across the face of the dam during the 2016 Kaikōura earthquake were also used to validate the model. The reservoir has been explicitly modelled together with the opening, closing and sliding of contraction joints and the foundation interface. This allowed the modelling of permanent displacements and the redistribution of loads within the dam under SEE loading, which had been shown to be an important behaviour from the previous stages of analysis.
Population at Risk (PAR) estimation involves quantification of people who could be exposed to flooding in the event of a dam failure. Conventionally, estimates of PAR involve manual and subjective assessment of individual structures located downstream of dams. To reduce the reliance on subjective judgement and better leverage publicly available population datasets, an automated method of PAR assessment was developed. This approach used the Geoscape dataset of building representations to disaggregate Australian Bureau of Statistics 2016 Census data for a study area around Gawler, South Australia.
Representative day and night spatial distributions of PAR were constructed to characterise the diurnal movement of people between homes and workplaces or other day activities. Flows of people were directly quantified to reduce reliance on high level assumptions regarding exposure. A Random Forest model was used to filter sheds and other unpopulated structures from the Geoscape dataset.
The largest deficiency in this approach is the lack of high detail data to classify building usage. It is recommended that the potential for automation of PAR assessment be continually revisited as more datasets become available.