Safeguarding Lake Eppalock1 February 2002
Keeping Lake Eppalock reservoir operational during a drought was a key concern for the safety upgrade at the Australian dam. Goulburn-Murray Water and URS Corporation worked together on innovative design solutions
THE safety of Lake Eppalock dam on the Campaspe river in Victoria, Australia was becoming an increasingly urgent issue. Conventional remedies were either unaffordable or likely to worsen the effects of a serious drought. An innovative and aggressive remedial strategy was required for a safety upgrade while keeping this valued community asset in operation.
When the dam was originally built in 1962, contemporary practice was for relatively steep slopes and uncompacted rockfill construction to achieve cost savings. More than 30 years later, Goulburn-Murray Water (G-MW), the new owner and operator of the dam, discovered that significant cracking had been taking place on the main embankment of Lake Eppalock for many years. G-MW engaged URS Corporation to undertake a comprehensive evaluation of the structure and to design appropriate rehabilitation measures.
The main embankment at Lake Eppalock has a maximum height of 47m and consists of a clay core, upstream and downstream filter zones and a rockfill shell. Intermittent cracking of the road surface, combined with an accelerating rate of settlement of the rockfill, alerted G-MW to the problem. Prolonged drought conditions aggravated the situation and ongoing monitoring surveys indicated that each succeeding low reservoir level was causing a dramatic increase in settlement and cracking, resulting in a real risk of failure.
Further investigations revealed that the dam's principal line of defence was inadequate because the filter contained too much fine material. If a flow path developed through the core, piping could lead to a catastrophic breach failure of the dam, resulting in serious damage downstream and possible loss of life. With extensive cracks already observed in the crest and accelerating settlement occurring during the most severe drought in the past 20 years, the reservoir had dropped well below historic levels. The safety risk was judged to be unacceptable and the dam was in urgent need of remediation.
A serious situation of this type would usually require draining the reservoir and taking the storage and roadway out of service while the dam was completely rebuilt. However, Lake Eppalock dam is used for a wide variety of purposes. It serves as a recreation facility and tourist attraction in addition to its primary function as a source for urban water supply and rural irrigation. So, keeping the dam in operation was considered essential for the customers who rely on the structure. As a consequence, it appeared that the only way to address the deep-seated settlement slip in the embankment was to place a huge amount of fill to stabilise the whole embankment. Such a solution can be exceedingly costly and cause significant environmental problems.
G-MW worked with URS to find a way of solving stability problems within the main embankment that would be cost-effective and could be carried out without disrupting the downstream community. This required understanding complex failure mechanisms and the design of remedial works that could be built quickly, taking advantage of a temporary low storage level that resulted from low inflows.
Following the identification of the problem in the investigation and design review, a streamlined programme of option studies, design and documentation was undertaken in preparation for the stabilisation of the most vulnerable portions of the embankment section. Close surveillance of the dam performance was conducted during the falling reservoir period, and the dam safety emergency plan was updated to maintain a safe operating condition.
At the start of the design review process, a preliminary risk assessment was carried out for all the Lake Eppalock structures. This covered the range of possible failure modes, including flood capacity, seismic hazard, mechanical, electrical and geotechnical issues. The risk profile was developed within a framework of a comprehensive dam risk assessment based on field investigations, engineering review, historical data and expert risk workshops. It showed that the main embankment failure was within the intolerable risk range of Australian National Committee on Large Dams risk criteria, confirming the urgency to proceed with remedial work on the embankment.
The design analysis employed a two-pronged strategy to examine the unusual deformation behaviour of the embankment, where either stability analysis or deformation analysis alone would have produced a misleading picture of the problem. Both stability and deformation analyses were performed to simulate the historical behaviour of the embankment, and the resulting model was used to reliably predict embankment performance during and after construction.
A major challenge for the design team was the development of a strategy that balanced the many competing project objectives and constraints. The solution took into account embankment stability, incorporation of filters and the potential for future deformation. Stability analyses provided a practical means of optimising the configuration of the new upstream and downstream rockfill sections.
Placement of an enlarged section on the upper bench dramatically increased stability for shallow failure surfaces but decreased stability of deeper surfaces. As a result, a balance was struck between adding filters and rockfill confinement, reducing the width of the core in the upper portions of the embankment and maintaining adequate stability within the section.
The primary challenge downstream was to establish a safe construction slope in the clay core while the filters and rockfill were excavated and replaced. It was critical to limit the extent and exposure time of the temporary clay slope, and this put constraints on the timing of the project and the construction methods that could be employed.
As the project progressed, the proposed embankment deformation mechanism was verified. It became clear that with each successively historic low reservoir level recorded, additional embankment deformation would occur and would likely be reflected on the crest road through crack formation. For that reason, the upper section of clay core was reworked to provide a ductile cap on the core, and a multi-layered crack-reducing pavement was installed above the core to evenly distribute any stresses arising from future deformation (and to prevent the ingress of water).
The final design required substantial works to the upper 10m of dam embankment on both the upstream and downstream faces. The works, which were carried out concurrently, involved closing the public road across the dam crest. The downstream works included the excavation of the upper 10m of uncompacted rockfill and filters while the storage was in operation. The upstream works consisted of the addition of rockfill and new clean fine and coarse filters.
The need for environmental preservation was taken into consideration even before the project commenced. A comprehensive environmental management plan (EMP) was prepared for the main embankment remedial works project, and an independent environmental management committee and auditor were appointed to review the plan.
The EMP ensured that existing environmental aspects were safeguarded and, where possible, improved upon. A community of rare striped legless lizards was identified in the preliminary studies, and steps were taken to shield their natural habitat. Also, an extensive revegetation programme improved the coverage of flora at the site, and an old quarry site was revegetated as part of the project.
G-MW managed construction of the project directly, which allowed an earlier start and made it easier to respond to local community requirements. It was recognised early on that quick, accurate placement of filters was the key to a short works programme, and large six-by-six articulated off-road trucks were set up with Flocon bodies to place the filters at the required rate.
In order to expedite construction, excavation of the clay core was carried out in two stages. However, this process left the core open to degradation from the effects of weather with the potential of contaminating the material being placed below. As a protective measure, the clay core was covered by hessian, which was then sprayed with a bitumen coating, effectively preventing the ingress/egress of moisture during construction without causing long term harm to the core.
Since a degree of uncertainty is inherent in geotechnical work, some modifications to the design assumptions were anticipated as construction proceeded. A number of technical checks were initiated to ensure that the design would tolerate some variations in its initial concept. The checks proved critical when, for example, the extent of cracking in the core required much more core to be removed than was originally intended.
During construction, the most significant risks were associated with the possibility of dam failure while the remedial works were partially completed. The risk was mitigated by carrying out the work during a historically low reservoir level, which entailed fast-tracking the final design and construction process.
Once the decision was taken to implement the project, a communication plan was established to inform the local community of its importance and what the effects might be. Noise, archeological and social heritage studies were performed along with those affecting the environment. Residents of the area were kept abreast of the projects progress through informational brochures. There also were regular site visits for students, senior citizens and local service groups.