This paper will present the use of Root Cause Analysis (RCA) as a means of evaluating the causes for failure modes and is based on work completed for an upstream tailings storage facility (TSF) raise where significant transverse and longitudinal cracking was observed.
The design of the TSF was based on the use of a starter wall with perimeter discharge from spigots spaced at about 25 m centres along the upstream crest. The TSF was raised using an upstream design and during routine inspections two years after completion of the raise, transverse cracks of up to 30 mm were noted on the crest and longitudinal cracks up to 40 mm width were noted on the downstream slope of the raised embankments. Concerns were raised over the extent and depth of the transverse cracking and the risks they pose to piping, seepage and containment.
Field investigations including test pitting and material testing were completed to evaluate the depth and extent of the cracking. The findings from field investigations, together with a review of the historical aerial photographs and superposition of the cracks and the locations of the spigots were then used in a Root Cause Analysis workshop.
Discussions on all causes for the cracking, asking the question “why did the problem occur?”, and then continuing to ask “why that happened?” until the fundamental process element that failed was reached”.
During the workshop, the most significant contributors for the transverse and longitudinal cracking and the likely location, extent and size of the cracks were evaluated. This identified the potential for traditional structural hog and sag bending moments causing the transverse crest cracking with the potential for transverse cracking at the interface of the raise and the original tailings. This was not previously identified as a potential piping location. The longitudinal cracking was considered to be mainly owing to settlement of the upstream tailings.
Thomas Fritz and Peter Lilley
A challenge with managing any diverse portfolio of structures is ensuring that expenditure is targeted at achieving the greatest overall improvement while also safeguarding against individual deficiencies. It is also important to ensure that expenditure is predominantly targeted at achieving outcomes rather than lost in over-exhaustive analysis.
Trustpower is a New Zealand based power generation and multi-retail company. Its dam portfolio contains 47 large dams which spans the whole range from low PIC to high PIC structures with a large variety of different dam types.
In 2014 Trustpower collected all available dam safety information on its large dams in a comparative database. All dam safety relevant structures were split into a number of categories for example stability under EQ loadings. Each category was divided into three sub-categories with a resultant total of 2,739 individual sub-categories which were individually rated based on a rating table with 7 ratings ranging from “desirable” to “deficient”.
All new information as it becomes available is being fed into the database and subsequently individual ratings updated as appropriate.
Annually identified tasks get ranked based on a maturity matrix and the tasks that achieve the highest portfolio wide risk reduction costed and put forward for the following’s year budget for execution.
Ryan Singh, Bob Wark
For existing dams built before modern theories and understanding of soil mechanics were fully developed, it was often the case that comprehensive investigations into the properties of the embankment and foundation material were not carried out. With more stringent dam safety requirements and engineering criteria, and a better understanding of soil mechanics, it is necessary to undertake embankment and foundation investigations on such dams, with the view to gain a better understanding of the embankment and foundation conditions.
This paper details the method used for a risk-based assessment of a dam’s stability against slope failure for steady-state seepage conditions, based on a probabilistic assessment of differing interpretations of the material properties for the foundation. To achieve this, several separate interpretations of material strength models were developed for a foundation, using various subsets of available tri-axial data. The mean strengths of these models were used to assess the stability, and to account for the variation in strength properties of each model, the sampling distribution of the mean was used to assess the likelihood of failure.
Finally, an event-tree type risk analysis was used to calculate a value for the probability of slope failure.
A case study has been presented using this method.
K.A. Crawford-Flett, J.J.M. Haskell
Dam inventories can provide a comprehensive understanding of a region’s dam population; from dam quantity, type, age, height, and purpose; to ownership profiling and broad-based regional risk assessment using GIS applications. Historically, New Zealand has lacked a comprehensive inventory of dam assets, instead relying on local and industry knowledge to characterise the dam infrastructure and its key properties, issues, and risks.
This paper presents a cross-sectional characterisation of dams in New Zealand, based on the recent compilation and analysis of a New Zealand Inventory of Dams (NZID). The NZID is the first inventory of its kind for NZ dams, comprising almost 1200 unique structures over 3 m in height. Inventory data was sourced from existing publications, NZSOLD, and regional authorities. The analysis of anonymised inventory data provides an understanding of the number and distribution of assets, along with characteristic physical properties (construction material, height, age, purpose).
Statistical comparisons are drawn in relation to published international dam inventories. Similarities and differences in the international dam populations are noted, particularly with regard to construction era and type. The NZ portfolio is unique in that dams are typically shorter in height, and a significant proportion of structures serve the hydroelectric and energy sectors.
Analysis of the new NZID confirms the need for research that is focused on the long-term performance of aging earth dams, particularly those exceeding 40 years of age. In addition to informing research needs and foci, the new NZID provides statistics on the dam population with far-reaching industry and management applications
Peter Simson, Deryk Foster
Fairbairn Dam is an earth and rockfill embankment dam with an ungated, concrete-lined, spillway, located at AMTD 685.6 km on the Nogoa River, approximately 16 km south of Emerald in Central Queensland.
Following the flood of record in 2011 it was decided to repair a number of areas of spalling concrete which uncovered a collapsed transverse drain and a large void beneath the chute floor. The spillway chute is designed with subsurface drainage system of floor slabs consisting of alternate strips of concrete footing and gravel bed to aid in the control of uplift. The gravel was flushed from under the spillway floor into collapsed earthenware pipes of the drainage system resulting in an unsupported floor slab. Further investigation was carried out using Ground Penetrating Radar (GPR) which identified additional locations of possible voids. Concrete coring was carried out at selected locations to confirm the voids with some being over 250 mm in depth.
Investigation of the sub-surface drains was carried out using CCTV and showed many of the open jointed earthenware collector pipes had cracked and/or collapsed causing the drainage gravel and founding sedimentary rock to be scoured out by spillway flows entering the system through open contraction joints.
Following the discovery of the foundation scouring it was decided to expose a number of anchor bars in the chute floor to undertake a pull-out testing program. Of the ten anchor bars that were exposed, six were found to have corroded completely with the remaining four noted to be partially corroded and subsequently failed under loading.
A geotechnical investigation of the foundation materials was planned to determine the condition and strength of the founding sedimentary rock. In addition, the investigation also included sampling of seepage and reservoir waters to characterise the hydro-geochemistry and its contribution to the deterioration of the anchors.
Artesian conditions also occur within the spillway area, driven by the reservoir, with water passing through an extensive network of pervasive defects in addition to permeable flat-lying strata.
Coal seam gas is also known to occur, providing a further contribution to aggressive water geochemistry.
Mark Arnold, Gavan Hunter and Mark Foster
Following the dam safety risk assessment for Greenvale Dam in 2008, Melbourne Water implemented a 3.0 m reservoir level restriction on the operation of the storage as an interim risk reduction measure. The 3.0 m restriction coincided with the ‘as constructed’ top of the chimney filter in the main embankment. This interim action reduced the dam safety risk to below the ANCOLD limit of tolerability.
Dam safety upgrade works were undertaken in 2014/15 to bring the dam in-line with current risk based guidelines and to enable the removal of the interim reservoir restriction, bringing the storage back to full operating capacity. Greenvale Dam was required to remain operational throughout the works and this required careful consideration of the dam safety risk during construction.
Deep excavations were required within the crest and downstream shoulder of the embankments, that,, without adequate management, had the potential to increase risk to the downstream population. Excavations up to 18 m depth were required into the wing embankments for construction of full height filters from foundation to crest, excavations up to 7 m deep were required in the main embankment to expose and connect into the existing filters and secant filter piles up to 13 m deep were used to connect the new chimney filter of the wing embankments with the original chimney filter of the main embankment.
A key element of the design and construction of the upgrade works was managing dam safety during construction. Dam safety considerations included (i) design based decisions to manage the level of exposure; (ii) implementation of further restrictions on reservoir level by the owner Melbourne Water; (iii) construction methods to manage exposure; (iv) an elevated surveillance regime during the works and (v) emergency preparation measures including emergency stockpiles and 24 hour emergency standby crew. The construction based dam safety requirements were focused on early detection and early intervention, and were managed via the project specific Dam Safety Management Plan.
This paper focuses on dam safety management including the decisions made, actions taken and construction requirements and touches on how these relate to the key project features.