Since their development, rock mass classification systems have used and manipulated various populations of geomechanical data to allow a rock mass to be divided into different domains or engineering ‘masses’ with the aim of assisting in the geotechnical design of underground openings, excavations, foundations and ground support systems.
Each of these methods consider different characteristics to generate a material classification; including rock strength, joint weathering, defect spacing, in-situ stress and groundwater. However, none of these systems cater for classification of the rock mass based on whole rock weathering, whole rock strength and incipient defect spacing along a borehole.
This new classification system, the Rock Condition Number (RCN), has been developed to reduce the human factor of variability in interpretation when collecting data to classify the rock mass, as other methods, such as Rock Quality Designation (RQD), are prone to significant variability based on the experience of the person logging the core. RQD provides an indication of rock quality over the length of the cored interval, which varies depending on the drilling equipment and ground conditions. This value may typically be calculated over an interval of 1.0, 1.5 or 3.0 metres. The RQD system does not allow for the rapid identification of thin, though important features in the subsurface.
Using data captured electronically in the field, the RCN calculates an instantaneous classification of the rock mass at any point along the borehole, highlighting variations within the rock mass by assessing a combination of characteristics, allowing rapid identification of potential hazardous zones within the rock mass. This allows for significant improvements in efficiency during the assessment and design process/es. Resolution is greatly improved over RQD, with thin, though important, zones of weak material highlighted using this new process.
Comparison between existing classifications and the RCN using real field data indicates the RCN provides greater resolution when identifying deficient zones within the rock mass.
Keywords: Rock mass characterisation, RQD, Rock Condition Number, rock quality, dam foundations.
J.H. Green; C. Beesley; C. The and S. Podger
Rare design rainfalls for probabilities less frequent than 1% Annual Exceedance Probability (AEP) are an essential part of spillway adequacy assessment as they enable more accurate definition of the design rainfall and flood frequency curves between the 1% AEP and Probable Maximum events.
Estimates for rare design rainfalls were previously derived using the CRC-FORGE method which was developed in the 1990s. However, as the method was applied on a state-by-state basis, there are variations in the approach adopted for each region. Differences in the cut-off period for data, the amount of quality controlling of the data undertaken, the base used for the 2% AEP estimates, gridding settings and smoothing processes have created inconsistencies which are particularly apparent in overlapping state border areas.
The Bureau of Meteorology has derived new rare design rainfalls for the whole of Australia using the extensive, quality-controlled rainfall database established for the new Intensity-Frequency-Duration (IFD) design rainfalls. These data have been analysed using a regional LH-moments approach which is more consistent with the method used to derive the new IFDs and which overcomes the limitations of the spatial dependence model in the CRC-FORGE method. In particular, the selection and verification of homogenous regions and the identification of the most appropriate regional probability distribution to adopt relied heavily on the outcomes of the testing of methods undertaken for the new IFDs. However, to focus the analysis on the rarer rainfall events, only the largest events have been used to define the LH-moments.
Keywords: Rare design rainfalls; Intensity-Frequency-Duration (IFD); Annual Exceedance Probability
Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
Chris Topham, Andrew Pattle, David Tanner, Oliver Giudici
Many owners around the world have dams that rely on grouted, post-tensioned rock anchors for stability. The anchors were installed during the original construction of the dams or retrofitted to improve stability during their operational life. The use of fully grouted post-tensioned anchors spanned the period of the 1960’s to 1980’s. The main issue with these un-sheathed grouted rock anchors is the question of integrity of the grout column protecting the anchor and concerns about possible corrosion of the high tensile wires from which the cables are constructed. While some of these anchors have corrosion monitoring systems installed, it is difficult to validate such data and there is considerable uncertainty over the condition of such anchors. To compound the problem, replacement of the anchors is technically complex, extremely costly and difficult to justify in the absence of known condition. For example, Hydro Tasmania has recent experience of work to cease reliance on such anchors at Catagunya Dam that cost $41m in 2009. With fifteen dams relying on some form of post-tensioned anchors, Hydro Tasmania has recently taken the unusual step of over-coring and extracting three post-tensioned rock anchors from operating dams in order to assess their condition. In what is believed to be a world first, a 42m long 70 strand high tensile anchor was overcored and removed from Meadowbank Dam in 2014. A further two anchors were successfully extracted from Repulse Dam in 2015, in conjunction with a group of international sponsors with similar anchors. This paper uses the 2015 work to illustrate the methodology used to extract the anchors, outlines the information gained from this unusual work, and presents the results of the condition of the extracted anchors. The paper concludes with some inferences for other owners with similar anchors and suggestions for further work.
Keywords: Grouted, post-tensioned rock anchor, ground anchor, corrosion, over-coring, extraction, dam safety.
T. I. Mote, M.L. So, N. Vitharana, and M. Taylor
This paper explores the sensitivity of selection of earthquake design magnitude to liquefaction triggering in Australia for ground motions typically used for dams. The low seismicity of Australia creates a situation where liquefaction triggering is marginal at design hazard levels and this low level of seismic hazard makes the liquefaction trigger analysis very sensitive to the derivation of the seismic inputs. A methodology is presented that couples the probability of liquefaction triggering with the distribution of earthquake contribution to the hazard from the magnitude-distance deaggregation. The results show that for the “typical” soil profile and input ground motions approximately equivalent to the maximum design earthquake for Australia, the probability of liquefaction triggering varies significantly with the design magnitude selected. Using the maximum credible earthquake or mean magnitude may provide significantly different liquefaction triggering implications. Combining the probability of liquefaction triggering with the contribution of varying magnitudes to calculate liquefaction probability is a useful method to understanding the sensitivity of liquefaction to design magnitude.
Keywords: Liquefaction Assessment, Design Magnitude, Probability of Liquefaction, Magnitude-distance deaggregation, Australia
Susantha Mediwaka, Nihal Vitharana, Badra Kamaladasa
Nalanda dam is the oldest concrete gravity dam on the Island built in the 1950s by the Ceylon Department of Irrigation. The dam was built in 9 monoliths having a dam crest length of approximately 125m and a maximum height of about 36m. The spillway consists of: (1) a low-level uncontrolled ogee-crested horse-shoe section with a crest length of 46m, and (b) a high-level broad crested weir with a crest length of 43m.
It was designed and constructed according to the then standard practices adopted throughout the world. Over the years, Nalanda dam has been showing signs of deterioration which is suspected to be Alkali-Aggregate Reaction (AAR). The dam was also shown to be deficient with respect to the stability levels required by modern standards. Under a program of dam safety improvement of the dams throughout Sri Lanka, it was decided to stabilise Nalanda dam as the first step in addressing a series of issues affecting the dam.
This paper presents the construction history, current issues, design assumptions and salient construction features in the upgrading of the dam to modern dam safety requirements.
Keywords: Concrete dams, dams Sri Lanka, concrete buttressing, upgrade, horse-shoe spillway