A proposed framework for characterising uncertainty and variability in rock mechanics and rock engineering

This thesis develops a novel understanding of the fundamental issues in characterising and propagating unpredictability in rock engineering design. This unpredictability stems from the inherent complexity and heterogeneity of fractured rock masses as engineering media. It establishes the importance of: a) recognising that unpredictability results from epistemic uncertainty (i.e. resulting from a lack of knowledge) and aleatory variability (i.e. due to inherent randomness), and; b) the means by which uncertainty and variability associated with the parameters that characterise fractured rock masses are propagated through the modelling and design process. Through a critical review of the literature, this thesis shows that in geotechnical engineering – rock mechanics and rock engineering in particular – there is a lack of recognition in the existence of epistemic uncertainty and aleatory variability, and hence inappropriate design methods are often used. To overcome this, a novel taxonomy is developed and presented that facilitates characterisation of epistemic uncertainty and aleatory variability in the context of rock mechanics and rock engineering. Using this taxonomy, a new framework is developed that gives a protocol for correctly propagating uncertainty and variability through engineering calculations. The effectiveness of the taxonomy and the framework are demonstrated through their application to simple challenge problems commonly found in rock engineering. This new taxonomy and framework will provide engineers engaged in preparing rock engineering designs an objective means of characterising unpredictability in parameters commonly used to define properties of fractured rock masses. These new tools will also provide engineers with a means of clearly understanding the true nature of unpredictability inherent in rock mechanics and rock engineering, and thus direct selection of an appropriate unpredictability model to propagate unpredictability faithfully through engineering calculations. Thus, the taxonomy and framework developed in this thesis provide practical tools to improve the safety of rock engineering designs through an improved understanding of the unpredictability concepts.Open Acces

A comprehensive approach to real time power cable temperature prediction and rating in thermally unstable environments

Electricity utilities are being forced to maximise the use of their major assets while maintaining or improving availability of supply. Most urban environments have ageing underground cable networks that are extremely expensive and disruptive to replace. Original steady-state ratings are often being approached or even exceeded. As is generally known, transient rating methods that acknowledge that load transfer is usually less than the peak value on which steady-state ratings are based can give extra transfer capacity and thus extend the useful life of cables. Real-time temperature prediction based on actual loads can increase the utilisation of cables still further. It is imperative, however, that real-time temperature prediction methods allow for changing environmental parameters such as overall moisture content, the movement of moisture away from highly loaded cables, ambient temperature and the effect of external heat sources, because the inherent safety margin of steady-state rating is lost. This thesis takes a direct and inclusive approach to these issues, using a real-time formulation of a summation of exponential terms to provide a simple but consistent framework to model cables and their installed environment. Methods are given to extend the use of a thermal ladder circuit to cover the entire environment rather than just the cable itself because the nodal solutions in the environment support the prediction of moisture movement in a transient adaptation of the 2-zone approach to moisture migration used in the standards, where the backfill or native soil surrounding cables is assumed to dry when a stipulated critical temperature rise has been exceeded. One feature of the work is that the movement of moisture can be slowed down, an especially important attribute when cables are cooling after extended high temperature operation. Measurements from a cable-scale heating tube validate this approach. The main content of the thesis is implemented in an algorithm that consists of two parts. The first part analyses the environment of a buried cable system and generates the governing exponential equations. The coefficients and time constants of these equations consist of moisture and moisture migration dependent polynomials. The second part of the algorithm consists of the real-time implementation, with full dependence not only on the position of the critical isotherm delineating dry from wet regions during moisture migration but also the overall moisture content of the environment. The algorithm is validated by comparison with Finite Element Method simulations and standard based computations. The thesis also contains overviews of how the approach can cope with installations in composite plastic tubes and external sources. While the main application of the algorithm is to predict conductor temperatures in real time from current measurements and a realistic knowledge of the thermal environment of a cable, it is realised that operating margins can be more safely reduced if temperatures are monitored. A full thermal analysis of the installed cable system can lead to highly accurate algorithms predicting the conductor temperature from current and surface or sheath temperature measurements, and these plus less accurate but 'universal' algorithms are also presented, as a development of earlier work.reviewe