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Curtin University (2011)

Effect of climate change on the performance of clay core dams

Nasir, Ahmad Bilal

Titre : Effect of climate change on the performance of clay core dams

Auteur : Nasir, Ahmad Bilal.

Université de soutenance : Curtin University

Grade : Doctor of Philosophy (PhD) 2011

Résumé
Hydraulic fracturing can occur in the clay core of an earth and rock-fill dam if the vertical effective stress in the core is reduced to levels that are small enough to allow a tensile fracture to occur. This situation may arise if the total stress and strain in the core is increased by the "loss of moisture" and the core settles relative to the rock-fill shoulders of the dam. During first filling of the dam, the water pressure increases in the core and reduces effective stresses further, and if they reach low enough values, a fracture will occur. A proper design of earth dams requires an additional resistance to hydraulic fracture (especially those dams with thin vertical or near-vertical central cores), as there have been several dam failures in the past that have been attributed to hydraulic fracture. This thesis presents a method to calculate loss of moisture content in the clay core of earth embankment for early prediction of hydraulic fracture. The model was developed by using a numerical procedure based on the finite element method. The finite element procedure makes use of special joint elements that allow fluid flow and fracture to be modelled which is well advanced, over previous methods. The commercially available software PLAXIS ; was fully capable for producing a heat model that allowed changes in the moisture content in the core of a dam. A study of the behaviour of five selected dams ; Beardmore, Bjelke-Petersen, Coohnunda, Teemburra and Blowering Dam was conducted on the assumption that the long term drought in Queensland and New South Wales may initiate hydraulic fracturing on clay core dams. A non-linear finite element analysis ofloss of moisture content and its behaviour was carried out under plane strain conditions, simulating its construction, reservoir filling and steady seepage loading stages. Material parameters are determined from triaxial tests on the rockfill, core, filter and transition materials, compacted to field dry density and moisture content. All materials show non-linear stress-strain behaviour, with rockfill exhibiting work softening at large strain. Mohr envelopes for rockfill are slightly curved but linear approximations are acceptable. When compared with stresses and deformations at maximum cross section, the analytical results show good agreement in most areas. A non-linear model for pore pressure response was used successfully to predict the high pore pressures in the lower region of the core. A stability analysis performed for the end of construction stage, based on finite element stresses, shows that the design safety factors are slightly conservative. The current research has provided a valuable interpretation on the loss of moisture content and its effects on a large earth and rockfill dams due to climate change. The loss of moisture and highest heat occurred in the top thirds of the dam embankment core which happens to coincide with where the designers sought to place earthfill materials with the greatest water content with respect to optimum moisture. A base model was prepared using average material propelties of the five selected dams. The predictions from the numerical model were significant and closely relate to the field parameters. The numerical model was further developed for each individual dam and loss of moisture content was predicated specific to the local conditions. Time histories of excess pore pressure, accelerations, and the displacements data from Sun Water and State Water were compared, in order to understand the effect of climate changes on the clay core of embankment dams. For the dam displacements (lateral and vertical), the difference between the computed and the recorded results were within 25% to 35%. The computed loss of moisture content and pore pressure matched reasonably well with the available data at most locations. The principal finding of the research is that the change in the climate condition does not seriously affect the loss of moisture content in the clay core of an embankment dam if it is constructed according to the design and with proper quality control. The maximum recorded loss of moisture content at the top layer was between 30 - 35%. These limits are within the safe range and the probability of initiation of hydraulic fracturing is very low. The res1 !lts suggested that the rock fill materials on the upstream and downstream slopes of the embankment dams have been acting as an insulator. The grading and settlement of rockfill acted in such a manner that suited to control the effect of climate changes.

Présentation

Page publiée le 29 novembre 2012, mise à jour le 1er juillet 2017