SEEDSMAN GEOTECHNICS PTY LTD ACN 082 109 082 Telephone 0417279556 Facsimile 0248722535 Monday, 10 June 2013 REF: BBUGS2013.docx Mr M Melville Chairman – Bowen Basin Underground Geotechnical Society mmelville@geotechnicoal.com Dear Michael, I refer to “Guidelines for presenters at BBUGS meetings” as downloaded from your website on 8 June 2013, and in particular to the dot point - Reporting negative results of other suppliers’, consultants and/or companies’ products / methodologies are not acceptable Also downloaded from your website on 8 June 2013 was a copy of the 28 February 2013 presentation by Dr Frith entitled “Geotechnical Challenges in a Lower Margin Underground Coal Industry”. Whilst Dr Frith was very careful not to mention my name in his presentation, it should be very obvious to all your membership that the dangerous ideas that he summarised on Slide 55 are all mine. It is my opinion that the statement that they are nonsensical and in fact this whole slide is contrary to the spirit of your guidelines. It is apparent that Dr Frith and I will never agree of the validity of what he calls my “dangerous ideas” and nor should we, as the diversity of opinion encourages debate and the advancement of knowledge. Dr Frith’s opinion (Slide 19) that this divergence of technical views is a barrier to change should be challenged – certainly my clients have the completely opposite view. However, there should not be any disagreement on established facts, be they either specific measurements or the application of standard engineering theory. The Frith presentation Page 1 of 5
contains a number of incorrectly asserted facts. I believe that your membership should be made aware of these instances so that they can adequately assess the validity of some of the Frith conclusions. Slide 23 – Low horizontal stresses in coal. Contrary to Frith’s assertion that horizontal stresses were not measured, the stress field was determined. The Seedsman (2002) paper explicitly states that “The vertical stress was approximately half that expected on the basis of the depth of cover and the major horizontal stress was approximately 0.45 times the vertical” (page 3, Invalid design , 4 th paragraph). Project confidentiality at the time prevented the publication of the full data set. Through Dan Payne (BMA Manager Geotechnical Services), I have been given permission to reveal that the data is that quoted subsequently for Seam A – depth 170m, sub vertical stress = 1.3 MPa, major sub-horizontal stress = 0.6 MPa, minor sub-horizontal stress = 0.3 MPa. A copy of the test report sheet is attached to this letter. Slide 29 – Are there other stress measurements in coal that suggest another story? There are different arithmetic calculations that can be made using the lesser of the “horizontal” and the “vertical” stresses. For the first set, 0.05/0.33 = 0.15, 1.2/0.9= 1.33, and 1.34/3.34=0.40. The ratio of the lesser horizontal stress to the vertical is within the “anomalous” coal stress regime. In addition Dr Frith is quoting the Kirsch solution for a circular hole, whereas a more correct approach would be to quote it for an ellipsoid with a width and height equal to the roadway dimensions (Brady and Brown, Rock Mechanics for Underground Mining, 3 rd edition, page 178). The stress ratio for the onset of tension for such an ellipsoidal roadway (h= 3.5m, w =5.5m) is 0.44. I should also add that since the publication of my compilation of stresses in coal in 2009 I am aware of 2 new data sets – Broadmeadow and North Goonyella - that are totally consistent with the trends seen at GEAP, Moranbah North, Newlands, and Ulan. Slide 30 - Does it fit mining experiences? Frith suggests that the stress regime is something to do with gassy coal. As discussed in Seedsman (2004), the model for the coal stresses is related to the removal of water pressures – all coal seams can be considered to be aquifers. Slide 32 - TG’s with adjacent goaf unload horizontally? At the most fundamental basis, scientists and engineers know that it is never possible to “prove” theories; the scientific method is to seek to disprove theories. It is perhaps of value to refer to the original paper by Shen, King, and Guo (2008) on the tailgate stresses at Ulan. They state on page 679, first column, line 9: “It is easy to understand that the horizontal stress in the direction parallel or 45 o to the roadway axis BBUGS2013.docx, 10/06/13 Page 2 of 5
decreases with mining progress because the longwall mining activity in these directions creates goaf, which releases the horizontal stress” Slide 33 – Poisson’s Effect (Ko) Poisson’s Effect is not a horizontal strain effect, and to apply the effect requires the assumption of no bulging. This may seem to be at odds with conventional thinking in the Australian underground sector but it is certainly correct. Brady and Brown, 3 rd edition, page 143 clearly state that there must be a condition of complete lateral restraint (= no bulging). Or you can refer back to Jaeger and Cook, Fundamentals of Rock Mechanics, 3 rd edition, page 113, chapter entitled Linear Elasticity, section entitled Special Cases, where the so-called Poisson’s effect is clearly stated to be derived for the case of uniaxial strain (no bulging). Frith is correct to note that whether horizontal stress gets generated in the roof is another matter entirely. For this, the appropriate approach is the determination of stress about a hole, of which the Kirsch solution is the simplest analytical step. Slide 36 - TG Roadway Being Monitored The problem here is that the model in the diagram has previously been offered by Frith in the context of Poisson’s Effect. Models invoking elastic stresses about holes do not support the concept of the horizontal stresses increasing. The elastic stress analyses have tensile strains not compressive strains in the roof. Slide 42 – Comments The entire theory does not hang off this single monitoring point. Almost identical data can be found on pages 38 and 39 of ACARP C1301 project report (Gale and Mathews, 1992, Stress Control Methods for Optimised Development and Extraction Options). Slide 49 – Is this a credible response to refute the argument? Dr Frith is selectively quoting from the original data. The margin figure is reproduced in full from Hutchinson and Diederichs and this was presented in full in my presentation to BBUGS last year. Firth only provides the top figure. The fact that there is bond slip with the plain strain cable is not disputed. But inspection of the data for nutcaged and bulbed strands reveal that the full tensile capacity of the cable was in fact mobilised, and these 2 data sets show conclusively that the full tensile load was mobilised for all orientations. There are differences in the stiffness response. BBUGS2013.docx, 10/06/13 Page 3 of 5
Slide 50 – Newton and Roof bolts The statement about mechanical advantage and Newton’s Third Law was actually made by Dr Colwell. I happen to agree with Dr Colwell on this matter. Slide 54 - Roof Bolting Scenarios The diagrams in the figure on this slide correctly show the action and reaction within a roof bolt as required by Newton’s Third Law. However this presentation is at odds with that published in Frith (2000) which clearly shows the upward directed force F without a reaction as required by Newton’s Third law. Note that U v is a deflection. Please note that I have restricted my comments to where the facts presented by Dr Frith need correction. I have not (nor would it be appropriate for me to do so) commented on the many interpretations with which I disagree with. This letter has been prepared on the understanding that you may wish to send it to your members or post it on your website. Yours truly, Ross Seedsman, PhD, FAusIMM, CP, RPEQ BBUGS2013.docx, 10/06/13 Page 4 of 5
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