| Letters
to the Editor: In response to the article, An alternative method for checking the kenematic admissibility of wedges by Dr. Laurie Richards (RocNews Fall 2003), we received the two responses below. This piece concludes with a final comment by Dr. Richards. RocNews welcomes and appreciates responses to our articles as well as to those from contributing engineers and academics in the field of geotechnical engineering. An exchange of ideas and points of view is always interesting to our readers. We look forward to offering more Letters to the Editor in the future. Hi, A couple of comments regarding Laurie Richards suggestion that the well known graphical method for analysing wedge instability, which he discusses in the following paper, be incorporated into Dips. http://www.rocscience.com/library/rocnews/Fall2003/WedgeKinematics.pdf An appreciation of discontinuity orientation data is a crucial part of the analysis and design of rock slopes. Hemispherical projection methods (HPMs), as detailed in Stephen Priest's 1985 book 'Hemispherical Projection Methods in Rock Mechanics' offer a powerful graphical technique for plotting and analysing discontinuity orientation data. Practitioners must, however, appreciate that the proper analysis and design of a rock slope must consider a wide range of rock properties and apply the rigorous principles of mechanics. The influence of complex factors such as discontinuity roughness, cohesion, water pressure, rock support systems, seismic effects and kinematics cannot be accommodated in graphical techniques, nor indeed in direct analytical techniques. Moreover, the application of the friction cone concept has substantial limitations. In addition, HPMs provide little information on the probability for particular FOSs to be achieved as will more rigorous probabilistic based techniques. We have no doubt that Laurie Richards is well aware of these points. For some time we have been concerned that practitioners not specialised in Rock Mechanics are basing their slope designs solely on the use of HPMs. HPMs are very useful for pointing the Engineer in the direction required for the next stage of an analysis; they should not however be used as the only tool for the analysis. Instead the designs should be based on the principles of mechanics, kinematics, vector algebra and probability theory as implemented in programs such as Swedge. Dips is an excellent piece of software for data interpretation and visualisation. If it also becomes a pseudo-analysis tool there is danger that it will be used by some professionals inappropriately which could lead to less than optimum designs. Best wishes, Dr Tony Meyers Senior Geotechnical Engineer and Director Mining Engineering Research Group. Professor Stephen Priest School of Geoscience, Minerals and Civil Engineering University of South Australia Mawson Lakes SA 5095 Dear RocNews: I have just read with great interest the note by Dr Laurie Richards entitled "An alternative method for checking the kinematic admissibility of wedges". The basic concept of using "wedge poles" was described in a 1987 paper by Richards and Atherton (see footnote on page 3) but, at that time, analyses using this concept had to be done manually. The present note demonstrates how this analysis can be performed with the help of the Rocscience program DIPS and suggests that future versions of the program could be enhanced to make the analysis even easier. The method described in this note falls into the category that could be called "filter analysis". This is the type of analysis which is carried out during the preliminary design stage of a project to determine whether or not there are stability issues along the proposed route of a highway or in the proposed slopes for an open pit mine. This type of analysis is extremely important in defining slopes in which are there are no problems so that the limited geotechnical resources available on such projects are not wasted on meaningless data collection and number crunching. Those slopes that are identified as having potential structural stability problems can then be categorized into "deal with now" or "study later" groups, depending on the severity of the problem and the consequences of failure. Severe problems will obviously require more detailed analyses than the simplified friction cone technique which forms the basis of the method described in this note. However, from my own experience, such problems normally comprise only a very small percentage of the total number of potential stability problems identified during a preliminary design study. Laurie Richards is well qualified to write on this topic since his international experience covers a wide range of problems in which this filtering of problems into different categories has been important in terms of the utilization of resources are the focusing of the attention of those involved in the projects. The note is also an important contribution to use of stereographic projections because it demonstrates a simple and elegant method for interpreting a large amount of structural data in which it is easy for an inexperienced user to get lost. I hope that more practical geotechnical engineers will find the time and take the effort to contribute notes such as these from their experience. Yours sincerely, Evert Hoek Letter to RocNews Editor: Tony Meyers and Stephen Priest have commented on my note that was included in the Fall 2003 issue of RocNews. In their letter captioned Hemispherical methods for slope design, they discuss the use of graphical methods for analysing wedge instability. They note that graphical techniques have substantial limitations since roughness, cohesion, water pressure, rock support, seismic loading and kinematics cannot be taken into account and point out that the friction cone concept has substantial limitations. They are concerned that inexperienced practitioners are basing slope designs purely on hemispherical projection methods rather than on the principles of mechanics, kinematics, vector algebra and probability theory as implemented for example in Swedge. They conclude that Dips is excellent for data interpretation and visualisation but is inappropriate as a pseudo-analysis tool. My note was entitled An alternative method for checking the kinematic admissibility of wedges and outlined a method of identifying kinematically admissible wedges by using wedge poles rather than their lines of intersection. There was no suggestion that this should have been interpreted as providing a full-blown analysis of wedge failures incorporating the factors noted by Messrs Meyers and Priest and where the use of Swedge would clearly be appropriate. Detailed analyses using Swedge should not, however, be undertaken until a full assessment of kinematic possibilities has been carried out. For the small data set of 100 points in my example, there are 4945 possible wedge combinations. A detailed analysis of the wedges formed by the means of specific joint sets may give quite misleading results and may not identify the wedge combination that is most critical for a given slope configuration. In some situations, the combination of kinematic checks plus friction cone stability assessment may provide sufficient information for a particular design requirement. This is particularly the case in open pit mines where bench angles need to be designed to minimise daylighting planes and wedges. Messrs Meyers and Priest somewhat surprisingly say that the influence of kinematics cannot be accommodated in graphical techniques. The technique in my note demonstrates how graphical techniques can be used to speedily evaluate kinematic possibilities and therefore identify appropriate failure mechanisms. For mining projects and highway slopes, this may be all that is required for design. For large civil engineering or other high slopes, the identified failure mechanisms may need to be analysed more fully using Swedge or other methods. The comments from Messrs Priest & Meyers and Priest misinterpret the intention of the writer's note and seem to indicate that detailed stability analyses can be undertaken without first carrying out checks for kinematic admissibility. As the title and content of the note indicated that this was concerned with kinematic assessment only, I had assumed that it was not necessary to elaborate on the overall process of data analysis, identification of failure mechanisms and detailed stability analyses. Their comments indicate that it would have been useful to have provided more detail on these matters for practitioners not specialised in Rock Mechanics and it is helpful that Messrs Meyers and Priest have raised their concerns in RocNews. Laurie Richards The Old School House Swamp Road Ellesmere RD5 Christchurch New Zealand Ph: +64 3 329 5663 Fax: +64 3 329 5661 email: laurie.richards@attglobal.net Again, RocNews thanks Dr. Tony Meyers, Prof. Stephen Priest, Dr. Evert Hoek and Dr. Laurie Richards for their contributions. |