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Hi Guys please keep this going for no rational reasons I have avoided joining geo-tectonics and so I have only just joined and it has been really helpful coincidentally I started my level 3 (UK) module in Engineering Geology for this year and I am getting directed to lots of useful web based material which undergraduates tend to believe in preference to me as regards to experimental work I would offer the observation that "good sand" is wind-blown sand the grain size and or grain size sorting produces material that behaves in a repeatable way wet or dry! Graham Potts ________________________________ From: Tectonics & structural geology discussion list [[log in to unmask]] on behalf of Scott Johnson [[log in to unmask]] Sent: 29 September 2011 13:00 To: [log in to unmask] Subject: Re: Thanks! and another question The angle of internal friction is a constant material property. It is only measured by the repose angle in the case where no shear stress is applied to the base. So, slowly build a pile of sand, and at the point where the surface is at failure – that slope is the internal angle of friction. If you push the pile along you will form a wedge, and the slope of the wedge will be lower than the angle of repose and will depend on the basal angle of friction and the slope of the interface. When the wedge reaches the critical angle, it will continually deform internally and along its base to maintain that angle. Annoyances like focused erosion, material strength heterogeneity, etc in the Earth lead to variations of course. I attach a portion of a lab I use in a class here to show the equations relating the various angles and frictional properties. You can also find some good sand (and other) experiments here (careful, they are student projects that contain some errors): http://www.geology.um.maine.edu/geodynamics/AnalogWebsite/html_files/crustal%20dynamics.html There is a vast literature on critical wedges. You will finds lots or references in the projects at the URL above. Cheers - Scott ********************************************** Scott E. Johnson Professor and Chair Department of Earth Sciences 5790 Bryand Global Sciences Center University of Maine Orono, ME 04469-5790 USA email: [log in to unmask]<mailto:[log in to unmask]> phone: (207) 581-2142 fax: (207) 581-2202 web: www.umaine.edu/earthsciences/faculty-staff/faculty-and-staff/scott-johnson/ ********************************************** From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Gines, Jorge Sent: Thursday, September 29, 2011 6:58 AM To: [log in to unmask] Subject: Re: Thanks! and another question Dear Simon, Faramarz and Aydýn Very interesting topic, especially the effect of the basal interface on the repose angle. I think we can expect the internal friction angle to be different to the repose angle, and I reckon it may be higher. I understand that the angle of repose should be related with the equilibrium of the interface between a solid and just the atmosphere around it (so, unconfined), whereas the angle of internal friction should represent the equilibrium between grains in internal interfaces (in a confined environment). So, basically, they represent an equilibrium situation of two different types of interface. Probably for practical purposes these values are very close, but I think the fundament of them is different, although in my opinion the analogy is valid and very graphic. I think that could be extended to explain the repose angle as a special type of "external friction angle". But I am out of my depth here (and probably from the very beginning!), and I am also curious now about this. Regards, Jorge Jorge Ginés Geologist, MSc Fugro NPA Limited Telephone: +44 (0) 1732 865023 / Fax: +44 (0) 1732 866521 E-mail: [log in to unmask]<mailto:[log in to unmask]> / Website: www.fugro-npa.com<http://www.fugro-npa.com/> Crockham Park, Edenbridge, Kent TN8 6SR, United Kingdom