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Dear All, The mountain of sand experiment is nice, but not perfect, too many variables affect it to get consistent results. The demonstration cannot be used to determine the internal friction angle of various types of sandstones, let alone a limestone or a crystalline rock. The internal friction angle, as I understand it, explains the difference between the angle of a newly formed shear plane in a rock and the maximum principle stress. This angle is not 45 degrees, which would correspond with the angle between the maximum shear stress and the maximum principle stress, but is 45 degrees minus phi (the internal friction angle)/2. [a=45-phi/2] Phi is a rock property. The relation ship a=45-phi/2 is a very powerful tool to determine for example stress orientations from tectonic structures. Phi is not a constant as is often thought. With low stresses Phi becomes larger and increases to 90 degrees in proper tension situations. This is why the angle between a tension fracture and the maximum principle stress is 0 (zero): [a= 45 - 90/2 = 0.] The best way to determine Phi is to do a series of tri-axial tests at different confining stresses. The resulting series of Mohr circles at failure, connect to the Mohr/Coulomb failure envelope, the slope of this envelope is the internal friction angle Phi. The tensile strength can conveniently be estimated using the Griffith fracture criterium, according to which the tensile strength is half the cohesion. The cohesion was obtained with the triaxial test results. Surprisingly, phi is close to 30 for most rocks. Due to the convenient relationship a = 45-phi/2, estimates of phi do not easily result in big errors, as the estimates are divided by two. Regards, Dirk Nieuwland On Sep 28, 2011, at 11:00 PM, Faramarz Nilfouroushan wrote: > Hi Simon, > > I am just wondering if angle of internal friction defined as angle of repose, then it will be function of basal friction which is the friction between sand and the flat surface (here referred as External friction). Therefore, if we measure the angle of repose on different surfaces with different basal friction angle, we will end up by different angle of repose (referred as angle of internal of friction) for the same sand . Is it right or I am missing something? > > Best Regards > Faramarz > > On 28 September 2011 22:30, Simon Woodward <[log in to unmask]> wrote: > Hi Aydin > > Angle of internal friction can also be thought of as the angle of Repose. > > ie, take a dry granular material like sand, and pour it onto a flat surface. The cone of sand will form up at an angle of repose determined by the internal friction of the material ie the friction between the particles. > > By External Friction, I infer you mean the friction between say that same sand and another material, such as the rear face of a concrete retaining wall. This will obviously be greater than the friction between that sand and something smoother like say glass. > > ============== > > Best regards, > Simon Woodward > Chartered Professional Engineer > > > > -- > --------------------------------------------- > Faramarz Nilfouroushan (PhD) > > Research Associate > Department of Earth Sciences > Uppsala University > Villavägen 16, SE 75236 > Uppsala, Sweden > Tel: +46 (18) 4712559 > http://www.geo.uu.se/mpt/default.aspx?pageid=1523&lan=1 > http://www.researcherid.com/rid/A-7630-2008 > > >