They are the same material and therefore will have a coefficient of friction of 1. The smoothness is something that you will need to judge. Conservatively you could try a value of 0.75 or less.
Although I don't disgree with the coefficient of friction of 1 for rough stone on rough stone I do disgree with the premise that the coefficient of friction between two of the same material is always 1. According to Mark's handbook for mechanical engineers here are some tested static coeffients of friction for same materials. I am sure other references will give other numbers.
hard steel on hard steel 0.78
Aluminum on Aluminum 1.05
Teflon on Teflon 0.04
magnesium on magnesium 0.6
oak on oak parrall to grain 0.62
oak on oak perpendicular to grain 0.54
cast iron on cast iron 1.10
I cant argue with your figures except I dont understand how the cof can be greater than 1? I found this definition: -
The coefficient of friction is an empirical rule that is associated with the force required to move one object rubbing against another relative to the force with which the two objects are being pressed together. The rule
is that the required force for motion is linearly proportional to the normal force, and the ratio between the two is the coefficient of friction (always between 0 and 1).
There are different coefficients for different types of motion - there is a coefficient to get the motion started, and another to keep it going, and yet another associated
with "rolling" as opposed to sliding.
Friction comes from the electrical interactions between the two surfaces at the level of the atoms and molecules, and can often be significantly reduced by interposing a liquid (a lubricant) between the two surfaces, because then the upper surface slides on a layer of lubricant which can move freely over the other layer of lubricant
attached to the lower surface.
I don't think the coefficient of friction has to be less than 1. By the way, for cof=1, the sliding angle is 45 degrees. In other words, if you tilt something on a surface until it starts to slide, the angle will be 45 for cof=1.
If this is all about building a stacked stone wall then a batter of 1:6 will be fine, ie. 6 vertical and 1 horizontal.
Consequently I'm not sure the coefficient of friction is going to come into the problem, since you now have a "gravity wall", ie. it's leaning backwards and as long as any loads from behind, vehicle impact, water pressure, direct UDL on top etc. etc. are less (by a factor of safety of about 2) than the wall reaction, then you'll be OK.
Calculate the above loads v's the actual weight of the wall. Anthony Tugwell
Project Director & Consulting Engineer - now in Australia
