Mass Wasting and Slope Stability (Continued) amazonaws com

Mass Wasting and Slope Stability (Continued) -For a particular slope that you are worried about, carry out a safety factor analysis 1. Slope steepness This determines the Probability of failure of that slope (Ph) -Safety factor: a measure of stability of the slope Safety factor= (resisting force)/(driving force) SF=N/T -W=weight, N=resisting force, T=driving force. The longer T gets, the shorter S gets until angle alpha get to 90 and N will equal zero -sin(alpha)=T/W -cos(alpha)=N/W 2. Nature of the material -Soil/aggregates (loose stuff, debris), or solid rock/ice -How much friction? -how much internal strength? -Normal stress vs shear stress (Review GEOL 104) -S is directly proportional to N; the bigger N is, the bigger S will be -Phi=agnle of internal friction, C=cohesion -Tan(phi)=a/b (opposite over adjacent)=the slope of the hypoteneuse, or slope of the line Tan(phi)= coefficient of internal friction -C and than(phi) characterize the strength of the material; it is a straight line! -So, the strength is C+(N*tan(phi)) But, replace N with w*cos(alpha): total resisting force=C+(w*cos(alpha)*tan(phi)) SAFETY FACTOR= [CA+(w*cos(alpha)*tan(phi))]/(w*sin(alpha)) A in this equation is the area of contact between the potential failure block and the potential failure plane beneath it Safety Factor Measures -Safety factor greater than 1 means that the slope should be stable, but real answer is maybe/maybe not -Ex: if safety factor of 1.0001, would you be 100% confident that it was stable? -SF of 1.0 = 50% probability of slope failure -SF of 1.5 = 5% probability of failure -SF of 2.0 = 2% probability of failure -Graph of probability vs safety factor is the opposite of an exponential graph

-What safety factor would you be happy with? depends on the situation in an open pit mine would probably be willing to accept greater risk, SF closer to 1 ON a highway cut rock where cars and school buses drive, much more conservative (willing to accept less risk), SF greater than 3 -So, we can now carry out safety factor analysis on slopes to determine how stable/unstable they are