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197_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011

197_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011 -...

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SL23 Critical Slip Surface - Minimum Factor 01 Safety a Analysis of circular faHure surfaces must be repeated for different circles until the-critical lip sudace is found, i.e~the:slip surface with the smallest factor ofsafety. a First choose a radius value or fix a starting point for the slip circle (say the toe). o Then develop a contour map, using a grid, which shows the calculated safety factor for each slip circle plotted at the circle center (similar to an elevation contour plot) o Identify zone of minimum safety factor from FS contours. Moment axis "b" (FS '" 1.5 lor momont aXiS\"b") Moment axis "a" ., (FS = 1.1 for momont rb •••• \ ....... ' IDeIS "a") ~ C:;-~~'""T''''-','~' "'"" ... ~ .' , . , , \ :"a . , , . , . .. .. " , Slip surface fO( moment axis "a" Most critical slip surface (least FS) , . . , , . Moment axis for slip surface having least FS (a) location of moment axis arlecls FS (b) Use of grid 10 establish FS contours to help Iocato most critical slip surface Sand
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Unformatted text preview: Sand Drain Effects of moment axis location on calculated factor of safety and melhod to establish mOst critical slip surface. o Choose another radius or staring point and repeat the FS contour map. o From a series of FS contour maps, identify a critical zone where FS is minimal. o Calculate the minimum safety factor for a slip circle centered in this critical zone. Typically require FSmin = 1.5. o Computer programs use a search technique to locate the critical zone. • Water Pressure by Flow Nets (use with method of slices) o Draw slope to scale and divide into slices o Construct tlow net with tlow lines and equipotential lines (they are perpendicular) ~-.. \ . ..... '/- FlowLines .. "(. . \ f .. \ \ \..-~ __ J,--~ Equipotential Lines , ,/1/1'_ ~~::;;'0>;r};;N;:~;.: l I I __ .J - - -----_.--_ ..----...
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