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• <br /> greater the number of N <br /> discontinuities a cluster <br /> contains, the greater the <br /> likelihood that a failure will - <br /> involve members of that =:_ ~ =~ <br /> cluster, given the opportunity: <br /> A :dope should thus be designed 3 - <br /> to minimize unfavorable <br /> orientations with respect to <br /> large or dense clusters. Slope w --- E <br /> investigators must keep in mind ---•:====:=- z i~~~• <br /> that the results of population ~ ` =- <br />`_`-= '~ ^~~' =° <br />~ <br />'E` ~ <br /> contourin are heavil <br />g Y _ <br />' <br />"`- <br />`~f _::_ <br /> dependent on data collection - - <br /> procedures. Biases may be <br /> unknowingly introduced by the. •~'~~ <br /> orientation of the slope and by S ;i.:: <br /> collection methods. For <br /> example, discontinuities <br /> <br />a;rallel or nearl arallel to <br />P Y p 5 <br /> the slope face are often under- . ar rtn~a.. • wo <br /> represented in the data as they Fiqure 6. Computer projection of <br /> do not appear in the face as contoured data as poles. <br /> often as discontinuities <br /> perpendicular to the slope. Yet, those which parallel the face <br />• of'ren control the stability. <br /> D. DISCONTINUITY SIGNIFICANCE ANALYSIS <br />Watts' Discontinuity Significance Index (DSI) is a rating of the <br />importance of each discontinuity in a rock mass to the stability <br />of any slope that could be cut into the rock. The concept is <br />described in detail in Watts and West (1985) and Watts and West <br />(1986). In some cases, one major discontinuity (like a fault) can <br />be more important to the stability of a slope than a large, dense <br />cluster of rough joints; yet, they all appear equal as "dots" on <br />a :standard plot. On plots where DSZ values are indicated in some <br />manner, a highly significant discontinuity will stand out clearly. <br />It may or may not lie hidden within a cluster of other <br />discontinuities. DSI values are extremely valuable for comparing <br />discontinuities at a site. They are less useful when comparing <br />between sites. <br />One simple type of DSI may be estimated on the site as data are <br />co:liected. The slope investigator records a value based on his <br />experience and on his understanding of the roles of various <br />discontinuity characteristics. In this exercise, simple DSI values <br />of 1, 2, or 3 will be assigned based on specific parameters in the <br />daL'a set, Although extremely useful, it is important to remember <br />that DSI values are subjective. <br />More precise discontinuity significance indices can be calculated <br />• directly from the field data if appropriate parameters have been <br />recorded. Parameters such as dip value, discontinuity length, <br />Stereonet Analyses 7 Appendix A <br />/h R-3 y <br />/~PPRoveD 3/24/oy <br />