PERMFILE55590 - Laserfiche WebLink

Home Browse Search

1 ~ -5- ' (iv) Rating for SD3CinQ of Discontinuities ' As for the previous parameters, t.'~e discontinuity spacing is determined from the "handled" core. For care with sticks of R1 or harder material and no weakly cemented joints there is little difference between the handled and unhandled values. The rating for handled core reduces where sticks of soils like hardness can be broken up by hand. ~: ~. ,.. (vl RatinQ for Condition of Discontinuities ' This rating is unchanged except that all core with a rock material hardness of less than R1 is assigned to the lowest category with a rating of zero. This prevents the assignment of ' high rating values to extremely weak rock material (SS say) with clean closed rough joints. Rack material with a hardness of R1 has a maximum rating of 10. ' METHOD OF APPLICATION OF SRR-RMR FOR ROCS SLOPE STREINGTH ESTL*SATION Core taken from very weak rock masses tend to be very ' variable as illustrated by the typical core sample on Figure 2. ` Portions of a rock core may have SRiR.R ratings of 50 or higher and be immediately adjacent to portions having ratings of 20 and ' lower. A method of logging and analysis is required to determine "average" rock mass strength but which distinguishes substantial zones of weaker mass from zones of stronger mass, ' as, for example, in the cored borehole illustrated in Fic;-ure 3. The practice used by Steffen Robertson and Rirsten .,s to ' assign a SR.~t to each one foot interval of the core. Th:,.s assignment can be done rapidly once representatwe sections of the core have been SRFC-~ rated in detail, and the core logger has developed a set of 'reference' ratings. A profile of rock mass rating values is then prepared by calculating the mceving ~ 'f average of the SRMR values for a 20 section of the care. A typical profile is illustrated in Figure 4. Also shown an this ' figure is the standard deviation of the SRMR values deterained Por each 20 foot string of values used to calculated the moving average. ' A failure surface passing through a weak rock mass will tend to be selective and pass preferentially through material which is weaker than the average encountered in any borehole core. This selective process is illustrated in Figure 2. Thus it is appropriate to use an SRMR value, less than the average value, to be representative of the material along the failure surface. The practice developed at SRIt is to subtract one standard deviation of the rating values from the mean. This value of the SRMR is also shown on Figure 4. The significance of this reduced value of the SRMR is that approximately 8+~% of ' core encountered in the 20 foot sample length will have rating values greater than the reduced value. 1