PERMFILE108040

• Permit No: C1981019
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: Sections 10 - 16
• Section_Exhibit Name: Exhibit 12 Experimental Part 3
• Media Type: D
• Archive: No

- 2-
Case I - Spoil angle of internal friction (p) •
Holding all other variables c
(a) ~ichanged from 390 to 350
(b) pchanged from 370 to 350
Case II - Spoil density ( ~' )
(a) x'changed from 105 to 125
(b) 1S chanced from 115 to 105
(c) ~ changed from 115 to 125
instant, we r"i nd
v FS= -4. 1
oFS= -1.8%
pcf oFS = -5.9%
pcf oFS = -r3.4%
pcf ~ FS = -2.4%
For these two variables, ~.ve find the maximum change in .the factor of safety
of 10.0 percent. By using the mid-point of the range of variables, !i.e.,
0 = 37 degress and ~'= l15 pcf), the maximum variation in the factor of
safety is reduced to the range of -4.2% to +1.6%. Considering the accuracy
of the known data we consider this range do be satisr`actory and indicative
that these variables do not signir"icantly influence the stability for the
conditions studied. The remaining variables, namely the relative location
of the r•rater table and angle of internal friction of the colluvium are con-
trolling factors in the analysis, as stated in the report. Analyses v!ere
conducted for four water table locations and both colluvium angles of in-
ternal friction. ~ '
The analysis was prepared io stuoy a wide range of possibilities and then •
make a judgment based on the results. Simple comoarision of numerical values
of the computed factor of safet~r is not pepper evaluaticn. The engineer and
she r•evi.e~.ver must look beyond the numeers and evaluaie the physical meaning
of the analysis. In the case of the Colowyo slope stability problem, all the
analyses indicate a factor of safety of more than 1.5 except for high ~•aater i
table conditions. The implication is that the slope is probably stable ender
dry conditions, but should a high water table develop, there may be failures.
6y carefully examining the critical circles presented, rde find the ground
water will cause small slips at the toe, mostly within the spoil. Experience
shows this to be reasonable, especially for granular material where the
strength depends entirely on the normal forces. The addition of water reduces
these farces and causes the failure. Further examination of the water table
cases, r•rhere the strength of the colluvium cnanges from 26° tp 400, indicates
the failure mode chanq_es from a shallow seated toe slip circle to a deep
sewed failure ',vith a much higher factor of safety. The interesting feature
is the location of the critical circles. As in Case 4 (lo~~rest '.rater table,
colluvium ki = Z60), all the critical circles for fall mostly within the
spoil and the bedrock and do not lie '.vithin the colluvium. Considering the
probable strength of the bedrock, it is our opinion that a deep seated failure
is unlikely. The analysis basically indicates for dry conditions the slope
is table and that failure is most likely to occur in the spoil if a high water
table develops.