Yes it is possible for the drained FOS to be lower than the undrained FOS. In may soils, the drained FOS is usually lower, sometimes much lower.

Slopes should be checked for all conditions that can occur. Sometimes drained parameters will not apply. For example an excavation only open for a few days to a few months depending on the material.

Mike Lambert

Thanks for your opinion. This time the type of soil is sensitive and at the toe of slide there is debris. When we continue our calculation, is it reasonable to regard debris as undrained materials and just use undrained shear strength (whcih is shown in the paper) and unit weight

What I "try" to remember is that all slopes should be analyzed using effective strength parameters. The reason that one uses the undrained parameters, Su for instance, is that it is very difficult if possible at all, to accurately determine the porewater pressures at any one point in which to apply to the effective stress parameters. Secondly, heavily overconsolidated soils, when cut, will want to suck in water (the stress path is to the right of the drained stress path) and the strength will decrease as it approaches the drained effective stress condition. Similarly, for normally consolidated clays, the strength will increase as the water is expelled.

How sensitive is the soil? What is the natural moisture content vs the liquid limit? You said that you were using residual undrained shear - do you really mean remoulded undrained? The two analyses alluded to above suggests that your "undrained" remoulded (residual) shear strength, Sur, gives a condition that does not necessarily reflect the actual soil conditions because of the porewater pressures noted. For me, the author for his effective stress analysis had a groundwater (porepressure) regime that was set but wouldn't be equivalent to one that is presumed in the Su analysis.

You could use undrained conditions for the debris but is should be lower in strength values due to the fact that it is "uncompacted" . . .

The question I would ask is "has the debris been in place long enough to consolidate under the imposed load?". If it has then drained parameters apply. Otherwise the actual strength will be somewhere between drained and undrained depending on the amount of excess pore pressure that has disapated.

Note that the above is simplified, but hopefully you can apply the thought process.

Mike Lambert

It is possible but you would normally get higher safety factor for the effective stress analysis (ESA) because of the porewater pressure(PWP).

PWP used for ESA are normally based on historical data from other stable dams which are not affected by large embankment shear strain, while the the pore pressure for the total stress method (undrained strength) are those measured in the soil at failure i.e., during laboratory testing. Of course, this will also depends on the soil type, you may not see much difference in safety factors for fine grained clayey soils assuming that you use a reasonable pore pressure estimate for the ESA...

As BigH rightly pointed out, for stability analysis, I preferred to use ESA at least for dams...

for the case of stiff-fissured clay, effective stress design would likley include the use of fully-softened shear strength and zero cohesion. So, you could end up with a friction angle of 24 degrees and zero cohesion. for infinate slope design you could also consider steady state pore pressures with saturation to the ground surface.

Contrast that to what could likely be an undrained shear strength of 5 tsf and in the near surface, the drained analysis would likely be more critical.

f-d

¡papá gordo ain’t no madre flaca!