A history of slope stability safety factors
A history of slope stability safety factors
(OP)
Hello again,
I am looking for a good summary on origins of FOS for short and long term slope stability. What is the main reason for the "short-term" FOS to be lower than the "long-term" one?
Thank you in advance,
CEMAB
I am looking for a good summary on origins of FOS for short and long term slope stability. What is the main reason for the "short-term" FOS to be lower than the "long-term" one?
Thank you in advance,
CEMAB
RE: A history of slope stability safety factors
My opinion is more or less the following:
1. The intent of any specific FOS is to account for unknowns, variability in material properties, variability in groundwater conditions, variability in loads, and provide some degree of assurance that the slope is stable.
2. With short term conditions, the amount of expected variability in some of the conditions above is less, just do to less passage of time. Therefor a lower FOS results in the same degree of assurance.
3. With long term conditions, the amount of variability is much greater in most if not all of the variables listed above, therefore the assurance is less.
Mike Lambert
RE: A history of slope stability safety factors
Something tells me that variability in material properties, groundwater conditions, loads etc. can be applied more or less equally to both short-term and long-term conditions. May it be related to the probability of loading conditions and consequences of failure instead?
RE: A history of slope stability safety factors
Consequences of failure are generally the same with long and short term.
Mike Lambert
RE: A history of slope stability safety factors
RE: A history of slope stability safety factors
Can you please elaborate a bit more on variability of material properties between short term and long term conditions?
Regarding the consequences of failure, you say that there is no difference between consequences of dam failing right after construction (before it fills up) or dam failing 2,3,4 years later with a full reservoir behind it?
RE: A history of slope stability safety factors
Slide during construction may not cause major loss of life or great property damage, so we are justified accepting lower safety factors than would be for when the dam is in operation i.e., when there is water in the reservoir.
For fine grained soil foundation/embankment, the short term stability is mainly governed by the dissipation of construction pore pressure. Since this pore pressures are likely to exist only during the first few years of the life of dam, it is desirable to use less conservative design than otherwise required. Though you may want to inform your client on the risk and the cost involved.