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# Time period from undrained to drained

## Time period from undrained to drained

(OP)
In slope stability analyses, we generally use drained parameters (c', phi) for long term assessment of slopes and undrained parameters (Su) for short term assessment.

After how many days/weeks/months/years does a material go to a "drained" conditions.

let's assume for this question that we have clayey SILT and it is a cut slope (as opposed to a a fill embankment). Also, i am just dealing with static conditions (no seismic). Am using limit equilibrium method to calculate a factor of safety.

### RE: Time period from undrained to drained

In slope stability analysis, a little cohesion goes a long way. Often the more critical analysis is the drained analysis. Therefore, if the long term, drained safety factor is OK, the short term, undrained safety factor is usually higher and the actual drain time is not a factor. But, we should check both.

### RE: Time period from undrained to drained

From Duncan and Wright "Soil strength and slope stability":

A practical measure of this time is t99, the time required to achieve 99% of the equilibrium volume change, which for practical purposes, we consider to be equilibrium. Using Terzaghi’s theory of consolidation, we can estimate the value of t99:
t99=4*(D^2)/Cv
where t99 is the time required for 99% of the equilibrium volume change, D the greatest distance that water must travel to flow out of the soil mass (length units), and Cv the coefficient of consolidation (length squared per unit of time).

Do you have the properties of the silt?

### RE: Time period from undrained to drained

A qualifier on PEinc . . . if the soil is heavily overconsolidated, the long term strength may be lower than the short term . . . not that helps in how many "days" it takes to transition from "short term" to "long term". The problem lies in that effective stress analyses should be used regardless but the porewater pressure responses are difficult to estimate reliably by normally practicing engineers.

### RE: Time period from undrained to drained

(OP)
i have been using c' of 6 kPa and phi' of 31 deg and su of 100 kPa. The c' probably sounds high but it is a reasonable value based on local geology. When i run the undrained with su of 100 kPa the FoS is very high (>3). The contractor wants to cut the slope and build his structure which may take about 3 months. So if i can determine that undrained conditions exist for 3 months, then stability will be ok (FoS>3). The permanent (long term) slope stability is fine as there will be a wall constructed to 'do all the work'.

I have never heard the t99 idea for estimating the time to get to drained. that is a good idea. Stoyan - can you provide some more details on the reference please?

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