when does short term become long term? 1

[ac4u2nv]

Dear all
At waht point do you stop using undrained parameters and start using drained paramaters? wouuld a recomendation of anyting after 6 months is considered long term and drained parameters are need to be used.
I have a slope which is stable using undrained parameters and fails using drained parameters. If the slope (cutting) is to remain unsurrported for a week as opposed to a year (no time frame mentieond yet) how do decide what is long term or not?

Please advise?

thank you

 

[GeoEnvGuy]

Terzaghi one dimensional drainage equation can be used to estimate the dissipation of pore pressure. I also find it odd that you are indicating the drained strength is less than the undrained strength, this goes against the common saying that the undrained strength is half the drained strength.

 

[ac4u2nv]

Why does my slope keep failing using drained paremeters as opposed to undrained parameters?
clay undrained: density 19kn/m3 Cu 55kpa
2nd clay undraind: 19.40kn/m3 cu 170kpa

Clay drained: 19kn/m3 phi' 25
2nd clay draind: 19.40kn/m3 phi' 24

 

[geomane]

If it’s a cut slope in overconsolidated clay, drained strength can be lower than undrained due to dissipation of negative pore water pressure

 

[geomane]

Refer to Soil Strength and Slope Stability by Duncan, Wright and Brandon for a discussion on when to use undrained or drained strengths.

 

[Mad Mike]

I love the title of this post.

I'm absolutely tired of my "temporary" excavations being left to stand open for months and months, to the point where I'm now inclined not to give any leeway whatsoever. I should qualify that in my country, temporary works are rarely inspected or supervised by a geotechnical professional (nobody wants to pay), which is how this situation arises.

Unless I were supervising the construction works very closely, I would be inclined to disregard the un-drained analysis or moderate the values somewhat. Alternatively, disclaim your design / recommendations by stating clearly "provided the duration of exposure does not exceed a period of ?1? month(s)"...

I'm done with taking chances on these marginally stable excavations that get left open until they collapse...

All the best,
Mike

 

[GeoPaveTraffic]

geomane has it right, in overconsolidated clays the undrained strength can be many times the drained strength. Best to always check both conditions.

As for how long it takes to transition from one to the other; much harder to determine. Consolidation tests can help, but you need to remember that natural deposits have a tendency to drain faster than small samples. Unless you have close control of the site or are working for a contractor who understands the risk, best to design the slope for the worst of the two conditions.

Just my 2 cents worth.

Mike Lambert

 

[ac4u2nv]

Thanks
my slope has a decent factor of safety in the undrained and completetly fails in the drained. My thinking is for it to become completely drained where cohesion is no longer a strength factor will most likely take a long time.

Do i even need to consider drained scenarios if there is a reasonable time the excavation will remain open - say a year? at what point do i need seriously consider drained conditions?

 

[PEinc]

Counting on continued cohesion is a gambel. You never know when the soil will get "tired" and decide to fail. Check drained and undrained. IMHO, most times, in average or better soil, drained will control.

http://www.PeirceEngineering.com

 

[mikeofBelAir]

Short term---you were able to cash the check.
MANY years ago I attended a seminar on landslide mitigation where the presenter discussed his firm's solution to a landslide in a residential area. He was very positive and the analysis was very much in-depth. Six months later, I was installing slope indicators on the same (recently failed) slope.

 

[HENRYZAU]

The following factors to consider:
1. unsaturated clay with suction and/or excess pore pressure due to cutting in saturated clay. 2. For the latter, drained parameters (c',phi')and effective stress (ground water table at long term balanced level) apply when excess pore pressure dissipates to near zero; undrained parameters (Cu or Su,phi_u = 0) and total stress apply when there is little dissipation of excess pore pressure. The real case is somewhere in between or we may call it consolidation.
3. To check if it's close to drained or undrained, Terzaghi's 1-D consolidation Theory may be used to check T = k x E_oed x t / (gama_w x h_D^2), if T is less than 0.01, it's undrained; if Tv is more than 0.4 then it's drained.
4. In theory, if you have realistic parameters of permeability,c',phi'and ground water level within the project timeframe, consolidation using transient ground water flow would give the right answer.

 

[fattdad]

. . . and then there's the question of long-term cohesion. We don't like it!

In the case of fine-grained soils, I'm mindful of softening and will consider softening effects in the upper 15 to 20 ft (i.e., that the cohesion will revert to zero over time).

For slopes, I'd require drained and undrained. I'd also require circles and non-circles show a safety factor of 1.3 or 1.5 depending on the height.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

I would relate short and long term conditions to the rate of loading. I also recall that Prof. Duncan has a chart that relates long/short term to the drainage time.

For using shear strength parameters in the analysis, undrained parameters are more critical for contractive soils (soft clays and loose sands) and drained parameters are more critical for dilative materias (stiff clays and dense sands).

 

[ac4u2nv]

ok so there is now rule of thumb i.e. if a cut slope remains for less than say a year then use short term parameters, and if longer than long term?
still unclear about how to work out when to decide when "long term" happens.

the cut slope im working on fails in drained paramters and the mititgation for that is quite costly. However if we only use undrained paramters it passes.

 

[GeoPaveTraffic]

Best of luck if you decide to proceed on the assumption that short term will last at your site for a full year.

I hope you have good insurance if your argument if/when the slope fails is "well these guys on this internet form said it was a rule of thumb that sometimes it should take a year for the slope to go from short to long term strengths..."

Mike Lambert

 

[HENRYZAU]

It's dangerous to adopt this: if a cut slope remains for less than say a year then use short term parameters. I would check both drained and undrained cases if I were you.