Pore pressure increase in slope analysis

[Okiryu]

Hi, I am reading Professor Duncan's book ("Soil strength and slope stability") and it is indicated that undrained and drained conditions depend on the time of application of the load. For example, undrained conditions are related to rapid loading (in this case the application of the load is too fast so it does not give time for water to dissipate thru the soil) and drained conditions are related to slow loading (loading is slow enough so it lets the water to dissipate).

My question is: I have a slope as a result of cut operations. The slope is quite above groundwater table. 5m height 1:1 slope. Overconsolidated clay. Su approx. 35 to 50 kPa. Not loading is planned for this slope. However it failed after several days of consecutive heavy rain. I understand that shear strength (effective stress) decreased since pore water pressures increased. Is the buildup of water pressure from the heavy rain considered as a rapid loading case, so undrained conditions (total stress) govern the analysis in this case?

If it is not considered as a rapid loading case, so I will need to check under drained conditions (effective stress), I understand that I will need pore pressure information for the slope which is not easy to obtain...

Thanks for your advice in advance.

 

[moe333]

I think your soil is only lightly overconsolidated and that this was an undrained failure. What are the Atterberg Limits and in-place moisture content?

 

[Okiryu]

Hi moe333, I am still waiting for lab results, but typical values in the area for W% are between 25% and 30%, LL=40~50, PL=20~30. Typical OCR values are between 2 and 3. BTW, what are the ranges for considering "lightly" and "heavily" overconsolidation?

 

[moe333]

Typically an OCR<4 would be considered lightly OC. Normally to lightly OC soils contract when sheared and generate positive pore-water pressures.

 

[Okiryu]

Moe, thanks for the reply. A question: If the soils are unsaturated, do they still generate positive pore-water pressures? Since they are unsaturated, I was thinking about some amount of suction and negative pore-pressures.

 

[fattdad]

lots of clays below the water table are unsaturated. Lots of clays above the water table still have high moisture contents. Lots of clays above the water table are soft and want to contract when sheared. Permeabilty affects the relief of excess water pressure whether you are above or below the water table. Just this week, I was talking to Mike Duncan about a consultant that made the argument that clay above the water table would not have undrained loading. We both had an informed chuckle.

f-d

ípapß gordo ainÆt no madre flaca!

 

[moe333]

I would consider undrained loading for saturation greater than about 80%. I typically do not rely on suction for saturation less than about 80%.

 

[Okiryu]

Thanks for your responses. It is more clear now. Talking about saturation of clays, I have a related question: even if you do not have 100% saturation, do you still considered consolidation analysis for settlement calculations? At what grade of saturation, the 1-D Terzaghi theory of consolidation does not work?

 

[fattdad]

Running a consolidation test on an undisturbed sample will return Cc, Cr and data to determine Cv. These values will be for the sample in its in-situ condition. Water is present to surround the sample, but for most clays there is no guarantee of saturation. So, the Cv that you get from the test results will be for that physical state.

Please recall that we are designing earthwork to be weatherproof. So, if the in-situ saturation (i.e., from a July exploration) is below 80 percent, it may not be there in January.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

fattdad, when calculating settlements using the 1-d terzaghi consolidation equation, do you use the buoyant density (to calculate initial stresses) even soils are unsaturated?

 

[fattdad]

Okiryu, The insitu state of stress would require some knowledge of the phreatic surface, irrespective of saturation below the phreatic surface.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

fattdad, in my area the phreatic surface is known to be located at relatively deep locations (approx. GL -10m / -30ft). Since most of our fine-grained soils are located above the phreatic surface and have typical saturation values of 90%, my question was related to this type of conditions. Do you think that under these conditions, the 1-D consolidation theory still work? I understand that soils may get saturated in certain period during the life of the structure though.

 

[killswitchengage]

you should know that water not only develop an excess PWP on soils ,but also is responsible for chemical alteration , seepage forces and freeze and thaw cycles.
some OC clays tend to revert back to its mud like state NC when subjected to many cycles of watering and dehydration , hell i even had a marly shale destroyed in 10 min in pure water . So you should add those in mind , plus Terzaghi and al suggest using the same OC clays and adding water to it to convert it into a slurry provided the amount of water added does not reach the liquid limit . Then , a DSS test is performed , or you could use their formula that inject a OCR to the power of 1-m in the Mohr-Coulomb shear strength formula . with care that is .

 

[Okiryu]

Thanks for your comments. Our site is composed of OC clays. We do not have shales which in our area are heavily OC. Yes, wetting and drying processes affect shales, we call this process here as "slaking". I am interesting to see the equation which you are referring to. Is that indicated in the Terzaghi and Peck book?

 

[killswitchengage]

yea like i said it inject the OCR to the power 1-m inside the Coulomb equation , the constant M is a function of the state of the material

 

[killswitchengage]

i should add the geological events into this subject, if a given oc clay or its overlying layers ware subjected to ersion in any given geological era , chances are that the material is heavily fissured which highly effect its intercept C , otherwise the angle of friction stays practically the same