slope stabilty of sides of canal in clay

Question

slope stabilty of sides of canal in clay

puddleclay (Geotechnical)

(OP)

6 May 11 05:48

I want to the check slope stability of canal section which is excavated in expansive clay. At present soil profile shows stiff to hard clay but after excavation soil will be loosened up and shear strength will be reduced when water will flow throught it, as water will enter in pores and develops pore water presuure (in submerged case). I have soil parameters for present stage but no exact corelation for reductions in cohesion and friction once it is in submerged state. Another concern is that canal have a daily fluctions of abot 10-12 m in water level (maximum canal height is 16 m approx), I cannot provide lining to canal. Also available software with me does not take care of sudden drawdown case (in that case water will be present in pores and starts flowing back to u/s and thus add to driving moment but due to pore water reduced shear strength of soil is experienced) which may be experienced once canal works for a year or so. I have some literatures for methodology and principles but its not possible to check it mannually, please suggest me software which takes care of long term stability of side slope in clay under sudden drawdown case (which is expected the worse case).

moe333 (Geotechnical) · Answer 1 · 2011-05-06 12:26:11

SLOPE/W has an automated rapid drawdown method based on Duncan Wright and Wong (1990) methodology. You need Effective Stress and undrained (CU) strength envelopes as input (you can get both from CU w/pore pressure TX). SLIDE and other software probably have this capability as well.

You may want to look at fully softened shear strength for long term static conditions.

fattdad (Geotechnical) · Answer 2 · 2011-05-06 12:52:30

How did you confirm that the existing clay does not have a phreatic surface?

f-d

¡papá gordo ain't no madre flaca!

puddleclay (Geotechnical) · Answer 3 · 2011-05-11 07:59:22

thanks (moe333) for your reply but with all these inputs i still do not have any corelations for predicting reduction in value of cohesion in long term when the present soil will be in submerged condition for long time since the value of cohesion i am having at present is for dry soil because i have a borelog data which shows water table at 8 m depth or so and test in done on same soil.

(mr fattdad) i have borebog data for the same site and same place where i am going to construct my canal and water table is found more than 7-8 m deep so pheratic surface is not at top.

moe333 (Geotechnical) · Answer 4 · 2011-05-11 11:45:15

You can estimate fully softened (effective stress) shear strength based on LL and %clay from paper by Stark (2005). It is more difficult to estimate total stress phi and c for rapid drawdown. A very conservative method for RDD is to assume phreatic surface is on slope face and by using effective stress strengths.

fattdad (Geotechnical) · Answer 5 · 2011-05-11 17:07:43

did the borehole data include a piezometer (pipe or vibrating wire)? I just know how borehole data can be misinterpreted 'cause you can move a lot of auger cuttings out of a clay hole faster than the water can enter from the sidewalls and field staff may conclude the hole is "dry" when given a few days you'll get a water surface in the hole.

Not trying to stir trouble, but. . .

f-d

¡papá gordo ain't no madre flaca!

oldestguy (Geotechnical) · Answer 6 · 2011-05-11 20:52:45

I'd also suggest you make some contact with the United States Bureau of Reclamation, by doing a Google search.  They have numerous canals and have had all sorts of experience in sides soughing in.  I'd bet they have many publications to help you solve your problem.   It may not be just running tests, but it may be more deeply into the side slope angles needed, etc.

My Internet search on Google shows they have lists of offices as well as a "contact us" page. Asking a few questions and you may be directed to some useful help.  They have many canals in south-western states.

I once knew one of their engineers describe him and his buddies sitting on the bank of a canal with the slope paving all destroyed by expansive clay.  They know about those things from experience.

puddleclay (Geotechnical) · Answer 7 · 2011-05-12 02:15:44

dear (moe333) i think i can also go for a UU/CU test for a fully saturated sample, as i am worried about rapid drawdown so i think undrained test & total stress approach will best stimulate the field condition. Theoretically in UU test for fully saturated sample failure envelop of Mohr's circle should be horizontal line but in case if it is not horizontal means it is not fully saturated eventhough i may be close to filed condition as stability depends much more on c rather than phi and i may get that reduced vale of c.

(fattdad) yes we had piezometer reading for 4 months and shows water table approx. 6 m deep.

fattdad (Geotechnical) · Answer 8 · 2011-05-12 23:06:04

O.K., now the next question is whether the clay below the water table is saturated. If you do a UU test take one tube and have the lab carefully carve three small-diameter samples from the one tube. I can't remember the platen size for the small-diameter sample - it's like 1.4 inches or so. Run the UU samples at the effective stress, the total stress and then go some increment greater. If the shear strengths are the same, then voila! It's saturated. If you get a c-phi value, then the sample is not saturated. You can use the shear strength obtained from the total confining stress sample.

Hope this helps. I've been recently learning more on this through a project I'm working on for my DOT.

f-d

¡papá gordo ain't no madre flaca!

moe333 (Geotechnical) · Answer 9 · 2011-05-13 00:24:42

If you want to run the 3-stage rapid drawdown analysis you will need to run a CU strength envelope with pore pressure. Preferably three confining stresses spanning the effective stress range of your project. This test will give you both effective and total (undrained) stress envelopes needed for the analysis.

fattdad (Geotechnical) · Answer 10 · 2011-05-13 09:43:42

I agree with Moe for sure. I was focused on the UU topic, but CU testing would be essential for rapid drawdown analysis.

Sorry if I missed the bigger picture. . .

f-d

¡papá gordo ain't no madre flaca!

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slope stabilty of sides of canal in clay