Determination of the strength gain of soft soils due to consolidation.
Determination of the strength gain of soft soils due to consolidation.
(OP)
I am evaluating roadway embankments over soft soils C=200 to 700 psf. The maximum embankment height is 86 feet. Using a 2.5:1 slope, the factor of safety is 1.15 < 1.3 = FS required. I do not want to go to a 3:1 slope due to right-of-way issues.
I was going to look at staged construction. The embankment is stable (FS > 1.3) up to a height of about 40 ft. Following consolidation of the foundation soils due to the 30 ft of embankment, the strength of the foundation soils will be higher.
Any methods/references on how to determine the gain in shear strength?
Thanks for your help.
I was going to look at staged construction. The embankment is stable (FS > 1.3) up to a height of about 40 ft. Following consolidation of the foundation soils due to the 30 ft of embankment, the strength of the foundation soils will be higher.
Any methods/references on how to determine the gain in shear strength?
Thanks for your help.
RE: Determination of the strength gain of soft soils due to consolidation.
RE: Determination of the strength gain of soft soils due to consolidation.
RE: Determination of the strength gain of soft soils due to consolidation.
Holtz and Kovacs, An Introduction to Geotechnical Engineering
Ladd and Foot, NEw Design Procedure for Stability of Soft Clays, ASCE Getech Journal, 1974
Ladd, stability Evaluation During Staged Construction, ASCE Geotech Journal, April 1991,
RE: Determination of the strength gain of soft soils due to consolidation.
RE: Determination of the strength gain of soft soils due to consolidation.
First stage embankment height is 30 feet. I typed the wrong height in the second sentence second paragraph. I am using UTEXAS3 for the analysis.
I am/will look at the undrained analysis.
In my literature search I found the Ladd, 1991 paper, but can not find a copy. Any help?
Thanks for the responses.
RE: Determination of the strength gain of soft soils due to consolidation.
http://pubs.asce.org/WWWdisplay.cgi?9100971
RE: Determination of the strength gain of soft soils due to consolidation.
I tend to think that 30 ft is rather on the high side of what you will be able to construct unless you are dealing with the higher end of the range. If I remember correctly, Tchebotarioff in his 1973 book had a chart of embankments vs undrained shear strengths showing typical heights at which local yielding would occur - and ultimate.
RE: Determination of the strength gain of soft soils due to consolidation.
I agree that 30 feet seems high for undrained construction. If the soft layer is not very thick, the stability analysis, may indicating that the factors of safety are acceptable, if you only check circular mode. If this is the case, you need to check the wedge failure mode, and check it with Spencer method or Morgenstern Price.
RE: Determination of the strength gain of soft soils due to consolidation.
The soft layers are thin. There are two low strength layers in the foundations. Silt and Clay (C=700 psf) layer about 2.5 feet thick underlain by a Silt (C=200 psf) layer about 2.5 feet thick. Silty Sand is above the Silt and Clay layer and Silty Sand is below the Silt layer. I have assigned a phi of 28 degreed for the Silty Sand layers.
Wedge Failure
I have only used a wedge analysis when I have geometry that would dictate that the failure surface is a wedge failure, usually on side hill fills, etc. How do you select the failure surface? I would seem arbitrary for an embankment fill.
Thanks,
RE: Determination of the strength gain of soft soils due to consolidation.
Second, if the "soft" layers are thin (how thin?) and the surrounding soils are sandy; drainage should occur very quickly. Have you estimated the time for these layers to consolidate? Can the embankment be built quickly enough to generate significant pore pressures?
Lastly, with respect to wedge failures and "picking" the failure surface. You should run many many trial failure surfaces. The search routines in UTEXAS are very good for circular failures, but not so good for wedge failures. What this means is that you need to select several different starting wedge locations and run the analysis. Then take the critical results from those runs and use them as input starting wedges for even more runs. During this process you will need to adjust the resolution input parameter for the wedge analysis, I think it calls it the starting shift or some such thing.
If you have not done this type of analysis, you really should get someone to check you results.
RE: Determination of the strength gain of soft soils due to consolidation.
RE: Determination of the strength gain of soft soils due to consolidation.
Removing the 5 ft of soft material will be an option that will be provided. I would like to have an accurate determination of the "actual" factor of safety so the cost of overexcavation is justified.
Thanks to all for the responses.
RE: Determination of the strength gain of soft soils due to consolidation.
B. Big H is quite likely correct about removing the foundation material. However, if you can tolerate consolidation settlements and sequence the fill operations properly (so you don't get bearing failures within the fill area), you may be able to leave some of the material in place by excavating a trench around the perimeter and backfilling it with good stout compacted fill. Maybe 15 m wide at the invert, and put it up under the fill slope a way so it has some overburden to give it confining stress.
C. Gosh, GeoPaveTraffic, I always liked the search routines in the UTEXASn family.
RE: Determination of the strength gain of soft soils due to consolidation.
dgillette:
I like the select overexcavation idea. The embankment is quite large. This would save a lot of excavation. Thanks.
I have never felt confident in the UTEXAS search program either.
Thanks again.
RE: Determination of the strength gain of soft soils due to consolidation.
I also cringe at the thought of building 30-ft in one fell swoop - BUT it won't be built instantaneously. NACFAC DM7-1 has guidance on estimating consolidation for gradual load applications. With silty, rather than clayey soils, you may get the consolidation you need, but your Su/P ratios are typically much lower (0.1 - 0.15) than for clays. You may not get the strength you need anyway, without ground improvement.
I agree that excavate and replace may be the way to go and I have used the method of targeted over excavation to create a "key" of good soil through the soft subgrade.
RE: Determination of the strength gain of soft soils due to consolidation.
RE: Determination of the strength gain of soft soils due to consolidation.
Considering that these are thin layers (with respect to the aspect of the fill), rotational failure is not likely the critical concern - wedge failure would be. Have you done any direct shear testing?
I'd think some of these quesitons would be important for this group to provide you with any meaningful feedback. To answer your question, however, I'd use 0.2 (you can also refer to Jamilkowski's (sp) research on this topic.
Good luck. . . .
f-d
¡papá gordo ain’t no madre flaca!
RE: Determination of the strength gain of soft soils due to consolidation.
If you could drill at the site, the best option is vane shear tests.
If the project is worth it consider doing some CPTs,
good luck!
RE: Determination of the strength gain of soft soils due to consolidation.
In January, my firm decided that our field vane shear equipment was not being utilized when it should/could be. It was decided to try to use this testing method more often, when applicable. Had the equipment by just did not use it.
It is a test we can do ourselves and will most likely will be one of the ways we will verify when stage 2 construction can begin. Nothing like measuring instead of estimating.
Thanks to all.
RE: Determination of the strength gain of soft soils due to consolidation.