Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay 1

[edwardbd]

I am wondering if anyone has had any experience with undercutting under a MSE wall that sits on a very weak clay.

The clay's cohesion has been determined by both N-Counts (~0 bpf) and by a CPT test to be 500 PSF. I am using the unit weight as 120 PCF.

It is a lean clay, suseptible to flooding, at a bridge end bent near a river. This clay is roughly 20 feet 30 feet deep.

Slope stability is met with a 14 foot undercut with aggregate (140 PCF and phi=36). But, bearing capacity using Meyerhoff's two layered approach, doesnt even come close. Plus, Meyerhoffs two layered approach does not take into account the water table, which needs to be taken into account according to LRFD. This usually cuts the bearing capacity in half. I have attached a .pdf of my two layer bearing spreadsheet.

Below the clay is a hard sand, (phi 36 deg). I have also attached slope stability runs without and with the undercut.
(The FOS to meet is 1.3.)

I am wondering if anyone has any research material on how to deal with such a soft clay where a MSE wall abutment is to be built, and how to calculate the bearing capcaity for such a situation.


Thanks in advance,
BE

 

[edwardbd]

Slope w/out UC

 

[edwardbd]

Slope with UC

 

[fattdad]

If slope stability is met, what's the purpose of bearing capacity? Meyerhoff's method addresses shear failure (rotational) owing to an applied surface load (bearing pressure). Isn't that akin to rotationial shear failure as divined from the slope stability equations?

Consider how you would actually address "bearing capacity" equations: Would you consider the "footing width" the entire width of the reinforced zone? (I would.) What about the depth of burrial? Would you consider the 18 or 24 inches of the first few blocks below the lower grade of the ground surface? Here it get's more complicated. . . You see a 2 ft depth of burrial implies that the entire width of the "footing" is at the depth of 2 ft. Well, that's just plain incorrect. On the opposing side of the reinforced zone you may have a depth of burrial of 10 or 20 ft. That's a lot of shearing resistance to throw away!

What represents acceptable settlement? MSE walls are quite settlement tolerant. So, is there something on the high side of the wall that will be vulnerable to settlement?

A 14-ft cut for subgrade improvement seems like a major effort!

f-d

¡papá gordo ain’t no madre flaca!

 

[edwardbd]

I see what you are getting at with not calculating the bearing given the slope stability being met.

As far as the burial depth, the face of the wall has a 2ft burial depth, and from the back of the reinforcement, a 1:1 slope is tieing back into the existing ground line (for constructability).

I guess the main concern with the bearing capacity, was really the settlement that would insue.
The lean clay PI is 8. I guess we were just overall concerned about the weak clay.

Also, the undercut option is for a packet of alternatives.

Are there other alternatives to consider?

I have tried to find good literature to pre-loading, but cannot find anything good.

Other alts are lime-stabilization, soil cement colums, and 'geo piers'.

Thanks for the help.

 

[oldestguy]

This site appears to be rather extensive. If it extends any distance up and down the shore line, I would suspect other area have had to deal with the same situation. Have you checked out the area? How about local earth moving contractors with their experiences? Sometimes in situations like this, others have experience with the same situation. If there have been slip failures, you might get an idea or two from them. Examining aerial photos stereo scopically also may help. Try Google Earth if there are no other photos.

 

[BigH]

I have been involved with similar conditions ( 6m of Su = 18 to 20 kPa). I have posted several times on this site the details. The design did not address the issue of the soft clay and it was handled during construction. Basically, we used pvd wick drains at 1.5 m spacings and stage loaded. 4 m of MSE construction, wait 2 months, 3 m and wait, 3 m and wait etc. Walls settled up to 1100 mm for 11 m high walls with no misalignment, etc. If you are in design, this can be done as well but design an overloading near abutment and then remove before paving to build out longer term settlements. Fairly straight forward.

 

[edwardbd]

Do you have any literature on staged construction and how to reccomend a procedure?

 

[fattdad]

(I'll try this again. . . My first writup got lost in cyberland.)

Staged construction is related to the placement of fill on a subgrade that's too soft to receive the entire fill mass in one go. If the undrained shear strength is 500 psf then you'll realize general shear failure at 2,500 psf (or thereabouts). This is related to an Nc value of about 5 for phi=0 conditions. So, you place 20 ft of fill and you will likely have shear failure.

Settlement is perhaps your bigger issue. You have a soft clay on bedrock. Now if that clay is 20 ft thick and your bedrock doesn't allow drainage then the drainage path is 20 ft. If you have a consolidation test, know Pp, Po, Cc, Cr, e0 and Cv you have what you need to look further into these details. You also have to target the percent consolidation that you'll accept, i.e., 90 or 95 percent). Once you have this target, you can then calculate the time to realize settlement. Please note that the biggest influence on the time calculation is the length of the drainage path, which is squared. So for a drainage path of 20 ft you'd the calculation would have the value of 400. If you have wicks on a spacing of 5 ft, you'd have a drainage path just under 3 ft, so then you have a value of 9 in your time calculation. That's a reduction of about 97 percent on the time to realize consolidation! That would make the difference of like 60 months to 1.2 months.

Consider the undrained shear strength. If you add 5 ft of fill in one go, the time to realize 95 percent consolidation would be the same as if you added 10 or 15 ft in one go (don't forget to check general shear failure though). So, let's say you have concern with general shear failure. Add 5 ft of fill, wait 1.2 months and calculate the increased shear strength after the consolidation has occurred. This requires that you have undrained triaxial compression testing with pore pressure measurements and that you use consolidation pressures in the range that the soil will realize by your project. From these data you should get a site specific Su/P value. That said, often folks use Su/P=0.2. So, after you place five feet of fill (and you are dealing with moist unit weight) you're undrained shear strength would increase by the unit weight of the soil, let's say that's a supplemental 125 psf.

It's all pretty simple and it's all found in a first-year geotechnical engineering textbook.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

It sounds to me that without experience in wick drains or preloading that you probably should ask for some help from another firm, preferably not a competitor. On any of the different geotech procedures, it is better to work into those gradually to gain experience. Considering the risk, while the advice given here is helpful, there is nothing like a guiding hand from a more experienced person. In my past, even though I thought I had all the answers, getting that more experienced person involved makes for a more confident operation. It doesn't cost that much, since I (we, the firm) did all the grunt work and all the calculations as well as all the plans and specs and construction control on the first big job over deep soft stuff, probably larger than what is described here. Also I can think of three other jobs (in my lifetime) that that extra person helped considerably to be sure all bases were covered. One of those mentors was a famous friend of H. Golder (Big H)

 

[edwardbd]

Fattdad,
First off, I looked at your profile and saw you and I are both Rams. Small world.

When you are refering tothe "reinforced zone" and you including the embankment fill behild the wall straps?

Also, the burial depth around teh entire wall does seem like I am using an equation for a footing rather than a retaining structure.
How can I estimate bearing capacity for a retaining structure?
I asked "my boss" about the bearing being represented in the slope stability and he told me how we are looking more at "global stability" of the wall and how this includes "additonal factors".... is he just saying that because it is a concept that he hasnt used, or is it correct? If so, how an I explain this to him?

I appreciate your help on this!
Thank you,
BE

 

[fattdad]

you need to look at the NCMA (National Concrete Masonry Associations) design manual for MSE/SRW walls. The concept of global, external and internal stabilities is fully defined. Global stability is defined by slope stability.

I consider the reinforced zone as that portion of backfill that includes the straps or geotextile. So, if you have a 10-ft high wall and the reinforcement is 0.7 L (i.e., 7 ft), your reinforced zone would be the length of the wall and the length of the reinforcement.

Studied geology at CSU. Engineering at Virginia Tech.

f-d

¡papá gordo ain’t no madre flaca!