Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
(OP)
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
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





RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
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!
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
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.
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
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!
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
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
RE: Undercut to Meet Bearing Capacity for MSE Wall Over Weak Clay
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!