Maximum swelling pressure behind retaining walls

[muuddfun]

What would be the maximum pressure that a retaining wall would see from an expansive backfill?

If you backfill a wall with a highly expansive clayey soil, say with an EI above 120, would the maximum swelling pressure against the wall top out at the at rest pressure? Or could it go as high as the overburden pressure?
Assuming a cantilevered wall? What about a keystone or MSE wall?

What earth pressure would you give for the following soil?
phi = 23
C = 250 psf
gamma = 120 pcf
EI = 136
LL = 75
PL = 31
PI = 34

 

[FixedEarth]

With the soils that you have described, I would estimate it at a range of 6,000 to 9,000 psf swelling pressure. To get the exact value, you can run ASTM D 4546 test.

If we get any backfill with E.I > 50, we recommend 18" wide backfill of 3/4" open graded gravel. With soils like yours, we would probably opt for 36" wide backfill of open graded gravel. With the 23 degree soil, I am obtaining Active equivalent pressure of 53 pcf and At rest value of 73 pcf. Keystone wall would not be suitable for this soil.

You may also consider asking the wall designer to check for minimum heel pressure of 2,000 psf to counter the swelling pressure. Since 2,000 psf is a lot less than the 6 to 9 ksf swelling pressure, you will need granular bed under the retaining wall footings. Something like 2 ft of Class II AB.