Can footing surcharge increase passive pressure on key?

[thejonster]

I designed a restrained masonry retaining wall and I am taking another look at my wall and I'm taking another look at it.

I pinned the wall base at the top of my footing for the DD/bid, and that looks like a bad idea to me now. I'm thinking I should run the same pressures on down into the 2'6" deep key as well.

Three things:

1) I have to use a 1.6 factor on the at rest soil pressures for LRFD load combinations, I wanted to confirm that I cannot also increase the passive pressures by the 1.5 factor of safety I was given with it. (boy I would like to right now

2) Could I increase the passive pressure on the key by adding a percentage of the bearing pressure above it on the 3' wide footing?

3) My other idea is to take some of the extra moment out beneath the wall by using the P/A+M/S bearing pressure of the 3' wide footing

Values from the geotech
Site class E
Bearing 3000 psf
Restrained pressure 55 pcf + 50% surcharge
Passive resistance 250 pcf

Any thoughts?

 

[thejonster]

I should clarify, I'm getting 30% greater moments in my retaining wall if I don't pin it at the top of the footing, and design it as if it was a sheet pile. But if I can take moment out from the p/a m/s of bearing portion of the footing, it would help the most

I guess I was just curious about the passive pressure questions, I don't really expect to be able to mess with those values, and it might not even help me a great deal if it was allowed (maybe get the key a foot shallower at best.

 

[FixedEarth]

I don't see the connection between higher moment and soil's passive resistance. If your PhiMn is greater than Mu at the base of the stem, you would be fine. If not, you need to add a new cmu wall in front of the existing until your moment capacity exceeds the moment demand. On a side not, most allowable passive resistance are only factored by about 1.5. So increasing it by 150% brings you back to ultimate passive resistance with no room for error.