Skin Friction on 30' Deep Concrete Wall

[tenguy23]

I'm analyzing an old environmental lift station (effectively a huge basement). It was designed in the early 1960s as a lift station with building on top, but now it will be demo'd with the basement portion only being reused for a future lift system.

Dimensions are 31.67' deep from ground level with a square plan dimension of 24.5' x 21.83' (15" to 18" walls). Borings taken adjacent to this "basement" are primarily clayey/silty soils with Q,u ~ 1 to 1.25 TSF (N values ranging from 6 to 10).

I'm concerned about buoyancy since this structure is next to a small creek adding about 22' of buoyant force on the structure. I want to consider effects of skin friction of the side soil (on all (4) sides) to help out with adding to my total DL weight counteracting buoyancy, but don't know how to go about calculating it. If I were to consider this as a huge pile with perimeter of 92.67', the alpha-method with alpha~0.5 gives me a total skin force of 2,471 kips, which seems to big.

Any ideas of how to calculate?

 

[fattdad]

2/3rds of the shear strength is where I'd start.

f-d

¡papá gordo ain’t no madre flaca!

 

[ishvaaag]

This worksheet deals with subpressure failure of excavations in clay as per Thronburn, Hanson, Peck. You see that even without a containment structure, the risk of failure is estimated small.

 

[ishvaaag]

... without a BOTTOM containment structure, should say above.

 

[ishvaaag]

So it seems you have enough safety factor by even a moderate assumption of the available adhesion at the interface.

 

[tenguy23]

The buoyant force I'm estimating is:

gamma * H * (bottom slab area)

= 62.4 * 22' * (24.5' x 21.83') = 933 kips upward.

My downward forces (walls/top&bottom slab/misc equipment after the demolition of the top building) = 905 kips down.

ACI for environmental calls for a 1.25 FS for uplift, and this scenario is giving me only FS = 0.97. To satisfy the 1.25 FS, I need an additional 260 total kips acting downward.

I'm confused by the attachment...I'm not seeing a comparison to any water forces.

 

[tenguy23]

Also, I'm assuming that when this was built, they excavated back, formed up the walls, and backfilled. If I'm calculating friction of the soil against the concrete walls, is there some mu-coefficient I can use? It's not going to be like a footing on soil since it was not poured directly on the soil, so I'm assuming its going to be smaller than 0.3...

 

[ishvaaag]

You are right, no water forces in the attachment. Just insight there.

su=qu/2=0.625 TSF

260 kip / ((2·24.5+2·21.83)·31.67) ft2=
0.088 kip/ft^2= 0.044 TSF << su

If infill assume friction factor able to transmit at least su, average. If not wanting, for an aged situation like this the K0 should have developed already between soil and wall, make a tentative evaluation of the lateral pressure and then a commensurate friction. Or, for a more conservative evaluation estimate the silo pressure between backwall and some notionally stable vertical face of soil, you will need some assumptions. If uncozy anyway with the result, anchor it, be it with bell tip piles or micropile anchors.