Retaining wall holding 16-17 feet of soil

[abusementpark]

I'm designing a retaining wall to hold about 17 feet of soil. For a traditional cantilevered wall design, I am getting a stem thickness of about 18 inches, which seems very large.

Should I start considering a counterfort wall or some other option that may be more economical?

 

[dcarr82775]

Doesn't seem unreasonable to me.

 

[bridgebuster]

Sounds about right. All thickness~ h/10. Counterfort walls, in my experience, are not economical until 30 to 35 feet.

 

[Ron]

ABP....you're in the ballpark...keep swinging. My rule of thumb is similar to bridgebuster's....height in feet = base thickness in inches. Taper accordingly or keep same thickness all the way up.

 

[abusementpark]

Thanks guys. Just wanted to make sure there weren't other options to consider.

Is tapering the wall generally worth it?

 

[hokie66]

Agree with Ron and bridgebuster about the thickness at the bottom. Also agree with tapering. Serves two purposes. Saves a bit of concrete, but more importantly, tapering the outer face allows for some rotation and bending deflection, so the face never goes beyond vertical.

 

[abusementpark]

This one is serving as a wall for building that is being built into an embankment. I'd have to taper on the heel side.

Also, I'm going to have to run a drain pipe along the wall that will drain around the building, which is 200' feet long. Any ideas on how to calculate the required slope and size of the drain pipe?

 

[hokie66]

A cantilevered wall as support for a building is generally not practical, as the wall will lean and/or deflect. If you can tie it at the top to the building structure, then that might work, but often a more practical approach is to batter the soil behind the wall. The battered undercroft might help with your drainage scheme as well.

 

[connect2]

hokie66,
Little confused by your statement 'not practical'. Stability and bending deflection are surely controllable through design.

 

[hokie66]

Not really. Cantilever walls do tilt. Most are designed assuming active pressure conditions, and for active pressure to develop, the wall has to move. We don't know much about the building above in this case, but movement of the top of the wall has to be considered.

 

[oldestguy]

Looks more like you should consider it part of the basement wall where no movement is allowed. Where no movement is allowed, active is no longer your design pressure. At-rest pressure may apply. How is the wall going to be backfilled? Assuming some form of backfill compaction is involved, you can go well above the at-rest condition. Many a wall has been pushed over. To be really careful you install pressure measurement devices on the wall and adjust your work accordingly. My rule of thumb, is that no compaction at all is applied within 2 feet of the wall. You may have to increase that, as you measure pressures. If you get questions about backfill settlement, tell them "check out what the silo effect may be".

 

[abusementpark]

oldestguy,

The wall will be built and backfilled before the building is erected.

 

[oldestguy]

No argument from me. Not my wall. Near the corners there is no give, thus not active for some length.

You didn't mention slope of ground up hill. Any slope there increases the pressure by either pressure criterion.

Most drains are laid level and set low enough in elevation to allow some higher position for the hydraulic head mid-length. Size generally is "what ever is common" in the area, because most of these drains never run full unless you are tapping a spring in gravel. 4" diam. in the states is common. See some threads elsewhere dealing with "French Drains". Be aware of possible plugging if only gravel is the backfill (not a filter).

 

[dik]

I always batter the wall into the retained soil... that way, when it tilts, it becomes closer to vertical...

Dik

 

[abusementpark]

Dik, how much do you typically batter?