Opinions on design

[shacked]

I have been hired to design the retaining wall system for a hillside residence. Basically the owners house sits about 20ft behind the top of a 2 horizontal to 1 vertical slope with a height of 20ft.
They want to extend the backyard to just beyond the downslope location in addition to a zig zag set of retaining walls.
A soils report was provided to me and in order to achieve the required daylight and support the new retaining wall foundation into competent soil I am utilizing a grade beam and caisson system.
I have come up with what I believe is a good design but it is a little different then what I typically design. Since both walls are relatively closely spaced the grade beam is 4'-4" wide x 18" thick in order to support both walls.

To account for the torsion, I am designing for the full 6ft retained up slope as well as the weight of the shorter wall on the opposite end of the G.B., and the weight of the backfill between walls and weight of walls also contribute to the G.B. in bending.
I am not accounting for the soil in between the walls with respect to additional torsion from the front wall.

Just curious to your guys thoughts. Would you consider additional torsion?

Thanks

 

[BAretired]

I agree with designing for just the six foot height, provided you are not counting on passive pressure on the other side. It may be advisable to add stiffener walls in some locations to force the two walls to act as a unit.

I don't know anything about the soil, but I would suggest you consider a slight batter for the piles as shown below. Otherwise, the top of pile is going to deflect outward which will not please your client.

 

[shacked]

Thanks BA. Thats a good idea to stiffen up the area between the walls to work as a unit.

The soil is fill down to 50ish foot, but it is considered competent with passive values for the caissons of 150pcf and twice that for isolated caissons.

I have never speced out a battered pile like that before. Are they drilled full depth at an angle?

 

[BAretired]

Yes, the shaft is straight for the full depth, so batter is constant throughout.

 

[CrabbyT]

I came here to recommend batter piles, looks like BA has it covered. Settlement will definitely cause them to shift. Alternatively, you could put some deadman anchors in the tops of the caissons.

Depending on what type of soil it is and what its angle of repose is, you may consider using geotextile to mitigate surcharge loads from higher walls to lower walls. And I don't mean yard fabric, but real geotextile - the kind with structural properties.

 

[shacked]

Thanks I appreciate the recommendations.

 

[PEinc]

If that pool is already existing, caissons and bearing piles may be hard to install with so little space and an existing hill. Instead of caissons or piles, consider micropiles which can be installed with smaller equipment?

http://www.PeirceEngineering.com

 

[shacked]

BA, I'm curious how do you go about determining the degree of batter? Is there a rule of thumb? I thought about calculating the deflection but I'm not sure how accurate that would be in this situation.

 

[HouseBoy]

Curious about the soil type (of the existing fill). I am often required (in the soils report) to use plug piers so that soil does not "flow" out between the piers down a certain depth below the surface. Your piers look like they are spaced apart a good bit. Just curious.

Plug piers are often called "Secant or tangent walls" (if you need to search that term).

 

[shacked]

Just saw this question to my post awhile ago. The soil is artificial fill 30 to 50 ft above bedrock. The fill material is CL fine grain sandy clay

 

[BAretired]

BA, I'm curious how do you go about determining the degree of batter? Is there a rule of thumb? I thought about calculating the deflection but I'm not sure how accurate that would be in this situation.

Sorry, I missed this post; probably daydreaming. The way I would do it is to calculate the vertical and horizontal load, then slope the pile in the same direction as the resultant or perhaps even slightly more. If the slope is too much for the drilling contractor to execute, use a tieback at the top of pile.

 

[AaronMcD]

Why not helical piles? The footing is wide enough to get staggered rows and they are very easy to batter.