12' Retaining Wall Question 3

[Sokka10]

Hi everyone, after lurking around the forum here for a few years I finally made a profile. I recently broke away from the small structural engineering firm (almost exclusively light-frame residential projects) I was at to start my own firm. I hope to contribute a little back to the engineering community here since many of your answers have helped me on projects I've worked on. Anyway...

I'm designing a 12' tall retaining wall pretty much right up against the property line in the client's backyard. I have a geotechnical report for this one so I'm using all the values they gave me in the report. I have attached the design that I've come up with so far. I have two questions...

1) At what point does such a wide heel on the footing stop helping? Can I really count on a 7'-6" wide heel to act as one unit? Let's say I had a 20' wide heel, it seems obvious that at a certain point the heel is so far away from the wall itself that it's not doing anything to help with overturning, bearing, etc.
2) Are you aware of places in any relevant code books (IBC, ACI, etc.) that gives insights on drainage design for retaining walls? I feel confident in the drainage I have called out on the detail (drain rock, perforated pipe, etc.) but I'm wondering if there is any sort of standard to reference when designing for drainage?

Here's the link for the retaining wall detail: Link

 

[phamENG]

1) Sure. At some point it'll stop benefiting you, but it's going to depend on the size of the wall, thickness of the heel, soil conditions, etc. Can you depend on a 7'6" heel to act as one unit? Absolutely. You'll want to consider deflection of all of the components to determine the total deflection of the top of the wall - that'll tell you how well it's acting together.

2) I'm not aware of anything. I typically call those out as "drainage by others". Sizing the drain pipe is not something I'm comfortable with as it's more storm water design than anything else. You'd have to determine the area of the drainage basin that you'll be collecting, flow rates, etc. to get the size of the pipe. Then site design to slope the pipe appropriately to a discharge. Then, based on the quantity of the discharge, do you trip any special requirements for your locality's storm water regs? I'd have no idea. If it's for ground water, you'd probably need the geotech to give you the amount of water you'd have to move based on predicted ground water levels (which most reports explicitly leave out because it requires 12 months or more of monitoring) and permeability. Could I do these things? Yes, with enough study. Can I do them quickly, efficiently, and with a level of expertise that I think my clients deserve? No, no I couldn't. So I'll put on the drawings where we need to limit the water level, and let somebody who's an expert in storm water and hydrology size it.

 

[SteelPE]

I didn't run any numbers, but your wall seems a bit thin (at 8" thick and 12 high). I also know there was a debate on here a while ago, but I'm not sure if those #5 J bars have the proper "hooked" into the footing.

In regards to the drainage behind the wall, I typically provide weep holes in the wall every 8'-10' o.c. With these holes I am just looking to relieve the hydrostatic pressure that may build up behind the wall. I can't say that I was taught to have as detailed a look at the drainage that phamENG suggested, maybe I'm doing it wrong? I am not sure if weep holes would work for you as you are close to the property line and the adjacent owner may object to them.

 

[Sokka10]

Thanks for your input thus far. The design I have right now was calculated in RetainPro, not sure how everyone feels about that program, but what I have right now is working. I was wondering if my wall is a little thin. I do have 3 other heights of walls on this project (6', 8', and 10') so I was trying to keep the same 8" thickness throughout so it's easier to build. The active soil pressure in the geotech report was 35 psf so not too bad. I'm also interested in the discussion on the hooked bars because the distance I have them turned in the footing right now I calculated from the section on hooked bars in ACI

 

[RWW0002]

ohh get ready..

https://www.eng-tips.com/viewthread.cfm?qid=401855

Some great information included on stem bar embedment but you might as well sit down and get comfortable before digging in.

As far as retain pro- I have used it (along with the retaining wall modules in enercalc, which are similar) for years, but generally add some detailing/design from CRSI for taller walls.

 

[STrctPono]

Doesn't ASCE7 require 1.6 load factor for Earth Pressure?

Mu = (1.6 x 35 x 11.5[sup]3[/sup])/6 = 14.19k-ft
d = 4.6875 in
a = 0.912 in
Phi Mn = 0.9 x 0.62 x 60 x (4.6875 - (0.912/2) = 141.7 k-in or 11.81 k-ft

The wall is too thin. You need 3" clear cover on backside. I would definitely bump that up. It's not a big deal to change wall thicknesses along the length. I would do so at your control joint locations.

Furthermore, you have #5 dowels at 6" o.c. but your vertical wall reinforcing is at 12" o.c. Are you using the extra dowels for added reinforcing only at the base as it looks like you are running them 3ft up the wall.

 

[RWW0002]

@ StructPono - I do not know that 3" cover is required at the backside stem unless it is cast against soil. It would however be recommended at the footing. Currently that 12" thick footing looks suspect considering 3" clear to bottom steel and 2" clear to top steel. Embedment of stem bars may be an issue as well.

@Sokka10
I have not reviewed thoroughly, but here are some thoughts
-See above link for stem hook length and extension in footing - you may want to provide a bit more horizontal length to your hook..
-Generally stem bars are hooked towards the toe. For your case where there is little/no toe the condition may merit a bit more thought/detailing
-If you have access to CRSI design handbook you might take a look at the use of "D" bars at the stem base
- A 10" or 12" stem may be a bit better suited for your case, but the 8" might be able to be made to work
-Footing thickness looks a bit thin for 2 layers of reinforcement/stem bar embedment

 

[BAretired]

That is a pretty tall wall. Looks too thin, although I don't think you need 3" concrete cover on the backside; 1.5" should be enough as the surface will be formed, not poured against soil. I would have thought you would need a key to engage more passive pressure in order to provide adequate lateral resistance.

Drainage is an important issue which needs to be carefully considered, along with site grading. Weep holes in the retaining wall, if used, would drain on the neighbor's property which could raise an objection.

BA

 

[dik]

I've not run any numbers, but your footing thickness appears to be a bit light. Your vertical wall reinforcing looks light. You might want to taper the wall from 12" at the base to 8" at the top. You might want to hook the top layer of your footing rebar into the toe. I'm not sure if there are any issues with draining your wall onto your neighbours property.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

echo...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

SteelPE - weep holes are great for bulkheads and retaining walls within a property line, but I try to avoid them on property line walls for the reason you suggested. If you're not punching through every few feet to drain the water, then I think the more in depth analysis of the drainage is needed with a wall this large as the hydrostatic build-up would get significant in a hurry.

For the record, my comments earlier were generic - I wasn't in a position to look at the attachment (sorry, should have said as much).

I agree with what others have said - your footing is way too thin and the wall looks a bit spindly, too. The hooked bar may be the right dimensions for a hook, but you lack sufficient embedment in the footing to develop it. ACI 318-14, 25.4.3.1 (a) gives 11.86" development length for a #5 hooked bar. I'd make your footing a minimum of 18" thick.

This will also be very susceptible to deflection. To use active soil pressures, the wall has to move (otherwise it's at-rest, which are much higher). A wall that high with a small angle of rotation is still going to be noticeable. It's more work in building it, but if you have room you may want to consider a slight backward slope to the outer face.

 

[dik]

That's why you tilt the tapered wall on the outside face so that with rotation, it becomes more vertical. It may be that the drainage holes are prescriptive and that little or no water flows... I've seen lots of them that didn't 'leak'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

I will retract my earlier statement of 3" clr. cover on backside of stem. I agree with others, running some quick numbers, the heel is way under capacity.

Sokka10, are there any other structures (house) within 12ft of this retaining wall? If so, that surcharge load would have an additional affect on your wall.

Tapered walls CAN work but need to be carefully detailed. If the wall height is not uniform across the length then forget it. You're better off going with a tapered retaining wall and changing geometry at the joints. Several years back, we were working on a project where we were trying to value engineer another firm's bridge design. As part of the bridge design there were lots of cantilevered concrete retaining walls (some as high as 24ft). It was very obvious that the EOR had designed all of the walls using Retain Pro and copied verbatim what the program had spit out as its output detail without using any judgement. This included tapered retaining walls with stepped footings which would have required all the formwork to be warped. As part of our VE package, we redesigned all the walls using a stepped wall design as the original design was unbuildable. Long story short, VE doesn't get accepted for political reasons, and original design stays. Massive contractor change order ensues due to these retaining walls. Be careful with tapered stems on retaining walls if you don't know how to detail them.

 

[wrxsti]

@phamENG what fc and fy did you work with to get that dev length
oo ok tried 60ksi and 4ksi

but what about the 0.7 modification factor for cover

 

[phamENG]

f[sub]y[/sub]=60,000psi
f'[sub]c[/sub]=4000psi

l[sub]dh[/sub]=[(f[sub]y[/sub]*(A bunch of factors I assumed to be 1.0))/(50*λ*√f'[sub]c[/sub])]*d[sub]b[/sub]

l[sub]dh[/sub]=(60,000psi*1.0)/(50*1.0*63.25)*(0.625in)=18.97*0.625in=11.85854123in

 

[dik]

I've used them lots of times... and much easier to construct than you would think... just a matter of tilting the form... tapered on one side only... and snap tie connections are easily 'shimmed'. Rebar cover is automatically adjusted by the tilted form... it's not really difficult to do and 10' to 15' is a good height for it...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[wrxsti]

i think retain pro prolly uses the 0.7 factor and told him his dev length is ok

 

[dik]

That's why you taper it... when it rotates, the face becomes more vertical... if it rotates more than 4", you may have other issues...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

dik - that's why I suggesting sloping the outer face in the next sentence...

wrxsti - you could be right. I don't like using that in a footing where development is absolutely critical to performance of a 12' tall retaining wall. I'd still make it a bit deeper.

 

[RWW0002]

Regarding the hook dimensions - Take a close look at the thread referenced above for discussion of anchorage vs development of the stem bars. I will leave the argument in the hands of others on this forum to keep from rehashing old discussion, but you might want to seriously consider providing more than a standard 90° hook at the stem base, even if you thicken the footing to provide appropriate development of the hook. As mentioned, anchorage in your case will be a bit more complicated also due to the limited toe condition.

https://www.eng-tips.com/viewthread.cfm?qid=401855