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Reinforcing existing retaining wall

Reinforcing existing retaining wall

Reinforcing existing retaining wall

I have a 12' deep 24" thick concrete retaining wall (no tiebacks) that needs to go about 6' lower. I plan to add tiebacks and underpin with epoxied rebar so the whole thing acts together continuously. I'm a structural engineer, not geotechnical, but I want to do this safely. I've been reading about retaining walls for 2 days and couldn't find a definitive answer. How to I check the existing wall, global stability, and if this method actually works? I could just buy software to do it for me, but I don't function that way. I'm a hand-calcs kind of person.

Plus, there are a series of stepped wood retaining walls beyond this thing. Should I reinforce the wood retaining walls with helical tiebacks and angle anchors? I don't know how to analyze this; I'm at the point of just saying that I can't do this. Not to mention seismic concerns, which I can't even fathom. But I really want to learn this and do it the right way.

I'm at my wits end; I just need a starting point to do a code-justified design, and I want to be able to sleep at night so that nobody gets hurt from my design. A starting point, please.

RE: Reinforcing existing retaining wall

I'd post some photos, with side view along the contours for a general idea, as well as a sketch of what you would like to do. Remember, our usual problem with posts is that important information is missing. Include any soil data, even if old. What kid of backfill might you be proposing? What about ground water? Drains? surface drainage? Start off with plenty of info please.

RE: Reinforcing existing retaining wall

This sounds tricky. Underpinning alone is unlikely to work. You can install tie back anchors but you need some embedment at the toe or you will have some kick out movement.

Another option is an L shape with the L returning away from the retained soil. You would likely need a long base to get the frictional resistance and this would have to be constructed in a hit and miss methodology to maintain stability of the existing wall. Again very tricky and impractical.

More info needed as OG said but initially to me it looks like a demolish and start again job! Could sheet pile behind the existing wall (depending where the heel is) and then remove the wall and excavate down to the required level.

Or demolish and batter back and construct a concrete L retaining wall.

RE: Reinforcing existing retaining wall

Thanks for the replies. I'm not sure how to upload multiple files, so here are links to the existing conditions:


Summary: Soil is medium dense sand. There are stepped wood retaining walls with deadman anchors and then I need to do a deep excavation (about 33'). In one place it's temporary (Section B), in another place most of it is permanent (Section A). An existing concrete wall has visible drain holes but the wood retaining walls don't, or maybe I need to go back to the field and look for the drain holes. The walls look very old, and they must have some drainage if they're still standing.

Sorry about the initial description. Now when I drew it in AutoCAD, I realized that it's much deeper than what I was thinking.

EireChch: I'm proposing to keep the wood walls as is, and reinforce if necessary. In front of them, I'm planning to do soldier piles, lagging, and tiebacks down to the required level. For the permanent retaining wall, I want to cast concrete against the temporary wall. My concern is the soldier piles corroding from contact with soil, which isn't a good long-term.

RE: Reinforcing existing retaining wall

This project looks like a serious candidate for an embedded retaining wall. You could use diaphragm walls, secant or contiguous pile walls placed along the footprint of the new building and avoid the temporary excavations and walls and any underpinning work, besides the reinforcement of those wood walls. The embedded walls could be supported during construction either with temporary soil anchors or be built with a top-down method, using slab strips parallel to the wall as supports (harder to execute but an alternative nonetheless).

For the design of these walls you really should get a geotech involved for all the soil-structure interaction analysis, soil anchors design and definition of construction sequence. I'd analyze this with a 2D plane strain model (ADONIS [which is free], Plaxis, RS2, etc.)

You could get a feel of the design forces and soil anchors needed with the Terzaghi and Peck envelope pressure diagrams for multi-propped walls in sands (0.65*Ka*gamma*H + 1.3*Ka*q) for the right side of the excavation adjacent to the road, but for the left side, a 10m high wall retaining a sloped ground and with water on the base level, it's probably a crude approximation. The global stability of that wood-crib retaining wall slope is also of concern and needs to be assessed properly.

RE: Reinforcing existing retaining wall

The sections from the start would have been a big help, it’s completely different than I imagined. Quite a challenging job. I agree that a embedded wall is your best bet. That temp retaining wall is 29ft (~9m!), and is taking the surcharge from the tiered walls above and has reduced passive resistance due to the sloping ground in front. You’re going to need a beefy wall to control deflections. It’s definitely doable with a soldier pile wall, but I think a bored pile wall with a few rows of anchors would be better. These may go over the property line which could be an issue.

I also think the internal stability (over turning etc) of the timber walls need to be checked. If was building that house I would be concerned. How are they constructed?

RE: Reinforcing existing retaining wall

avscorreia: Thanks very much for the response. You're right, an embedded wall would be great. It would double as a foundation wall and remove the need for temporary excavations. I'm going to use secant pile design because I've done similar designs, whereas the top-down method is totally new to me. I'll probably get a geotech involved during actual design; this is preliminary. I'm not qualified to do an FEM soil-structure analysis, but for the preliminary feasibility design, I plan to use Terzaghi and Peck envelope pressure diagrams and an analysis based on active pressures from different soil strata. If I can get away with it, I want to do everything by hand and avoid FEM. I was thinking that instead of using beta angle to calculate active pressure, I can use a series of line load surcharges based on the depth of soil above.

EireChch: I agree, I should've made the sections from the start; I imagined it completely wrong until I drew it. I'm not sure what you mean that the bored pile wall might go over the property line. I'm planning to keep all the new work as far away from the property line and existing houses as possible. The house on the top of the slope is old wood construction. It's about 20' (6m) from the property line.

RE: Reinforcing existing retaining wall

I mean your anchors might go over the property line.

Also I would design for ko pressure. ( this could another debate going smile )

RE: Reinforcing existing retaining wall

I thought that Ko is for rigid walls and basement walls. I believe that your implication is that the soldier pile wall or diaphragm wall with anchors is stiff, so Ka doesn't apply. I'm not qualified to side with Ka or Ko, but it has huge cost implications.

RE: Reinforcing existing retaining wall

We use ka for temporary (in simplified or elasto-plastic analyses) and k0 for movement restrained basement walls for long-term conditions.

RE: Reinforcing existing retaining wall

I'd consider a shotcrete soil nail wall with under pinning of existing structures. I'd make damn sure your global model is conservative. Most importantly I'd hire a good geotech and do some additional subsurface investigations for this.... this has liability written all over and you want the security of sound due diligence before even touching that toe.

RE: Reinforcing existing retaining wall

I was originally thinking of a soil nail wall coupled with installing micropiles to underpin the concrete wall. They both can be designed for permanent conditions and the soil nail wall can be faced with whatever your client wants. I would only recommend it if you have very good geotechnical contractors in the area. If not, forget it.

Ka is fine for these walls. Ko would be overly conservative and costly. Not sure why Ko is always brought up on this forum when it's for walls that allow movement. Just increase your factor of safety if you're puckering your cheeks.

RE: Reinforcing existing retaining wall

I struggle to see how soil nails combined with micro pile underpin will work? What do you do, underpin with micro piles then excavate downbeside them and install a row of nails between the micro Piles?

If you construct a bored pile wall you shouldn’t need to underpin at all.

Re Ko and Ka, for the temporary wall ka. If the wall is to be incorporated as part of the structure (as the op indicated) then to limit/reduce deflection it has to be ko....imho. It gives a longer stiffer wall.

Another argument for ko is that the wall will be restrained due to the anchors or slabs

RE: Reinforcing existing retaining wall

Quote (EireChch)

What do you do, underpin with micro piles then excavate downbeside them and install a row of nails between the micro Piles?

Yes. Micropiles are typically slightly battered not plumb. It's been done on several projects and is cheaper in areas where I have practiced than doing bored pile walls. I could see bored pile walls being possibly cheaper in areas where they are utilized more often.

Keep in mind that this site is sandy. I only have experience with bored pile walls on clayey sites since sands typically collapse or slough when constructing large diameter bored piles. Casing will likely be be required driving up costs and potential construction issues. Maybe you or someone else can comment on this aspect since my experience with bored pile walls in sands is non-existent.

Quote (EireChCh)

Re Ko and Ka, for the temporary wall ka. If the wall is to be incorporated as part of the structure (as the op indicated) then to limit/reduce deflection it has to be ko....imho. It gives a longer stiffer wall.

Sorry, I read that conversation backwards (bottom up). For some reason I thought that you were talking about a temporary wall. I agree, Ka for temporary wall that is allowed to deflect and Ko if it to be incorporated into the final building.

RE: Reinforcing existing retaining wall

Thanks for the clarification MTN.

Yes bored piles in sand would definitely require casing to prevent collapse.

CFA piles could be an alternative to avoid casing requirements etc.

I think the options we have discussed are all do-able..just depends for how much money. (like everything)

RE: Reinforcing existing retaining wall

Thank you all for your responses. It has been very helpful in communicating with the owners and presenting all the options.

geojosh84: I already have about 20 borings, but I'm definitely going to get a few more at the high ends of the slope.

MTNClimber: That's an interesting and creative idea. The owner is leaning more towards making tall 20'-30' underpinning pits and using high-capacity helical tiebacks. I didn't know helical piles can get around 150k in tension, but they could. For some reason, he doesn't want to use any heavy machinery, like for a secant wall or micropiles. His solution is more expensive, but I think it could work.

I checked global stability and it is failing, as expected. I'm going to use helical tiebacks to reinforce the slope.

RE: Reinforcing existing retaining wall

How's he gonna excavate 20'-30' pits? they will need some retaining..?

RE: Reinforcing existing retaining wall

EireChch: I was thinking about making the pits with bracing. The soil lateral pressure should cancel out on each side, and then it becomes a calculation of the axial force in the bracing and the out-of-plane bending of the lagging. I'll have some tiebacks when excavation starts.

RE: Reinforcing existing retaining wall

Man that seems like a ton of work to me! Bored pile wall would be ideal. Regardless, its an interesting little job. Good luck and keep us posted!

RE: Reinforcing existing retaining wall

I talked some sense into the contractor. He finally agreed to take out the whole wall. Now we're doing a steel soldier pile wall with helical tiebacks. He's used to doing this, and wants to stay away from bored pile walls. Tangentally, there are some places near the top where the soil slope is no good in my opinion (and calculations), and I'm going to present the contractor with some kind of slope stabilization with plates or small piles.

Thanks for the reference!

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