Retaining Wall Design Solution? 2

[Redacted]

Hi all,

Just a bit of background and sorry for the long post :

There is an existing retaining wall that was built quite a long time ago that retains a road and the earth underneath it. The wall is made up of huge blocks of limestone about 1'x1'x2' deep blocks. See the attached image. There was some concern that was raised regarding the wall bulging out. Behind the bulge, there is about a 1m deep cavity, with the plan of filling this with concrete to ensure the road surface doesn't dip. This concrete fill will require the wall to be braced when placed.

I think that a few things are happening to cause the wall to bulge. The first being a tree root behind the wall is dislodging the stones and the second being that I think the wall is getting undermined by erosion, which is causing it to drop slightly. The wall is on coastal beach sand and doesn't look to be on a footing. We have had a surveyor down there and it was found that the wall bulged about 5mm in a month.

As a solution, I was going to design and construct a new reinforced concrete cantilever or gravity retaining wall with a shallow foundation and anchor the foundation into the adjacent rock formations.

Although if you can think of a more practical solution, please do share.

For the retaining wall design, I started to determine the horizontal forces but my boss mentioned that I am being too conservative? Essentially my assumptions are attached but I am assuming that the existing wall has failed and therefore this new wall needs to retain that wall and the road/earth behind it. I was assuming fully saturated soil and water pressure starting from the road surface because I didn't want to depend on a drainage system, which might get clogged. Not sure if this water pressure assumption is practical though. Would love to hear opinions and advice on this.

 

[r13]

I think this is an elevated roadway, the ground water table seems unrealistic. Check the local flood records and use, otherwise use moist soil is adequate.

It seems you have very good assessment, pay attention to the tree, the roots can spread very far, and destroy everything on its way. Remove it if you can.

Are you going to cast the concrete over the stone face, or remove the stones then cast concrete?

 

[Redacted]

@retired13

Yes, it's an elevated road-way, so the assumption of the water table at the top may be too conservative. I was trying to avoid having to put a drainage system in as I predict it won't be maintained properly and will get clogged up but perhaps this approach is cost-prohibitive.

The tree root may be a bit of an issue. The client doesn't really want to do any major works on the road surface itself other than filling cavities with concrete. To fully remove the tree they would need to excavate the road and pull it out (it's quite large). The road has quite a few services running through it as well. I was thinking we can cut the root flush with the stone wall and pump concrete into the gaps surrounding the tree, effectively encasing it in before constructing the new wall. What are your thoughts on that?

I was planning on casting the new concrete wall against the existing stone face.

 

[SlideRuleEra]

I think that a few things are happening to cause the wall to bulge. The first being a tree root behind the wall...
To fully remove the tree they would need to excavate the road and pull it out (it's quite large)...

Have you accounted for the weight of above-ground parts of the large tree as a point surcharge load on the wall?

http://www.SlideRuleEra.net

 

[BridgeSmith]

Depending on the soil conditions below the retained soil zone and interest in preserving the aesthetics of the existing wall, a non-gravity cantilevered wall (driven sheet pile) directly behind the existing may be an option.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Redacted]

@SlideRuleEra

I haven't accounted for the weight of the above-ground parts as a surcharge because it's quite a strange scenario but the tree isn't actually above ground. It grew sideways out of the wall and is resting on an existing rock formation. The main heavy portion (trunk and branches etc) were cut off at some point in the past. What remains is the smaller root system, which shouldn't account for significant load. The difficulty with pulling it out is that I think the roots are entwined in the rocks and fill so if it gets pulled out, a portion of the fill that is holding up the road will get pulled out as well. To remove the tree root safely I would've thought that the road surface would need to be removed and the tree roots dugout, which is why I was somewhat inclined to concrete the roots in. Although if they need to chop out the road to remove the tree for a structural reason that can be done but I am not sure if it is required.

See attached images.

 

[r13]

I would cut down the tree to the ground level, apply roundup, and get ride of the shallow roots. I agree, the concrete might kill. or prevent, the deeper roots that have no food source (the above ground tree) feeding them now.

For the ground water concern, proper road slope, sealing road-wall joint, or concrete ditch, is better solution. Also, well graded backfill and weep holes with PVC pipe can alleviate the headache of clogged drainage system, as they can be opened externally if necessary. Place larger pipe near the base, and smaller on higher elevation shall work.

 

[SlideRuleEra]

RStars - Very good. For backfilling the cavity, consider flowable fill instead of concrete. This may allow more options in the future if additional work is needed.

http://www.SlideRuleEra.net

 

[r13]

Make sure the roadway is properly drained, as it has not gutters near the edges. I think lime will kill the small roots. I think you need to apply surcharge load per use/traffic pattern of the roadway.

 

[Redacted]

Noted and thanks for the advice all of you!

 

[bridgebuster]

A few thoughts...

Since you're in coastal area, as was pointed out above, check the flood elevation because you may have to consider a rapid drawdown case. Assuming the water at the roadway level may not be that far from reality.

Cut the tree down, even if you can't pull up the roots. Filling the voids with flowable fill is much better than using concrete.

These types of walls move constantly, 5 mm in a month could be a lot for this wall or it could be normal. You have to have a baseline survey and regularly scheduled measurements, of multiple points, to decide if 5 mm is a reason to lose sleep. The wall looks like it's 150 mmm out horizontally and vertically in some spots, that's not good.

What is the pipe sticking out near the top? Too big to be a weep. The bottom of the wall looks like it was pointed. These types of walls were often built without mortar joints, over the years people point them even though it's unnecessary. Could the pipe be broken causing a build up of hydrostatic pressure? You need to figure out why it's moving, otherwise the new facing is going to move with it.

You also mentioned the possibility of a new gravity wall. Since you have doubts about a drainage system, maybe you want to look a T-Wall by RECO, or similar (no I don't work for them but I have used their system), no drainage system needed; the system is self-draining.

Another repair option - something I saw done on a 100-y/o rubble retaining wall. I wasn't involved in design; only inspected it. The designer used several rows of drilled in piles behind the wall to relieve some of the earth pressure. The wall height varies. Off the top of my head, I think the max height was 6-8 meters. Weepholes were also installed.

 

[STrctPono]

There are certainly a lot of options. Depends on how much of a quality the client is looking for, what your right-of-way limitations are, what your traffic control limitations are, and how much they are willing to pay. What is their budget?

Where is your Right of Way? In other words, how far beyond the wall does the client own the land?

I would not concrete the voids behind the wall. If you can remove some of the backfill then you could use what others have said... a flowable backfill. We call it CLSM out here.
If excavation is not possible then perhaps you could pressure inject a flowable grout through the wall to fill the voids. This would require filling the gaps in the block to avoid losing the material out the front.

To help stabilize the wall, you could drill a secant pile wall either on the frontside or backside (roadway side) of it. If on the frontside you would have to backfill between the secant piles and existing wall. Also kind of a pain since you could have to provide temporary casing above ground. If on the backside then you don't need casing but have potential traffic control issues. Kind of an expensive option but we've used this option before in areas where site excavation is not possible and there is major erosion or seasonal coastal wave issues.
As BridgeSmith said, you could also do sheet piles, which essentially serves the same purpose as the secant wall but uglier and not as durable since it will likely rust.
If the wall is long, then it might be cost effective to install tie backs. You can drill the tie backs through the wall, grout them, and then shotcrete the face using the existing wall as your backform.

Probably all of the above options are expensive so some more practical ones would be:
As you mentioned, construct a new cantilever or gravity wall on the frontside. I would opt for a gravity wall and then fill the void between the gravity wall and existing wall with flowable fill to make one large gravity wall.
Bridgebuster has a good idea with the T-Walls. We've used these before on a few emergency jobs where DOT highways have been falling onto the beach due to wave erosion. They are nice because they can be constructed quickly with minor earthwork required. Since they are precast, you piece them together like stackable blocks and fill the voids with CLSM. The end result is a gravity retaining wall.

 

[Redacted]

@bridgebuster

Yes, I will definitely take a look at the historical flood charts. Will also consider predicted sea-level rise as well.

I agree, flowable fill looks like a much better option opposed to standard concrete, I will look for a reseller.

Yes, 5mm movement doesn't seem too bad to me, I am more concerned with the 150mm horizontal and vertical drops, which I assume are due to the tree root pushing out the stones and undermining from erosion respectively.

That's a good question regarding the pipe and I am actually not 100% sure, the wall was built a very long time ago and there are no historical drawings. The pipe stretches all the way from across the road between the retaining walls so I think it may have been used to relieve pressure from storm surge. The pipe is broken, although we haven't had any storm surge within the month that the 5mm difference was surveyed.

Thanks for the alternative options, I will keep those in mind.

 

[bridgebuster]

@RStars - attached is a standard sheet from 1913, showing dimensions various dry stone gravity walls; I've found it helpful in evaluating old walls. You mentioned that the blocks in your situation are 1' x 1' x 2', if there aren't any cut blocks behind the face blocks, dollars to donuts there's loose stone rubble behind them. Something to keep in mind when selecting an alternative.

 

[r13]

Have you considered shotcrete?