Stabilizing / "fixing" a leaning concrete cantilever wall 3

[ptdgeo]

Does anyone have tips regarding recommendations for stabilizing or in the clients words "fix" leaning concrete cantilever retaining walls?

A client has an emergency spillway for a lake that was constructed of segmental concrete cantilever retaining walls that vary in height from 4 feet to 13.5 feet (stem height). Each wall segment is 13 to 15 feet long and from what I can tell from visual observations they are not tied together. The spillway has a concrete floor and is approx 50 feet wide. All the displacement is along the taller walls near the bottom of the spillway and they are leaning into the spillway. The last wall sections have displaced 7 inches at the top on the east side of the spillway and 5 inches at the top on the west side of the spillway. The displacement reduces to no sign of movement as you go back up to the spillway entrance.

Borings indicated the backfill behind the walls consists of Lean to fat clay with liquid limits in the range of 50 to 60 and PI's in the range of 33 to 43. The backfill is fairly level for a distance of approx. 15 feet behind the wall then angles up at approx a 2.5 to 1 slope. The backfill was underlain by native shale at footing level. Behind the backfill zone the native soils consist of Sandy lean clay (LL in the range of 33 to 36 and PI's in the range of 22 to 24) underlain by Shale. The Shale appears to start approx. 4 feet above the bottom of the footing for the wall.

No information available about the walls other than the tallest walls have a stem height of 13.5 feet and are18 inches thick. The thickness of the stem reduces to 8 inches when the wall height is less than 10 feet. From test pit info the heel appears to be only 5 feet long from the back of the 13.5-foot tall stem.

 

[civilperson]

The lean should not be a problem once stabilized. Measure the same points at 3 month intervals to see if the lean is progressing. If the movement continues, then a fix is necessary, if no movement continues then the wall is functioning as designed.

 

[cvg]

you could construct deadmen tied to the top of each wall to hold them from tipping. I have looked at this in the past for spillway training walls. soil or rock anchors could also work but might be more expensive. If your footing is only 5 feet wide for a 13 1/2 feet tall wall, it looks like the design may have too small of factor of safety. I doubt that the dam safety regulator will allow these walls to remain leaning or un-fixed... Monitoring at intervals is a good idea. Also, the failure modes should be evaluated to determine what caused the leaning. Is there excessive seepage through the dam embankment or abutment which is causing hydrostatic pressure or swelling of the clay behind the wall? Perhaps you also need to install better drainage behind the wall. You don't show any in your diagram.

 

[msquared48]

I agree with cvg.

The toe soil may be overstressed causing the rotation and an inadequate foundation drain may be an additional factor. At 13.5 feet of fill, I would expect a toe in the order of 3 to 4 feet and a heel ... maybe 4 to 6 feet to control the toe pressure. I would also expect a 10 to 12" thick wall and a footing 11 to 13" thick, f'c 4000 psi, grade 60 steel.

If ythe rotation continues, you might consider either reducing the level of fill behind the wall, or tiebacks grouted into the undisturbed material outside of the slip circle. You would have to carefully consider the reinforcing pattern in the wall to properly place any tiebacks.

Regarding the drainage - were 2" round weep holes provided, or a foundation drain? Weep holes are notorious for plugging up.

Another scensario, but less likely, is that in times of spilling, the spillway chute may be leaking water to the base eroding material at the base of the retaining wall footing. Just a thought.

Mike McCann
MMC Engineering

 

[Prepakt1]

Perhaps jacking off the far wall will enable them to be pushed back into position. It will need to be very slowly done. one or two cm/day .Once the walls are vertical. you will need to grout beneath the base to fill the resulting voids and support the base.Water flow at the back and below the base should also be checked. This is very important. If there is any and you block by grouting it will cause problems elsewhere.

Intrusion Prepakt /marineconcrete.com

 

[ptdgeo]

all good comments.

You are correct that Dam Safety has recommended to the owner that the spillway be investigated and any necessary repairs be made before they will renew their permit.

I’ve considered anchors but hesitant without knowing anything about the reinforcing steel in the walls. Also, a little concerned with trying to “push” things back into place – Could that cause additional damage to the walls? Again no idea if the stems are doweled into the footing or how they were constructed.

Also, there is additional info that I didn’t include in the op. There is groundwater concerns and there is the possibility that there is seepage under the spillway slab that could be causing some scour. There wasn’t enough money in the budget for installation and monitoring of piezometers, however groundwater was encountered in the sandy lean clay behind the backfill zone and at 11 feet below the top of the wall in the backfill zone. A test pit was also before behind one of the walls and there were no drains or gravel backfill behind the wall, all clay backfill. At the 11-foot depth, we encountered sandy lean clay behind the wall and water freely flowed into the test pit. Some sluffing started to occur so we quickly filled the test pit back in.

Another concern I have with anchors or deadmen behind the wall, is the need to remove the backfill. Concerned with a blow-in from the groundwater conditions or if somehow there may be a hydraulic connection with the lake, which lead me here searching for other thoughts on stabilizing or fixing the leaning walls.

Also wondering if the wall may have been underdesigned. From what I could tell from the test pit the heel seems short. A boring was performed 8 feet behind the wall and did not hit a footing. The walls were constructed with a short leg on the upstream side of the walls that extend back approx. 3.5 feet. (see pics)

Attached are a few pics of the spillway walls.

 

[ptdgeo]

pic 2

 

[ptdgeo]

pic 3

 

[ptdgeo]

pic 4

 

[ptdgeo]

pic 5

 

[cvg]

I have seen spillways constructed with struts which support the top of the walls. this might be another option you consider. See attached sketch

 

[ptdgeo]

cvg

I see what you mean from the sketch. Worth a look. Would you try to push the walls back? Also the bottom walls are nearly parallel but the spillway is wider at the entrance than it is at the outlet so the walls angle away from each other.

 

[cvg]

Struts or anchors will stabilize the wall. I think pushing them back might be ineffective unless you excavate the backfill material first. This would be costly and unless you plan to also install the wall drainage, would be unnecessary. However, your regulator may require it to renew your operating permit.

 

[PEinc]

I stabilized a similar wall several years ago. It had tilted up to 9 inches. I used Chance Helical Anchors. I also installed vertical drains behind the wall. I drilled 2' diameter holes vertically and immediately against the back of the wall to the top of the footing heel and placed clean stone wrapped in a geotextile filter fabric. At each vertical drain, I drilled weep holes that butted to the drain. I excavated a trench along the top back of wall and filled this with clean stone wrapped in fabric. This horizontal French drain connected to the vertical drains. It collected surface water from rain and melting snow piles in winter. For aesthetics, the front of the wall was then screened with treated wood slats which covered the tilted wall. We did not straighten the wall.

 

[cvg]

good idea for the wall drainage rehab! I have suggested the anchors before for spillway walls, but the geotechs I have worked with have always recommended against it. Generally (I assume) because the anchors would penetrate into the dam embankment and that would not be approved by the regulator.

 

[ptdgeo]

We have had speciality contractors come to our office and give presentations on the different applications where they have used Helical Anchors and I have been considering that option.

Did you design the anchor in-house our sub it out?

Debating over giving a local speciality contractor soil strength perameters and letting them design a restraining system with criteria input from us.

I like the type of drainage system you installed also.

Thanks for the comments.

Ptd

 

[DRC1]

I belive the footing may be under designed. This as well as the structural design of the wall should be cheked.Also, granular fill should have been used to tobackfill behind the wall. The wall should not be pushed back as the force to resre the wall will be several times the force required to tilt the wall. To restore the wall you would need to drive tied back, 2 levels,sheets behind the wall, excavate the fill, correct the footing and backfill with granular material.
If you chose to leave the wall in place, I would use bar anchors drilled into the rock. the bar anchors could be left with out tension; thus reducing premenat stress on the wall. Helical anchors in the clay may creep over time, worsening the problem.

 

[cvg]

the problem with the bar anchors is that in order to approve the design (and approve the plans for construction permit), the regulators will need to know the pullout strength. that cannot be ascertained until they are constructed and pull tested. So, for design you would have to estimate a very conservative capacity and install a lot of anchors. With drilling and grouting, retaining plates and excavation to install them, you are talking about a lot of money. Not to mention the fact that you are working on a steep slope on the edge of the spillway. You will also need to know if the footing is strong enough to handle the additional loading. I agree, the footing is likely not wide enough. I would have thought at least 7 or 8 feet wide, not 5 feet.

 

[DRC1]

The capacity of the rock anchors can easily be computed and the rock will provide excelent pull out strength. Proof testing will confirm the strength for the regulators. The helical anchors will be shalow and anchored in a clay that may have issues. In order to be effective, these anchors must be installed so that pull ou resitance is developed outside of both local and global failure zone, which in this case are known to be an issue.
If you only pin the wall, excavation would not be required. If you want to reset the wall, correct the footing and properly backfill, I was saying you would need to drive sheets behind the wall and excavate. That would be expensive.

 

[cvg]

how will you install rock anchors to pin the concrete footer at the back side of the wall under 13 feet of backfill without excavating it first? How do you know what the pullout strength in the rock is without testing? How do you test it without mobilizing your contractor, excavating the backfill and installing a few anchors? What happens if your pullout strength is not what you thought? So without knowing any of this, how do you design the rock anchor system?

Note that I do not support the use of soil anchors at all due to clay material and potential for anchoring close to or in the embankment.