Stabilizing / "fixing" a leaning concrete cantilever wall
Stabilizing / "fixing" a leaning concrete cantilever wall
(OP)
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.
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.





RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
Intrusion Prepakt /marineconcrete.com
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
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.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
I was called in by the project's architect/engineer to design a fix for them. I then hooked them up with a certified Chance anchor installer who handled the entire fix, except for the finished screening. I designed the anchors and the drainage system. The vertical drains were spaced at about 10 feet on center but the horizontal drain at the surface provided drainage along the entire top of wall. The stone in the drains was wrapped with the fabric to prevent contamination of the stone from soil fines. In order to minimize the hole through the concrete wall, I had the contractor core a small hole (about 4 inches diameter) and then excavate enough behind the wall at each anchor location so that the anchor helices could be set behind the wall and the shaft could be fished through the 4 inch hole to the front side of the wall. Then, additional extension shafts could be added while screwing from the front of the wall. Bevelled washers were used to provide the proper tieback angle with the tilited wall face.
The link will take you to a 1 page project report with a picture of the tiedback wall before the finished screening was installed.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
What type of soils did you have behind the backfill zone to anchor into? Did you have structural info on the walls (type of reinforcing steel, spacing, one or two curtains of steel, if one, what face, etc..) I do not have this info available for the walls on my project and nobody can seem to find a set of the original plans (owner or DNR, which oversees the Dam Safety in our State).
I'm concerned with causing cracks in the walls by changing the stress distribution along the wall without the structural info. I guess I could look at it as unreinforced, but that may call for very closely spaced anchors.
As mentioned above in other posts, is creep a concern for the tie-backs at the project you mentioned?
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
I think we are talking abou two different things. I think you are talking about excavating to the footing and installing anchors through the back of the footing into the rock. I am talking about drilling an anchor similar to a tie back anchor that the bonded section is drilled into the rock. Such anchors are similar in construction to grouted soil anchors. These anchors can develop in excess of 200 kips. Although you would have to drill and test an anchor, with a somewhat conservative design, one would have a fair degree of confidence of success. If the ancor does pull, it can be derated and subsequent anchors will be lenthen to achive capacity. However, to stabilize the wall, the required capacity is generally much less than the available capacity.
RE: Stabilizing / "fixing" a leaning concrete cantilever wall
Do you have the same problem here?