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.

 

[PEinc]

ptdgeo,

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.

 

[ptdgeo]

PEinc

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?

 

[DRC1]

CVG,
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.

 

[Bluefoot]

Studies by others suggested the main causes of retaining wall failures fall into two categories: 1) Improper design of the drainage system , 2) unsatisfactory dimensioning of wall base. This statistical study which is shown in the figure below (Techeng and Iseux 1972; Ireland 1964) also indicated that problem walls were most frequently backfilled and founded on clay soils.

Do you have the same problem here?