AboutRobert Cummings, P.E. Expertise I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.
Experience 30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.
Organizations Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.
Publications Mining Engineering, AEG Bulletin.
Education/Credentials BS and MS Geological Engineering
Question Thank you so much for taking the time to look into the retaining wall failure post and offering valuable information.
I have begun to weigh the different options to determine how to deal effectively with the "slope creep". I put in drainage and drilled holes in lower part of back wall(no water came out-appeared dry as a bone). From what you posted, the footer has sunk, causing the stair step cracks on the side to back wall failure and the single horizontal separation along the topmost row of blocks. To prevent more slope creep I will PROBABLY form up and pour a footer behind the existing footer along the whole length of the "retaining" wall, then put piers along the wall using #4 re-bar and tie them to the existing slab using anchor bolts this anchor would be L shaped and extend from each top most pier block to the slab where it would be bolted into the slab. I assume a hole could be drilled at a point level with the existing slab that would be large enough to accommodate. See: http://www.daytonsuperior.com/a02_movement13.html ...
My concern would be whether or not an expansion joint would be needed at this interface between the slab and the top most pier block.
Answer One thing to keep in mind when considering a supplemental footing and piers is that you are adding weight to the structure which can exacerbate settlement if the new footing is not designed to bear the weight of the old structure plus the new one. It will probably have to be considerably deeper than the old footing. Whatever you do, be sure the new footings bear on very firm soil, or, if practical, the underlying weathered bedrock, which you may be able to reach if you use helical anchor piers or drilled shafts. I would not necessarily recommend a strip footing, for this reason. It won't add much bearing capacity to the piers and will add weight. If you can, just deepen the pier excavations and place rebar cages before pouring concerete. If you dig for a new strip[ footing, depending on the steepeness of your slope you may expose the old footing and/or the bearing horizon when digging for the new one. In some cases the loss of lateral confinement of the bearing zone below a footing can produce settlements.
I am not sure I understand the connection between the pier and the slab. It sounds like you intend to bring the tops of the piers flush with the slab. As I imagine this, the piers are vertical with their inside edges against the retaining wall, which has the slab on top. Would that not mean the piers would be outside the slab perimeter? In that case, the structural connection you are talking about, between the slab and piers, would be accomplished by bolting horizontally from the pier to the slab? In this case the bolted connection is in shear, which is not ideal. You need to figure out how to get the piers beneath the slab so the pier is in compression. Perhaps you can offset the top rows of blocks to bring the pier under the slab. Keep in mind, when you do this, that the piers will have eccentric loading since the slab load is not imposed vertically down the center of the piers. This will put the piers in vertical bending so reinforcement will be required along the outer edges of the piers to provide tensile restraint.
If you stick with the shear design I would not use an expansion joint. It will only serve to reduce shear resistance between the pier and the slab -- you may get some cracking, but the shear resistance is more important. If you bring the pier beneath the slab then a means to reduce relative horizontal expansion stresses would be appropriate.
