Retaining Wall Issues: Inclined, Near Structure, in Limestone

[Mr. Skippy]

Attached is a picture showing the site conditions. The bag wall is not acceptable and needs to be replaced. The ground conditions consist of top soil over limestone. There appeared to be some leveling fill, but the corner of the house is resting on limestone rock mass.

My thoughts:
1. Global Stability of the rock mass supporting the corner of the house: The bag wall has no soil behind it and wasn't designed to take any structural loading. I would think that the engineer who designed the foundation of this house considered the subsurface exploration and took into consideration a safe offset distance. I will need to check into this further.

2. Lateral surcharge pressure: I have not done any rough calculations, and I have no testing on the limestone rock mass, and the slope of the cut between the house and the wall is about 45 degrees and about 15 feet way... Tentatively I am aiming to proceed with no surcharge affect from the house on the wall, due to the geometry and material.

3. Height of Wall: The height of the wall would need to be about 8 feet high at least. A Cantilever RC wall would need about 4 to 5 feet of foundation width. This would require, excavating limestone rock, cutting down trees, and cutting material out from around the base of the foundation of the house. This would cause a lot of difficulties and probably underpinning the house.

4. The incline of the R-wall Foundation: This would create complex loading and construction issues.


My plan:
Is to stair step the foundation of a modular block retaining wall, to its max height allowed of 4 feet (Assuming the design allows). Then above that retaining wall set back an appropriate distance construct another 4 foot modular block retaining wall. Using properly designed drainage and geogrid if necessary.

If the assumption that the geometry and material can handle the surcharge of the house, then the goal is really to control the erosion of that slope. In which case stacking modular block retaining walls seems like the way to go.

Any insight would be greatly appreciated.
Thanks

 

[Mr. Skippy]

The owner got the prior engineer for the house to write a letter stating the house is founded on bedrock. Bedrock is very shallow and the slope between the house and driveway is cut from leveling the pad for the house and consist of some top soil with limestone chunks.

The wall will be 2 feet tall at both ends and increase in height and meet being a max 5 feet tall. The city inspector wants a 3:1 slope between the wall and house and below that natural grade is ok. I have come to figure out that the HOA has been complaining to the city because the house is of much lower value than the neighbors. The inspector wants the open cut along the driveway to be covered and look nice essentially.

The shotcrete wall would have a key into bedrock to resist sliding. The toe would be increased to 5 feet wide so the joint would be down the center of the 10 foot wide driveway. Only about 3 feet is required to resist overturning. The wall stem will be 6'' thick and the driveway portion will be 6'' thick by the wall and slope to match the 4'' thick driveway. The toe will be reinforced.

The toe being apart of the driveway is not ideal. I am still calling companies to see if they could drill for solider beams but am having no luck, because its a small residential project.

 

[Mr. Skippy]

Or, a very short 1 foot toe can overlap on top of the existing driveway which would shorten the driveway to 9 feet wide, and a short 1 foot heel could be deepened into the key. Then hand held rock drills could be used to drill holes for grouted dowels that are embedded into the key/heel. The issue is the uncertainty regarding the quality of the bedrock. I could use a hydraulic Jack and a load cell I have to measure the pullout strength to verify them.