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Foundation/ Retaining Wall Assistant

Foundation/ Retaining Wall Assistant

Foundation/ Retaining Wall Assistant

Good day,
I have a proposed building (see sketch) on a site of 50'x200'. The problem I am faced with is the land being sloped and requires a retaining wall before construction can begin.
The challenges are: what type of foundation can I use that will have minimal surcharge pressure on the retaining wall to prevent overturning and also what type of retaining wall is best suited for this application. Keep in mind this is a residential project so there are budget restrictions.

RE: Foundation/ Retaining Wall Assistant

It looks like the new building will be quite close to the retaining wall and the retaining wall is quite high. In order to minimize surcharge pressure onto the retaining wall from the new building, the foundations will have to extend down to the level at the base of the retaining wall. The retaining wall is fairly tall (30'). Presumably the soil material behind the retaining wall will be backfill material. Therefore, I think the new building should be founded on piles that extend to a depth below that of the base of the retaining wall, combined with grade beams to support the foundation walls.

RE: Foundation/ Retaining Wall Assistant

MSE block wall with geogrid is my choice. Will require minimal over excavation, will require quality structural fill which will help your building foundation out as well, performs well with surcharge loads as the weight of the building helps the geogrid pullout strength, performs well with any differential settlement or other potential future issues, and design can be delegated to the wall designer selected by the contractor. Should keep costs low and allow you to use any foundation you desire. MSE walls in my region are typically cheaper than cast-in-place walls and are generally architecturally pleasing.

Donwsides, MSE walls require good drainage to function properly (not suitable for a badly drained site), MSE walls require attention to detail on the grid/soil installed to ensure it meets spec, dry cast (CMU) block walls have low durability and repairs to the wall will be difficult after the building is built but dry cast blocks are cheaper than wet cast (precast) blocks.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries https://americanconcrete.com/

RE: Foundation/ Retaining Wall Assistant

Can you support the building by some pillars extended down to footing of the wall? Or just sit it on the wall?

RE: Foundation/ Retaining Wall Assistant

Looks like an expensive problem, especially if this is a residential project. You may also need retaining walls at each end of the main retaining wall (u-shaped layout) if the two adjacent properties are also sloped. If you choose MSE walls, you may also need temporary sheeting to allow excavation for the geo-grids. If you choose non-gravity retaining walls, you will need tieback anchors or a 4th wall (i.e., 2 sets of parallel walls) so that you can use thru-ties between opposite walls. With either MSE walls or tiedback walls, you would need to carefully place bearing piles (if piles are needed) so that you don't damage the grids or ties. With high walls and a narrow site, make sure any geo-grids or tie rods do not interfere with underground utilities.


RE: Foundation/ Retaining Wall Assistant

I looked at your sketch and that is a lot of fill to import nevermind the cost of the wall. Also do you need the wall or can you just do piers? The amount of stress on the wall can be greatly reduced if you offset the building from the wall with a path that is at least the width of the foundation. As for the best type of retaining wall I guess that depends on what is downhill and the foundation conditions for the wall.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

RE: Foundation/ Retaining Wall Assistant

One other thing that was not mentioned - does the new building have a basement????? If so, that could interfere with geo-grids or tieback anchors.


RE: Foundation/ Retaining Wall Assistant

"...does the new building have a basement????? If so, that could interfere with geo-grids or tieback anchors."

Or provide anchorage for the tiebacks...

If using MSE, below the basement the reinforcing can go as far as it needs to. Beside the basement wall, the 'retained soil pressure' is zero; the reinforced soil mass only has to support itself.

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