Retaining wall ideas Needed - steep slope with only top access

[tractchores]

Ok got some time to make a quick diagram of the slope etc. Bear with me in the description as I didn't have the patience to draw a topo map. The entire property is on a slight tile, mostly going from top to bottom of the diagram, but with a slight tile from left to right as well. The house site was leveled when built, as was the pool. This means that in the diagram the left side of the pool is a much more gradual slope and to go from the pool area to where you get to the more gradual slope down to the water is probably 3 - 4 feet. On the right side of the pool the drop is MUCH steeper and probably 10' to get to the more gradual slope. That slope is so steep that you cannot walk up or down it (even on hands and knees), I'd guess maybe 50 degrees? The 'gradual' slope down to the water is probably 20 - 30 degrees and from the edge of the pool to the water is likely 30 - 40 feet vertically over a distance of ~100' horizontally.

The real challenge here is that for equipment access you would come in from the right (lawn), which is the steepest side. I can get over to the left side with my tractor (have to go down through the woods or up and across a very scary slope. I cannot work from the bottom up due to the wetland buffer, which is 6' beyond the edge of the flower bed and within the extremely steep section. Contractors were thinking of cutting in from the left with an excavator and using 20 - 30 yards of fill to build up a flat area about 6' wide, which would then accommodate the toe pressure of the retaining wall and give a place to work. Given the access that's a very expensive project.

Walls under 6' in my town do not need engineering. I could probably get a wetlands permit to work into the 100' buffer zone and then restore it, but that's quite a bit of money and far from a guarantee.

 

[Firemanbuck]

You don't mention what the slope is composed of. Soil? Rock? Side-cast fill from the pool excavation? Each of these materials has it's own properties - weight, shear strength and soil pressure - that need to be factored into designing a retaining wall. I don't care what you call it, landscape wall or retaining wall, but you are looking at retaining soil, so it needs to be designed as a retaining wall. If it isn't properly designed, it will simply topple over with time.

Call a local geotechnical engineer or engineering geologist to come out for a consult. They are the ones who have the education and experience to design a decent, lasting wall. It doesn't necessarily have to be expensive. I like the gabion idea, but they are constructed in place, not filled and then lifted over the side. Keystone blocks are simply decorative facing, when dry stacked they don't have enough internal strength to resist the active soil forces. That's where the geogrid comes in - it increases the internal strength of the backfilled soil and reduces the active soil pressures. Same goes for rip rap, unless it's made from BFR (big f*ckin rocks).

Lastly, I'd listen to the contractor. They are the ones with experience and know what is needed for access and material movement. Working on your tractor and crossing a "very scary slope" sounds like a recipe for an injury, or worse.

/just my 2 cents

 

[tractchores]

You don't mention what the slope is composed of. Soil? Rock? Side-cast fill from the pool excavation? Each of these materials has it's own properties - weight, shear strength and soil pressure - that need to be factored into designing a retaining wall. I don't care what you call it, landscape wall or retaining wall, but you are looking at retaining soil, so it needs to be designed as a retaining wall. If it isn't properly designed, it will simply topple over with time.

Call a local geotechnical engineer or engineering geologist to come out for a consult. They are the ones who have the education and experience to design a decent, lasting wall. It doesn't necessarily have to be expensive. I like the gabion idea, but they are constructed in place, not filled and then lifted over the side. Keystone blocks are simply decorative facing, when dry stacked they don't have enough internal strength to resist the active soil forces. That's where the geogrid comes in - it increases the internal strength of the backfilled soil and reduces the active soil pressures. Same goes for rip rap, unless it's made from BFR (big f*ckin rocks).

Lastly, I'd listen to the contractor. They are the ones with experience and know what is needed for access and material movement. Working on your tractor and crossing a "very scary slope" sounds like a recipe for an injury, or worse.

/just my 2 cents

Thanks. As described above the soil is mostly clay filled with boulders. It is well drained and stable (It's been like this for ~30 years). The slight sinking of the pool deck is probably from original settling VS the hill moving. Its well vegetated, so no erosion issues or run-off problems. My guess is that it is the excavated fill from the original house and pool construction, which means its just clay that got dug up and depending on the size of the excavator they used probably free of the car sized boulders I regularly find in other parts of the property.

Definitely worth building right. Since this is mostly aesthetic and hopefully will be small I'm trying to balance 'value' with 'doing it right'. Obviously none of it is worth doing anything that is unsafe on my tractor or with any other equipment. The contractors advice has ranged from major excavation and a face cut to pounding in a few steel poles and putting a 2x6 flat between them to hold up the last foot of soil to make the flowerbeds level (not considering this one...).

 

[CurlyDave]

... I don't care what you call it, landscape wall or retaining wall, but you are looking at retaining soil, so it needs to be designed as a retaining wall. If it isn't properly designed, it will simply topple over with time.

Call a local geotechnical engineer or engineering geologist to come out for a consult. They are the ones who have the education and experience to design a decent, lasting wall. It doesn't necessarily have to be expensive. I like the gabion idea, but they are constructed in place, not filled and then lifted over the side. Keystone blocks are simply decorative facing, when dry stacked they don't have enough internal strength to resist the active soil forces. That's where the geogrid comes in - it increases the internal strength of the backfilled soil and reduces the active soil pressures. Same goes for rip rap, unless it's made from BFR (big f*ckin rocks).

Lastly, I'd listen to the contractor. They are the ones with experience and know what is needed for access and material movement. Working on your tractor and crossing a "very scary slope" sounds like a recipe for an injury, or worse.

/just my 2 cents

I agree any wall needs to be engineered in order to resist soil movement. But, if you call it a Retaining Wall, the town will demand engineered drawings which can be very expensive. For a Landscape Wall, they will be satisfied with pre-engineered examples from the Keystone website and brochures. This is much less expensive than the custom drawings they want for a Retaining Wall.

An engineer such as you suggest is a very good idea, and will save money in the long run. Again a Landscape Wall can be built from informal sketches, which are vastly less expensive then formal drawings.

The Keystone blocks are much more than decorative facing. They have examples of the use of their blocks which will produce a wall good enough for almost any application under 4', which is higher than the OP needs. Used with their geogrid, and their recommended backslope (they call this "batter") you will hit a home run every time. The wall will be stable and safe.

If you use one of the canned designs, a strong, decorative and good-looking wall becomes a DIY project.

I built more square feet of wall than the OP with a crew of 2 to 3 day laborers and directions from me. I needed engineering because of the height of my wall, but with low wall like the OP is mentioning, a 3-4 hour visit from an engineer just as a sanity check, and a small crew will be sufficient. In many instances I have found that contractors will push an owner toward techniques the contractor is familiar with, and will almost always be more expensive then a DIY project. This wall is so small that a tractor is not needed. You would be amazed at how much material an experienced laborer can move.

* * * * *

Tip: Look at different types of walls and check to be sure the engineer you call can design with that rock. Companies like Keystone sell computer programs specialized for their product only. If the engineer already owns the program he will have a minimal charge for using it. If he has to buy the program he will either turn down your project or charge the full price (expensive) of the program.

 

[Streetcar]

I have built keystone walls over twenty feet in height. They were engineered and used geogrid.
In most states the height of the wall determines design required for plan approval
From description of site, it seems the wall will be located on a previously filled area. An engineer will be able to determine bearing capacity of the soil.
For short walls the design is fairly basic, I have used the blocks interchangeable to the design. The different types of blocks are slightly different in size

 

[timswi]

We used Keystones on my neighbor's wall. It's apx 8' high and probably 85' long with geogrid and has a slightly meandering design...it's holding back a wall of vertical waterlogged shale from the massive pasture above it...We
French drained and center and back filled with gravel..It has held up very well and looks spectacular.

 

[tractchores]

Thanks for the good ideas, keep them coming. How would I go about finding an engineer who could do something like tell me what the bearing load for the soil is? Do I just look up landscape engineers, or is there a specific type of engineer that specializes in this type of thing?

 

[CurlyDave]

... Do I just look up landscape engineers, or is there a specific type of engineer that specializes in this type of thing?

Look up geotechnical engineers.

 

[woodlandfarms]

On the wetland buffer, do you own the land to the water? Outside of the wall, I was wondering if it would be cool / appropriate to put stairs down to the waterfront.

 

[ultrarunner]

In my city permits and engineering are often required for the simplest of projects plus design review for astetics is often thrown it.

A very large home went in across the canyon... 12,000 square feet and a lot of grading and tons of piers required for the home...

I mention this because the project also required and extensive retaining wall... the owner had an excavator and trucks delivering large boulders for nearly two weeks... no permit or engineering required because it was natural stone... found it odd and the HOA did research and found the same...

Looks magnificent and blends in well... the contractor does a lot of Napa and Sonoma wineries.