Who knows about stacked-block retaining walls?

[airbiscuit]

That's quite a project! You will like having that extra parking space. I think it will last as long as you do.

 

[KennyG]

It's looking to me like this is a reasonable approach for a very difficult situation. There are so many variables in soil type and configuration, it's hard to guess. It may not last so well but it may be in fine shape 30 years from now. I do like that the overall slope is so steep that with any kind of drainage, it shouldn't stay too wet. The trees are worrisome but they are so big, I can't imagine they are growing very fast.

 

[ultrarunner]

The site slopes are typical of hillside development here.

A home built say 1960 compared to the one built 50 years later look very much the same on the surface.

The difference is 24 to 30’ deep drilled piers and shear walls plus code for energy efficiency, etc.

Engineer explained it like this… since the beginning of time foundations were designed to support the structure above… now foundations are designed to hold the structure down… slides, earthquakes, etc.

The “New” home had 200k expense just for the foundation…

 

[Sodo]

All I can say is I hope you have Gods good Grace looking over your shoulder!
I know I have rarely temped God with so many challenges and got away with it

Several location on my (steep) land are 'fills' that have been here since ?1960s? and they are still OK. They don't move.
And they have no geotextile reinforcement.
I have some other areas that I have filled even 12 feet thick, simply dumping the dirt over the slope. Areas where there's no consequence to soil slippage. They are firming up, not moving.
There's no water source above, and no earthquakes (yet).

I don't think many have a concept of how substantial geotextile reinforcement is.
A lot of folks understand "roots".
Geotextile is like permanent, horizontal roots, but much stronger because it's 2-axis. And it doesn't die then decompose.

The "foot" of my geotextile layup is near the base of the slope, so there's nowhere for this layup 'to go'.

Here is a quick sketch.
I expect the topsoil to completely cover the (sand) lifts and protect them from erosion.
I do not expect enough water to erode anything.
In April, vegetation will grow with a vengeance.

 

[2manyrocks]

We have some 50+ year old T posts. I suspect, but can't prove, that the newer T posts aren't as thick and are more brittle.

One concern with anchoring into sand is whether T posts would work their way out of it over time. The helical ties are designed to bore into soil and then have resistance to withdrawal. I don't know which one would resist movement and withdrawal better over time in sand. I know I can pull a T post from soil using a manual t post puller, but I've never tried pulling a helical anchor out.

I suppose you could drive both into sand and then see if either one anchors better. But I also think you don't want to make any unnecessary holes in your existing Geofabric if it's only going to weaken what you already have.

 

[EddieWalker]

I'm struggling with understanding how your soil is "sand" and why it doesn't wash away when it rains. That slope next to your retaining wall is very steep. How does it remain there with your soil being sand?

 

[ultrarunner]

It looks like the slope is angle of repose from graded material pushed over the edge?

Where in Washington are you… I remember Oso where 43 died in a massive mud slide.

A few years back the slide made world wide news taking everything for a ride and nothing in the path survived.

2014 Oso landslide - Wikipedia

en.m.wikipedia.org

 

[Sodo]

Engineer explained it like this… since the beginning of time foundations were designed to support the structure above… now foundations are designed to hold the structure down… slides, earthquakes, etc.

Good to keep this in mind.

Geotextile is one of those significant changes.
But how long does geotextile last?
I doubt "planned obsolescence" has been engineered, like a plastic trash can or plastic rake.

It needs to be placed such that it does not create a slip-plane though.
Again, I am 'hopeful' that I have not placed this mass on a slip-plane. It's sand below.
ALL of the trees are growing perfectly straight up vertical.
The trees do not indicate that any of my slopes are moving.

Highly doubt poking a few holes in the geotextile will weaken it. They will be very few.
I would cut the square corners off the T-posts leading edge.
Making a sharp point, to divide the fibers, not cut as many.

 

[Sodo]

I'm struggling with understanding how your soil is "sand" and why it doesn't wash away when it rains. That slope next to your retaining wall is very steep. How does it remain there with your soil being sand?

It's glacial. Imagine sand/with a little soil mixed in, with the crushing weight of 500 feet of ice above it.
"Legend has it" that the weight of the ice flow compressed the entire land 200-300 feet creating the channel for the water body of Puget Sound for example.

It has ~ 20,000 years of organic material ontop (looks like about 1-2 feet).
NO question it would "wash" if there was water movement.
If where was water movement here, it would have been gone perhaps 19,000 years ago.
I do have to be mindful with my driveway water and roof water.

I definitely remember Oso!!
That was a slope with a river undercutting it.
Look at the topographical map, that slope above the neighborhood was simply the "next in-line" to fall.
Unknown "what century" it might come down, but its demise was inevitable.
The re is a memorial at the site. Anyone driving US Hwy 2 should allow time to stop there and read about the tragedy. Oso Landslide Memorial - Wikipedia
The lawyers tried to get in on it but ultimately the USGS determined that it was inevitable but not clearly human-caused.
The lawyering wasn't successful in absconding with taxpayer funds. But the taxpayers of the county had to buy the now-condemned land.
And the logging companies were very vulnerable, and had to pay some judgements as expected in our jury-controlled lottery system.
The wikipedia article Ultrarunner linked explains it.

Yes the slope pushed over the edge from the geotextile layup, is angle of repose.
I will push topsoil over that (next).
If it slips down a bit there is no consequence.
When that happens, it makes a real firm 'toe' at the bottom,
then I'll just fill again with little more topsoil, and the vegetation grows.

 

[2manyrocks]

Fwiw, there are some internet papers discussing the holding power of helical tie backs placed in sand. But starting to get so deep into many factors that it looks like something that requires an expert which I am not.