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.

 

[Sodo]

The helical anchors will arrive today Sunday.
Crazy what Amazon has done to projects, with one-day delivery.
I cannot IMAGINE they will hold as well as 6ft (or even 8ft) T-Posts.
Maybe I'll just toss them in the toybox and go get 12 T-posts.

I'll know how easily (or difficult) a 5 foot post drives in, fairly soon.
Driving 12 T-posts could be a chore, but I have 2 days....

Here's a pic of a sharpened T-post and my two drive methods.
I bought the gas-powered driver for another project.
Intended to sell it after that project, but didn't.
All you tool-guys understand how that goes.

 

[LouNY]

Good luck with your retaining wall, I see a lot of them around here. Many go from sloped back to getting ready to fall down in a few years. I expect much of that is from frost heaves.

 

[Streetcar]

Place your geotextile on vertical face of soil and have it turn horizontally to extend in between the blocks. Weak point of tie backs will be attachment to blocks

 

[Flogger]

We installed appx 180 feet of wall that ranged from 2 to 8 blocks high backfilled with crushed stone. The base was level contoured with at least 1 block below ground level. We installed 2 drains in the lowest spot and topped with 8” of topsoil graded and lapped over the top of the blocks. No tiebacks of any kind. Looked absolutely fantastic. We would listen to the boaters ooh an aah all summer long. Right or wrong 20 years later when we sold the place the wall hadn’t moved a bit.

Also we stacked them 8 to 10 blocks high on the back side of the garage and made a parking pad that sloped to the open end. We had 3 boat trailers stacked and parked on the pad. It had no drains and no tiebacks and stood solid for the same 20 years.
With a solid base these blocks are amazing.

 

[Sodo]

Thx Flogger that's good to hear.
Three significant details that influence success / failure !
------> (conditions, conditions, & conditions).
It's "the unknown", "changes", and "the future" (seismic?) that should be the impetus for overkill, not internet glory.

From my (very simple and limited, engineer but NOT geotech,....) understanding,
the geotextile layup is a 6-foot thick retaining wall,
and the block wall leaning against it is mostly a decorative facing.
But this block wall supports the topsoil bank above it--- that will hide the geotex layup and I want it to remain visibly straight.
The topsoil bank sitting up higher than the block (the row count ! )..... wants to settle in behind the wall and push it out.
per Curly Dave's explanation. But there's only a little drained fill that wants to get to its angle of repose.
It's not mud trying to go 'level'.

----- streetcar mentioned a way to use the weight above to secure some "added geotextile".

How much that big (16 foot) sand layup is gonna settle though, -----> is an unknown for me to discover in time

I'll tie this wall back to 'stack the deck' because it is 4 blocks higher than the mfr recommendation.
And because this retired guy has two days to fiddle while the wall-builders are away on another job.

===========================

The helical anchors available quickly on Amazon.....were un-impressive (no suprise).
I'll put them in my toybox for some other project.

Driving 6 foot fenceposts in with a gas-powered fencepost driver.

Well they didn't just fly in.
But I think tight enough,
and splayed apart they will resist pullout.
I think this arrangement is much stronger than plastic geotex deadmans could ever be.
If I'm bored tomorrow I'll screw some of those little helical anchors in too, I'm curious.

I'll weld this #5 Rebar frame all together.
That one up at the right was my "test-pole" to see how hard they drove in.
I'll attach to it simply because "I can".

Here's 3 drilled blocks for row 6
and 3 drilled blocks for row 10.
Rebars poked thru a hole with a welded dogleg to retain it in the block.
I'll weld tie-bars from the blocks to the frame as the wall goes up.

 

[airbiscuit]

Mice job. Get back to us in about 40 years, and tell us how it looks!

 

[Sodo]

I welded the eight 6ft T-Posts and rebar rectangle together.
It's really solid.
No way in h*ll is a plastic deadman as strong as this method.

From the other buried steel I find here, I'm sure this will last 40 years and more.
I doubt this little house will still be here in 40 years though.

I lengthened a couple 2.8" helical anchors with rebar and screwed them in.
Mostly was just curious “how they go in”.
They got tangled in geotextile, but eventually got them in, and welded.
Every little bit helps (helps the overkill !)

 

[goeduck]

Same state, and maybe county, as you. No tree protection laws would stop you from taking out a few trees. I do it on a regular basis. If any of those trees are western red cedar, they do not like their surface roots covered, they typically die. I can't tell on my phone what type of trees those are.

 

[Sodo]

They're Redwoods, planted in 1985.
I would like them better if not so close to the house.
The 40" is 11 feet from the house.
Someday they have to be taken out.
It's a 1 bdrm, decent condition, but low-value house (rental).

Here's row 6, welded to the tieback structure

Here's row 9 welded to the tieback structure.
This wall ain't gonna tip over.

Just about done, and ready to backfill the dirt bank.
I'll put some of that black woven fabric over the gravel before covering it with soil.

Pavers tomorrow.

 

[RayIN]

I do know one thing about constructing a block retaining wall.
Never construct in winter months. I hired a landscape company to construct 2 retaining walls. Both have sank about 1" in one area, with that 1" void clearly visible.
Called them back to inspect the void. A new man showed up and immediately asked if there was drainage tile behind the walls. No there is not I replied. He said there's the cause, no way to install it now but remove everything and start over, and the owner said he will not do that since it's been 4 years.
Now I've complained every year, but nothing in writing, my bad.