Who knows about stacked-block retaining walls?

[Sodo]

WOW some real attention! Thank you.
Not that I didn't appreciate the earlier responses, I'm taking it all in.

Water is it biggest enemy on a retaining wall.
Lots of good advice in comments which you seem to want to disregard.

Hold on there fella the pics above are still current.
Nothing more has been done. Still gathering info.

This level area where the mini-ex and tractor sit does NOT gather water from acres uphill.
It's about 400 sqft of levelish surface that can only collect water from it's own 400SF.
The levelish (new) surface drains ------> away from the wall.
I am concerned that with settling, some of this 400sf could begin to drain towards the wall, that would change things a little.
I haven't considered the 400SF as "a lot of water" risk. I have doubts that it will create a mud-pressure behind the wall.

My advice is to try to have your top ground layers made with clay that slows the water absorbsion somewhat.

The top layer (behind the 4x12 wood) is clay-ish. Water is currently pooling ontop and takes days to go away.
The geotextile layers that you see are filled with sand.
If I use clean gravel as backfill behind the wall, I'm pretty sure it can get down to drains below.

A driveway drainage problem could feed it with driveway water.
Pretty sure I can build a reliable method to prevent driveway water from the 400SF.

Driveway water (and one gutter) goes into that white 4" pipe, and exits far beyond the wall.
If the white pipe plugs up, the overflow will be chosen to go on down the driveway, not to this wall.

The house basement wall is stout, 6" thick concrete wall. 8 feet tall. The paver walkway, if shoved by retaining wall blocks, will be pushing 3 ft above basement floor.

OK that's addressing some of the concerns. I'll get thru them all.

 

[lifedata]

I'm thinking you should put some kind of drain close to the bottom. They make "thin" rectangular drains for tight spaces. JDrain is one brand.

 

[Sodo]

I think that for this to work, you need to remove the trees. It's probably the hardest thing to do, but there is no such thing as a good tree next to a house.

Hardest thing .....

As the saying (might) go...
The best time to (remove) a tree (next to the house) was 30 years ago.
The second best time is "today".

I am gonna punt on this one. They're huge.
Gonna build the wall, and punt on the trees.
I think they're liable to get bigger.

The one 11 feet from the house is 40 inches diameter. The farther tree is 34inches.
Redwoods, planted 40 years ago.
You old farts may need to be reminded that 40 years ago was 1985.

On your wall I think you should excavate more so you have room for drainage pipe (6" clear behind the wall would be enough to get a wrapped pipe in there buried in crushed rock
......
If the additional excavation is not an option you could also move the wall forward 6". Probably wouldn't notice.

Don't wanna cut my geotextile 'cuz the fill could squirt out and push on the wall.

I first planned to build the wall with 8" wide blocks,
but then decided 12" wide would be better,
THEN 12" at a good price came available,
so I lost 4" on the bottom course.

I think a drain 2 courses up will be good enough.
Maybe add a 1 1/2" drain at the bottom too.... why not?

The engineers here can correct me if I'm wrong but to me it looks like the way you're doing it, essentially the geo grid dirt bundles are your retaining structure and the blocks are a going to be the second line of defense, albeit with a couple asterisks.

The concern about the patio pushing against the foundation of your house is also something to think about if you're relying on that to keep your wall in place. That's more math than I can do counting on my fingers.

I thought that a geo-textile layup IS the retaining wall - it's about 6 feet thick.
Where the stacked blocks are mostly a "facing".
Where most of the job of the stacked blocks .... is to hold up the backfill (between the blocks and the geotextile).

It would be great if some engineers with Geotextile experience would weigh in.

 

[jigs_n_fixtures]

It seems like you're using the fabric to keep the dirt back. I believe it's intended to hold the wall in place, with the backfill being securely compacted in layers. That said, I don't think those blocks are designed for a wall tall enough to require geotextile fabric.

I've seen them in engineered walls of over twenty feet. But, that required tiebacks.

 

[jigs_n_fixtures]

There are walls like that, question is how good is the compaction as it is built. There is also a wall like that known as Hilfiker which is wire. He is putting a wall over a wall. Probably should have started differently way before now. I agree about the trees they should also have been removed long ago. Sorry.

Backfill is clean granular material, which falls in at 100% compaction. And is free draining, with enough pore space to allow any retained water to freeze in the pores.

 

[Sodo]

yes free draining. Sand would not be free draining.
I highly doubt there's enough water to soak the sand, (but "famous last words....")
And we don't freeze that hard here.
But I'll use clean gravel, not sand (I have lots of sand nearby).

 

[jigs_n_fixtures]

Ok, reading through this, I have the following comments:

For backfill, see if you can get reject intermediate concrete aggregate from a local Redi-Mix supplier. Some have it, some don't, but it is a suitable backfill which is relatively inexpensive if they have it. Back fill as you go. Don't have more than a couple of courses in the air before you backfill. If you have a concrete stinger, it would be a good idea to run it along your backfill, but it isn't critical.

Drainage: I have seen a perforated poly drainage pipe which was 1-1/2-inch diameter. But, have no idea where to buy it. I try not to specify anything a contractor can't buy through a big box like Home Desperate or Lowers. But they may have it. I've never looked to see. Unless you use a washed aggregate for the backfill, do not install drain lines with a sock filter. The number one cause of failure for drains is fines migrating in and clogging the sock. If you use a good drain material for the backfill, even with the drain line several courses up, the gaps in the un-mortared blocks will weep water through the gaps.

If you install tie backs, do not use rebar. It will rust through, and you will lose the strength. If I had designed the wall, I would have installed geotextile as the tie-back and brought the block up as I installed the geotextile lifts. Not sure of an effective but inexpensive way to tie back at this point. But typically, the block manufacturers don't call for tiebacks until you get to about 8-foot. But that depends on the seismic zone. If your growing redwoods, I'm guessing coastal northern California, as a location. So, seismic could be critical. If I were doing the facing block, I might glue the courses together with one of the high stretch urethane adhesives. Not a good substitute for tiebacks, but will keep the block aligned in most instances, and in reality, if the big one hits, nothing will hold up if your close to the epicenter.

 

[neighborman]

years ago I built some retaining walls using a variety of large rock and mortar they were all over 5' high. The walls that have drainage are still standing after over 40 years.

 

[Sodo]

OK I'm smelling an engineer (or 3)..... Thank you jigs for reading thru! And the other engineers too!
....and the 100-walls experienced too

I'm quite capable of perforating a PVC pipe
....onsite,
in a fraction of the time it takes to walk the aisles at Home Despot.

What I'd like to know is,,,,, (knowing you're not able to look at the site in person...)

THIS -----> block wall leaning 1:6 into a geotextile layup (of sand),,,,,
Do I need to concern myself (at all) with tiebacks?

 

[ning]

Hardest thing .....

As the saying (might) go...
The best time to (remove) a tree (next to the house) was 30 years ago.
The second best time is "today".

I am gonna punt on this one. They're huge.
Gonna build the wall, and punt on the trees.
I think they're liable to get bigger.

The one 11 feet from the house is 40 inches diameter. The farther tree is 34inches.
Redwoods, planted 40 years ago.
You old farts may need to be reminded that 40 years ago was 1985.

If those trunks are buried (and it sure looks like it), the death warrant has been signed on the trees.
Good thing is, they'll look fine for a decent number of years. Can't predict how many, though; most likely the water & oxygen uptake from the roots has been reduced substantially as well (burying the trunks results in the trunks rotting out... below grade).

So strongly consider how those trees are going to be taken down at some point, and try to make sure that happens before a storm knocks them down after they've been compromised.

 

[jigs_n_fixtures]

OK I'm smelling an engineer..... Thank you jigs for reading thru! And any other engineers too!

I'm quite capable of perforating a PVC pipe
....onsite,
in a fraction of the time it takes to walk the aisles at Home Despot.

What I'd like to know is,,,,, (knowing you're not able to look at the site in person...)

THIS -----> block wall leaning 1:6 into a geotextile layup (of sand),,,,,
Do I need to concern myself (at all) with tiebacks?

Depends on our seismic zone. The entire Pacific Coast has a pretty high-risk level. If I had done a design, I would have included them. I'm a belt and suspenders, with tight pants kind of guy.

At this stage, it would be difficult to do right with any of the inexpensive systems. Drive toggles might work, with your sand filled geotextile lifts. Something similar to these, which I have used to install aircraft tiedowns. I've driven them with an SDS-Max driver and a ground rod driver attachment to drive the rod:

Duckbill® Earth Anchor

 

[Sodo]

The trees are buried.
Anything that slows their growth is fine by me.
They are too big already, and too close to the house.
We have tree canopy protection laws here, If the tree wants to show distress, I need that (for the death-warrant).
I wonder how susceptible Redwoods are to trunk-rot.
Redwoods are not natural here, but these 1985ers sure like the growing conditions (in sand).

They are beautiful trees .....if they were farther from the house I would admire them greatly.

Speaking of beautiful, that's a beautiful suggestion !
I can build duckbill anchors.

Another idea...I have a gas-powered fencepost driver.
What about T-posts as tiebacks?
I can drive two T-posts, splayed out at 45° (with some effort....) and one vertical.
Weld three together where they meet.
I bet that would hold more than a whole flock of duckbills.

The biggest Q I have..... remains...
Here asked a little differently.
Does a geotextile layup exert pressure against a retaining wall??"
If it does not, tiebacks seem unnecessary.

 

[jigs_n_fixtures]

The trees are buried.
Anything that slows their growth is fine by me.
They are too big already, and too close to the house.
We have tree canopy protection laws here, If the tree wants to show distress, I need that.

They are beautiful trees .....if they were farther from the house I would admire them greatly.

Speaking of beautiful, that's a beautiful suggestion !
I can build duckbill anchors.

I have a gas-powered fencepost driver.
What about T-posts as tiebacks?
I can drive one at 45° and another vertical, and weld them together at the top.
Or two, splayed out at 45°, with one vertical.
I bet that would hold more than a whole flock of duckbills.

The biggest Q I have though, does a geotextile layup exert pressure against a retaining wall??

I'd be concerned with t-posts rusting out over time.

 

[CurlyDave]

Does a geotextile layup exert pressure against a retaining wall??

Even if it doesn't, the fill between the geotextile layup and the wall will exert exactly the same pressure on the wall as if the geotextile layup were not there.

Think of it this way, if I have a 30' vertical piece of 1/2" pipe and fill it with water, the pressure at the bottom will be exactly the same as the water pressure 30' beneath the surface of the largest lake on earth.

No matter how thin the fill layer is, the pressure vs. height relationship is the same. AND, no matter what you do, the fill will always be there. You can either choose what the fill will be by placing it there yourself, or allow it to form from whatever debris is washed out of the geotextile layup, or washed into the space behind the blocks from above. Over time a void behind the wall will always fill up. It is far better to have a material you select than whatever random collection of debris forms naturally.

 

[Redlined0002]

I'm putting up a stacked block retaining wall (6x17x12") blocks.
25 ft long, 7 courses, 42" high.
It's going up against an embankment of geotextile layers.
The lower 3 geotextile lifts are (very) sandy soil.
The dark layer is sandy topsoil.
Inn the end I'll just slope soil down to the 42" top of the wall.

Do I need tiebacks to the wall blocks?
Can I backfill it with sand?
You can see a test-stack of 7 blocks.
View attachment 2423010

I have built many bulkhead. I don't think this will hold the way you are doing it. The best way is to either use plastic sheet piling or do concrete telephone poles stepped back as you go up pinned with 3/4" rebar every 30".
Plastic sheets will require a tieback system 25' from the wall. You will also need to put filter cloth behind the bulkhead.

 

[AnotherBueller]

Your geotextile looks impressive!!!

The main reason for retaining walls to fail is water building up behind them. The water creates pressure, adds weight and loosens the soil. Another issue is when there is water behind the wall and the ground freezes. The ground expands and moves the wall. When the ground thaws, the wall remains in the moved location and either fails or is a lot weaker.

A drainage pipe at the bottom solves a lot of future issues. Rock is used between the soil and the wall so water will drain to the bottom and then exit through the pipe. Weep holes do this for smaller walls, but it's not as effective as a perforated pipe. Remember, the holes for the pipe go on the bottom.

Anything up to 4 feet tall generally doesn't need any engineering. Just stack them and be done.

Usually, the first row is buried in the ground to lock the entire wall in place.

I looked into doing this type wall (40W X 4H) by my shed. When I priced it out and added in the amount of manual labor I would need to do - - I went with Lego blocks from a local concrete place, made of non-continuous pour left overs at about 1,500 lbs each and $300 per block. All said and done cheaper, quicker and much easier on my 74 year old back.

 

[aircommuter]

Do you have a cross section drawing of the existing geotex you can post? Were the layers of compacted material tested? Sand isn’t a good fill for that purpose. Sand fill works best in a trench where it’s trapped and can be compacted and it stays in place. What will the slope be above the proposed wall?

 

[Sodo]

Impervious plug at the top is a must as you don’t want to introduce surface water to a subsurface drainage system.

Got "claylike" soil as a top layer. Currently it drains away from this wall. Fingers crossed that it will continue to drain "away"as the whole stackup settles over time. I will have to pay attention over the years and deal with it.
This is not like an engineering job that a responsible party signed "responsible" then has to come fix it if it doesn't work.
It's mine and I live here.
I will have to pay attention over the years and deal with any problems.
I think the main focus will be ensuring water goes the other direction, doesn't dump over the edge to get behind the wall.

Before you start using the fabric you need to study the different types and use the appropriate type in the location/s as needed. Every type has a purpose.

I chose the fabric based on availability and price.
I had to pay $400 for a 600 ft roll. Which I thought was cheap.
I got it from a guy who bought it as surplus (for a song).
It has no current pedigree (purchase invoice etc).
It's "Huesker Fortrac20".

You also should protect the area between the proposed wall and the house from any water from other areas (like downspouts) and make it drain away from the house and the wall.

In the 7 years I've lived here I don't see any water problems in this area.
I'm capturing all the water that I can (downspout and driveway) tight drainpipes.

Seems like you may need to backup a bit in this project.

Can't do that.
I need to get the best info that I can and move forward.
This project can't sit or it won't get done 'till next winter and my wife aint havin that.
She wants it landscaped and growing cover ----->this spring.
And I agree, I want it looking like it was "existing" and ASAP.

Tomorrow an experienced wall builder is coming to help.
He's not a contractor, he's a laborer but he's done some pretty wild projects.
Young guy recommended to me by a 'driveway contractor'.

Here's a pic I took of a freeway overpass construction that I took while driving by.
Absolutely _Vertical_ layup of geogrid. Appx 22 lifts, maybe 18-24" each.
I'm guessing they will assemble a veneer of concrete panels over it.

 

[Sodo]

Were the layers of compacted material tested? Sand isn’t a good fill for that purpose. Sand fill works best in a trench where it’s trapped and can be compacted and it stays in place.

There is no test.
Compaction is just what I can do with the excavator track.
Sometimes I was able to use the tractor with a full bucket load and its small diameter front wheels.
The sand has a little 'dirt' in it and packs very nicely.
And it's winter, and has a bit of water in it.
I expect that it will settle (drop) over time, but probably not as much as dirt.

The guys who brought it told me that it was similar to what the city used for roadbeds.
It was all free, and delivered for free too. Many 11yd truckloads.

Do you have a cross section drawing of the existing geotex you can post?

Here's the first 8 lifts.
No fill dirt dumped over the fabric (yet) in this pic.
There's a lower level 8 foot long retaining wall (48" high) too.

The 9th lift is the "claylike layer" ontop.
The noses of the geotex fabric is covered by fill-dirt dumped over the side.
It has to have dirt covering it - because any water running over it will scour the sand out of the geo-tex layers quickly.
The final dirt layer will be dark topsoil so vegetation will grow on it.

What will the slope be above the proposed wall?

This final slope of the 'dirt bank' will be topsoil.
I suppose it will be a little shallower than 1:1.

Hope this answers some of your GREAT questions!
Definitely appreciate experience looking over my shoulder.

I know this is not the engineering way.
I definitely appreciate the engineering way, and there's a place for it, and its responsibility
but this is on the "git'er done program".

 

[FullMetalBucket]

Lots of good info in this thread. Looks like I will be joining the wall building club. Our “new” place we moved to has a retaining wall on one side of the property line that’s about 55 feel long. It starts at 5 feet high and goes down to 2’ heading to the back of the property. The current wall was just stacked rocks and it finally collapsing. No idea how old it is. Cheapest quote to rebuild was 22 thousand, and that’s reusing the existing rocks. So yeah I will be following this for sure.