Who knows about stacked-block retaining walls?

[Sodo]

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.

 

[camshawn]

By the look of the picture, your wall is not very long. I would put some tieback in just a cheap insurance. Is there any drainage issue in that location? My buddy didn’t put enough in and had a 4’ wall start to move in one of our Pineapple Express events.

 

[5030]

I used 'Perry Blocks' for my grade retaining wall for the grade change on my Clearspan truss arch building, however, they are very heavy and require a substantial machine to even move them.

Perry Blocks are what they use on the highway for concrete dividers and can be physically interlocked as well. The ones I purchased weigh about 3000 pounds per block.

 

[airbiscuit]

I thought they usually put crushed rock below and behind a retaining wall.

 

[2manyrocks]

If these are just dry stacked, what keeps them from sliding apart under pressure ?

 

[EddieWalker]

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.

 

[dodge man]

Eddie is right about drainage. If you go a few blocks high you don’t usually need tie backs but yours looks high enough you do.

 

[LD1]

I do alot of excavation for retaining walls for a local company that specializes in retaining walls.

First...not sure what you mean by tiebacks. If its what I think.....thats where you use the geo-grid sandwiched between the blocks and runs behind the the wall and ties into the dirt.

Sand is not a good choice for backfill. It moves, settles, and retains water which is a freeze-thaw problem.

Here in Ohio, we call it #57 limestone. That is whats to be used. Its a uniform washed and screened stone....roughly the size of marbles (~3/4"). Not sure what its called in other parts of the country.

The "footer that the wall sits on is usually filled with ~8" of #57 and compacted. Wall stacked on top of that....backfilled with #57's and drainage at the bottom as well.

Depending on the height of the wall...1 or 2 blocks are usually burried. Keeps the bottom of the wall from pushing out.

Given the overall cost and labor of the project, doing it right adds very minimal cost in comparison to having to do it right later

 

[Sodo]

If these are just dry stacked, what keeps them from sliding apart under pressure ?

They tip back one inch for every 6” up.

Thanks very much for all the suggestions and wisdom.

As you can see I’m building this from old blocks, and not solely because they’re cheaper. They’re almost “not” cheaper because of the physical labor to get them. But I didn’t have to pay 10% tax either. Including other reasons for it to look like it was done “long ago”.

It would have been wise to sandwich some geotextile flaps between the layers, leave them hanging out there for the retaining wall but that’s a first-timer for ya.

For tiebacks I was thinking of driving rebar rods and then figger a way to attach to the blocks.

I don’t have space for a drain pipe at the bottom. Have to put it at the 3rd course. There could be water here if my driveway drains “go unmaintained”. I think everyone knows that driveway drains eventually ALL “go unmaintained.”

The bottom row can’t kick out because there will be a paver-floor against the house holding it. And if what I read about geotextile soil retention is true- the bank won’t push. What do you guys think about that? Have any of you observed geotextile lifts behavior (or failure) over “years”?

 

[2manyrocks]

If these blocks are rectangular, you might get some tie back by setting some of them lengthwise. Weep holes and rock backfill will help drain water.

 

[KennyG]

I've had good luck with those blocks and no tie backs. I put a drain tile below them and backfilled rock behind them. It's important to get the base layer really level, so they don't end up tilted out.

 

[Sodo]

62 lbs per block. 2 is 124 lbs

It was no fun at all for a guy my age.... palletizing 170 blocks.
10,540lbs.

I hired one helper from HomeDepot for the first (5000lbs) load.
5,000 lbs off the ground and onto pallets on my trailer, in one hour.
ugh.
Unloaded witht he tractor forks, then took a nap.

For the 2nd load (5500 lbs) I hired 2 guys and just counted blocks.
This was just to get the blocks on-site.
I haven't unloaded the 5500 lbs yet.

planning the project,,,,
I thought:
Just do 10 blocks in the morning
then
10 blocks in afternoon,
then go in the house,
....take a nap.

It will be done in a week.
With ease.

But looking at the mini-ex just sitting there
..... I entertained the idea ......

Soft-jaws for setting blocks at the bottom of the stairs.

And I get to use my favorite toy.

 

[airbiscuit]

Clever. It would be cool if you could do a video

 

[Streetcar]

The blocks are typically designed to build a 48” high wall without additional reinforcements. The first layer is buried to key the wall and provide lateral resistance. The first layer should be installed on a well graded(not same size) gravel. Behind the wall the gravel should be free draining.
Wall building looks good

 

[HawkinsHollow]

I have very little knowledge of walls like this. But it doesn't seem like it wouldn't hurt to put some tiebacks. What about stainless wire that is staked back into your geotextile and then sandwiched between layers of block. Maybe make a anchor by twisting the wire around a piece of rebar or something. Seem like a really cool project you got going. I think the end product is going to look really good. Hard to see the bigger picture of what is going on but it seems like what you are doing will really maximize space. Take your time and do it right and it will serve well for years to come. Following!

 

[LD1]

Second or third row down from the top looks like you would have room to do a "tie-back" with geo grid. Its cheap insurance.

While the geo lifts may not push....you still have alot of material to backfill. And dirt or sand absolutely will push. Its imperative that the backfill be with washed and screened stone. "free-draining" as mentioned earlier.

the top 6" or so can be topsoil if you choose to plant grass or flowers there. A layer of geo-fabric before the layer of dirt wouldnt hurt

 

[2manyrocks]

The additional picture really shows the issue much better.

Because of the amount of labor involved and never wanting to do this again, I'd check for any YouTube videos that discuss this. Fine Homebuilding might have some articles, too. Might even try calling a manufacturer's rep for whoever made the block and certainly look at their installation instructions.

From what I quickly saw, some YT videos strongly advocate using geo grid to tie the block back to the fill material, but I didn't watch enough or read enough to be confident this is the best way for your situation.

 

[HawkinsHollow]

Are those 2 big trees I am seeing on top of your landing there? Do you worry about them being compromised during this process? Or affecting your wall in the future?

 

[jonsstihl]

I Like your soft jaws very clever. I really would try to get a drain in behind the wall even if it is a smaller two inch drain at the bottom of the wall and then a 4 inch drain higher up. the large drain would handle most of the water and then the small drain would handle whatever gets below the large drain. you may have to make a 2 inch drain. not sure they are available commercially

not sure if you get freeze and thaw where you are.

 

[Sodo]

I doubt enough water could back up 2 blocks enough to cause a problem. I bet water has soften halfway up with muddy backfill to cause trouble. Thats a guess.

Its all sand, with geotextile reinforcement.

A bunch of sawcuts on a small plastic pipe would be easy though. Maybe there’s space. I can extend it out so I can see if it ever drips water then I’ll know theres water behind the wall. And have to divert it or something. Thx for the suggestion of a small perf pipe.