Questions about geogrid and cinderblock wall

[Sebculb]

Hey everyone,

So my buddies and I built this retaining wall for a customer whose driveway fell off the side of the mountain. 8" thick cinderblocks, probably 12' high. Got counterfort anchors (is that what they're called in english? We build in Spanish...) extending perpendicular back into the hillside every 10 feet. 3/8 Rebar vertical every other void and horizontal every other course. All voids filled with concrete. Footing is 2.5' wide and 2' deep, with 1/2 rebar cages.

Now i gotta backfill this mess and I'm terrified. We're in SW Costa Rica, rainforest clay. Lots rain. I am aware of the existence of geogrid but have never used it. Is not a common building material here. There's plastic fencing here that looks the same, or could likely be geogrid but everyone just uses it for chicken coops and to keep their chihuahuas in their yards.

My main question is, what do I do regarding anchoring the geogrid to the block wall? All the YouTube videos I watch are with interlocking landscaping blocks and the grid is lain in between courses of block. But this wall is cinderblock and they're already glued together.

Is it even worth putting in at this point? Should I just lay it flat even though the YouTube videos say it's a common mistake and don't do that? How about folding up the last six or eight inches against the wall and packing dirt up against it like that?

I'd like to use i cuz I'm convinced of its effectiveness but this is a little uncommon circumstances.

Also, how often should I put down layers of grid?

I don't have a proper tamper. Just a rear backhoe bucket that can sort of pack it down, and a big fat farmhand that we can put on walking back and forth on it all day.

Any advice or experiences to share?

Thanks!

 

[Sebculb]

Here's some pics, the anchors go back about 6 feet at the bottom

 

[Raul-02]

cinderblocks,

haven't heard that word in a very long time. I've actually owned a few of them.

 

[Sebculb]

Concrete masonry units?

 

[hosspuller]

Twelve feet high !! Back fill it and run away. Maybe get into Witness Protection. I doubt the wall will hold more than a few years. Blocks are fine in compression. Yours are in tension and will crack & separate under wet clay pressure. Perhaps, backfill with clean stone to allow water to drain. Hillside will still exert a bunch of pressure.

 

[Sebculb]

Twelve feet high !! Back fill it and run away. Maybe get into Witness Protection. I doubt the wall will hold more than a few years. Blocks are fine in compression. Yours are in tension and will crack & separate under wet clay pressure. Perhaps, backfill with clean stone to allow water to drain. Hillside will still exert a bunch of pressure.

I know, I know, but I got advice from a lot of grumpy old masons and they all said more or less the same thing. And there's a lot of walls like this around built in this manner that are still holding. I'm listening about the gravel but there's plenty of drainage holes in the wall. And gravel is expensive in this economy so it's typically not used for that.

Anyway the question was about geogrid...

 

[CADplans]

My son-in-law and I each have a property with a hill next to part of the driveway.

Over about a 4 year period, the hill will "flow" down, and fill the ditch next to the driveway.
I helped my SIL clean out his ditch this past weekend.
Mine will need cleaned out in 12 to 24 months.

If that were my block wall, there would be VERY strong anchors going into that hillside prior to backfill.

I would dig a trench in the hill, install a "T" shaped anchor for whatever metal that would go to the block.
The trench would be backfilled with concrete.

If you wanted to try to do it cheap, put auger-in anchors in the hillside, like the anchors used to tie down mobile homes.

If you want to see some ideas, look at how dams for holding back water are built.
The backfill behind the wall will end up acting like water, only slower.

A dam is very wide at the bottom, and tapers up.
The wide bottom gives LOTS of weight to hold back the weight of the water/dirt.

 

[Have tractor did travel]

I like what Cadplans had to say. I'll add - While you say you've got lots of drain holes, I can't see them. I'd like 2" diameter. May we assume you drilled through the block AND the poured filling cement (grout) ?
I'd lay geotextile fabric against the earth wall that remained after you excavated, and on the top of the stones (see next sentence). Then, the space between the geo fabric and the block wall, I'd fill with washed stones - bigger the better. What you're aiming for is water coming down the hill (or falling on the top of the rocks) to quickly flow between the rocks and out the weep holes you have drilled in the concrete block walls.
Incidentally, the name "cinder blocks" comes from how they were first made - from the cinders left from the burning of coal, and likely ground slag from production of steel.

 

[amhicks21]

So actual geogrid is kind of an overpriced ripoff. Unfortunately I dont think the type of plastic fence you're talking about would be very effective in clay since it'd basically flow through it over time. I think you could use a solid woven landscape fabric with better success. Its a bit unfortunate that you wont have a good way to anchor it to the wall but putting a layer down, backfilling a foot and doubling it back over top would be better than nothing.

Is there any way to keep the backfill permanently dry after its done? since you arent dealing with any frost heaving that wall would never move if the clay behind it could be kept dry. Its a little hard from the picture to tell how the project is going to look completed so i dont know if thats even remotely possible

 

[Sebculb]

While you say you've got lots of drain holes, I can't see them. I'd like 2" diameter.

We installed a half block on its side at regular intervals. The drain holes are like 6" diameter.