Questions about geogrid and cinderblock wall

[Rockbadchild]

I would listen to erikjohn67 ...

I would also install drainage fabric on the inside of the wall over your drainage holes so they don't wash out and create sink holes ....

 

[Rockbadchild]

No, do not do this. I will preface my comment to you by stating that I am an engineer and have have been a contractor in heavy civil construction for 25+ years building roads, bridges and complex civil works. I have years of experience with mechanically stabilized earth walls which includes geo grid walls. We use geo grid walls as temporary walls with a wire face for phased construction.

The geo grid tie back is absolutely necessary even though it may seem like to the layman that it is not. You would be surprised how much force that geo grid will resist as it may not seem strong but when wedges between dirt layers it resists a lot of force.

1) Using filter fabric can reduce the drainage behind the wall which will creat unnecessary head pressure against the wall and can cause it to blow out, unless it was specifically designed that way. The geo grid is free draining and does not have this issue.

2) The fact that you have a solid wall is only that, the wall is solid. It could be a foot think steel plate standing up but unless there is something keeping it from tipping over(tiebacks like the geogrid or chance anchors) the stiffness of the wall means nothing when it is being used to retain earth because their is nothing to resist the moment(overturning force) the wall is feeling.

Install the geogrid per the manufactures specifications for a wall that height. Use the proper backfill. Clay and silty material does not work as it has slip planes in the material(hence why clay is so slippery when wet). It need to be free draining sandy material without organics.

Do not compact to hard next to the wall as you it is not the hard to move the wall out of place. Consider some minimal hydro compaction within 3 foot of the wall after a very lightly compacting mechanically. Out side of three feet you should be good but start at the middle of the geogrid and compact away from the wall and then come back and compact towards the wall.

Im sure there is lots of videos and literature online to watch but you you absolutely have to properly tie back that wall or it will fail.

the manufacture specs is to install and secure the geo grid between the cinder blocks Op is past this point, how do you suggest he secure the geo grid to the wall ?? anchor bolts with steal flat bars would be my go to ...

 

[erikjohn67]

the manufacture specs is to install and secure the geo grid between the cinder blocks Op is past this point, how do you suggest he secure the geo grid to the wall ?? anchor bolts with steal flat bars would be my go to ...

Yes I figured that but was getting awfully long winded in my first post.

I agree with your suggestion. The grid needs to have a continuous attachment in order to take the load without failing/tearing itself apart when loaded. I would suggest two bolts per block to anchor the flat bar but absolutely at least one per block.

This is going to be a lot more work than if the geo grid had been installed correctly from the get go and there really isn’t going to be any easy shortcuts.

 

[erikjohn67]

I would listen to erikjohn67 ...

I would also install drainage fabric on the inside of the wall over your drainage holes so they don't wash out and create sink holes ....

You actually want those holes to have some drainage through the wall because if the water builds up behind it in major rain event, it creates a lot more load on the back of the wall than dry moist soil. The wall wall essentially becomes like the walls of an above ground swimming pool try to hold back that load.

Usually the way around keeping that buildup away but also keeping soil from migrating out the face of the wall is that you put 1’-2’ thick of clean rock directly behind the wall as a buffer between the dirt and the wall. Taking that a step further is to install an under drain system at the bottom of the rock that will allow the water to flow down through the rock and then out the under drain to a ditch or drainage structure. It all about keeping that hydraulic pressure off the backside of that wall which can easily be 2 to 3 times the load feels when the material behind it is dry.

Nothing will take out a wall quicker than not correctly designing to eliminate this hydraulic load. When I say quickly I mean it will be a catastrophic failure that happens immediately as opposed to a failure due soil pressure which will usually slowly push the wall out over time until it just falls down.

Again I am sure a quick google search will show some of these techniques.

 

[Rockbadchild]

You actually want those holes to have some drainage through the wall because if the water builds up behind it in major rain event, it creates a lot more load on the back of the wall than dry moist soil. The wall wall essentially becomes like the walls of an above ground swimming pool try to hold back that load.

Usually the way around keeping that buildup away but also keeping soil from migrating out the face of the wall is that you put 1’-2’ thick of clean rock directly behind the wall as a buffer between the dirt and the wall. Taking that a step further is to install an under drain system at the bottom of the rock that will allow the water to flow down through the rock and then out the under drain to a ditch or drainage structure. It all about keeping that hydraulic pressure off the backside of that wall which can easily be 2 to 3 times the load feels when the material behind it is dry.

Nothing will take out a wall quicker than not correctly designing to eliminate this hydraulic load. When I say quickly I mean it will be a catastrophic failure that happens immediately as opposed to a failure due soil pressure which will usually slowly push the wall out over time until it just falls down.

Again I am sure a quick google search will show some of these techniques.

I hear ya but I am talking about a permeable fabric simply to hold solids and only let the water go through.

 

[erikjohn67]

I hear ya but I am talking about a permeable fabric simply to hold solids and only let the water go through.

Yes I agree with you, it’s filter fabric and is used the way in which you explained behind walls all the time to do exactly what you explained……when it was incorporated in the engineering of said wall. Although it protects from soil migration out the front, it also limits how quickly the hydraulic pressure is released out the front face of the wall depending on the permeability of the particular fabric’s weave.

Unfortunately it’s not a one size fits all situation. The 100% sure thing is the rock and the under drain and you wrap the rock in fabric except at the top (think of a sling) and the under drain pipe is in the bottom of the rock encompassed in the fabric also and it carries the water out away from the wall at the bottom.

The OP, IMO as a business, really needs to consider what their liability is if this wall fails and how they are going to make this wall right.

 

[dodge man]

Keep in mind the op is in another country and it sounds like he didn’t have complete control of the project

 

[71bronco]

Don't think it will last. Should have been a 6' wide footer with min 5/8 bar every 16'' stuck in concrete. Min 12'' CMU's anchors going back to the dirt. Been a bricklayer for 40 years and have repaired and built a lot of retaining walls and I don't like the looks of what you have.

Jeff

 

[Rockbadchild]

on the other hand I have seen retaining wall built with old tires filled with clay without issue…

 

[Bentrim]

Recommend being sure to put drainage in the bottom then refilling 2/3 to 3/4 height with stone. Then add minimum soil on top. Stone will leave water pass to drainage without adding pressure to the wall. be sure to add retaining rods back to undisturbed soil.