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Old 08-27-2019, 08:35 PM   #1
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Building a retaining wall for raised bed, which comes first?


Do I dump the soil on the bed, raising it by a foot, or do I build the border first?

Is there any kind of polymeric sand or substance I use to join the blocks together side by side (just 2 rows), or will their weight be enough to keep the soil in and potential roots/weeds from growing through..
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Old 08-28-2019, 07:27 AM   #2
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Re: Building a retaining wall for raised bed, which comes first?


If you want it correct, you dig a small trench, add paver base and lay up the wall portion and then fill the area.


If you mean 2 block in height the base can be thin and the weight of the blocks should be enough for foot traffic if the are installed plumb and level.
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Old 08-28-2019, 11:02 AM   #3
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If you are building it two layers (or more) high, your joints should be staggered, so each block onthe 2nd sits half on each of two blocks on the first row. You can, but do not have to, use some construction adhesive between the first and second row blocks. This will add stability to the wall and if applied correctly, the blocks will not come apart at all after the adhesive has cured.
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Old 08-28-2019, 02:29 PM   #4
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Re: Building a retaining wall for raised bed, which comes first?


It sounds like the 2 rows of CMUs is more decoration that a full-on retaining wall. I suspect the first course will be just set on the grade-level dirt, so place the blocks before you backfill.

I am not aware of a retaining wall block style that would hold a jointing material on the sides. If you desire to use landscape block adhesive for the horizontal, that is fine there.

You will get some washout on the face, just like a flagstone or boulder stacked wall. You will also get some weeds through it. Since its just 2 high, I would probably not glue them which would allow them to be repositioned every so often as needed. A periodic hose wash and power wash every year or two should keep it nice.
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Old 08-28-2019, 05:38 PM   #5
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Re: Building a retaining wall for raised bed, which comes first?


Quote:
Originally Posted by Colbyt View Post
If you want it correct, you dig a small trench, add paver base and lay up the wall portion and then fill the area.


If you mean 2 block in height the base can be thin and the weight of the blocks should be enough for foot traffic if the are installed plumb and level.
Really... I just filled the beds with a mix that includes manure, dirt etc. and marked out the borders. The height of the wall will be 15 inches high... all it's holding in is the soil that shrubs will be planted in. It's essentially holding a 10x15 raised bed inside.

Do you still recommend putting compacted granite as a base below the retaining wall stones, or just placing them on level ground?
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Old 08-28-2019, 05:41 PM   #6
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Re: Building a retaining wall for raised bed, which comes first?


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Originally Posted by Brian Famous View Post
If you are building it two layers (or more) high, your joints should be staggered, so each block onthe 2nd sits half on each of two blocks on the first row. You can, but do not have to, use some construction adhesive between the first and second row blocks. This will add stability to the wall and if applied correctly, the blocks will not come apart at all after the adhesive has cured.
For a 15 inch high wall holding a 150 sq ft raised bed with shrubs in it (shrub roots can get quite out of contorl if not cared for properly), do you recommend concrete adhesive?
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Old 08-29-2019, 10:05 AM   #7
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Re: Building a retaining wall for raised bed, which comes first?


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Really... I just filled the beds with a mix that includes manure, dirt etc. and marked out the borders. The height of the wall will be 15 inches high... all it's holding in is the soil that shrubs will be planted in. It's essentially holding a 10x15 raised bed inside.

Do you still recommend putting compacted granite as a base below the retaining wall stones, or just placing them on level ground?



It depends on where you live and the soil under the wall. Laid on packed clay and leveled with sand or rock dust where frost heave is not a problem it will probably be fine.



Laid up on a loamy soil you probably will have some settling and work to do in a few years.



I have both in my yard. They were built with two different objectives in mind. One 3' tall at the high point built on a 6" well compacted gravel base and one that is a simple 9" high laid on well compacted clay with a 1/2" +- of rock dust to level the first course.
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Old 08-29-2019, 01:28 PM   #8
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Re: Building a retaining wall for raised bed, which comes first?


@NatureFly , welcome!

Where are you? What city/county and state/province? That might matter a lot. I'll accede to the obviously excellent experience of @Colbyt , @Brian Famous and @3onthetree about the actual building of a wall. Except you want to be careful, in my opinion, to have a footer at the base to avoid frost heaving.

Back in Ohio where I'm originally from, we had that problem. In California, we don't, so that's why I'm asking where you're at.

Also, be careful about the soil you back-fill behind the wall with. Don't do as a buddy of mine did and just mix together planting mix, manure and sand, which was basically just a big mass of extremely humus rich potting soil. The humus "evaporated" and decreased in mass by about 95% in a few years.

Instead, use some good "topsoil" and mix plenty of extra humus in that, with sand, etc. for drainage if you have to. Regular soil has "stone" elements in it that won't decay and will retain their mass. Maybe 1/3 humus and the rest soil. If it's nasty clay topsoil, you can go heavier on the proportions, like half and half.

In the event you already know all this, I offer it for the benefit of others who might be lurking.

Hope this helps, maybe post a picture? And in any event let us know what you do and if this helps.
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Old 09-01-2019, 12:10 AM   #9
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Re: Building a retaining wall for raised bed, which comes first?


Quote:
Originally Posted by Colbyt View Post
It depends on where you live and the soil under the wall. Laid on packed clay and leveled with sand or rock dust where frost heave is not a problem it will probably be fine.



Laid up on a loamy soil you probably will have some settling and work to do in a few years.



I have both in my yard. They were built with two different objectives in mind. One 3' tall at the high point built on a 6" well compacted gravel base and one that is a simple 9" high laid on well compacted clay with a 1/2" +- of rock dust to level the first course.
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Originally Posted by DoomsDave View Post
@NatureFly , welcome!

Where are you? What city/county and state/province? That might matter a lot. I'll accede to the obviously excellent experience of @Colbyt , @Brian Famous and @3onthetree about the actual building of a wall. Except you want to be careful, in my opinion, to have a footer at the base to avoid frost heaving.

Back in Ohio where I'm originally from, we had that problem. In California, we don't, so that's why I'm asking where you're at.

Also, be careful about the soil you back-fill behind the wall with. Don't do as a buddy of mine did and just mix together planting mix, manure and sand, which was basically just a big mass of extremely humus rich potting soil. The humus "evaporated" and decreased in mass by about 95% in a few years.

Instead, use some good "topsoil" and mix plenty of extra humus in that, with sand, etc. for drainage if you have to. Regular soil has "stone" elements in it that won't decay and will retain their mass. Maybe 1/3 humus and the rest soil. If it's nasty clay topsoil, you can go heavier on the proportions, like half and half.

In the event you already know all this, I offer it for the benefit of others who might be lurking.

Hope this helps, maybe post a picture? And in any event let us know what you do and if this helps.
Appreciate the well thought out responses!

I am in South Texas so no... frost heaving is not a problem. Extreme heat, humidity and rain are but the house is at a high elevation so no flooding issues. The soil beneath is very hard clay, in fact, that's why I had to raise the beds in order to plant new shrubs that could eventually expand their roots into the ground.

The blend I use does consist of manure, because quite frankly I need it to break down the clay beneath, but it also has alot of other soil elements that won't degrade away over time and I overfilled at the point anyways. The retaining wall stone arrives next week, so I will post a before after picture when done. However I did build a 15 inch high wall in the back (not visible to the outside) to allow for a walkway behind the shrubs and it isn't perfectly aligned.. so I'm wondering if leveling the ground with a tamp is all I need to do for the retaining wall.
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Old 09-01-2019, 01:11 AM   #10
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Re: Building a retaining wall for raised bed, which comes first?


Quote:
Originally Posted by NatureFly View Post
Appreciate the well thought out responses!

I am in South Texas so no... frost heaving is not a problem. Extreme heat, humidity and rain are but the house is at a high elevation so no flooding issues. The soil beneath is very hard clay, in fact, that's why I had to raise the beds in order to plant new shrubs that could eventually expand their roots into the ground.

The blend I use does consist of manure, because quite frankly I need it to break down the clay beneath, but it also has alot of other soil elements that won't degrade away over time and I overfilled at the point anyways. The retaining wall stone arrives next week, so I will post a before after picture when done. However I did build a 15 inch high wall in the back (not visible to the outside) to allow for a walkway behind the shrubs and it isn't perfectly aligned.. so I'm wondering if leveling the ground with a tamp is all I need to do for the retaining wall.
Do you have the clay that expands with water?? I have heard it can be as bad as frost.
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Old 09-01-2019, 10:59 PM   #11
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Re: Building a retaining wall for raised bed, which comes first?


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Do you have the clay that expands with water?? I have heard it can be as bad as frost.
Not that I know of... it just turns black and muddy, and then turns into a rock like ball when it dries again.
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Old 09-01-2019, 11:07 PM   #12
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Re: Building a retaining wall for raised bed, which comes first?


You are going to have 7 yard of dirt in there, if you are having that delivered and can dump it in a pile in the middle and then build the walls around it. that would be the best.

I would want 1/2 of a block below ground level sitting on about 4 or 6" of compact gravel.
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Old 09-15-2019, 01:21 AM   #13
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Re: Building a retaining wall for raised bed, which comes first?


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You are going to have 7 yard of dirt in there, if you are having that delivered and can dump it in a pile in the middle and then build the walls around it. that would be the best.

I would want 1/2 of a block below ground level sitting on about 4 or 6" of compact gravel.
Really... 4 to 6" of compacted gravel for a foot high retaining wall? I spread out 7 cubic yards among 2 areas, so it's really about 3.5 yards per 110 square foot section (final area ended up being smaller).

This is the set up I have now, about 2 inches of torpedo sand to be compacted on top of landscaping fabric with a tamp... what is the method to ensure it is level? Hoping for the best without digging deeper. =]
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Old 09-15-2019, 01:26 AM   #14
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Re: Building a retaining wall for raised bed, which comes first?


When you have some of it below ground level , the dirt adds support for the wall.
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