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Landscape Design

Remove 3'+ of existing sand for raised patio?

7 years ago

last modified: 7 years ago

I am building a raised paver patio with a 2-4' block retaining wall. When I began excavation I was surprised to find that there is an excessive amount of sand underneath where a deck used to be. I dug a test hole and am already 3' down with nothing but sand. I am not even sure how deep it goes.

The patio will be 13' out from the house foundation and 17' long. Plan to terrace the one side that is 4' into 2 x 2' walls and build stairs. Planning to compact 4" of road base (class 6, 3/4 minus) then a small 3/4" layer of sand for leveling. The pavers are the 12" x 12" and the wall blocks are 16" x 6" x 8". I also plan to install a perforated drain behind the wall sandwiched with landscaping fabric and 4" of backfill that will drain on the lowest part of the wall.

I already have 14 tons of the road base which I thought I was going to use as the base material and backfill behind the wall. I realized (after it was delivered of course!) that this is not the right material at least for the backfill behind the wall. Now I'm thinking of ordering something else for the 4" behind the wall such as plain 3/4" gravel (supplier recommended) and using the extra road base to replace the excessive sand instead. I live in Colorado Springs where we do have periodic sustained freezes.

1) Do I need to remove all of the sand and replace it with the road base?
2) Can I use the road base under the retaining wall blocks and the pavers?
3) Should I use the plain 3/4 gravel for backfill behind the wall or something else?

I know I'm in over my experience but could not afford to hire someone. The deck was rotting out and causing the base of house to rot with it, attracting carpenter ants and such. Any advise is welcome and appreciated.

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Comments (9)

PRO
Yardvaark
7 years ago
Sorry, but some of your descriptions of the details have me mystified. I generally don't understand your vision of how you intend to arrange materials. This would be a lot easier if you actually show in pictures what you are doing. Start with pictures that show the whole relevant area so we can understand the context. If grade is important, frame the pictures so that you convey information about it. You might add a simple sketch in order to clarify any points, if needed.

If the sand you are digging is well compacted (which I supposed it is) you would not need to remove it. Show pictures of the fill materials, too.
freshid
Original Author
7 years ago
last modified: 7 years ago
Here are some pics and design of what I had in mind:

cm2148's ideas · More Info

cm2148's ideas · More Info

This is the test hole I dug up with 3'+ sand:

Top view design:
cm2148's ideas · More Info

Front view design:
cm2148's ideas · More Info

Side cut-out view of planned retaining wall and patio:
cm2148's ideas · More Info

This is the " brown road base" material planning to use under pavers and bottom row of wall blocks. It seems like there is a lot of dirt/clay mixed in with this stuff:
cm2148's ideas · More Info

This is similar to what the 3/4" gravel that I plan to use behind the wall over the drain:
cm2148's ideas · More Info
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Yardvaark
7 years ago
I reread and see that you're planning to backfill behind the wall with 4" thick layer of 3/4" clean, stone material. The gravel for drainage is commonly called #57 stone in places where I've been. The road base is fine for the compacted patio fill.

Where the lower tier of the patio ties to the upper tier, it seems like it will be a very awkward connection. As shown, it seems like a busier look than if you had it match outer wall of upper tier.

Where I'm asking "what's over here?" is because it seems your grade rises along the house at that side. I'm wondering if it makes sense to tie the steps to the lower level or to the upper level (fewer steps) along the side. Would be good if your picture pivoted to show that area.
freshid
Original Author
7 years ago
last modified: 7 years ago
There are tall lilac bushes up against the rounded corner marked in red. That is why I wanted to step it in to leave more space for a path but not totally necessary. The shovel in the first pic below would be near the center of the red curve. At the bottom of the stairs is level area I may expand the patio or path later on. In the upper circle is lawn a step below the new patio level that is held up by an existing railroad tie wall which I plan to replace with wall block in the future (second pic below)

.
freshid
Original Author
7 years ago
Can I use the road base material under the bottom row of blocks?
PRO
Revolutionary Gardens
7 years ago
Assume you're talking about segmental retaining wall blocks, not regular concrete block? depends on the road base material. Here what's sold as 21A (also called 3/4" minus elsewhere in the US) is compacted to create a footer/leveling pad. But without knowing exactly what you have I can't say for sure. Ask the supplier, and if needed ask the block manufacturer.

4" of clean stone behind the wall? I'd use 12".

I've never heard of compacted sand, at least compacted enough to build on. I would dig until you hit undisturbed soil.
PRO
Yardvaark
7 years ago
Where I live -- Florida -- everything is built on compacted sand because most of the state is that. If you dig down farther, you hit more sand. (Dig even farther and eventually you will hit a sinkhole.)

Even though you are rebuilding the timber wall later, you will need to build the portion of it that will support the steps. I would widen the raised portion that wraps the house, making circulation easier, and eliminate the lower tier for ease of construction and cost savings. (I don't think it adds in a positive way.)
freshid
Original Author
7 years ago
last modified: 7 years ago
Yes, segimental retaining wall blocks made by Pavestone and sold at Home Depot. The "brown road base material is labeled as 3/4" minus.

As for the sand, it is packed as much as sand could be, at least what I have not yet disturbed. Little worried that the Bobcat will mush it up. Not sure what I'd do with it all besides getting rid of it a little at a time via regular trash service.

The main reason for the lower tier is to not exceed the 3' max wall height spec of the block. Another reason is that I was not planning on adding any railing. The height of that section is 42" with out the tier. Could the spec be exceeded if it is well built with 12" of back fill instead of 4" and a deeper base? I also plan to bury a full block minimum.

The width marked "make wider here" is 64" but looks narrower in the pic. Also not sure if I have enough block to replace much of the tie wall. Likely only the stairs in original plan.
PRO
Yardvaark
7 years ago
Don't exceed the spec. I don't think the way you are showing the tier intersecting midway in the radius corner of the larger wall is going to work, on account of the specs and the mechanics.

If I dig down in my yard beyond the topsoil, I hit a layer (which could be many feet deep) of hard packed sand. Digging in it is almost like chipping into soft rock. For us, it's the subsoil. They build buildings on it.

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