Hillside Wood Retaining Wall: Are we on the right track?

kingfisher2 · 09-07-2012, 10:22 PM

We are on the side of a hill very close to a lake. We are replacing a retaining wall that was here when we moved in 20 years ago. Five walls were built on a terraced area (very narrow layers/levels) using only 4x4 posts & 2x4s. We do not have freezing problems.
Unfortunately, we started working on the top level.... thinking it needed immediate help... and we've decided to finish the top level.
We have already sunk 6x6 'green' wood posts every 4 ft. They were each buried 4' in 5-600 lbs of concrete.
Here is our plan:
1) Dig a trench about 2' out from 6x6s
2) Lay a couple inches of 1.5" drain rock (free from fines)
3) Secure 2x12 treated boards onto the 6x6s
4) Place roofing paper over boards to keep moisture off (waterproof?)
5) Place landscape cloth (mirafi) over dirt bank, rock, & up boards
6) Place flexible, covered 4" drain next to boards close to bottom of wall
7) Backfill area with 1.5" drain rock and tamp down every 3 inches
8) Cover rock with mirafi landscape cloth
9) Put more clean drain rock on top of everything - no soil whatsover

If the wall will be 5' high, where should the deadmen be placed... at what height and how far apart? Should we use vertical 4" steel pipe sunk in concrete & attach cables to the 6x6s? Or should we us 6x6s in a T-shape and bury it in the hillside with rebar... if so, how would we attach the deadmen to the 6x6s or wall?

Do you have any suggestions on our current plan?
Once the top row is completed, would you recommend starting at the bottom & working back toward the top? Or should we do something midway? The bottom level is actually under water during the winter... so that may have to wait until late spring.
If we're not in over our head now.... the rain is coming soon.

We are on the side of a hill very close to a lake. We are replacing a retaining wall that was here when we moved in 20 years ago. Five walls were built on a terraced area (very narrow layers/levels) using only 4x4 posts & 2x4s. We do not have freezing problems.
Unfortunately, we started working on the top level.... thinking it needed immediate help... and we've decided to finish the top level.
We have already sunk 6x6 'green' wood posts every 4 ft. They were each buried 4' in 5-600 lbs of concrete.
Here is our plan:
1) Dig a trench about 2' out from 6x6s
2) Lay a couple inches of 1.5" drain rock (free from fines)
3) Secure 2x12 treated boards onto the 6x6s
4) Place roofing paper over boards to keep moisture off (waterproof?)
5) Place landscape cloth (mirafi) over dirt bank, rock, & up boards
6) Place flexible, covered 4" drain next to boards close to bottom of wall
7) Backfill area with 1.5" drain rock and tamp down every 3 inches
8) Cover rock with mirafi landscape cloth
9) Put more clean drain rock on top of everything - no soil whatsover

If the wall will be 5' high, where should the deadmen be placed... at what height and how far apart? Should we use vertical 4" steel pipe sunk in concrete & attach cables to the 6x6s? Or should we us 6x6s in a T-shape and bury it in the hillside with rebar... if so, how would we attach the deadmen to the 6x6s or wall?

Do you have any suggestions on our current plan?
Once the top row is completed, would you recommend starting at the bottom & working back toward the top? Or should we do something midway? The bottom level is actually under water during the winter... so that may have to wait until late spring.
If we're not in over our head now.... the rain is coming soon.

Hi, Thank you for your responses. In order:
1. We realize NOW that we have created more work for ourselves... ow. We have already hand dug & placed 16 6x6s along with the trench on the top level. The lower levels will also be hand dug... equipment is not feasible.
2. Yes, the entire slope has been shifting over the years. We'd like to finish the top & then address the bottom asap...
3. Unfortunately, flying by the seat of our pants has gotten us where we are. We are really hoping someone from this forum will be able to give us suggestions on completing this first stage.
So here are some direct questions:
1)We want to put deadmen on the top level. Can you share ideas on what to use & how to install them knowing that we have 6x6 posts and 2x12 boards?
2) Should we put down 4" of clean rock (no fines) before we put down our landscape cloth & drain? Or would it be better to have compacted rock (with fines) below the drain to help water stay in it?
3) French drains usually have the perforations facing down, but if we're on a slope and there is drain rock above & below the drain, wouldn't it make more sense to face the holes up to catch the water flowing down?
4) Should we cover the boards with roofing paper to ensure water goes down to the drain rather than flow through the gravel & out between boards or, worse yet, under the wall?
By the way, the tensar geogrid looks great. I would love to call someone up & have it put in... but we are in a remote area of southern Oregon. I can't see that happening.
Thanks for any help you can give us. Also, we're doing this on weekends...

RE: top level of tiered retaining wall
Hi,
Should the gravel base UNDER landscape fabric (marafi) be clean rock (no fines) or crushed rock (with fines)? We will be putting clean rock on top of the landscape fabric but don't know if we need to create a firmer base under...
Thanks.

allthumbsdiy · 09-08-2012, 02:00 AM

I don't know where you live, but my town has a set of very specific building codes I must follow for any retaining wall structures that are over 3 feet tall.

Have you checked to see if there are any requirements where you live?

Regarding your approach:

1. I think you are making your work lot more difficult by starting from the top because you will most likely end up hand digging for the lower levels

2. Based on your picture, it looks like the entire slop is sliding down. Addressing the top level may have some immediate impact but I think the eventual erosion from below will possibly make you re-do your current level that you are working on.

3. I am not sure but you are mixing a bunch of different types of building materials (6x6 posts, 2x12 treated boards, 4" steel pipe sunk in with cable, etc) which tells me you are kind of flying by the seat of your pants. The most common (and economical) retaining walls I've seen are made from PT 6x6 timber, laid and stacked horizontally, secured with rebars.

It also sounds like you are about to hurry because of impending rain which is never good when working a project.

If I were you, I would create a swell at the top to direct water away from the area and cover the entire thing with a tarp to buy you some time to plan our your project.

Good luck

KevinPh · 09-08-2012, 10:11 AM

I have two suggestions .....

1) If you want to continue securing the slope with PT wood - now that you have the top somewhat secured, start working at the bottom, then build back up.

2) For such a severe slope you could use other ways to retain it - slopes even steeper than this, almost vertical, are secured along highways with tensar geogrids that allow grass or other plants to grow through it. Here's a link. http://www.tensarcorp.com/uploadedFi...A_BRO_5.08.pdf Normally these are used along highways but they can also be used in landscape projects.

You could also try terracing with ledgerock (3 ft. wide x 2 ft wide x 6 ft. long). Each one weighs a ton so it must be installed by crane - you could combine this with the geogrid to create a really attractive slope.

Kevin
www.home-additions-startup-guide.com

KevinPh · 09-08-2012, 10:52 AM

I have two suggestions .....

1) If you want to continue securing the slope with PT wood - now that you have the top somewhat secured, start working at the bottom, then build back up.

2) For such a severe slope you could use other ways to retain it - slopes even steeper than this, almost vertical, are secured along highways with tensar geogrids that allow grass or other plants to grow through it. Here's a link. http://www.tensarcorp.com/uploadedFi...A_BRO_5.08.pdf Normally these are used along highways but they can also be used in landscape projects.

You could also try terracing with ledgerock (3 ft. wide x 2 ft wide x 6 ft. long). Each one weighs a ton so it must be installed by crane - you could combine this with the geogrid to create a really attractive slope.

Kevin
www.home-additions-startup-guide.com

sdsester · 09-08-2012, 04:04 PM

The length and degree of that slope concerns me too and you can see what the forces are doing to the existing structure. Did you pass your plans past a civil engineer or landscape architect? They really are not that expensive and could save you quite a bit short term and long term on something like this. I was a landscape designer with a landscape architect as partner. I would have gotten him involved in a project like this without blinking twice. Among other things I would have need drawing sign off for a dramatic retaining structure and grade "change" like this.

I agree that timber structure may not be your best option, no matter how robust the timbers you choose. I also agree you need to shift gears and work from the bottom up now or you will just see the slope slipping under or folding over your upper structure.

Good luck. Post how you decide to proceed please?

kingfisher2 · 09-09-2012, 05:22 PM

Re: We're gonna finish the top level of the tiered retaining wall

Can someone tell me if the rock UNDER the landscape fabric should be clean rock (no fines) or crushed rock (with fines). I know we're putting clean on top of the drain and covering it all with the landscape fabric...

But is the base/ground (under the fabric) suppose to be more solid like what would occur with crushed rock (w/fines)?
Thanks.

KevinPh · 09-10-2012, 07:45 AM

Crushed rock and clean rock have the same strength, the only difference is whether you want to allow drainage or not.