Replacement of failed retaining wall on hillside


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Old 12-28-13, 10:27 PM
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Replacement of failed retaining wall on hillside

Hi,

My house is on a canyon with a huge deck sitting on posts down a hillside. The grade of the hill varies between 20' on up to 45+. I was repairing the deck due to a sloping issue at the corner of the deck, and noticed the retaining wall under the deck which is next to the post footings is failing. It is badly slanted and cracked. I'm going build a new retaining wall.

First question: Which is stronger... the interlocking block retaining wall pictured here: [ATTACH=CONFIG]23656[/ATTACH]

Or traditional retaining wall with the cinder blocks and rebars.

Second question: I can knock out the old retaining wall and most the dirty above should remain in tact while I build the new one? Or can I build the new one a few foot from the old one, and leave the old one intact (and fill the inbetween with the crushed stone, etc?
 
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Old 12-29-13, 12:00 AM
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Hi,

Also, I'm reviewing the past posts about retaining walls.. I see a lot of recommendations on keystone walls which appear to be dry.. you just use the lip or the pin.. are these as strong as buying doing the concrete with rebar/cinder block setup?

Sorry, dont know much about retaining walls, my first research into it
 
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Old 12-29-13, 07:30 AM
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Personally I prefer dry stack retaining systems like Keystone because they tolerate settlement and shifting very well without failing and their dry stack design helps promote good drainage through the wall. That said either type, dry stack or CMU and concrete, is not stronger or weaker than the other. It all depends on how they are designed and built. You just mention that the hill is 20-45 feet high but don't mention the slope or distance back all of which will affect the wall and did not say anything about the height of the retaining walls so it's impossible to offer any specific advice.

Many block system walls already have much of the engineering done and you simply need to follow their existing installation instructions. Search their websites and you will find instructions for handling different wall heights and back slope situations.

Will the hill stay in place if you remove the retaining wall??? That all depends on the hill and the weather and soil conditions and you did not provide any information about the slope so again it's impossible to stay. Remove the wall and the soil behind may stay mostly in place but can you get the new wall installed before a big rainstorm?
 
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Old 12-29-13, 02:53 PM
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You really need to contact the LOCAL code enforcement official in your area BEFORE doing anything. Generally speaking, any retaining wall over four feet high (and often lower) requires a building permit and to get the permit you need to supply drawings with enough detail to allow the building department engineer to ascertain what kind of a wall is necessary.
 
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Old 12-29-13, 04:19 PM
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I would never consider using conventional concrete blocks (CMUs) to construct a retaining wall more than a few feet high. The disadvantages far outweigh any advantages:
1. A reinforced concrete footing is required, requiring significant time and $$$.
2. Particularly for taller walls, installing CMUs in a vertical plane will tend to make the wall prone to leaning outward when soil pressure is exerted.
3. Offsetting each course of CMUs rearward (to assist in resisting soil pressure) will compromise the wall's integrity, as the surface area of horizontal mortar joints will be reduced. Approaching 33% reduction for an offset of just 1/2", resulting in a weaker wall.
4. CMU walls will show mortar line cracks when subject to differential loads acting on the back side of the wall.
5. Filling CMU cells with reinforced concrete can overcome some of the foregoing disadvantages, but requires more time and $$$. Far better to just skip the CMUs, and form and pour an adequately-reinforced solid concrete wall.

A dry-stack wall such as Keystone will perform far better than CMUs, will be quicker to build, and probably can be done at comparable cost.
 
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Old 12-29-13, 10:06 PM
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Thanks for all your help everyone! I'm going to go with the keystone compac bricks with the fiberglass pins. The wall is only 3 feet high (including the base layer) in most parts and 4 feet high in the highest point. Going to get some help (muscles) lifting these 100#+ bricks but I really like the design of the system.

I wanted to use geogrid but cant remove the natural soil because it hits the deck posts. Wish me luck!
 
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Old 12-30-13, 06:53 AM
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Whatever you do... do not skimp on the base preparation and setting the first row. The compacted gravel base is the foundation for the wall and properly burying the first course is what anchors the wall and prevents it from being shoved by the back pressure. That beginning stuff that gets buried underground is the most important. After that building the wall goes very quickly and easily. The more level you got your first course of blocks the easier it is.

Also, buy a few pairs of good work gloves as the surface of the blocks, especially the face is very aggressive. And always wear safety toed boots when working with the blocks. As you know they are very heavy and can really mess your feet up if one slips out of your hands.
 
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Old 12-30-13, 10:16 AM
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Hi,

With the way the keystone blocks are pinned, I assume the bottom of the block has horizon groves that allow the blocks to be placed on a curving style?
 
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Old 12-30-13, 10:50 AM
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The top and bottom of the blocks are flat so rows are always horizontal. The back of the stones is narrower than the front so you can have them touching in the front but angled to form a curving wall. Then the pins can be inserted in one of two holes. One set makes the wall vertical while the other gives it a slight setback into the hill.
 
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Old 12-31-13, 07:18 PM
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So started this project today, got the blocks delivered and our trench is done. We had time to lay out about 5 blocks before it got dark and boy these blocks are much more difficult to lay out and make perfectly level due to the sheer weight of the blocks.

But ran into a little snag and I dont know how to solve it... So we are installing a 4" perforated drain pipe for the wall, the problem is, the the grade of the slope of the wall from left to right runs high to low in the center to high again at the end. We are doing a block step formation where the center of the wall's blocks is one block lower then the sides. How do I set up the pipe so that gravity will direct the water out?

The only way I can figure out how to do it:

Option #1: Lay the pipe from right of the wall to left of the wall but the right side of the pipe will be up to 3-4 blocks high and the left side will have the pipe on the base level. This doesnt sound like its optimal cause any water that drains below the 3rd block ont he left side wont be directed properly?

Option #2: Lay the pipe so that the right side is high and goes to the center which is lower. Also lay the pipe from the left side high and goes to the cneter which is lower as well.... Next, but a pipe at the center going down through the wall and have it exit under the wall in the front... This is a lot of work and I'm not sure if the integrety of the wall is compromised

Option #3: Similar to option #2 but make a hole in the pipe at the lowest point in the center of the wall and allow the water to seep through the dry stack. I can leave a bigger gap on the blocks towards the center to help with this?

We do live in San Diego so we only get a few inches of rain a year... Maybe its not even a problem?

Not sure if I was able to explain the problem correctly heh.
 
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Old 12-31-13, 09:06 PM
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Eh... i think I figured it out.. I need to use a perforated pvc drainage tile, and couple it with an exit point in the lowest area, punch it out under the block.. like the spec described:

Correct?

 
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Old 12-31-13, 09:10 PM
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Best perforated pipe performance is achieved by setting the pipe as close as possible to the lowest course of pavers. While at the same time maintaining a steady pitch to outfall. I've seen flexible perforated pipe often perform poorly because of its tendency to slink like a serpent (undulating up and down) during installation, such that fines tend to collect in the low spots. The result is everything working at first, but then very gradually slowing to a trickle before completely plugging up. Best to use rigid pipe, with perforations at 4 and 8 o'clock. And filter fabric under and around the trench, with a bedding of washed rock on top of the fabric before placing the pipe. Followed by more washed rock and then the (continuous) fabric over the top before dumping in wall backfill.

Regarding your situation, I wouldn't recommend dumping water behind the wall near the middle of it (even though you don't get much rain). All it takes is one gully-washer to ruin all of your hard work. I think using a "T" near the center low spot, and exiting pipe out through the wall at that location, sounds feasible. I'd be tempted to sawcut and remove a 4-1/2" section of the center block, to enable running the exit pipe through the wall without messing up the course sequence and pin alignment locations. Don't forget some mesh inserts wherever the pipe exits, to keep any rat, mouse or gopher populations from colonizing the place.
 
 

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