[jtswilson]

I am trying to begin a project and I am in the planning stages. Here is what I want to do.

12 degree allan block
55 feet wide
4-6 feet tall (varies)

I want to put in a concrete footing rather than crushed stone. The neighbor put in the same type of wall with a crushed stone footing and it is now sagging after less than a year.

Questions:

what should the dimension of the footing be?

should the bottom row of block be covered 1" for every foot in height?

Should I put down the bottom row of block while the concrete is still wet to create an even more solid base?

Do I need rebar?

Thanks for your help

[AZTLANLC]

Unless you're going to put a concrete footing below frost level it will not be such a good idea, compaction and the right base materials are the key for a successful wall installation, also that wall should be engineered due to height, check with your town regulations.

The 1" per foot height rule is correct but also it has a minimum of one full block.

Another thing to consider is geogrid and good drainage behind it.

Building a successful retaining wall takes knowledge and skills if not build correctly it will fail I would highly recommend to look for a respectful company around your area to tackle this job.

[Bill Schwab]

In most states, any wall over 3' tall requires permitting. If it is an SRW wall, there will be 2 thing necessary in order to opull that permit. First, a comprehensive soil anaylsis, and in some cases such as when wall sit on slopes decending from the leveling pad, a global stability anaylsis. The report then can be given to a civil engineer, where they will plug in the soil numbers and calculate the loads on the wall, and, design how the wall will be built. From those documents, you should be able to aquire a building permit for the wall. Since Alan Block 12° has a design limitation of 3' from the top of the leveling pad or footing to the top of the wall if it is used as a gravity wall (no strata grid). If you go over 3', you will need to cut back at least 60% of the wall height from the back of the wall so you can install strata grids as per design requires. Then as the wall is going higher in elevation, the soil and clear gravel is installed with the soil being compacted to acheive 90% hardness. Most cities will call you on this one and require a construction hardness testing company to report and test soil compaction as you build the wall.

99.9% of the time, use of a concrete leveling pad is never required so long as you compact the base to that same 90% I mentioned. If you use a footing situation, AZTLANLC makes a great point. You need to have that footing below the frost line, and, your wall height is now going to be considerably higher, because that is measured from the top of the footing. So, you might add 3' or more of wall hieght that will need to be buried in order to meet the frost line requirements.

If your neighbors wall is sagging, there is some other reason than a gravel base going on with it, unless of course he did not compact or put enough base in the first place. We have built 25' tall walls on gravel leveling pads as per design and no issues.

I don't want you to take this the wrong way, but frankly, a segmental wall, designed by a supply house, sold to a homeowner can be one of the most dangerous things in the landscape. To further confuse the issue, some manufacturers general instructions are too vague to build a wall over 3' tall.

I'll use a couple examples. A guy in Solana Beach built a wall to expand his property space. And, this winter, we got hit with lots of rain. The wall accumulated water (hydrostatic pressure) and, during a downpour, blew out. He did not have the right weight grid in the wall, nor had it been drained well, and he did not have the soil he used as backfill tested to see if it would hold what he was trying to achieve. The 115 lb blocks fell on top of a perfectly restored 1942 Dodge Powerwagon, crushing 3 years worth of work. That was bad enough, it could have been his kids, and in other cases, people have been injured when these walls blow apart.

In another case, a garage slide down a 5' slope because an SRW wall failed, that wall was only 4' tall. Live surcharge ans wet soils killed that one.

These walls can with stand hundreds of years of elements, the Roman aquaducts are a segmental design if they are done right. Too many people want to get in and reinvent the wheel and it usually is recipie for disaster.

I don't know your abilities, and, most people with basic common sense home improvement skills can do segmental walls, if you are given the right instructions through engineers and soil professionals and you follow what is written to the letter. That means you need concrete saws, soil compaction equipment and it usually helps to have a skid steer or some type of loader. and you have to line someone up for soil compaction testing during construction.

If there is any way you can keep the wall to 3', it will elimninate most city needs to pull a permit. And you don't need concrete. Any more questions, fire away!

[jtswilson]

A little more Info I left out.

I have 2 rolls of 3x50 Geogrid. My plan was to use the Geogrid every two rows - which will probably be only 2 rows of geogrid.

The city I live in only requires a permit above 4ft

I have already put in a footing of crushed rock which is about 4inches deep. Then I rented a compactor and compacted.

If I just stick with the crushed rock, how deep should it be.

I am planning on putting some perferated pipe behind the wall, but haven't figured out how to get the pipe to fresh air.

I am scared about this wall, everything I have readd about walls on this site scares me into doing it.

I plan on backfilling with fill dirt. The wall is in my backyard, I am not putting anything on top of it as far as buildings or cars.

[Bill Schwab]

What is the soil like? Do you have a slope behind it, or is the ground flat? Leveling pad depth should be one full block depth, plus 6" of 1" minus compactable stone.

As far as taking a pipe to daylight, is the footing lower than any other grades? If so, bury the bottom and fill the cells with the compactable stone, then use 3/4 clear gravel (washed gravel) in the cells and behind the wall. Drill 4.5" holes every 50' through the face of the wall just at grade to daylight the pipe and put a grate over the hole so varmints don't build nests there.

As far as grid goes, 3' of grid on a 4' wall is bare minimum requirement, but, I would use 4' of grid a call it a day. You will need a jumping jack style compactor (wacker) to get the soil hardness you need in the grid zone. You will prbably need another roll of grid to build your wall.

You will need at least one row of grid between the bottom course, and the second course, then between the 4th course and fifth course assuming you have good soils and the ground is flat.

Keep in mind wall heights are measured from the top of the gravel leveling pad to the top of the top course or in some cases, the cap. That will effect your grid requirements.

[jtswilson]

The soil is somewhere between clay closer to the wall and regular dirt as you get closer to the house, I used a jumping jack for the gravel I put in already.

I think I will have to redo the footing because the top of the footing is level with the ground around it.

The wall will only be 55' feet wide.

We have dug out the slope so that there will be about 4-6" of flat ground behind the wall.

Not a problem to buy more grid.

Question though - people are saying not to poor a footing. Is the reason being that it is overkill, or is the reason being that it wouldn't work. The reason I wanted to poor the footing was so that I knew the footing was completely flat and to prevent sagging.

I should run the grid perpendicular to the wall, correct?

One end of the wall steps down twice, first step is about 1.5 blocks and the second step is another 1.5-2 blocks down, does this pose any other issues?

Thanks, for all your help.