For walls up to 3ft, 5.5 inches thick.
There you will find out how to actually lay brick and blocks onto a solid foundation. The basic rules governing the building of a garden retaining walls are simple. Firstly the foundation you place should be a minimum of 150mm (6 inches) thick. When you have measured how deep the garden will be you simply divide it by 4 to get the required thickness of wall. If your retaining wall is holding back a 1m depth of garden then it needs to be 225mm thick. The minimum thickness of a garden retaining wall should be the width of a standard brick @ 112mm. C. Modular concrete facing units must have footings as required by the manufacturer, but not less than a base of compacted crushed 57 limestone aggregate with one course below grade. 2. Hollow concrete block units shall not be used for the construction of curbs, walls or retaining walls. 3. A granular backfill shall be placed behind all walls and shall extend from an elevation coinciding with the finished grade at the face of the wall to within 8 of the top of the wall.  The width of the foundation should be twice that of the width of the wall you are building and the wall should be placed in the centre of the foundation. If you need to build a wall 225mm wide the foundation should be 450mm wide. This will give you a foundation overlap of 112mm either side of the finished wall. To work out how thick the wall needs to be (A) you must first work out how deep the garden will be (B) at the point you plan to build the wall. When you have measured how deep the garden will be you simply divide it by 4 to get the required thickness of wall. If your retaining wall is holding back a 1m depth of garden then it needs to be 225mm thick. The minimum thickness of a garden retaining wall should be the width of a standard brick @ 112mm. There are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes. Poured concrete foundation walls that are less than 8 feet tall and have soil outside that is 6 or 7 feet deep against the wall can often be 8 inches thick and function quite well. As soon as you go higher or have greater depths of soil pushing against the wall, you need to increase the thickness to 10 inches. Don't forget that the concrete comes in different strengths depending upon how much cement is added at the ready-mix plant. I would use a minimum mix of 3,500 pounds per square inch(psi) mix. You can upgrade to 4,000 psi concrete if you desire, but I would only do this if it was specified by the structural engineer. Another consideration is whether a concrete truck can get to the wall location. Most homeowners should leave walls higher than 3ft to the pros. A poured concrete wall with rebar does not need to be thick to be strong. For walls up to 3ft, 5.5 inches thick works fine. As laag points out, the wall needs a sufficient footer.  I did take pictures of pouring the concrete. We are building a fence that will go on top of the wall. The area around the wall will be lined in rock and back-filled with dirt. The retaining wall is 60 feet long 20 1/2 high and 7 1/2 wide and we ordered 5 yards of concrete but only used 4 1/2 yards. We used 2x 8 for the concrete forms because we are using them in another project after the fence is built. You can use plywood instead. The 2x8's were used to keep the concrete in the forms 3. Height (vertical thickness) of the concrete footing. I pour my concrete footings thick when building mortared stone walls; that is, I make them at least 12 inches in height. If I’m building a really big wall, I’ll pour them thicker (taller) because the concrete footing will need to support more weight.  Retaining Wall Cost. Poured retaining walls with elaborate curves require complex formwork and will drive up cost for a wall. The total cost to have a concrete retaining wall installed will be a combination of material and labor prices. For a basic wall you can expect to pay between $30 and $40 per square foot. If you add a decorative finish, curves, steps, lighting or other upgrades, the cost will rise. For Ralston, $39 per square foot is the average cost he charges for a wall under 4 feet tall. For walls below 4 feet, he says his prices are often competitive with finished block. A block wall has to be stuccoed or finished with veneer, while a concrete wall does not need an added finish. One cubic yard of concrete will pour 80 square feet of a 4-inch-thick floor, 40 square feet of an 8-inch-thick wall, or about 30 linear feet of 8 x 16-inch footing. A satisfactory mix for basement walls, footings and floors is a six-sack mix containing 6 gallons of water per sack and a maximum aggregate size of 1 inch. Concrete mixed with 6 gallons of water per sack will be 40 percent stronger than the same mix made with 8 gallons per sack. Concrete for your basement will probably be delivered to the site by a local ready-mix company in a truck-mounted mixer unit. The thickness of the foundation walls you are looking at are not the same. The wall below and the one to the left of the hose bib faucet are only 8 inches thick. The other walls are 10 inches thick even though there is less soil pressing against them. Don't forget that the concrete comes in different strengths depending upon how much cement is added at the ready-mix plant. I would use a minimum mix of 3,500 pounds per square inch(psi) mix. You can upgrade to 4,000 psi concrete if you desire, but I would only do this if it was specified by the structural engineer.
http://www.diydoctor.org.uk/projects/retainingwall.htm http://www.cincinnati-oh.gov/planning/linkservid/13A0F0AE-BC5D-C6C1-DBEA420103745648/showMeta/0/ http://www.diydoctor.org.uk/projects/retainingwall.htm http://www.askthebuilder.com/thickness-of-poured-concrete-foundation-walls/ http://forums2.gardenweb.com/discussions/1416919/poured-concrete-retaining-wall http://www.instructables.com/id/Retaining-Wall-And-Footer-pouring-concrete/ http://landscaping.about.com/od/Building-Stone-Walls/a/concrete-footings.htm http://www.concretenetwork.com/concrete/poured_concrete_retaining_walls/cost.html http://extension.missouri.edu/p/G1700 http://www.askthebuilder.com/thickness-of-poured-concrete-foundation-walls/
how thick does concrete need to be garden wall