Is a concrete slab required to support a block wall?

Question

I'm starting to plan my build of a backyard grill station / pizza oven. I plan to build a concrete block wall about 4 feet high. I'll put either concrete or stone surface on top, as well as a 1600 lb brick pizza oven, so it will be supporting a fair amount of weight.

I'm a complete novice to masonry projects, so I'm learning as I go. I have two questions:

1) Part of the wall will be built on what is currently dirt in a planter area; the rest of the wall will extend onto existing pavers. Do I need to pour concrete in the planter as a foundation, level with the pavers? Or can I get away with compacting crushed gravel as a base in the planter?

2) The grade of the pavers angle slightly down, toward a drain. What's the best way to level out the base of the wall that will extend onto those pavers?

Hopefully I provided enough info to get a few suggestions.

Thanks! David

Answer 1

1) Part of the wall will be built on what is currently dirt in a planter area; the rest of the wall will extend onto existing pavers. Do I need to pour concrete in the planter as a foundation, level with the pavers? Or can I get away with compacting crushed gravel as a base in the planter?

You'll need to do 'something' to deal with the overturning and sliding forces on the wall, not just from the soil pressure but also that significant 1,600# surcharge. A 4 feet tall is generally the start of the retaining wall design height and even below that height it's assumed that the wall has a standard footing. You will need to provide a concrete footing and tie it into your wall with rebar (steel reinforcing). A typical footing (for 4' of soil) is 16" wide and 8" deep w/ (2) #4 rebar continuous. You need to have the bottom of the footing below frost depth to avoid issues with frost heave, and for soil in front of the footing to prevent it from sliding along with the entire wall. Provide a footing drain at the back or weep holes at the base of the wall because any water building up creates a lot of additional force that isn't considered

2) The grade of the pavers angle slightly down, toward a drain. What's the best way to level out the base of the wall that will extend onto those pavers?

The footing should be below the pavers so you can have the pavers butt right up to the finished wall

Answer 2

That sort of weigh precludes building on any of the top, humus laden layers of soil. You'll need to dig down to undisturbed subsoil. This will be compact and able to support the weight without sinking. Depth of subsoil suitable to build on varies hugely from area to area so I can't give you a figure. In my region (in the UK) it's typically around 1m deep.