Compacting Monolithic Garage Foundation

I am preparing to construct a carriage house with residence above on a monolithic foudation (42 inch deep, 8 inch wide footings). It will be accessed by the existing alleyway. The ground it will be constructed on is about 6-10 inches lower than the alley. I obviously will need to add compacted material to raise up the area below the slab. What is the best process for doing this? My concern is keeping the compacted material from rolling into the trench footings if I dig first. I have had the following suggestions:
1. Add and compact the entire area prior to digging. This adds significant cost to the job and is somewhat wasteful considering a large portion of that material will be removed.
2. Dig. Compact material in the center using a temporary form around the interior. This temporary form would be removed during the pour once the footings were filled. Keep in mind I will be using wire mesh, rigid insulation and radiant tubing making the quick and clean removal of the temporary form tricky.
3. Any other suggestions?
Thanks
Replies
This is what I do for a living. Filll complete area compact with a jumping jack style compactor, sand sledge are not woth using. then dig your footing. use a material with some clay not a heavy clay but a sandy clay. this will keep the footing open.
Not to hijack the thread, but how's the house coming along, Johnny?
Its haunted, looking for a exocist, now.
1. Add and compact the entire area prior to digging. This adds significant cost to the job and is somewhat wasteful considering a large portion of that material will be removed.
Building temporary forms adds significant cost also, so does having to be extra careful around any trenches and other "stuff" in the way. The added cost of precompacting is only the cost of the extra material (6"-8") over the proposed trenches.
NOT precompacting will leave a soft area around all the footings as it will be impossible to compact that area without damage to any existing trenches/constructs.
Preparing the ground under the foundation is critical to building a good structure that will withstand the tests of time. If you MUST scrimp due to budgetary concerns, don't start saving till AFTER the foundation is poured. Anything above the foundation can be relatively inexpensive to upgrade/repair later...Doing anything to the foundation (including the soil part of it)...well, it's "set in concrete" after all.
IMHO, the ground prep and foundation MUST ALWAYS be done to the highest standards attainable, anything (good or bad) done at this stage will be reflected throughout the structure for the lifetime of the structure.
Be
Prepared
SamT
Only one comment I'd like to make...why use wire mesh in a monolithic foundation slab?...especially if this slab is pretty large and will need to hold a large structure. USE REBAR period. IMHO, WWM should only be use for sidewalks. It has no place in large foundation slabs.
What some posters refer to as "rat" slabs or "raft slabs" is what we call, "reinforced slabs"...that is comprised of 1/2 inch dia rebar layed out 12 inch on center . Another layer of rebar is layed out 12 inch OC perpendicular to the first layer. The rebar is tied together using tie wire, and the entire mat is raised off the ground 2 to 3 inches, depending upon thickness of anticipated concrete pour. We use metal "chairs" (others refer to this as chair rails) to raise the rebar off the ground. Chairs are layed out on the ground first, then the first layer of rebar is set down. rebar is tied to the chairs. Then 2nd layer of rebar laid perpendicular and this layer is tied off to first layer. Rebar does not need tied off at every cross junction...we usually teid off "every other one."
Rebar, chairs, and tie wire can all be purchased from reputable masonry suppliers. This material is very inexpensive, though a bit labor intensive to install. BUT, it is cheap insurance toward insuring a solid slab foundation.
Also, I would place some rebar in the actual footing trenches as well.
LOL with your project.
Davo
what about digging for footings then forming inside with expanded metal then filling and compacting after metal is in place. metal does not have to be removed and you can use wood forms on top of metal to build up concrete sill above pad. you may have an extra wide footing thickness through the area of grade that is fill but concrete is a relatively cheap material. in other words you may want to hold expanded metal back away from edge of footing excavation 6" or so depending on how the soil holds the metal stakes that hold the expanded metal and fill, also a pea gravel or sand fill that requires little compaction could help from caving in the stakes and metal