New Garage Raft

Postby Bun » Mon Apr 09, 2012 9:42 am

I am replacing a single skin brick built garage with something similar only slightly larger, 6m x 3.5. The existing garage slab failed (built about 1954) because one end was built over an assortment of buried rubbish including chimney soot, timber and other soft crud. The slab cracked and dropped across from the pedestrian door and up the other side, affecting about a third of the floor space. I've taken down the affected end and constructed a stud wall as a temp measure to continue using the unaffected end as a store.

The existing slab is about 20cm thick at the perimeter and 10cm in the middle. This seems to have been adequate for the most part as there is no sign of any problems with movement over most of the old garage.

My intention is to take down existing garage and remove slab and start again with a reinforced slab/raft.

I have dug a trench at the perimeter affected by the rubbish to sound clay (80cm to 1m deep, 45cm wide), and where clay is a bit soft I have used a trench compactor to ram in hardcore and filled with reinforced concrete. I now have a L shaped concrete foundation at one end that is about 30cm below finished level. The remaining perimeter ground is sound I will remove any soft material left that will be under new raft and replace with compacted hardcore.

My question is regarding the construction of the new raft. I have found a diagram of a keeled raft here but am wondering if this is the example to follow. Looks like it may be over kill compared to other examples but I would rather over construct.

Any advice or examples of raft construction and placement of rebar would be most welcome.
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