A blanket raft is a type of solid slab raft consisting of deeper ‘toe’ sections running around the edge of the foundation as well as a central deeper ‘beam-like’ section which offer rigidity to the slab. The blanket raft in question is 250 mm deep with top and bottom A393 reinforcing mesh supported by 400 mm deep 600 mm wide toes with 4 no. top and 4 no. bottom 20 mm diameter bar reinforcement and 16 mm diameter shear links (see below).
This blanket raft is to be extended to accommodate additional plant, however, the average loading per square metre is to be less. Consequently, the same depth of 250 mm and reinforcement detail can be used with two layers of A393 mesh top and bottom. The primary difficult depends on where to place the deepened ‘beam-like’ sections and how to achieve fixity between the existing foundation and its extension. The proposed method to achieve this is to scabble back the edge of the existing slab (to achieve good bond with the existing concrete) and to drill and dowel the new rebar into existing slab. By way of a concept design, I have proposed the location of the deepened 400 mm deep by 600 mm wide sections of the slab extension.
Is this a good layout for the deepened sections of the extension to the blanket raft?
Is it necessary to put a deepened toe on the extension immediately adjacent to the deepened toe of the existing slab?
Should the dowels be epoxied in or greased in (do they only need to function in shear)?
Can you suggest what are the primary considerations during the concept design stage?