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).

slab elevation

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.

plan on proposed 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?


2 Answers 2


We don't see the magnitude and layout of the load. That will determine how the slab needs to work. Your layout seems fine to me, but someone may disagree. Also at the top left corner, too narrow of a slab left 80 cm (2000 - 600 - 600 mm = 80 cm)in between, you might as well do that section thick all the way, for ease of construction - just an idea. Whether you need new deepened toes adjacent to existing or not, will depend on your load analysis. I believe the dowels should be epoxied, why would you want movement between two parts anyway?

  • $\begingroup$ Details on the idea behind allowing movement can be found here. The idea being that since it is a slab-on-grade it is not spanning therefore no moment transfer is required and, consequently, the connection only needs to function in shear. The concrete in the extension will likely shrink so this is why it may be a good idea to allow movement. $\endgroup$
    – egg
    May 11, 2020 at 14:34
  • $\begingroup$ As it says in that post, "if the slab is a rigid pavement, we do not epoxy the dowels". That is why I wanted to know what this slab is made for, in other words, the loading. I understand what you say about it is not spanning and not structural slab, and you are probably right and answered your own question then. What is being supported by the way? $\endgroup$
    – upstream
    May 11, 2020 at 14:52

Foundation design starts from soil and geology and ends to the structure, live and dead loads.

First one needs to investigate the soil, its competency, stratification, any subterrainian water level, proximity to fault lines. Underlaying geological conditions, etc. Then you can start to thing of what type of foundation best answers the particular situation your project has.

Many cities and town's and building authorities require an applicant for a building permit to hire a soils engineer to investigate the soils and advise the owner on the type of foundation required.

In some cases they have their own geology departments and will check the site and recommend the type and details of the foundation.

Your first step is to check with your building department.

  • $\begingroup$ I believe the soil is assumed to have sufficient strength here. The original poster wants to know the slab structurally. There is also an existing slab newer the near slab, which that same soil carries. So he doesn't have a geotechnical but a structural question. $\endgroup$
    – upstream
    May 11, 2020 at 22:12
  • $\begingroup$ he needs allowable soil pressures, in numbers. he needs to know what codes is being used, what is the specs on concrete and what are the factored loads. what if any is the concentrated load. do we have seismic or overturning moments that need to be resisted? $\endgroup$
    – kamran
    May 11, 2020 at 22:49
  • $\begingroup$ your answer referred to only geotechnical aspects I believe, that is why I made that comment. Otherwise, above I also asked about the loads and what is being supported to OP. $\endgroup$
    – upstream
    May 11, 2020 at 23:20

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