From a typical ground investigation borehole log the tarmac thickness is recorded but no tests have been carried out on the road surfacing or formation level. I'm after an approximate bearing capacity of the road surface, including the contribution of the surface itself and the sub-formation under it.

Obviously heavy vehicles can be resisted, can I take a typical heavy vehicle and assume a minimum bearing capacity from the bearing stress under the vehicle or is there another/better more reliable method?

I can't carry out further insitu tests.

  • $\begingroup$ Are you looking for concrete strength or an overall bearing capacity of the tarmac based on the contribution of the sub-base and subgrade? $\endgroup$
    – grfrazee
    Mar 4, 2016 at 15:20
  • $\begingroup$ I'm looking for the overall bearing capacity including the contribution from the sub-formation. only an approximate conservative value. $\endgroup$
    – steinbock
    Mar 8, 2016 at 8:39
  • 1
    $\begingroup$ I'm not particularly familiar with tarmac construction, but are there any standard specifications for installation and preparation of the subgrade? Or possibly a project-specific one that you can reference? $\endgroup$
    – grfrazee
    Mar 8, 2016 at 13:56

1 Answer 1


You can't use the pressure from the heavy vehicle as your bearing capacity. The reason is that the pressure from small dimension (from tyres) is not equivalent to that of a large dimension (building footing) even though the net pressure immediately under them is the same. The pressure bulb penetration depth into soil is a lot bigger from the latter. Therefore, in road site investigations they only investigate the top metre of the soil and treat it if required. They hardly bother to see what's below that. While for a one storey to say 10 storey building you will drill down to 3 m to 10 m (depending on many things to work out this depth) respectively. Therefore, if you are using them only for the vehicle load, you should be fine. However, for bigger structures you need to investigate deeper.


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