Is a shale material can be used as selected subgrade materials

[defg]

Dear Sir/Madam
I'm concerning to one road project which require placement of selected subgrade materials (capping layer of 150mm thick) prior to subbase construction. In principle the characteristic materials requirement by the specification is CBR>15 soaked 4days, maximum swell<1.5 and plasticity index<25.However the above said shale materilas has meet the specified requirements. In practice selected subgrade materials have similar properties like subbase materials, but no grading envelope is specified. But the oversize materials limited to 50mm shall be removed and discarded during placement. In addition to this, the selected subgrde materials must contain hard particles which do not break under repeated load/wet and dry cycles. These characteristics shale materials doe not have. The Contractor has placed these shale materials on road claiming that provided it meet the specification for CBR,P.I and swell has right to use them. Could you please assist or giving some references regarding the characteristic requirements for selected subgrade materials?

 

[BigH]

defg - you have a bit of a dilemma with respect to technical and contractual points. Normally, one wouldn't use shale as a select capping material as it easily weathers and by weathering, the PI can increase as the weathering process turns the shale more and more into a clayier-like material. In your area, I would check out if any state/provincial/territory or local governments have used the shale similarly with good (or bad) results. Having said this, the same can be said for residual soils in the tropics as well - and they are both used. Your specifications clearly has indicated the properties that the capping material is to have - the CBR soaked for the saturated strength, the swell to ensure that the material (and shale) does not swell (i.e., have unwanted clayey minerals present - and you don't mention the definition of swell that you are using) and the PI to ensure, too, that the material behaves in the manner. Unless the contract specifications forbid the use of shale or there are other issues other than than CBR, PI and Swell, then the contractor does, in fact, have the right to use shale. If you don't want him to use the shale, then you will need to write a field instruction (or change order) to forbid it - and then, of course, the contractor will be able to claim against the contract. Final comment is that the successful or detrimental use of broken down shale or other residual type soils will also depend on the characterisics of the highway useage (design traffic count).

 

[Ron]

I would only add one point to BigH's excellent answer....if the water table is high, your problems will worsen. If that's not a consideration, then the contractor will likely push harder for acceptance.