CBR, Compaction, Pavement Thickness and Cost

[CivelRocks]

One thing I have been noticing is that ASTM D 1557 and D 698 are used interchangeablely and I am trying to figure out the cost implications between using one or the other. If you use 698 to find the OMC opposed to 1557, your CBR is going to be lower. But is 1557 worth using because then when you have to compact in the field you have to do more runs of the compactor, but you might have a higher CBR which saves you money on pavement thinkness. I was wondering if anyone out there has developed a practice or standard for what compaction criteria to use. We are assuming that there are no town/local government ordinances/laws that tell you what compaction to use.

 

[cvg]

The two methods are not interchangeable, your geotechnical engineer should advise which to use, as well as the necessary percent of maximum and the moisture content for the type of soils on your site.

this topic has been discussed at length in the geotechnical forums. For more info, suggest you do a search there or post your question in that forum. Try the following threads:

thread261-37564
thread274-59980

 

[BigH]

cvg is right - they are not interchangable. But, doing CBR to AASHTO standards, you will get a curve of CBR vs dry density. If you use modified Proctor at say 95% MDD, you might have a dry density of (2x.95=1.9 as an example (using g/cc). From this you get your CBR for that density. Similarly, for the same soil, if you use the 1.9 g/cc dry density for your CBR, you will find that you have achieved, say, 99% MDD of the standard Proctor. Same density/CBR but noted as different % of the various forms of MDD.

You are correct in that the CBR at 95% MDD Standard is less than the CBR at 95% MDD Modified. But, the choice is almost semantic - pick your desired dry density and then choose your modified or standard % to get it.

 

[dirtsqueezer]

Cost implications will amount to manpower. ASTM D1557 is a slightly higher standard, and will take the contractor more passes to reach, thus more man hours, thus slighly more cost- for the contractor. If they bid the job to a certain budget, it's on them to get the job done, regardless of the quality required.

I will say it's possible that compaction criteria more with settlement or failure issues than with budget in mind. If it were me, I would find out what the local jurisdiction's doing. Standards are sometimes based on structures that have historically been a problem. For example, the Covington water district requires 95% standard for utilities under roadways, but for detention ponds they accept no less than 95% D1557. Why? Most likely they experienced berm failure in the past. No surprise to me really, their soils are mostly >30% silt.

Hope that helps.

:>