When to use T-99 or T-180 Comp Spec?

[skidot]

Are there any guidelines related to soil characteristics (granular, clayey materials)which favor the use of T-99 or T-180 compaction specs? I have looked in the references I have available and can't find any type of guideline concerning this query. Any help would be appreciated.

 

[Ron]

skidot....generally, T180 is used for sands and well graded materials; whereas T99 is usually used for materials with more fines such as clayey sands, silty clays, silty-clayey sands.

 

[rochplayer]

The difference between T99 and T180 is compactive energy applied to the soil, applies to both fine and coarse grained soils. T99 was developed when equipment was smaller and lighter. T180 was developed to better model the energy input by more modern compactive equipment. Specifying T180 requires the contractor to do more work, get heavier equipment, etc. so in my experience they don't particularly like seeing it in the spec and may push back.

What I recommend is to look both at soil type and what is being supported by the fill. If the structure (be it an earth structure, e.g. berm, or other structure, e.g. building, bridge, etc.) can tolerate deformations that will occur when the fill is not as well compacted, specify T99. If deflections cannot be tolerated to the same magnitude, specify T180.

A great book to read is "Construction Of and On Compacted Fills," by Edward J. Monahan. He explains in very plain terms the Standard vs. Modified compaction issue, specifications, and field investigation.

 

[BigH]

rochplayer gave a nice reply and I agree with him on the book. Sometimes, though, if I wanted to ensure that the contractor would 'pay attention' to his compaction, I would specify the modified value. I typically use modified if under a foundation (especially an important foundation or one that is touchy on differential settlements) and in subbase and base course for roads.

 

[GeoPaveTraffic]

Rochplayer, very nice response. I would add that when the modified value is specified for relatively weak crushed stone, some breaking of the stone can occur both during the test and during field placement. So if you are using a weak rock and are concerned about increasing the amount of fines in the material, you may want to stick with the lighter method.