Aggregate Base "Cap" Placement

[loweengineer]

I work for a GC. We were contracted to place 4" of aggregate base on compacted subgrade for a site access road, and then, after construction was complete come back, and place another 2" of aggregate base as a post-construction "cap." We initially agreed with the Owner to just place the 2" Cap with water and a roller for compaction. Well, now the Owner wants us to rip the bottom 4" of base and lay the 2" on top and re-wet/re-compact. Is there actually any value to this? Is there a chance that we don't acheive the same compaction that we had on the first 4" lift?

 

[EireChch]

Not really sure what you are asking, the owner now wants to just lay 2 inch of gravel on the subgrade?

 

[coloeng]

Yes and yes.

Because the first 4-inch base course was already compacted when you placed the 2-inch base course the top 2-inch base might not bind to the bottom base course. That is probably why he wants the bottom ripped and then place the top course. As far as compaction, there is always the possibility that you won't get the same compaction, but if done properly, you should be able to achieve adequate compaction.

 

[EllenaIsra]

Placing an additional 2" cap of aggregate base over a previously compacted 4" layer can be effective if done correctly. However, ripping the existing 4" base before adding the cap is generally unnecessary unless there are specific concerns about bonding or uniformity. Typically, re-compaction of the existing base can disrupt the compaction integrity achieved initially. It's often better to place the 2" cap directly, ensuring proper moisture content and using adequate compaction to achieve the desired density and stability. Always consider consulting with a geotechnical engineer to address specific site conditions and requirements.

 

[EireChch]

Sorry I understand now.

I think you are all completely over thinking it. Placing 2 inch of gravel over 4inch of previously compacted gravel is absolutely fine. Its done all the time.

 

[darthsoilsguy2]

I'm curious why this was considered. was there a testing question driving this? as in... if the bottom materials and the top materials are different, it makes the reference proctor invalid or you set the bar at the higher proctor. Or did they want to proofroll the subgrade again before placing the 6" and fixup some weaker spots... I guess to find a way to remove the in-place 4" from change order work?