From the GMA Techline
Subject: Arizona road I am a design engineer working on a road construction project in Arizona. The existing road is to be rebuilt, with 6 inches of asphalt concrete (AC) over about 12 inches of existing base. The city is concerned with groundwater and has instructed us not to remove more than 2 or 3 inches of the existing base material. The subgrade R value is about 8.
We have finished the design plans and they have been let for bid. We are calling for removal and full replacement of the AC pavement, and removal and replacement of 2 inches minimum of the existing aggregate base under the AC.
In an attempt to gain base reinforcement, we are specifying geogrid below this 2-inch minimum new base, but above the existing base. Again, the city has asked that we not expose the subgrade on this project.
The contractor is now asking about two things: (1) Should we be installing a separation geotextile fabric for the project? (2) Is it possible to construct 2 inches of base over a geogrid without damaging it?
My response to item 1 is: “Probably”—that it is my understanding that a separation fabric is utilized between the subgrade and the base material to prevent intrusion of the base into weak subgrade. Since we are not exposing the subgrade, if a separation fabric was used, it would actually be adjacent to the geogrid, sandwiched between the old base (to remain) and the new 2-inch-thick base on top. Therefore, it does not seem logical to utilize it in this instance, unless the project was modified to require removal of all base material.
My response to item 2 is that a careful placement and compaction of the 2-inch lift of base over the geogrid is possible if no abrupt braking, turns, or steel-track equipment is used on it. I am hopeful that you can help us through this situation because we are in a bit of a quandary. (Rick in Arizona)
Reply: You are completely correct in that the use of a separation geotextile between the remaining base and the to-be-placed 2 inches of base is useless. It might even be detrimental in that a potential weakness plane might develop. Do NOT use a geotextile in this situation.
Conversely, do use a geogrid. It will embed itself between lower and upper base aggregate sections and provide reinforcement without the creation of a plane of potential weakness.
So you are batting 2 out of 3 so far.
Hey Rick, to place 2 inches of stone that is 1-2 inches in diameter with a piece of construction equipment is simply not possible. You will be tearing the grid up so much that it will be poking up through your 6 inches of AC.
I guess you could bring the stone on site with wheelbarrows and hand broadcast it but I am not volunteering. My suggestion is to cut down 6 inches of the existing base, then the grid, and then the remaining 6 inches of base. (Do it when it’s not high-water level!)