Flexible Pavement Subgrade Question

[ChipperB]

We have high clay soils in this area of Central Texas and this is a high volume of traffic street (arterial). The contract was awarded with the contractor having Option A- 2.5" HMAC over 8" crushed limestone compacted base course over 6" lime stabilized subgrade or Option B 2.5" HMAC over 12" crushed limestone compacted base course over MC 30 prime coat or an emulsion applied to raw subgrade.

The contractor is going with Option B. We are having doubts about Option B.

Has anybody had experience with the Option B pavement section? For this area, lime stabilized subgrade is the standard, so for me, I am out of my comfort zone with Option B.

Thanks

 

[ishvaaag]

The question is that the contract is awarded under specs allowing for option B, really the contractor can proceed along it. Any authorized party may however make their objections standing in paper in front of the other parties just in case; and if with authority enough one may object heavily upon the commissioning or receiving party, upon stated reasons. That is part of the personal responsabilities of the involved parties in order to ensure a final good result, and may develop in some change even with economical impact.

Depending upon your position one may sometimes elect to walk away of some work where good practices are not being followed, but if not the convenient thing a written statement with copy receipt may do something to salvage future liabilities if the other parties are not wanting to follow some sound warning.

 

[ACtrafficengr]

Will the prime coat satisfactorily prevent migration of fines up into the subbase and vice-versa? I think I'd consider a separation fabric.

Did you ask TX LTAP? (

http://teex.org/teex.cfm?pageid=EUprog&area=EU&templateid=1263)

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[ChipperB]

ishvaaag-We have a standard 1 year warranty. In my experience, even mediocre paving will hold up pretty well- for the first year. After that is when deterioration really begins. We have a chance to avoid future liabilities now, but what we pay extra now will be far less than we would have to pay to fix a long strectch of bad pavement.

ACtrafficengr- Your answer is more in line with what I am looking for. You have confirmed my feelings that a prime coat is not enough to stop the migration.

 

[ACtrafficengr]

Thanks, but Ish has a point. The contract has been awarded, and unless you want to issue a change order, you may be stuck with it.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[BigH]

Notwithstanding the contractual point, I am presuming that the clayey subgrade is expansive as found many places in TX. In this case, it is imperative to keep the water out of it. Presumably, this is what the emulsion prime coat is for. I am not sure I would want to rely on it alone. Water has a propensity of screwing up your world. Might I suggest, if you are going with Option B, that consideration be given to putting in drainage and ensuring that any water that might seep down through the pavement (cracks, natural filtration, etc.) has a way to escape. You might try reading the section that Cedergren has in the bible of water in soils "Seepage Drainage and Flow Nets" that deals with pavement. Very interesting. You can permit Option B - and contractually you'll be right but maybe it would be better to bite the bullet (financial) and make the change to Option A if this has proved, in your area, to work well. Getting it wrong at this stage will lead to maintenance nightmares. My take on it - for what it's worth.

 

[Ron]

As with the others, I would be uncomfortable with Option 2. Yes, the increase in thickness of the high modulus base helps to mitigate load issues on the subgrade, but expansive clays can easily lift and crack this layer as well.

As AC noted, fines migration won't be stopped by the emulsion...it is a feeble attempt at a water resistive barrier, that doesn't really need to be there...the filter fabric would be more effective.

I agree with BigH...go with what is known to work. Changing things in areas of known difficult can be a very expensive experiment.

 

[dik]

For high volume of traffic, both specs appear to be light, else you have smaller trucks in Texas...

Contractually, if the contractor has been given an option, you can issue a Change Order for a different spec... Contractually, it's your only recourse.

Dik

 

[TDAA]

What is your position in the project?

Who is your client (or are you the client)?

Is there an actual issue with the pavement section, or is it that YOU are just uncomfortable with it?

Is that section used on other projects that you have not been involved with?

Have you reviewed the actual design, that I would assume has been engineered by a consultant (or 2) to meet the standards of the governing agency?

Has the agency who is getting the road reviewed and accepted the design options?

While there are some valid concerns, there are also some assumptions that the design does not meet the standard of practice in the area just because it is not widely used. Is this true, or is it that it doesn't meet YOUR standard of practice?

By the way, what are the subgrade soils? Plenty of advice being given without the basic site information. We too have expansive clays, but they do not cover 100 % of the area. On our expansive clays, we prefer full depth asphalt; however, lime/cement treatment may also used, as well as aggregate base (without a tack coat, fabric or subgrade treatment).