Runway Replacement, Rehabilitation?

[lovethecold]

I am a relatively new geotechnical engineer. The project I am now working on involves giving some recommendations regarding the pavement and subgrade for a rural general aviation airport. The airport would rather be able to rehabilitate the runway if at all possible. The airport was built in the mid to late seventies, since then there has been an overlay placed on the runway. No one is certain of the dates. The overall pavement thickness is approximately 6 to 7 inches. There is noticeable oxidation taking place. While coring, it was noted on several the pavement was raveling. On one core the technician only got through the overlay (1 ½ inches), the rest raveled. The overlay appears to be 1 ½ to 3 inches thick.

The civil working on the airport said that during the summer the runway is relatively smooth with only some cracking. Alligator cracking is negligible. During the winter the runway develops significant heaving. There will be 4 to 6 inch heaves every 4 to 6 feet.

The soil borings show frost to 3 ½ feet. Below the pavement is approximately 1 ½ to 2 inches of sand and gravel base (more in some areas). We are unsure of how much of this is raveled asphalt and actual base material. Fill appears to extend from 1 to 4 feet below grade. The fill is mixed and consists of lean clay with sand, lean clay, and some topsoil. Below the fill the soil is primarily lean clay with sand. About 10 feet the soil is sandy lean clay. The water table appears to be below 11 feet (the depth of our borings). We have been told the water table is at 15 feet.

The moisture density for one of the lean clays was 30% and 90 pcf, at 5 feet (this seems typical of CH soil, not CL soil). Others were higher, but lower than would be expected for lean clays. The way the clay crumbles, it seems like there may be a significant amount of silt mixed in the clay. I am waiting on the Atterberg tests, and am thinking of running a hydrometer.

The runway is generally level with the surrounding ground surface in most areas. In some areas though, the runway is below the surrounding ground surface.

My analysis. Although there is no significant cracking, the pavement is old and has become very porous. As such, there is a fair amount of water seeping to the subgrade. With the runway below the ground level in areas, there is no place for the water to go. I am also thinking the subgrade is not crowned, and is perhaps even concave in some areas, allowing water to pond below. After freeze up, the runway heaves. Although I would expect some heaving, this seems excessive and why I am thinking there is water ponding below the pavement.

It is my opinion the runway and subgrade should be replaced. The subgrade should at minimum be scarified and recompacted, and crowned to allow drainage out from under the pavement. Also, the higher ground surrounding the runway should be cut back and have drainage ditches installed. I am thinking of recommending the subgrade be cut to 3 feet and replaced with non frost susceptible sand (if it is in their budget). The biggest issue will be how the airport has to spend.

Is there something I am missing? What recommendations might you have, are there other questions I should ask? Thank you for any help.

 

[BigH]

lovethecold: Interesting project. Can I ask a couple of questions:
1. Can you give the general local - state?
2. What is the "frost depth" that is normal in your area.
3. You seem to have the general subsurface condition profile. It may be possible that the moisture content could have been influenced by the water used in coring - if the technician (field and lab) didn't scrape off obvious "muck" on the outside of the core. Did you actually do Atterberg limit tests on each of the layers? heave tests?
4. What is the type of aircraft loading? commuter jets or heavier planes? Current and historic.
5. Are there any plans to the original runway - did they put in subsurface drains?
6. Can the airport still operate if you totally redo the pavement structure (including removal of what is described as "poor" fill)? - i.e., does it have two runways or only one?
7. Is the worst area in the landing, taxi-ing or sitting area? In other words, does it affect the whole of the runways, taxiways and tarmacs or is it geared towards one or the other. If the runway is okay, this helps keep the airport open during rehab. (Note that the runway receives the "lowest" loading condition - usually the pavement in the runway is the "least strong" of the three areas noted above.)
Don't really like the idea that you have fill with topsoil mixed up.

 

[msucog]

one thing you might consider if full depth reclamation by somebody like the Miller Group. the upper layer would likely need to be taken off and the underlying soils stabilized. the good thing is that AC millings make a great soil cement constituent. you might even remove say 4" for stockpile, then perform FDR on the 10-12" below (for stabilization only to reduce the cement content needed) since this wouldn't require as my cement versus using 100% soil. then, you could throw the stockpiled millings back on top and add graded aggregate base to perform FDR again except making the next 10-12" a full blown soil cement design. the AC millings and GAB make a jam-up soil cement with minimal cement required. this layer could possible be your base course layer then new AC placed on top. by doing this (essentially raising the finish grades), this might reduce the amount of soil that must be removed from next to the pavement.
i can post a few links if this sounds interesting to you...

 

[BigH]

msucog - I would have headed to the use of cold-in place recycling of the existing poor sections - and using this as a new base course - then put a HMA overlay (maybe 4 to 5 inches thick) over it. But - before suggesting that, I thought it prudent to find out some details as to the nature of the airport, etc. Some solutions that might be used for small low-load, low capacity airports might not apply if the airport took larger planes.

 

[msucog]

indeed good point bigh. apparently, my cart is pulling the horse this evening.

 

[lovethecold]

Thanks for the fast replies.

BigH-
1,2) Frost depth at the time of drilling was 3.5 ft, typical frost depth is 5 to 6 feet. Location is central North Dakota.

3) No water was used in the drilling. Hollow stem auger with split spoon sampler was used. The first samples were grab (off the auger flights), due to the frozen soils. So, getting a real clear picture of the soil is difficult. But the grab samples do seem much softer than typical for frozen soils.

4) The airport is rural and handles Cessna's and other light aircraft. No heavy aircraft. At most a plane will be 6000 lbs.

5) No original construction plans, and no drains of any type are present. Most likely some farmer scraped some of the topsoil off, leaving some and flattened the area out. Tossed some sand and gravel down, then paved the runway.

6,7) There is a crosswind runway, so should be able to remain operational. Only the main runway is affected. The crosswind runway is apparently made of grass with only a small strip of asphalt, for access to the main runway.

Atterbergs all show lean clays. LL/PL = 39/21. The LL ranges from 38 to 42, the PL ranges from 20-22.

msucog -
I don't mean to sound like a noob, what is an FDR?

Sounds like some good ideas. Thanks.

 

[msucog]

full depth reclamation (FDR). it won't get you below the frost line without removing several feet of material first since it should only be done in about 12" layers since you have to place it back in compacted lifts. perhaps fixing say the upper 2' might get you want you're looking for (relative to the expected performance of partially remediating the area). others from that part of the country (or eskimos) can likely provide better insight in to the difficulties and necessities to perform remediation in such a cold climate. my opinions likely would not hold up too well since you're about 12 states north of me.

 

[lovethecold]

12 states north msucog, where are you at, South America

I had never heard of this much heaving that was taking place. Thank you for all the assisstance.

We recommended full depth reclamation, 6 to 8 feet, backfilled with non frost susceptible sand. We don't expect them to follow through with this (very expensive). We told them they could make the base layer thicker and that would help. The thicker the better.

 

[BigH]

lovethecold: I would question the wisdom of such deep reclamation for a small load/small plane airfield. You have indicated that the field was built in the 1970s and was overlaid once (?). I really would consider using cold in-place recycling to a depth of 200 mm (Wirtgen has a machine to do this - as do other manufacturers). What you basically do is add a bit of cement on the top of the asphalt layer, then let the machine chew down the 250 mm, add some emulsion asphalt, mixing the existing pavement-emulsion-cement up and leaving it in place. Compact this. This forms a good asphalt stabilized base and then place a 100 mm or so HMA on top. Would be a whole lot less disruptive than digging out 6 to 8 ft. Was involved in its use on a major highway in India (see attached file of a paper I was involved with - no copyright noted for the proceedings). Also, the pavement, to be honest, lasted/behaved well over a "long time" - some 30 to 35 years - says that the pavement structure (regardless of the underlying materials) was reasonable and prudent.

 

[CarlB]

Look into the Corps of Engineers pavement design methods for pavements in seasonal frost areas. Two basic methods -limited frost penetration & reduced subgrade strength. I don't believe under either method you would have to replace the entire active layer with NFS (non-frost susceptible) material. I'm thinking your initial suggestion, about 2 feet of NFS fill, is in the ballpark. Use a separation geotextile at the bottom to prevent mixing, both during construction and later frost/traffic action. The problem with the treated base methods is they don't address the frost heave. Well they might, if they were 4 feet plus thick. And yes, do whatever is reasonable (ditching at least) to keep water away.

 

[JML4TCM]

If full depth reclamation is too expensive, look at resurfacing with a paving fabric to create a moisture barrier to minimize the amount of water seeping into the subgrade soil.

 

[BigH]

Again I caution that this is a low volume, low load airport - that most planes can land on dirt tracks if needed. The existing runway has appeared to behave "well enough" over the last 30 odd years with only an overlay. Tailor the solutions to the needs.

 

[lovethecold]

We gave them 2 recommendations. If they wanted to be as certain as possible no frost heaving would occur, then excavate to frost depth and replace with non frost susceptible sand.

The other recommendation was to cut to 2 feet below the surface of the final elevation of the pavement and provide for proper drainage.

The final design will be up to the civil engineering firm working on the project. They are in contact with the airport authority. My understanding is they were concerned with the amount of heaving that was occuring and wanted all possible solutions.

Thanks for the responses. Was quite interesting, and inline with the senior engineers' ideas. Sometime I like to ask others and get their inputs.

Take care.