Runway Replacement, Rehabilitation?
Runway Replacement, Rehabilitation?
(OP)
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.





RE: Runway Replacement, Rehabilitation?
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.
RE: Runway Replacement, Rehabilitation?
i can post a few links if this sounds interesting to you...
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
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.
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
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.
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
RE: Runway Replacement, Rehabilitation?
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.