Gravel runway - turbo prop operation - design criteria for runway surface

[tribo]

Anyone who knows where to find design criteria for the surface layer of gravel runways?

Grain size distribution, preferable for arctic gravel airports with turbo prop operations.

[SNS3Guppy]

I've never heard of design criteria for a gravel runway. If you use gravel, you're going to risk engine damage and prop damage, no matter how you slice it...unless you use fairly large stones, in which case it isn't really a gravel runway.

If you're looking to minimize FOD damage, the best thing you can do with gravel is hard pack it and make it long enough that slow power advances can be made. This means the aircraft needs a fair amount of room to get rolling before full takeoff power is applied, which minimizes gravel intake, flap damage, and propeller damage.

[call100]

Guess the Canadians would be your best bet. Or try in the military forum, I think they build a lot of them.

[FlightTester]

A lot of the data has to do with CBR (California Bearing Ratio). I believe that the spec contains the details you're after. If memory serves me correctly, you'd be looking for a CBR of around 30 for a gravel runway. Hydro Quebec operates several gravel strips, they should have some details.

Cheers

FT

[OverRun]

tribo,

Yes of course; no problem. Typical sources are FAA, general roads department gravel road criteria, TRH 20 design manual, and the old Australian DCA ALOP manual. Four things to look at:

- design thickness (including frost protection)
- strength of gravel
- functional properties (oversize stones, slippery, erosion)
- coping if only poor quality gravel exists

Design thickness
Assuming a Dash 8-400 with 900 kPa tyres (low pressure). 230 departures per year for 20 years.

Subgrade D (CBR 3 = pretty soft). Total pavement thickness = 620 mm (of all layers including gravel; this is made up of various poorer material and good gravel)
Subgrade C (CBR 6). Pavement thickness = 400 mm
Subgrade B (CBR 10). Pavement thickness = 280 mm
Subgrade A (CBR 15 = sandy, firm). Pavement thickness = 220 mm

The Dash 8-100 would be some 50mm thinner because it is lighter. Frost protection adds a need for 50-100mm extra total thickness.

I’d tend to assume that the insitu soil was poorish quality. It is also likely to be affected by frost and weakened in the first thaw. Subgrade C is a good starting point (unless you walk on site in spring and find your elbows dragging through the mud whereupon you might assume it is subgrade D). And we’ll allow freezing of the subgrade so we’ll assume reduced strength during the period of thaw. We’ll need about 460 mm total thickness (400mm as designed above for the Q400 + 60mm frost protection), and we’ll use several layers.

The first (lowest) layer should be something cheap, and a poorish gravel or gravel sand (which is less frost susceptible than clay or silt); maybe 260mm thick. Then the wearing course good gravel can go on top – some 200mm thick.

Strength of wearing course gravel
Generally the same as gravel roads. For gravel pavements, the surface gravel has closely controlled grading to avoid dust problems while maintaining proper binding characteristic of the material. This is controlled by particle size distribution (needing a range of big, medium, and small particles which pack down to high density), not too much clay but enough to bind together (Atterberg Limits: PI, LL, PL, LS) and adequate strength (CBR or California Bearing Ratio).

The particle size distribution varies with the maximum stone size of the gravel, but generally 85%+ shall pass the 19mm sieve, and 100% shall pass the 37.5mm sieve. For turboprops, I might even make that 100% shall pass the 19mm sieve, so there aren’t bigger stones that can cause problems at higher speeds. This controls oversize stone.
Plasticity index 2-10%. California Bearing Ratio (wet condition) >= 60% at 95% mod. AASHTO compaction.

Functional properties (oversize stones, slippery, erosion)
We’ve done the oversize stones bit. For the other functional properties, they are generally the same as gravel roads. The breakthrough in the roads world, which is new to the aircraft world, is the selection guidelines for unpaved roads in TRH20 (1990) which has been adapted around the world. The ideal gravel wearing course for aircraft can be considered by modifying public unpaved road requirements. It should have:

- The ability to provide a safe and aircraft friendly ride without the need for excessive maintenance.
- Adequate trafficability under wet and dry conditions.
- The ability to shed water without excessive erosion.
- Resistance to the abrasive action of traffic and propellors.
- Freedom from excessive dust in dry weather.
- Freedom from excessive slipperiness in wet weather.
- Low cost and ease of maintenance.

This is done by choosing the gravel which has the right combination of properties. For the discussion and calculation spreadsheet, go to Pavement Engineering (http://www.geocities.com/profemery/pavement.html#gravel)

Coping if only poor quality gravel exists
There is also a CRREL nomogram that can be used to relate very weak (earth, gravel, snow, ice) runways to aircraft weight, tyre pressure and number of passes (caution: only a senior airport pavement engineer has the additional knowledge needed to use this in practice). The link is: http://www.geocities.com/profemery/aviation/low_strength.pdf

Not sure how far this takes you. Any further questions, just ask.

PS - I just remembered learning that the South Africans were putting their manuals online. I found TRH 20 and that is located here (http://www.nra.co.za/content/Blogin/22.pdf).

[tribo]

Thanks OverRun

This was what I asked for and ++++