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I'm under the impression that some large craft are rated for "unpaved strips", which appear to be either some sort of gravel or perhaps dirt. The 727, and this runway, being a good example of this:

enter image description here Picture taken from this question, which is also a good reference on how large airplanes can operate on "rough" fields.

What I'm wondering is how these fields are prepared? I assume the ground is required to be able to hold up a certain amount of weight, and I also assume that ability must be uniform, or you'd be risking a wheel poking into the runway and basically ruining everyones day...

So how do they prepare/create these runways? And how do they ensure that they maintain the ability to be used for operations with craft that are rated for "unpaved" but probably not rated for "random, somewhat grassy field."

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Advisory Circular (AC) 300-004 from Transport Canada has detailed information regarding the maintenance of unpaved runways.

The purpose of this document is to outline methodologies for the measurement and reporting of surface shear strength for unpaved runways. In addition, the document outlines recommended practices for condition inspection, maintenance and repair of airport gravel surfaces and turf landing strips.

Runways should be designed with consideration for drainage (slope, crown, etc), susceptibility to frost heave, vegetation, roughness, etc.

Maintenance can be based on surface hardness measurements made with a penetrometer. A schedule for periodic inspection is created when the runway is new. The AC recommends a maximum of three years between hardness measurements. One type of penetrometer measurement is described as follows:

The Boeing High Load Penetrometer (Reference Boeing Document No. D6-24555 - High Load Penetrometer Soil Strength Tester) consists of a hydraulic cylinder with a cone point test probe mounted at the rod end. The hydraulic cylinder is normally positioned against the frame of a heavy vehicle which serves as a reactive load. In the test procedure, the probe is driven at a steady rate to a 100 mm (4 inch) depth into the surface by the application of pressure through a hand pump. Generally a pressure reading is taken about 30 seconds or less after movement of the penetrometer has stopped at the 100 mm (4 inch) depth.

Once maintenance is deemed to be required...

Gravel pavement surface maintenance primarily involves periodic grading to remove the surface irregularities developing with time and to re-establish grades for drainage purposes. Occasionally, new gravel has to be added to replace lost material. Dust suppression measures may also be needed during the summer months.

The recommended aggregate sizes are shown in this table from the AC:

Gravel sizes

The recommended grading method is shown here:

Grading method

Also note a wealth of information in the reference documents listed in section 2.1 of the AC.

This presentation (.pdf link) from a company that provides airport maintenance services has additionl useful information and photos.

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First off all pilots are trained for soft field landings and I assume the procedures for large craft are not all that different from small craft. The main goal is to keep weight off the nose wheel as much as possible.

How the field is prepared will depend on where it is geographically. In the desert or other dry hard areas this is not really that hard for obvious reasons. In a grassy area you will need to mow the grass and possibly clear some trees or other low brush. The area of PA I fly in is littered with private grass strips that are hard to differentiate from the fields next to them. Many of them look like this

enter image description here

As for the uniformity of the ground beneath them, as far as I know they are not altered in any way. This factor is mainly altered by the weather. If it has just rained the ground is going to be muddy and slick, possibly not useable. Check out this question for more info on that.

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  • $\begingroup$ Interesting information, though mostly I'm wondering how this is handled for runways that deal with much much larger aircraft. $\endgroup$
    – Jae Carr
    Commented Aug 28, 2015 at 14:26
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    $\begingroup$ Dirt strips intended for large aircraft are typically only found in very arid areas, and preparing/maintaining the strip is a fairly simple matter of running a grader or bulldozer over it every so often, possibly followed by a heavy roller. That's still cheaper than trying to maintain an asphalt or concrete strip, especially in very sandy locales where wind erosion can bury a concrete strip in days. A grass strip, as in this answer, is found in more temperate areas to prevent soil erosion, and while a larger airliner might be able to set down here in dry weather it's never ideal. $\endgroup$
    – KeithS
    Commented Aug 28, 2015 at 16:58
  • $\begingroup$ @KeithS If you could expand that out and make it into an answer. The situation you are talking about is the exact situation I'm curious about. $\endgroup$
    – Jae Carr
    Commented Aug 29, 2015 at 13:57
  • $\begingroup$ @KeithS Especially if you know of any equipment they might use to test to firmness of the ground... $\endgroup$
    – Jae Carr
    Commented Aug 29, 2015 at 14:04

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