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:
The recommended grading method is shown here:
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.