Ross...
From the Boeing tech website:
"Whatever surface was to be used, certain guidelines had to be observed. The surface had to be smooth with no bumps higher than 3 inches in 100ft; good drainage with no standing water or ruts; and the surface material had to be at least 6 inches thick with no areas of deep loose gravel. Boeing offered a survey service to assess the suitability of potential strips. If a surface was not particularly hard it could still be used by reducing tyre pressure down to a minimum 40psi in accordance with a chart.
The Boeing "unpaved Strip Kit" components include:
Nose-gear gravel deflector to keep gravel off the underbelly.
Smaller deflectors on the oversized main gear to prevent damage to the flaps.
Protective metal shields over hydraulic tubing and brake cables on the main gear strut.
Protective metal shields over speed brake cables.
Glass fibre reinforced underside of the inboard flaps.
Metal edge band on elephant ear faring.
Abrasion resistant Teflon based paint on wing and fuselage undersurfaces. Strengthened under-fuselage aerials.
Retractable anti-collision light.
Vortex dissipators fitted to the engine nacelles.
Screens in the wheel well to protect components against damage."
More info can be found at:
http://www.b737.org.uk/unpavedstripkit.htmI
I was aboard a PWA B737 conducting tests of the effectiveness of the kit components at various unimproved strips in the Pacific Northwest waaaay back in 1971.