Everybody here gave us a lot of good ideas and we are very grateful for all those advice.
In return, I will pass on a tip on what to use not to sink in the ground:
In spite of Mike telling me to ask my father, I postponed doing so, because after 25 years in the States, I couldn't find french words for drain field,....etc. My father is nearly 80 years old and lives in France. He operated a road building company for 40 years, and before that was trained as some type of engineer in a similar field. So, I just called him and he didn't hesitate one second - it takes me longer to sort all this between 2 languages:
On small airports where the ground is too soft or muddy, they use metal plates, perforated with about 4" diametre holes. These plates are about 20 to 24" wide, 80" long. They are very light and strong, and are hooked end to end to make any length. They are used too to move vehicles in the sand.
He advised me to look for similar products at a metal recycling place. I believe that what they use in factories and similar buildings, for loading platforms or pedestrian bridges will be similar, maybe too heavy.
an on line dictionary taught me that what I am looking for is called: PERFORATED (STEEL) LANDING MAT or PIERCED STEEL PLANK(ING)
for you history buffs, I found on the Net: "By the end of April 1943,.... It took just 14 days to build an operable airstrip on Adak. (in the Aleutians)...The first bombers were flying for runs over Kiska after the perforated steel landing mat
was laid down."
It makes sense that something similar to that would still be used in third world countries, where temporaries roadways are needed by industries, military,...? In the States, could that be used by loggers, oil companies setting up temporary air fields,.....? By the US Army Corp of Engineers ? Now it sound overkilled !!!
The moral of this story: When he was still in his 40s, my father hanged a plaque on a wall, saying something like: "when we are kids: dad knows everything..... teenagers: it's amazing how little the old man knows..... when old ourselves: if only dad was still here so we could ask him." I guess he was right.
Thanks again for your input.