Airfields in a Hurry

The big test of a new and speedy way to make airfield runways came when a New England airdrome for fighters was used for Flying Fortresses. The runways, made of a mixture of soil and cement, came through handsomely, stood up under the beating of B-17 landings as well as they had taken the lighter swoops of P-40s. Thus a short cut to hard-surface roads as well as airdromes was war-proved.

Never intended to replace reinforced concrete, soil cement has far exceeded engineering expectations. Engineers had known that the ground where cement bags were shaken out soon hardened. But soil cement stayed in the rule-of-shovel stage until the mid-'30s.

Since then engineers have found, first by trial & error, then by more exact methods, that almost any soil may be used. On small jobs, little special equipment is needed. The ground is plowed, harrowed and cleared of larger stones. Bags of cement are spotted in a checkerboard pattern. Spread evenly, the cement is mixed dry with ordinary farm machinery, (disc and spring-tooth harrows are good), then sprinkled with water and mixed wet until an even color shows that the mix is right.

Big rush jobs like airfields use machines in tandem which make scarifying and mixing a continuous operation. Most important is thorough tamping: rolling the surface hard is not enough; the wet mix must be compacted from the bottom. This is done with the well-named "sheep's-foot" roller (see cut), whose hundreds of small steel projections pound down into the mixture. Once tamped hard, the surface is graded smooth, then protected against scuffing by a thin bituminous top coat.

Costs are a fraction (about 20%) of concrete construction. But the most spectacular saving is in time. Soil-cement enthusiasts boast (and deliver) "a runway in a week; an airport in a month."

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