In 1940 the Iowa State Highway Commission made use of an experimental terraced type of design for highway side slopes along a 4-mile east-west route through the loess of Harrison County, Iowa. Relief along the route is 250 feet, and road cuts as deep as 80 feet were required. Terraces were 15 feet high and 15 feet wide; treads were sloped along the length and toward the base of the risers. It was believed that this design would have advantages over the conventional side slopes of 1-to-l to 3-to-l ratio. The risers, being in loess, would tend to maintain verticality, but the hazard involved in single vertical cuts of as much as 80 feet would be avoided. Such slopes would be less subject to raindrop and rivulet erosion.
Spalling of risers, principally the result of changing stress conditions, commenced within a year. Almost all risers have now been affected to some extent. The greatest changes have been the result of rivulet erosion down the risers, presently developing as a consequence of the accumulation of sloping piles of material on the treads. Deterioration has been greatest on the slopes of the south side of the road; the loess does not dry out as readily, and freezing becomes more effective; more debris accumulates on the treads, and slope wash increases.
Although the slopes now have a ragged appearance, the design is considered reasonably successful as far as maintenance requirements are concerned. Further experimentation is being considered.