Washboarding

Mechanism

Washboarding or corrugation of roads comprises a series of ripples, which occur with the passage of wheels rolling over unpaved roads at speeds sufficient to cause bouncing of the wheel on the initially unrippled surface and take on the appearance of a laundry washboard. Most studies of washboarding pertain to granular materials, including sand and gravel. | article-number = 061308 | access-date = 2014-12-25

Highway department experts in the mid-1920s were aware that traffic volume and speed were primary causes of corrugations on gravel roads and cited the role of drive wheels tossing material as a factor. | chapter-url = https://books.google.com/books?id=7Wg4AQAAMAAJ&q=road+corrugations+cause&pg=RA2-PA85}} | chapter-url = https://books.google.com/books?id=czs5AQAAMAAJ&q=road+corrugations+cause&pg=RA18-PA7}}

Laboratory-scale studies of the phenomenon typically employ a wheel or a blade, which is towed behind a pivot point, tracing a circular path through a pan of the material under examination. These studies have investigated a variety of granular and viscous, even fluid, materials. In the laboratory, washboarding has been studied for a range of parameters, including the thickness and grain size of the material for varied wheel sizes, shapes, and masses. Experiments produced ripples for each parameter, above a threshold speed, when the wheel (or blade) began to bounce. Experiments also show that the pattern can move either against the direction of motion or in the direction of motion. They also show that a passive, non-driving wheel suffices to create corrugations and that displacement of material, rather than ejection, is the dominant mechanism.

Several articles about real-life washboarding on roads cite South Dakota Local Transportation Assistance Program (LTAP) Special Bulletin #29, "Dealing with Washboarding," by Ken Skorseth. ;Examples of corrugation in different soil types File:Road_with_Washboarding_in_Baja_California.jpg|Fine-grained soil particles on a sandy road in Baja California File:Corrugations.jpg|Semi-cohesive soil on a road in Kalbarri National Park, Western Australia File:Corrugated Road - Fremont California.jpg|Coarse-grained soil particles on a gravel road in Fremont, California

Maintenance

Guidance from various highway departments suggests that choice of gravel can be key to mitigating washboarding. They cite "sieve analysis" tests that use a series of screens or sieves to characterize the sizes of particles contained within a gravel sample. Highway department guidance suggests a range of particle sizes from stones that are in the 1 in range, mixed with progressively finer particles to include a small fraction of fine particles that bind the larger particles together. They also mention the role of equipment that can re-blend and smooth surfaces that have corrugated.

In 1925, the Nevada Department of Highways in the United States advocated mitigating corrugations with crushed pit-run gravel, using material 1 in and smaller, including only the fines from crushing. The maintenance advice from Colorado was to drag or grade the road frequently, applying light volumes of new gravel with minimal sand content and providing good drainage with a crown. The same source advises reduction of traffic speed.

Guidance based on South Dakota LTAP Special Bulletin #29 and US Federal Highway Administration guidance from the same source in 2000 suggests that the surface gravel "should be a blend of stone, sand and fines that will compact into a dense, tight mass with an almost impervious surface." It emphasizes the proper gradation of gravel—100% passing the 0.75 in) sieve—to have fractured stone to "interlock" and 4–15% fines passing the #200 (75-μm) sieve to act as a binder and create cohesiveness in the gravel; substituting other binders, such as clay is also recommended. Alternately, reclaimed asphalt can be incorporated in a half-and-half blend with quarried gravel to improve the binding properties of the surface. For existing washboarded surfaces, the bulletin recommends using a grader to cut and blend existing material to a depth one inch or more below the bottom of the washboarded segment and then add the new material into the top layer. Useful equipment includes a blade with rotating scarifying teeth or a replaceable bit-type cutting edge attached to the moldboard blade of the earth-moving equipment.

References

References

1. Subcommittee: E29.01. (2013). "Nominal Dimensions, Permissible Variations for Wirecloth of Standard Test Sieves (U.S.A.)". ASTM International.
2. Subcommittee: E29.01. (2013). "Standard Specification for Woven Wire Test Sieve Cloth and Test Sieves". ASTM International.
3. (November 2000). "Gravel Roads Maintenance and Design Manual". South Dakota Local Transportation Assistance Program.