• Blading and reshaping of a roadway or shoulder should take place in moist weather conditions if possible, the South Dakota DOT says. “When blading, material should be pulled from the in-slope area back up onto the roadway or shoulder surface and smoothened, watered, and compacted to the proper grade and crown slope. The grader should be operated at a top speed of between 3 to 5 miles per hour, and the grader moldboard should be operated at the correct angle and pitch to adequately move and mix the material.

Inventory, Plan in Washington County

Maintenance of unpaved or gravel roads in a jurisdiction can be addressed in a systematic method, as Washington County, Iowa – located just west of the Quad Cities – did recently.

Beginning in 2008, when a “perfect storm” of diminished cash flow, increasing traffic and loads, and excessive flooding severely damaged many unpaved roads, the county’s engineering staff prepared a $12 million, multi-year unpaved road improvement program funded largely by an $8 million general obligation bond issue that county engineers were able to justify with the plan.

Poor drainage results in ponding and aggregate loss in unpaved roads.

“High costs and flat revenues, increased vehicle weights, and increases in rural traffic volumes have created conditions on the gravel road system in Washington County that are quickly becoming unacceptable to residents,” said David Patterson, P.E., Washington County engineer, in September 2008.

“While any one of these changes – financial, increased weight, or traffic issues – could probably be handled by the system under its current allocation of resources, the combination of all three has tipped the system into a downward spiral that will become irreparable if changes are not made,” Patterson said in terms that are familiar to the pavement preservation community. “Under its current allotment of resources, the gravel road system has reached a point where damage is being done faster than can be repaired.”

This plea was buttressed by an unpaved roads inventory. “During the summer of 2008 the Secondary Roads Department performed a complete inventory of the gravel road system in the county,” Patterson said. “The inventory marked the condition, needs, and noted any special circumstances of every gravel road in the county.”

A comprehensive review of the available solutions was then performed to see what solution would best apply to which road. These solutions included adding rock surfacing, grading the roadway to repair deficiencies, upgrading a roadway for paving, or vacating/downgrading the road classification.

As articulated in 2010, Patterson outlined four outcomes for bringing county unpaved roads up to par:

• Fix the six to eight “hot spots” where the county spends three to four times its average maintenance dollar (average of $2,500 per mile, compared to $7,500 to $10,000 per mile in these locations)

• Make system-wide improvements to gravel

• Reduce long-term maintenance cost, and

• Rebuild and recover from disasters like the extensive flooding of May-June 2010, when it rained continuously.

Ultimately the plan would provide five miles of new paving, rebuilding of 80 miles of gravel roads, and additional rock surfacing for 40 miles of roads. “By improving these 125 miles of roadway, maintenance resources will be available to make improvements on the other 800 miles of our county roads,” Patterson said. “The new gravel roads would be 30- to 50-percent cheaper to maintain, trap less drifting snow, have proper drainage, and have an appropriate surface course for the traffic they are carrying.”

GPR and Gravel Roads

The use of motorized vans carrying ground penetrating radar (GPR) is well-established in characterizing the subsurface conditions of road networks as road agencies update their pavement condition inventories, but in 2012 researchers at the University of Illinois at Urbana-Champaign described use of GPR for unpaved roads.

In Denver, this unpaved alley was topped with a mix of straight RAP and neat rejuvenator, and then compacted; the result is an inexpensive unpaved driving surface.

In their TRB paper Assessment of Subsurface Deformation in Unsurfaced Pavements Using Ground Penetrating Radar, U of I graduate research assistants Debakanta Mishra and Zhen Leng, and professors Erol Tutumluer and Imad L. Al-Qadi describe the findings of full-scale accelerated pavement testing to evaluate aggregate quality in two unsurfaced pavement sections.