(605) Key Principles of Gravel Road Maintenance:

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1 Ken Skorseth Retired Program Manager SD LTAP/SDSU Gary, SD (605) Lecturer on Low Volume Road Maintenance Key Principles of Gravel Road Maintenance: Two distinct principles must be understood if gravel roads are going to perform well: 1) Correct roadway shape and 2) Selection of good surface gravel. If these are understood, gravel roads will generally perform well. Here is an overview of these two key principles as covered in the workshop. Roadway Shape: The use of the motor grader to establish and maintain roadway shape is the critical factor here. A motor grader operator who understands correct roadway shape and has the skills to use the machine to establish and maintain it is a valuable employee. Management must also understand and support the operator in shaping roads. The key word in understanding shape is drainage. Drainage begins on the surface. Roadway crown must be established and maintained to get water off the surface. How much crown? At or near 4% is good. This is very close to one-half inch per foot drop on the cross slope. Another way of defining this is to say a road with surface width of 24 feet should have six inches of crown on the surface. If drainage is still a problem, crown can be increased slightly, but should not exceed 6%. Crown should also be straight from the shoulder to the center of the road like the roof of a house. It should not be rounded or parabolic like a haystack. The next principle in roadway shape to manage is the high shoulder. High shoulders often form at the edge of gravel roads. This obstructs drainage to the ditch. Water can stand at the edge of the roadway which weakens the surface and subgrade. In hilly terrain, it will also cause erosion as it flows down the edge of the road instead of the ditch. Poor maintenance procedures, poor gravel that constantly shifts from the roadway to the shoulder and heavy loads can all cause the high shoulder to form. No matter what the cause, maintenance or reshaping must be done to eliminate high shoulders to get the best performance from gravel roads. 1

2 Yet another drainage matter to think about is the side ditch. This is often more than a simple maintenance matter and may be more than a motor grader can handle. However, when time allows, ditches need to be inspected and cleaned if necessary. It may require the use of an excavator to remove silt and debris. In dry periods, small scrapers might be used to remove silt and place it back in adjoining fields if feasible and landowners are willing. A motor grader can be used in some situations to reestablish a ditch. The point is: this is very important maintenance which will result in a better road surface. Finally, culverts and bridges must be maintained in good working condition to get water away from roads where streams, creeks and rivers flow under the road. It is hard to find time to inspect culverts and small structures to see if drainage is obstructed. It s even harder to find time to clean, repair or replace. But this maintenance pays in so many ways. Getting water away from roads always provides a stronger, more stable roadbed and surface. Good Surface Gravel: After roadway shape is established, the selection of good surface gravel cannot be emphasized enough. Many problems such as corrugation (washboard), loose gravel accumulating between the wheel paths and excess loose gravel at the shoulder are blamed on the motor grader operator. In reality, these problems are most often material related. Too often, base gravel is used as surface gravel. There are three key differences between surface and base gravel. Good surface gravel has smaller top-sized stone than base gravel. Generally three-quarter (3/4) inch top size stone is good. Smaller stone will remain embedded in the surface longer than large stone. The ideal is a sub-layer with larger stone as a base to establish strength and a layer with smaller stone on the surface, but this may not be affordable or practical. The second difference between base and surface gravel is more fines are needed. True fines in surface gravel will pass a No. 200 sieve. This is so fine, you cannot see the individual particles with the naked eye. This is the only way to get a dense and tight surface that will stay in place under traffic. Generally, between 7 and 15% of the material (by weight) should pass the No. 200 sieve. The final key ingredient in good surface gravel is to have fines that are cohesive in other words the fine material will bind the larger particles and help hold it together, especially in dry periods. The only way to define this is have a lab test done to determine the plasticity index, or PI. This test defines whether the fine material is primarily silt or crusher fines, or cohesive clay. Good surface gravel needs a minimum PI of 4 to 6. If a high percentage of clay is present in the gravel, a maximum PI of 12 to 15 should be specified. In some regions, good surface gravel is nearly impossible to find locally. It may be possible to mix material from different sources to make a better blend on the 2

3 road. There may be alternate materials that will work such as reclaimed asphalt or blends of asphalt with natural gravel or quarry aggregate. Crushed gravel with fractured stone is always superior to natural, screened gravel. Summary: Skilled operators and the support of management is the first key to get good performance on gravel roads. Thereafter, use the best surface material that can be obtained. In virtually every case, if this is done the life cycle cost of maintaining a gravel road is reduced. There are other special situations that will be covered in the workshop. Two examples of roadway crown. Top photo has little crown, bottom has 4% which provides excellent drainage. 3

4 4 Distinct contrast between two roads in the same community with similar traffic and climate. One with very good surface gravel, the other with poor gravel.

5 Current Specifications from MNDOT 2018 Standard Specifications: Table showing Class 1 and 2 Specifications only: Copied from page 562 of Minnesota 2018 Standard Specifications for Base and Surface Aggregate (Gravel). 5