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© Maximum Sustainable Trail Grades The 10 Percent Average Guideline advises that, generally,an average trail grade of 10 percent or less is most s sustainable. But what about maximum grade? Maximum grade is the steepest section of trail that is more than about 10 feet in length.When designing + a trail,it is essential to determine early in the process the precise maximum trail grades the trail will be able to 3' s sustain in your local conditions.This target figure will help guide your layout and ensure sustainability. Although maximum sustainable trail grade is typically about 15 to 20 percent, it is site-specific and fluctuates based on several factors.The variables to be considered when setting your target maximum trail grade include: F ;' • Half Rule A trail's grade shouldn't exceed half the grade of the sideslope. If the trail grade is steeper than half the grade of the sideslope, it is considered a fall-line trail. lNote:the maximum sustainable grade on a gentle hillside will be half the grade of the sideslope. u ' • Soil Type There are many types of soil and each has different qualities of cohesion and drainage. Some soils will support steeper trail grades than others. See page 84 for a more thorough discussion of soil types. • Rock Trail grades can be steeper on solid rock. However, steep earthen sections between rocks may need to be fortified or armored to prevent soil loosening and erosion. • Annual Rainfall Amount Trails in regions with either very high or very low annual rainfall may need to be designed with gentler trail grades. Lots of rain can lead to water-caused erosion.Low rain levels can lead to very dry and loose tread surfaces. • Grade Reversals A grade reversal is a short dip followed by a rise, forcing water to drain off the trail.It is an essential technique for preventing water from channeling down the trail. Frequent grade reversals will } allow for slightly steeper trail grades.We'll describe grade reversals more thoroughly in a moment. • Type of Users Trails restricted to relatively low-impact visitors such as hikers and mountain bikers can sustain maximum grades as high as 15 to 25 percent for short distances depending on soil and rainfall. Trails open to visitors with higher impact,such as horses or motorized users, should have more gentle maximum grades. • Number of Users Trails with high anticipated use may need shallower maximum trail grades. • Difficulty Level Trails with a higher level of technical challenge may incorporate steeper grades,but construction techniques such as frequent grade reversals and armoring may be necessary to ensure sustainability. Calculating the maximum sustainable trail grade is a complicated process that requires a high level of trail- building knowledge and experience.When in doubt, design trails with conservative grades until you have had the opportunity to observe the effect of a variety of trail grades in your local conditions. 66 Trail Solutions