Abstract
The low load-bearing capacity of subgrade soils is typical for forest roads. For the determination of the resilient modulus Mr of unbound natural as well as recycled materials, a laboratory triaxial test with cyclic loading is generally preferred. For low volume roads, including forest roads, an alternative method of the cyclic CBR test, which uses standard CBR devices for repeated loading, is being tested and applied in practice. For forest subgrade soils, the procedure for determining the modulus Mr based on cyclic loading of the specimen to a constant penetration depth of 2.54 mm was verified. This procedure was tested on an extensive dataset obtained from 11 forest roads in the Czech Republic, which was then statistically evaluated. The obtained results showed disproportionately high mean values as well as high random variability. Further data analysis revealed that the reason seemed to be the chosen test methodology. When using this procedure for forest soils, high values of plunger stress can occur, which for many types of soils greatly exceed their maximum load-bearing capacity. As result, the modulus Mr is determined at unrealistically high plunger stress values and in many cases on the disrupted specimen. The necessary solution to this shortcoming is to censor the results of the cyclic CBR test, i.e., to exclude unrealistic values of the modulus Mr determined at plunger stresses exceeding the limit values.
Highlights
The quality, durability, service life, and damage ratio of forest roads are fundamentally influenced by the subgrade soil quality described by its load-bearing capacity
The limit values of plunger stress were determined for individual soil classes, of which it was possible to assume with a high probability that the modulus Mr,eff was not determined on the damaged material
Forest road pavement design requires a proper estimate of the resilient modulus Mr, because it plays a crucial role in optimizing the road construction layers design The quality, durability, service life, and damage ratio of forest roads are fundamentally influenced by the subgrade soil quality described by its load-bearing capacity
Summary
Forest management efficiency and the accessibility of landscape areas require a huge forest road network. Forest roads provide access to landscape areas for economic and recreational reasons, as well as to ensure area accessibility outside of public transport. The quality, durability, service life, and damage ratio of forest roads are fundamentally influenced by the subgrade soil quality described by its load-bearing capacity. There are many uncertain factors or events of natural origin that directly affect subgrade soils, especially the humidity and unfavorable water regime as well as the variable material compaction level. The forest road pavement design requires a proper estimate of the resilient modulus Mr , because it plays a crucial role in optimizing the design of the pavement structure layers
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