Abstract
Forest roads are often constructed in environments with low bearing capacity of the subgrade. The subgrade then has an effect on their service life and damage. According to the methodology of the American Association of State Higway and Transportation Officiales AASHTO, the design of pavement is divided into three levels according to the intensity of the traffic load. For pavements with the highest load intensity, preparing the resilient modulus from a cyclic triaxial test is required. For other traffic load classes, including forest roads, the methodology allows the use of the estimate of resilient modulus value determined from other tests. In the laboratory at the Faculty of Forestry, Mendel University of Brno, the method from the Delft University 2009 was tested and subsequently modified, using a standard CBR machine for repeated loading. A total of 276 samples from various types of forest road subgrade from the Czech Republic were tested by the method of repeated loading on the CBR machine, from which the values of the Resilient Modulus were newly labelled Mr,CBR. The results of the statistical analysis showed a large variability of Mr,CBR values and wide intervals of its occurrence for individual types of subgrade. The variability was subjected to analysis and the influence of basic geotechnical parameters on the values of Mr,CBR was analyzed. A fundamental correlation was found between the value of Mr,CBR and the value of the plunger stress, which reached values exceeding the bearing capacity of the soil types using the Delft University method. It is necessary to limit the plunger stress during cyclic loading up to the failure limit or even better to the expected traffic load. The modified procedure results show a more consistent behavior of the modulus.
Highlights
Forest roads, which are ranked among low volume roads by the American Association of StateHighway and Transportation Officials AASHTO based on the transport intensity [1], are an integral part of the road transport system
Once the geotechnical tests were finished, the different soil types were classified according to Unified Soil Classification System (USCS)
The values of the plunger stress from the last loading cycle were compared with the stress limit value of the soils by Terzagi’s theory, according to which the maximum load bearing capacity ranges between 150 and 650 kPa based on the soil type
Summary
Highway and Transportation Officials AASHTO based on the transport intensity [1], are an integral part of the road transport system They are used to ensure access to the areas that need to be accessible for economic, social, recreational, and security reasons. These roads often need to meet criteria that are in conflict with each other. Forest roads in the Czech Republic are in the lowest road category with the smallest demands on the quality of input characteristics, they have to simultaneously meet the requirements regarding load bearing capacity according to TP 170 (2011) and design methodology for low volume roads (LVRs) [3]
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