Using Resilient Modulus to Determine the Subgrade Suitability for Forest Road Construction

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.

An Updated Cyclic CBR Test with Realistic Stress Values under the Plunger for Resilient Modulus Calculation

“…The method has been tested and used by a number of authors [15,16,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Although the influence of the state of stress on the modulus has been correctly mentioned in some papers, and some studies have even been performed under an intuitively lower loading force not set up according to the standard CBR test [25,29], the question of the magnitude of the stress under the plunger has been overlooked.…”

“…Although the influence of the state of stress on the modulus has been correctly mentioned in some papers, and some studies have even been performed under an intuitively lower loading force not set up according to the standard CBR test [25,29], the question of the magnitude of the stress under the plunger has been overlooked. In the first papers dealing with this problem [15,31], the effect of the plunger stress on the calculated resilient modulus was measured, analyzed, and discussed. The authors also discussed how the plunger stress affects the calculated resilient modulus and how these values are consistent with the reality of in-situ soil behavior.…”

“…First, it was found from 276 specimens tested [15,31] that the plunger stress values can be high, up to 3000 kPa, and when these values are recalculated from published available data from other authors (e.g., [20,25,26,30]), they can be as high as 9000 kPa. Second, when testing more specimens taken from one soil sample, the individual specimens are cycled under a wide range of plunger stress values, because when penetrating to a depth of 2.54 mm, a different loading force is required for individual specimens and thus a different plunger stress will arise.…”

“…Another consequence of high plunger stress values are high resilient modulus values too, as there is a high positive correlation between modulus and plunger stress [15,31]. That is, if cyclic loading is performed with high plunger stress values, we can expect with a high probability high resilient modulus values, and vice versa.…”

“…A statistical analysis of the results obtained using the new updated method for nine subgrade soil types according to the Unified Soil Classification System [13,14] is presented. The results are compared with those obtained using the standard cyclic CBR test [15] and the cyclic triaxial test.…”