The Influence of the Soil Treatment on CBR Test Values

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.

Section: Discussionmentioning

confidence: 86%

Section: Discussionmentioning

confidence: 99%

Section: Laboratory Analysis-cyclic Cbr Testsmentioning

confidence: 99%

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

Ševelová

Florian

Hrůza

2020

“…If we compare the plunger stress values with the assumed maximum soil load-bearing capacity according to Terzagi's theory, the maximum value of the soil load-bearing capacity is assumed to be between 150 and 650 kPa [34], depending on the soil type [35]. In many cases, these limit values are exceeded many times during the standard cyclic CBR test.…”

Section: The Standard Cyclic Cbr Testmentioning

confidence: 99%

“…The resilient modulus should consider these factors [9] and should be obtained from an appropriate laboratory test that realistically simulates expected loadings by repeated passages of transport vehicles and taking into account the expected state of stress in an actual structure. At the same time, the test must not violate the soil specimen being tested or exceed the maximum load-bearing capacity of the material [10,11]. After applying numerous loads, the irreversible plastic deformation in the last loading step becomes smaller compared to the total deformation.…”

Section: Introductionmentioning

confidence: 99%

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

Florian,

Ševelová,

Žáková

et al. 2023

The design and assessment of forest and rural road pavements made from unbound natural and recycled materials require the careful determination of their physical-mechanical properties. The fundamental parameter for characterizing the soil material property is considered to be the resilient modulus Mr. The generally accepted alternative solution for its calculation is the cyclic CBR test, which uses standard CBR testing equipment. To perform tests on intact specimens and under stress conditions corresponding to expected states of stress in a pavement structure, an updated cyclic CBR test is proposed. In contrast to the standard cyclic CBR test, the applied loading force for repeated loadings is not determined in the first loading step by the plunger penetration to the prescribed depth, but by the stress value that is expected in an actual pavement structure. The updated test was verified on 240 tested specimens taken from a total of 40 soil samples belonging to nine soil types according to the USCS system. The resilient modulus results obtained are compared with those obtained using the standard cyclic CBR test and the cyclic triaxial test.

Influence of Plunger Stress on Resilient Modulus of Forest Subgrade Soils Obtained from Cyclic CBR Test

Ševelová

Florian

Žák

2021

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.