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

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

Řízení podnikové ekonomiky

Prezenční výpůjčkaVŠB - Technická univerzita Ostrava. Ekonomická fakulta. Katedra (115) management

Study of the relationship of moisture and compaction on the modulus of resilience obtained by cyclic CBR testing in local soils for a quality rural tourism

Low Volume Roads are a key part of both rural and urban life and an accurate study of the parameters involved in their design is fundamental at an environmental level. In nature, due to the passing of the seasons, with changes in humidity and temperature, the density and compaction of the soil changes notably, giving rise to cracks and defects that directly affect their users and entail both economic and environmental costs in terms of vehicle efficiency and repair costs. Furthermore, this type of defect hinders the practice of outdoor activities and sports, which are essential for the development of rural tourism. For this reason, in this study, the Resilience Modulus of a nearby material has been analysed by cyclic California Bearing Ratio tests, thanks to Mendelu's specialised software, under different compaction and humidity conditions.Published Versio

Influence of Environmental Humidity on Mechanical Properties of Natural and Recycled Unbound Materials

Low volume roads are widely used all over the world. To improve their quality a FEM computer simulation of their behavior is proposed. The input information about mechanical properties of individual materials is crucial for obtaining results as exact as possible. Among others, the mechanical properties are generally dependent on the state of stress and on humidity conditions. For this purpose the cyclic-load triaxial machine testing of cyclic-load performance of materials seems to be a promising test method. The test specimens can be prepared with different amounts of water. Thus modulus of elasticity (Young modulus) of different materials including recycled ones can be measured under the different conditions of horizontal and vertical stresses and under the different humidity conditions. Using the proposed testing procedure the modulus of elasticity of materials used in the newly built low volume road is obtained under the different state of stress as well as humidity conditions set to standard, dry and fully saturated level. Also recycled materials which are able to replace the traditional materials in the pavement are tested. Obtained values of modulus of elasticity can be used in a FEM study of the newly built road

Influence of water content on the shear strength parameters of clayey soil in relation to stability analysis of a hillside in Brno region

Shear strength of soils is highly affected by moisture conditions (i.e. water content), especially if the soil contains clay materials. Usually the laboratory specimen, which are used to determine shear strength of soil are prepared at water content and dry density same as in the field conditions, without respect to the fact, that the conditions in the future might not remain the same. For the purpose of this study soil specimen were compacted and the optimum moisture content was identified. After compaction soil was tested at the dry side of optimum water content at w = 9 %, 10 % and 11 %. Parameters of shear strength were obtained and used for stability analysis with software GEOSLOPE/W 2012. According to referenced literature, it was expected for the shear strength of the soil to decrease with increasing water content. This hypothesis was not proven for clayey soil from Brno region. Development of values of friction angle and cohesion exhibited anomalous behaviour and such development was found also for values of Factor of safety (FOS) obtained from stability analyses. Results proved the necessity of taking moisture conditions into account, when processing stability analyses, in order to achieve reliable and safe constructions

Reliability Analysis of Temperature Influence on Stresses in Rigid Pavement Made from Recycled Materials

Complex statistical and sensitivity analysis of principal stresses in concrete slabs of the real type of rigid pavement made from recycled materials is performed. The pavement is dominantly loaded by the temperature field acting on the upper and lower surface of concrete slabs. The computational model of the pavement is designed as a spatial (3D) model, is based on a nonlinear variant of the finite element method that respects the structural nonlinearity, enables to model different arrangement of joints, and the entire model can be loaded by thermal load. Four concrete slabs separated by transverse and longitudinal joints and the additional structural layers including soil to the depth of about 3 m are modeled. The thickness of individual layers, physical and mechanical properties of materials, characteristics of joints, and the temperature of the upper and lower surface of slabs are supposed to be random variables. The simulation technique Updated Latin Hypercube Sampling with 20 simulations is used for the reliability analysis. As results of statistical analysis, the estimates of basic statistics of the principal stresses σ1 and σ3 in 106 points on the upper and lower surface of slabs are obtained. For sensitivity analysis the sensitivity coefficient based on the Spearman rank correlation coefficient is used. As results of sensitivity analysis, the estimates of influence of random variability of individual input variables on the random variability of principal stresses σ1 and σ3 are obtained

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

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

Simple Procedure for Probability Calculation of Tensile Crack Occurring in Rigid Pavement: A Case Study

Formation of tensile cracks in concrete slabs of rigid pavement can be (among others) the initiation point of the other, more serious failures which can ultimately lead to complete degradation of the concrete slab and thus the whole pavement. Two measures can be used for reliability assessment of this phenomenon - the probability of failure and/or the reliability index. Different methods can be used for their calculation. The simple ones are called moment methods and simulation techniques. Two methods - FOSM Method and Simple Random Sampling Method - are verified and their comparison is performed. The influence of information about the probability distribution and the statistical parameters of input variables as well as of the limit state function on the calculated reliability index and failure probability are studied in three points on the lower surface of concrete slabs of the older type of rigid pavement formerly used in the Czech Republic