Impact of sample size and clipping method on triaxial test results for coarse granular soil

Details

Supervisors

Rattez, Hadrien ; Vanden Berghe, Jean-François

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil des constructions, à finalité spécialisée

Abstract

In civil engineering, embankments are often bluit using a class of materials known as "coarse soils". They are characterised by the large size of their constituent elements, which are over-dimensioned compared with standard-sized devices. Assessing their resistance to shear requires the use of large devices, the testing of which represents a cost that few can afford. The value of the internal friction angle for this type of soil is not measured and is therefore assumed to be at least 35◦. This master thesis investigates the evolution of the shear strength characteristics of a soil as a function of the device’s size used and the sample clipping method, with the aim of increasing the slope angle of an embankment. After defining a coarse reference soil, the simple clipping method and the parallel gradation modelling method are presented and used to produce standard-sized samples. Tests under consolidated drained conditions are carried out using triaxial devices of different diameters ϕ = 38, 70 et 100mm. The influence of the size of the device and the clipping method on the values of the internal friction angle and cohesion are studied jointly. It has been found that both clipping methods show an increase in the internal friction angle as the size of the device increases. For the simple clipping method, increasing the diameter of the device from 38mm to 100mm implies a gain of 2,6◦ . For the parallel method, an increase of 11◦ is observed between the 38mm and 70mm samples. Cohesion for the first method is 13,08, 6,5 and 6,38 kPa for samples 38C, 70C and 100C. For the second method, the values are 30,71 and 11,67 kPa for samples 38P and 70P. This difference in shear strength characteristics shows the involvement of soil properties, such as fines content and coarse particle size, defined by the clipping method used. These shear strength characteristics are then used to define an acceptable embankment slope. Lastly, an analysis of the gains following this change in slope is carried out. A saving of between 19,9 and 24,84% compared with the cost of a reference embankment is achieved with the use of 38mm devices. This saving rises to between 22,9 and 28,59% for a 70mm diameter and increases again for 100mm to between 29,41 and 36,72%.