Keywords: computation model, geosynthetics, geogrid, ballast bed, embankment, subsoil, bearing capacity, stiffness, settlement.

The aim of this work is to verify the usability of numerical modelling for soil construction of a railway reinforced with geosynthetics when using the ANSYS programme. For the computation model of a simple ground body the process of model creation, choice of finite elements, definition of material features, application of contact elements, definition of boundary conditions and load and setting the parameters of computation were verified.

The constructions of railway embankments on less load-bearing subsoil are parts of many projects. By using geogrids (e.g. TENSAR) we can avoid the problems connected with the conventional way of extracting earth from the subsoil and its substitution with loose earth of good quality. The stiffening function of the geogrid lies in using the membrane effect, i.e. the forces in the area of the reinforcement, which have a favourable effect on tensile stresses in the soil construction. Combined with soil a composite is being created, which improves force and deformation properties of the soil construction compared with non-reinforced earth.

A volumetric parametrical three-dimensional model with plane deformation in the direction of the cross section of an embankment is used for modelling of embankment on earth subsoil with reinforcement of geosynthetics on the interface between both layers. For modelling the properties of embankment and subsoil material the classic Drucker-Prager model is used. The reinforcement of the geogrid TENSAR SS is modelled as a thin plate of constant thickness which is mostly stressed by normal tensile force and the bending stiffness is unimportant. The finite element model consists of two volumes among which a plate (membrane) is interlaid. Between the earth and geosynthetics there is (besides constant pressure) friction which sets off activity of the geogrid as a membrane. The embankment is loaded with a surface vertical uniform loading. The verification of the model is done by comparative calculations on two models. In first case we are dealing with an embankment on subsoil without any reinforcement, in the second case a reinforcing geogrid is put between the volume of railway bed and the volume of subsoil.

The bearing capacity of soil construction has great importance for practice. The reinforcement of a geogrid has a favourable effect on bearing capacity of the embankment. At the lower embankment the bearing capacity has increased by 46%, at the higher by 13%. Also the prestress of the reinforcement is favourable but only for lower values of prestress. At higher prestress there is a damage of cohesion on the contact of geogrid-soil.

The stiffness of the soil construction, which can be derived from the values of load, the total height and vertical displacement, is an important value. As for the model the influence of reinforcement on stiffness of the reinforcement at elastic solution is approximately 2.5%. For the Drucker-Prager soil model the reinforcement with the geogrid adds to increasing the construction stiffness approximately by 3.5% and 5.5% (for a lower and higher embankment). Also the introduction of the geogrid prestress has a favourable effect on the construction stiffness.

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