The paper deals with an analysis of the shear strength behaviour of coarse-grained materials using large-sized direct shear apparatus. The shear strength curve of compacted coarse-grained material allows for determining the three typical shear strengths, i.e. the shear strength for a maximum density of the sample tested (shear strength at point A), the peak shear strength (shear strength at point P), and the critical shear strength achieved for a large horizontal movement (shear strength at point C). Professional laboratories usually apply for determining the critical shear strength two different methods. The first method considers the critical shear strength at point A and the second method considers the critical shear strength at point C. The aim of the article is not to critically evaluate both methods, but to analyze the results of shear tests of gravels. The results of a large series of tests showed that these stresses may differ from each other in some cases, especially for poorly graded materials. In the case of sandy soil, the shear strength at point A and at point C is equal. However, in the case of poorly graded gravel, the shear strength at point C is greater than the shear strength at point A. The greatest difference between these shear strengths was measured in the case of poorly graded medium gravels. The results showed that the ratio of shear strength at point C to the shear strength at point A increases with increasing the grain size of the material tested. The peak as well as the critical shear strength parameters of poorly graded gravels is better represented by a multi-linear or a nonlinear failure line.
The paper deals with the laboratory testing of coarse-grained soils that are reinforced using a geogrid. The shear strength properties were determined using a large-scale direct shear test apparatus. The tests were executed on original as well as on reinforced soil, when the geogrid was placed on a sliding surface, which permitted determining the shear strength properties of the soil-geogrid interface. The aim of the tests was to determine the interface shear strength coefficient α, which represents the ratio of the shear strength of the soil-geogrid interface to the unreinforced soil. The tests were executed on 3 samples of coarse-grained materials, i.e., poorly graded sand, poorly graded fine gravel and poorly graded medium gravel. Two types of geogrids were tested, i.e., a woven polyester geogrid and a stiff polypropylene geogrid. The results of the laboratory tests on the medium gravel showed that the reduction coefficient α reached higher values in the case of the stiff polypropylene geogrid. In the cases of the fine gravel and sand, the values of the interface coefficient α were similar to each other. The shear strength of the interface was reduced or was similar to the shear strength of unreinforced soil in a peak shear stress state, but significantly increased with horizontal deformations, especially for the fine gravel and sand. The largest value of the coefficient α was measured in the critical shear stress state. Based on the results of the testing, a correlation which allows for determining the optimal grain size distribution was obtained.
Stone columns made of coarse-grained materials and crushed stone are one of the most-used technologies for soil improvement all over the world. Stone columns improve the strength and deformation properties of subsoil and reduce the time required for the consolidation of fine-grained soils. The impact of the improvement depends on the properties of the original subsoil as well as the properties of the coarse-grained materials used for the stone columns. The article deals with the effects of the properties of coarse-grained materials for stone columns on the settlement and consolidation times of improved subsoil for the foundation of a factory. Numerical modeling as a 2D task was performed using Plaxis geotechnical software. The numerical analysis included two methods of modeling stone columns in a plane strain model, i.e., one method often used by practical engineers in the region of Slovakia, and one modified method, which allowed for a more accurate determination of the final settlement and consolidation time. The method modeled stone columns as continuous walls, and the compaction of the soil between the stone columns was taken into account. The results showed that the type of coarse-grained material can significantly affect the final settlement and time of consolidation. Stone columns made of quarry stone were suitable in the given geological conditions regardless of the design of the mesh, while stone columns made of pebble gravel were suitable only with a mesh of 1.5 x 1.5 m.
The article deals with designing and analysing a wrapped geogrid reinforced structure (GRS) with a passive facing system. The analysis has been done using analytical calculation and numerical modelling. The analytical calculations were executed using FINE Geo5 geotechnical software, and numerical modelling was executed using Plaxis 2D software. The analysis is focused mainly on comparing tension forces in geogrids and the stability of the reinforced embankment determined using both computational methods. The deformation analysis was done only using numerical modelling. The numerical modelling allowed for a more detailed analysis of the wrapped GRS. Each construction phase was modelled step by step according to an actual construction procedure. Two complex road embankments supported by GRS were modelled and analysed. The first model consisted of three GRS, which not affected each other. In the second model, the GRS at each side of the embankment influenced each other. The analysis results showed that tension forces in geogrids, determined using both computational methods, can differ significantly from each other. The stability of the reinforced embankment determined using numerical modelling was within the range of 0.87 – 1.22 of the stability determined using analytical calculation. The numerical modelling results showed that the final horizontal deformation of the passive facing is about 2.8 – 3.8 times smaller than the deformation of the wrapped GRS, which occurs during the construction of the embankment.
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