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Tavakoli Mehrjard G, Motarjemi F. Effect of Soil Particles Size on its Shear Strength. Journal of Engineering Geology 2018; 12 (2) :277-292

URL: http://jeg.khu.ac.ir/article-1-2532-en.html

URL: http://jeg.khu.ac.ir/article-1-2532-en.html

The general failure mechanism of soil element in geotechnical structures is shear failure under static and dynamic loads. Therefore, assessment of soils’ shear strength parameters is very crucial in the performance of geotechnical structures, especially in slope stability. Tavakoli Mehrjardi et al. (2016) showed that by increasing soil grain size in unreinforced soil masses, bearing capacity of foundation increases due to increasing shear strength parameters of soil mass. Furthermore, Tavakoli Mehrjardi and Khazaei (2017) found out that generally, for all reinforced and unreinforced conditions, cyclic bearing capacity was enhanced by increasing the medium grains size of backfills. Taking into account the deficiency of studies on the shear characteristics of soil, a series of large direct shear test have been carried out to investigate and to compare effects of the soil’s physical properties such as aggregate size and relative density, besides of normal stress, on the shear characteristics of the backfills.

In this study, three types of uniformly graded soils as fill materials with the medium grain size (D

The curves of shear stress as a function of shear displacement and also shear displacement-vertical displacement for samples show that shear stress dropped down to a specific amount of residual shear strength after reaching maximum amount of shear stress . It was observed that increasing the particle size and relative density of the fill materials mostly fortify interlocking of the grains which in turn, resulted in increasing the tendency to expansion through the shear plane. On the other hand, the initial compression has decreased and dilation was started from a smaller shear displacement. This may be interpreted that as the soil particles size increases, more expansion is required to reach the maximum shear strength. Moreover, comparing the observed behavior, it is found out that unlike the effect of grain size and density, increasing the normal stress caused the materials to be more compressed, resulted in reducing expansion and increasing the initial compression of the soil mass. This conceivably means that increasing normal stress, transferred on shear plane, can change the failure mechanism of materials, from dilatancy failure to bulging failure under shearing. From the results, it was found out that increasing medium grains size of soil from 3 mm to 12 mm ended to improvement in the maximum friction angle at relative density 50 and 70% by the value up to 4.4 and 5.8 degree, respectively. In fact, due to increasing grain size, the grains interlocking have been fortified. In order to have a comparison, the maximum dilation angles of all fill materials, mobilized at the shear plane, have been derived. Accordingly, the maximum dilation angle was increased with the increment of the fill grains size and relative density of the material. Nevertheless, by considering variation of peak dilation angle with normal stress, it is found out that the normal stress had a negative influence on the advancement of interface’s dilation angle. These findings can be directly interpreted by considering the compression/expansion of the materials during the increment of shear displacements.

The current study, consists of 18 large-scale direct shear tests, aims to investigate shear characteristics of soil which influenced by different parameters such as relative density of the fill materials, normal stress at the shear plane and aggregate size of the fill materials. Eventually, the following conclusions are presented:

- Increasing relative density, soil particle size and normal stress have beneficial effect in shear strength improvement. But, the mechanism
~~s~~of each parameter in this enhancement is different. - The dilation rate of shear interfaces directly complies with changes in the ratio of applied shear stress to vertical stress. So, the maximum dilation angle and the maximum ratio mobilized at the shear plane have occurred around the same shear displacement.
- Maximum values of friction and dilation angels have been occurred around the same shear displacement. Moreover, compaction effort leads to increase the required shear displacements to approach the maximum shear characteristics.

Type of Study: Case-Study |
Subject:
Geotecnic

Received: 2016/05/16 | Accepted: 2016/10/29 | Published: 2018/07/4

Received: 2016/05/16 | Accepted: 2016/10/29 | Published: 2018/07/4

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