OICC Press Iranian Journal of Earth Sciences 2228-785X 16 1 2024 04 27 The effect of brittle fault performance on rock mass engineering characteristics (North Tehran fault), Iran 72 84 10.57647/j.ijes.2024.1601.06 EN Elahe Hassanbeigi Department of Earth Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran 0009-0005-7922-5871 Ali Solgi Department of Geology, Science and Research Branch, Islamic Azad University, Tehran, Iran Mehran Arian Department of Earth Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran 0000-0002-4574-5116 Ali Uromeihy Department of Engineering Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, Iran Journal Article 2024 04 27 Brittle faults have the most influence on the engineering properties of rock masses and they are of special importance. The complexity of the faults makes it difficult to survey their geotechnics, and even the type of fault, has a different effect on the fragility of rock masses. For this purpose, in this study, the geomechanical properties of rock faults and Bimrocks in the North Tehran fault zone, which is located in the southern part of central Alborz and north of Tehran, are investigated and analysed. Therefore, by using BIMROCKS investigation (Block-in-matrix rocks), we can reduce the high costs of boreholing for the construction of engineering structures within the limits of the fault zone. Bimrocks, which are a group of limited rocky blocks in the matrix, have irregular textures and properties, thus creating problems for civil works in the area and also causing slope instability. The cross-sectional measurement of the North Tehran thrust zone from the forehead to the end of the zone indicates some regular changes in volumetric proportion and uniaxial strength of the fault rock blocks, So that the amount of fragmentation of rocks decreases from the forehead to the end of the zone, and therefore the volumetric proportion and their strength increase. Having the result of this study shows that the resistive properties of the fault rocks depend on the presence of micro-faults, the spacing of shear-plates from each other and the amount of displacement created by the fault.