Golab water transmission tunnel which is 4.5m in excavation diameter and about 10358 m long is excavated for roviding water to the cities of Kashan and Aran Bidgol from Zayandeh Rood regulatory dam. This tunnel is located in a region about 120 K.m away from the west of Isfahan and 50 K.m away from the north of shahr-e-Kord. considering the reasons of crack in the segment of this tunnel with special attention to the estimation of the ratio of the ratio of in situ stress in excavating place of tunnel is the main goal of this study. the rock masses of the tunnel route is studied according to the samples taken from 10 drilled bore holes along tunnel and field studies. Morever, the critical cross section of the tunnel route was determined by kashida; TEXT-ALIGN: justify; LINE-HEIGHT: 90%; TEXT-KASHIDA: 0%; MARGIN: 0cm 0cm 0pt" UDEC discontinuos setting software was used for determining the load upon the segmental covering and modeling them because the route of Golab water transmission tunnel is through weak, layer and fracture rocks. The segments of the tunnel were simulated in one ring, four pieces and in three dimensional by ABAQUS continuos setting software. The amount of load upon the segemental covering was determined by UDEC software and performed on the segments simulated in ABAQUS software. The analysis of sensitivity was done on the in situ stress ratio, deformation module of rock mass and the thickness of the segment. The obtained results Showed that when the thickness of the segment increases from 25 cm to 30 cm, the amount of generated stress and displacement in segmental ring reduce 21.2%. However, the increase of the thickness didn’t prevent breaking the segment when the deformation module of rock mass was 1.5 Gpa and the in situ stress ratio was 0.5. Eventually, the reaction of the segments was studied according to change of these three parameters after considering the obtained results from the numerical modellings. Then, the reasons of the segment failour and the required strategies for preventing the failour of the segmental covering within mentioned limit, which had weak layers(schist) and was squeezing alot, were determined
