Stability Analysis of Fault Rock Heterogeneity on the Left Pressure Shaft Collapse ofSiah Bisheh Dam, North Iran

摘要

The Siah Bishe Pumped Storage Dam is located in the northern part of the Alborz mountain range in Iran.The investigation results and the effect of brittle deformation of two fault sets in the right pressure shaft ofSiah Bisheh Dam are discussed in this paper. The shaft is arranged in sedimentary and volcanic highlyjointed rocks of Triasic and Jurasic ages. The length of the shaft is 450m and its diameter is 6m with 65degree inclination relatively south-north trend. The results of geo-engineering surveys using boreholes andstatistical joint-graphics of stereograms reveal at least two sets of faults in the rock mass. Several transversalsections along the shafts were studied in order to analyze the effect of faults in shaft collapse. The findingsfrom the surface and underground mapping and investigation boreholes are described and interpreted in ageological map of pressure shaft slope including sections and stereograms. The site area and especially thearea of the pressure shaft are characterized by the presence of three important thrust faults that cut throughthe whole stratigraphic series. The Main Thrust Fault (MTF), fault Nr. 2, both dipping to the north and faultNr.3 that dips to the south. East of the site, faults 2 and 3 border a large ‘pop-up’ structure while the MainThrust Fault occupies a lower tectonic position. The results show that the collapse could be a localoccurrence which is limited to a critical, fractured and water-saturated rock zone at the boundary betweenMTF and fault 3. Stability of the shaft was analysed with Unwegde 3.0 software. Result of rock bolting ofthe shaft was not suitable because of depth of the tip wedges are longer than maximum allowable rock boltlength. The lowest safety factor occurred on wedge number 4 (SF=0.873). But using of shotcrete as a supportshows that safety factor of the wedge is increased to 3.219, when shotcrete thickness is 20 cm.