EFFECT OF BRITTLE-DUCTILE THRUST WEDGES ON THE LEFT PRESSURE SHAFT COLLAPSE OF SIAH BISHEH DAM, NORTH IRAN

7th International Scientific Conference - SGEM2007, www.sgem.org, SGEM2007 Conference Proceedings/ ISBN: 954-918181-2, June 11-15, 2007

ABSTRACT

The Siah Bishe Pumped Storage Dam is located in the northern part of the Alborz
mountain range. The investigation results and the effect of brittle deformation
of two fault sets in the left pressure shaft of Siah Bisheh Dam are discussed in
this paper. The shaft is arranged in sedimentary and volcanic rocks of Triasic
and Jurasic ages. The length of the shaft is 450m and its diameter is 6m with 65
degree inclination relatively south-north trend. The results of geo-engineering
surveys using boreholes and statistical joint-graphics of stereograms reveal
at least two sets of faults in the rock mass. Several transversal sections along
the shafts were studied in order to analyze the effect of faults in shaft collapse.
The findings from the surface and underground mapping and investigation
boreholes are described and interpreted in a geological map of pressure shaft
slope including sections and streograms. The site area and especially the area
of the pressure shaft is characterized by the presence of three important thrust
faults that cut through the whole stratigraphic series. The Main Thrust Fault
(MTF), fault Nr. 2, both dipping to the north and fault Nr.3 that dips to the
south. East of the site, faults 2 and 3 border a large ‘pop-up’ structure while the
Main Thrust Fault occupies a lower tectonic position. The results show that the
collapse could be a local occurrence which is limited to a critical, fractured and
water-saturated rock zone at the boundary between MTF and fault 3.

Keywords: brittle deformation, thrust fault, collapse, borehole, Siah
Bisheh
Dam