ویژگیهای زمین شناسی و ژئوتکنیکی پل بند باستانی گرگر، شهرستان شوشتر

In this paper , the geological and geotechnical characteristics of Gargar Weir being nearly two thousands year old, upstream Shushtar City , north of Khuzestan have been invetigated in order to repair and rehabilitate existing tunnels . The weir is consisted of three tunnels being excavated in Agha Jari sandstones of which , some parts of walls and roof of the left tunnel  have been eroded and collapsed due to various reasons . The sandstones are moderately grained and relatively well sorted with average bedding dip of ۵  NE . Two main joint sets trending N ۴۰-۶۰ E and N ۴۰-۶۰ W with nearly vertical dip and a bedding parallel one have formed . Regarding structural nature and results of geotechnical studies in adjacent areas , the tunnel walls are almost stable however , erosion of marly layers at water contact specially downstream , and solution of clayey lenses within sandstone layers caused some collapses . The permeability of equivalent sandstones and marlstones in the vicinity are measured as ۱ to ۱۵ and ۰ to ۳۰ Lugeons , respectively . It must be noted that heavy vehicles traffic on the weir amplified collapse . Wholely , based on the investigations and regarding archeological limitations , to stabilize the tunnels , the right confined one can be shotcreted and the left one can be shotcreted ( with sulphate resistant cement ) and steel anchored beneath the street .