Effect of the initial grout curtain and treatment works on hydraulic relation between the Tangab dam reservoir and downstream groundwater

The dams and reservoirs are usually designed with a certain capacity to meet agricultural, urban and industrial needs. Karst terrains are typically considered inappropriate for dam construction because of their complex geological and hydrogeological characteristics. The Tangab Dam with a reservoir volume of 140 million cubic meters was built to control river flood and supply part of downstream agriculture water. Geologically, the dam and reservoir are located at the entrance of a karst valley, on the northern limb of the Podeno Anticline, in the simply folded Zagros Zone. The Podeno Anticline is made mainly of Pabdeh marlstone, Asmari limestone, alternating layers of limestone and marlstone of a transitional zone, and Razak marlstone. By beginning of impoundment in 2009, the reservoir has faced with a leakage problem. In order to reduce water leakage, the additional grouting and treatment works were designed and executed at dam site. In this research, an attempt has been made to investigate the function of additional grouting and treatment works on reducing the hydraulic connection between the reservoir and downstream groundwater.

The geologic map of the study area was prepared based on a geological map provided by the Geological Survey and Mineral Exploration of Iran. Water level in the reservoir and boreholes were measured monthly during a five years period by the Fars Regional Water Authority. The grouting history at the dam site was determined based on the available reports. The dam site was divided into three sectors including the left abutment, riverbed and the right abutment. At each part, effect of the initial grouting and treatment works on the changes in the groundwater level was investigated. Pearson coefficient was used to determine correlation between the reservoir water level and water level in downstream boreholes.

The initial water tightness system of the Tangab Dam includes a cut-off wall implemented in the riverbed and a 180 m deep grout curtain with 256 and 216 m extension respectively into the left and right abutments. By starting of reservoir impoundment in 2009, discharge of the downstream springs was increased (from 1.4 to 3 m3/s) due to water losses from the reservoir. To reduce further leakage, two additional grout curtains with 300 and 570 m extensions were executed respectively in the left and right abutments. Moreover, additional grouting was done at different parts of the initial grout curtain. At the left abutment, similar fluctuations were observed in the reservoir water level and downstream groundwater. The calculated correlation coefficient between the reservoir water level and water level in boreholes located upstream and downstream of grout curtain varies between 0.80 to 0.96 and 0.70 to 0.89, respectively. About 50 m difference between the reservoir water level and water level in downstream boreholes shows an acceptable performance of grout curtain at the left abutment. At the riverbed, it seems that the cut-off wall and grout curtain works appropriate because the boreholes located in the dam foundation did not show considerable relation with the reservoir (correlation coefficient between 0.06 and 0.12). At the right abutment, the lowest correlation between the reservoir and the boreholes water levels was observed in the RG2, ARG2 and RG6 galleries. Remedial works in these sections have also cause several meters falling in downstream water level. It appears that the closest relation between reservoir water and downstream groundwater was observed along the 570 m long grout curtain constructed at the reservoir bank. Here, it was concluded that permeable sections were remained in the rocks located at the upper levels (especially more than 1420 meters) because a significant rising in groundwater was observed in response to the 25 m rising in the reservoir water level. Results of this research show that the initial grout curtain and the additional treatment have worked successfully in reducing the rock permeability and also increasing the length of leakage paths. However, reservoir water could be leaked by moving through, below or by passing the ends of the hanging grout curtain which is not connected into the impermeable rocks in depth and at the ends. To reduce leakage from the Tangab Reservoir, the additional treatment needs an extensive and expensive grouting works. Also, achieving a complete sealing of the reservoir is not guaranteed even with the execution of this grouting. Therefore, it is suggested that no other grouting operations be designed and implemented at this site, because the leaked water through the Asmari Aquifer is not unused. Deep percolation of karst water is limited by the Pabdeh marlstone at the bottom of the aquifer. If the reservoir water moves along the edge of the anticline towards the plunge area, it will recharge the karst aquifer and use in future. If the reservoir water moves to the southern limb of the anticline, it will discharge through the downstream spring and use for the local agricultural purpose.