Study on the Rapid Drawdown and Its Effect on Portal Subsidence of Heybat Sultan Twin Tunnels in Kurdistan-Iraq

The excavation of tunnels below the water table causes variations in the hydraulic level, pore pressure and effectivestresses. In this regard, rapid drawdown is considered as a destructive phenomenon as to the change in the flow regimewhich has mostly been studied for the reservoirs of embankment dams. The rapid drawdown occurred at the upstream shellof the dam gives rise to increase in the pore pressure at the upstream shell. This is as a result of the incompliance betweenthe water loss inside the shell and the reservoir water level. Hence, it would be more likely to have instability and slidingat the upstream slope on account of decrease in the effective stress. Lack of sufficient studies performed on this matter intunnelling projects on the one hand and the knowledge on the most important parameter for decreasing the destructiveeffects of this phenomenon on the other hand necessitates performing further studies on this matter. To this end, the reasonsfor the occurrence as well as the affecting parameters were studied by modelling the large subsidence of the inlet portal ofHeybat Sultan twin tunnels located in Kurdistan-Iraq making use of the variations of the groundwater boundary conditionsunder Phase2 code. The modelling results depict the importance of the drawdown rate and the permeability coefficient ofthe surrounding rock mass. In the interim, the rapid loss in the hydraulic gradient caused by the drainage of a considerablevolume of precipitations into the tunnels led to the rapid decrease in the pore pressure and increase in the effective stressesup to total stress. This has resulted in the consolidation settlement in the tunnel portal.

Keywords:

Rapid Drawdown , Subsidence , Pore Pressure , Effective Stress , Tunnelling