Collapse hazard zonation of qanats in greater Tehran area

Several cases of subsidence occurred in greater Tehran resulting from collapse of walls of underground water channel system known locally as qanat. Many mechanisms contribute to the qanat collapse, which add to the complexity of the problem. The present research focuses on identifying the main factors that influence qanat collapse and the parameters that should be considered in modeling their behavior. Some of the important factors that influence the stability and performance of qanats and considered in the analysis presented here include: qanat geometrical characteristics; loading due to surface structures; underground water level and geotechnical properties of the soil. A suitable model was chosen and deformational behavior of qanat walls was simulated. The stability of qanats was evaluated considering both elastic and elasto-plastic soil models. It was found that qanats in north and northeastern parts of Tehran have high strength and qanats in south and southwestern parts have low strength. A GIS-based qanat collapse hazard zonation map was prepared. It was found that the hazard of collapse increases for shallow qanats with any structural loads imposed at the ground surface. As expected, this hazard is much higher for qanats situated in weak soils. Furthermore, the hazard of collapse is much higher for qanats with larger diameter.