In the past, many underground water channel systems (Qanats) were used to extraction of groundwater in Mashhad and its surrounding area. Nowadays, most of them have dried out and the city has developed on them. The constructions of tall buildings in recent years increase the potential of Qanats collapsibility in this area. In this paper the distribution of Qantas system in West of Mashhad and their stability’s under different loading and support system conditions are investigated. For this, the locations of Qanats wells are mapped using the aerial photograph then the roughly depth of Qanats are estimated based on the few site measurements and existing documents. The stability of Qanats was evaluated based on the elasto-plastic soil models in four depth and different support systems. The results have shown that with increase of Qanat depth the extension rate of plastic zone decreases. Also the effect of the support covers in the plastic zone decrease with increasing the Qanat depth. As it is estimated, for every 1.2 m increase in depth, the bearing capacity of Qanat gallery increases as much as a load of one-storey building.

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