The Masjed Soleyman dam is a central clay core rockfill dam with the height of 177 meter and 490m long center core. Throughout the dam construction, reservoir impounding and operation comprehensive monitoring had been dam behavior was based on measurement data of instruments installed on the dam body and foundation. This research present research the first impounding and summarize instruments are in the field of stresses rotation of the principal stresses direction had been investigated. The rota points were generally constant, but slight increases in the angles after full impoundment ha observed in the downstream side of the core. This would lead to bulging of the cor permeability of the clay material, the dissipating process of the pore water pressure hadn’t been continuing with notable progress (in the past 13 years after the first impounding). So the clay behavior as an undrained material and volume change is not significant especially in the center area of core. This is the main reason for developing the lateral deformation in the clay core which called bulging.

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