The Bidvaz dam is an earth-rockfill dam located in the northeast of Iran. About seven and a half years after the start of the first impounding, a subsidence appeared on the surface of the dam. Evaluation of the dam behavior based on the data recorded by its instruments confirms the occurrence of internal erosion in the lower parts of the core. Hydraulic fracturing phenomenon has been hypothesized as a main cause of initiating erosion in this dam. Thus, the main purpose of this research was to evaluate the possibility of this hypothesis. To achieve this aim, first the behavior of the dam was simulated with a three-dimensional numerical model. Then, the safety factor against the hydraulic fracturing in the upstream face of its clay core was calculated. The results showed that being located on a steeply sloped narrow valley has caused significant stress transfer from core near the abutments. This has caused the safety factor of these regions against the phenomenon to be less than unity.

This paper presents the results of laboratory experiments on the effect of matric suction on the compressibility of unsaturated gypsum sand soil under various types of loading. The soil samples were obtained from Al-Najaf city in Iraq with gypsum contents of 14 %, 22 % and 29 %. A wetting-process tests were conducted in a modified triaxial cell on these soil. This procedure likes when a structure has been constructed and the degree of saturation of a foundation soil is increased (decreasing in matric suction). Four matric suctions were adopted; 100, 60, 30 and zero percentages of initial matric suction under two different mean net stresses; 100 kPa and 200 kPa. The changes in the degree of saturation may be due to rainfall, water table rising and/or leaking of sewage and water pipes. The results from this path revealed that the volumetric strains are increased with decreasing of the matric suction and increasing of the gypsum content.