|Study of the Excess Pore Pressure During First Reservoir Impoundment and Control of the Hydraulic Fracturing
Manager of Dam Projects, Dezab Consulting Engineers Company, Ahwaz-Iran
Mohammad Amjad Zadeh
Ms.C. in Soil Mechanics, Dezab Consulting Engineers Company, Ahwaz-Iran
Due to the fact that the clay core of an earth-dam has a very low permeability, the excess pore pressure results when external stresses are applied. Increase of the temporary or permanent pore pressure in the clay core during the time of the reservoir water surface raising results in a decrease of the effective stress in the dam body. This leads to a decrease in the shear strength and sometimes may results in a hydraulic fracture failure.
Increase of the pore pressure simultaneous with fracturing results due to decrease of the effective stress because of arching of the clay core with gravel shell may result in a failure in the dam body. Designers try to select an appropriate geometry for the clay core to avoid stress transfer and development of pore pressure and eventually controlling the failure due to this phenomena . In order to control the arching phenomena effect, plastic behavior of materials, geometry of abutment, implementation methods, optimum water content of materials, etc. should be considered.
Balaroud zoned earth-dam with a height of 77.5 meters in Iran is being constructed in the present time. The geometry of the geological formation, material plastic properties, geomechanical specifications of the foundation and abutments, lack of clay materials in the vicinity of the dam site and material behavior properties have produced constraints in the geometry selection of the clay core.
In this paper the hydraulic fracture and the time of its occurrence are analyzed and predicted using stress strain analysis. This analysis can predict the possible occurrence time of the phenomena during reservoir impoundment due to increase of pore pressure in the vicinity of the upstream slope of the clay core which is the worse location to produce and develop fractures. Based on these analyses a proper geometry for the dam may be considered.
Keywords: Pore pressure - hydraulic fracture - Balaroud dam - arching