Contents preface (VII) introduction 1—37

(iii) Sudden Drawdown Condition
This condition corresponds to lowering of reservoir level at a rate much faster than the rate of subsequent dissipation of pore pressure. The condition results in excess pore pressures and unbalanced seepage forces and may become worse when the materials of the upstream portion of the dam are not freely draining. If the coefficient of permeability of the shell material is less than 10^–4 cm/s, full pore pressure should be considered (12). If the coefficient of permeability of the shell material is greater than 10^–2 cm/s, pore pressure may not be considered in the analysis. For other values of the coefficient of permeability of the shell material, pore pressure values may be interpolated (12). These recommendations are based on a drawdown rate of 3 m/month.

522 IRRIGATION AND WATER RESOURCES ENGINEERING
The drawdown pore pressure in clay core (Fig. 15.25) can be determined using Bishop’s formula (12):

U = ρg [hc + hr (1 – n) – h]

(15.32)

Here, U is the drawdown pore pressure at any point within the core, ρ the mass density of water, g the acceleration due to gravity, h_c the height of core material at the point under consideration, h_r the height of the shell material at that point, h the drop in the head under steady seepage condition at the point, and n is the specific porosity of the shell material, i.e., the volume of water draining out from the shell per unit volume.
The resisting and driving forces for impermeable material are calculated using the submerged and saturated weight, respectively, if the pore pressures are not otherwise included in the stability analysis (13).

		Centre of assumed
			failure surface
			Phreatic
Water level			line
	before drawdown
		hr	h
Lowered water	Shell	hc	Core
level
Failure surface		Equipotential line