EXIT GRADIENT BELOW HYDRAULIC STRUCTURES FOUNDED ON THREE LAYERED SOIL

Abstract

Seepage considerations play an important role in the design of hydraulic structures founded on porous media. The water seeping through the media exerts seepage forces at the bottom of the structure. The problem is to keep the seepage forces within safe limits, eo that the structure is not damaged. The damage can take place due to excessive uplift pressures and unsafe exit gradient at the toe of the structure. For a safe, design of hydraulic structures these should be within safe limits. Solutions are avail able for determining exi t gradient and uplift pressures below structures founded on homogeneous and isotropic soil with various boundary condi-tions. In actual field conditions the subsoil may be strati-fled. Exact solution is available for determining uplift pressures under a floor founded on two layers. Experimental. solution for determining exit gradient and uplift pressures below structures with floor and end cutoff founded on two laye ra is given by U. P.I. R. x.(12) and Al am S3. ng13). rxp.eri-mental study was conducted by Rishikesh{14} to determine uplift pressures below structures founded on three layers. Analytical and model results have been combined to. determine exit gradient at the end of structures founded on three layers and presented in this dissertation. -iv- Results indicate that exit gradient decrease with the increase of the depth of cutoff and increase in the floor lengths. It is also seen that exit gradient increases with the increase in the coefficient of perme ability of the second or third layer i.e• k /ki and k3/'ki• The lowest value of exit gradient is obtained when k2/k1 0 and maximum value obtained when k2/k1 = ©©• It is also seen that effect of floor length on exit gradient values is insignificant with very large values of k2/k~ •and K3/K1.