EFFECT OF STRATIFICATION ON SEEPAGE PRESSURES ON HYDRAULIC STRUCTURES

Abstract

The design of hydraulic structures such as Weirs and Barrages on the permeable foundations has been standardised in the publication No.12 of the C.B.I. and P based upon the applicability of potential theory to subsoil flow, In this method the subsoil is supposed to be homogeneous and of infinite depth with similar extensions on the upstream and downstream sides. Conditions in the f9eld, however, do not conform to either of the above idealised situations. The sub-soil is scarcely homogeneous, on the other hand, stratification in to layers of different permeability and occurrence of clay or rock strata with in a short distance is quite common. In order to make a rational design, it is desirable to know the effect of the above factors on the uplift pressures and exit gradients acting on the structure as deduced from the ideal potential flow theory. Thus the study of uplift pressures below hydraulic structures with sheetpile, and the resulting exit gradient laid on pervious foundations with different degrees of stratification, appears to have maximum practical utility. Hence the detailed investigations of the more common ri case of hydraulic structures with the downstream end sheetpile has been carried out in this study. As exact analytical method for the solution of the above case has yet to be developed, attempt has been made to solve the same by two-dimensional electrical analogy method, which is the most simple and reliable of all methods. The study comprises of the analysis of the results obtained by the experimental. studies on potential flow of water through two layered soils-of equal depth and having different permeability ratios under lying (i) a simple floor (ii) a depressed floor and (iii) a flat and depressed floorp with end sheet-piles of different depths. The results have demonstrated that the uplift pressure below the hydraulic structure increases with the increase in the degree of stratification.