WITH PARTICULAR REFERENCE TO SEEPAGE CONTROL IN EARTH DAMS AND LEVEES

Abstract

SEEPAGE AND ITS EFFECTS The impounding of water by a dam across a valley or the holding back of river flood waters by a levee creates a head differential between the upstream and the downstream face of the dam or the waterside and the land side of the levee. Water seeps thru the embankment and the underlying foundations under the action of this head differential. This seepage of water exerts a mechanical drag force on the soil particles which is due to the friction between the percolating water and the walls of the voids. This force is conventionally called the seepage force or pressure. 1.011 When this force exceed)the resistive forces of the soil grains a mass body of the soil located along the downstream or landside toe may be moved or else movement of grain may commence leading to a gradual washing out of the finer materials. This removal of soil grains together with roofing action creates flow channels along which there is decreased resistance to seepage and hence increased flow. This erosive action can gradually work backwards and form cavities in the body of the embankment or foundation. This condition, termed as piping, needs to occur only in one local channel to be a threat to the safety of the structure. L.012 Again in stratified foundations, underseepage may result in transmission of excessive uplift pressures which may cause uplifting of the top soil cover in regions of inadequate weight of the overlying material. Seepage thru such openings and pockets of pervious material under excessive gradient may act as nucleous for progressive formation of springs. If the discharge is sufficiently large and the soil conditions favour underground erosion, the spring may create a tunnel by eroding along the line of maximum gradient, thus undermining the foundations and leading to collapse of the structure. 1.02 SEEPAGE CONTROL Uncontrolled seepage, even in samll quantities, thru the embankment or the foundation is therefore dangerous for the stability of the structure and it is absolutely essential to provide measure for its control so as to check its adverse effects. The aims of seepage control measures are three fold : 1) To control and limit quantity of seepage 2) To provide for a controlled release of uplift pressures. 3) To limit the exit gradient to safe values. The following are the measures adopted for these purposes: A) Embankment i) Filters ii) Rock toe iii)Drainage blankets