Q.2 Discuss the design criteria for earth dams.
Ans. The earth dam should be designed such that the failure of the dam or its foundation does not occur. The following criteria have to be satisfied for safe design of an earth dam –
(i) No Overtopping - (a) An adequate free board must be provided for wave action, frost action, wind set up and earthquake motions.
(b) Proper shrinkage allowance should be provided during construction.
(c) A suitable allowance in the height of dam should be made to account for settlement.
(ii) No Seepage Failure- (a) The seepage line or phreatic line should remain well within the downstream face so that no sloughing of the downstream slope takes place.
(b) Seepage through the body of the dam, foundations and abutments must be controlled by adopting suitable measures.
(c) The dam and its foundation should be safe against piping.
(iii) No Structural Failure – (a) The upstream and downstream slopes should be so designed that they are safe during and immediately after the construction.
(b) The upstream slope should be stable during sudden drawdown condition.
(c) The downstream slope should be safe during steady seepage under full reservoir condition.
(d) The whole dam should be earthquake resistant.
(e) The upstream and downstream slopes of the dam should be flat enough so as to provide adequate base width at the foundation level, such that the maximum shear stress induced remains well below the corresponding maximum shear strength of the soil, so as to offer a suitable factor of safety.
(iv) Proper Slope Protection - (a) Upstream slope should be protected against erosion by waves action. Rip-raps should be provided on the entire upstream slope and also on the downstream slope near the toe and upto slightly above the tail water so as to avoid erosion.
(b) Downstream slope and crest (Top) should be protected against erosion due to rain and wind. Turf should be provided on the downstream slope.
(v) Proper Drainage - The portion of the downstream of the impervious core should be properly drained by providing suitable horizontal drain filter, or toe drain. or chimney drain, etc.
(vi) Economic Section - As far as possible, the available material near the dam site should be used to reduce the cost.
All these criteria must be satisfied and in order to obtain a safe design and construction of an earth dam.