A Case on Innovative Method of Ash-Dyke Raising Beyond Ultimate Height

Coal ash in one of the thermal plant of NTPC is discharged in wet-form into ash pond created by constructing peripheral ash-dyke embankments. The ash-dyke has been originally conceived with maximum four raisings by upstream method. Further raising beyond 4th raising was not technically feasible by conventional upstream method of raising due to inadequate factor of safety in stability analysis. Also raising by downstream method was not feasible due to presence of water bodies adjacent to toe of starter-dyke. Due to above technical/practical constraints in conventional methods of raising, the alternative was necessitated to ensure the dyke raising for sustaining power generation. With various alternatives explored, it is observed that raising with offset method is technically feasible in instant case. Conventionally, the new raising-dyke is generally constructed abutting the existing dykes, however with evolved offset method of raising, the open offset space between two raisings may be severely affected by sub-surface seepage from proposed new raising. Therefore, the toe-drain of the proposed new raising is to be adequately designed to handle seepage water and continuously divert to toe of existing downstream dyke with cross-drainage arrangements. In NTPC, the 5th raising with above offset (15 m offset) method has been successfully designed, constructed and satisfactorily operated for ash disposal. Through this explored offset method of raising beyond ultimate height, the capacity of existing dyke is significantly enhanced with sustained power generation. This paper presents the details of constraints and adopted new method of design and construction by offset method of dyke raising.