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Experiences gained from expansion of storage capacity of a lined pond at Hindustan Zinc Ltd

Construction of embankments on soft sludge is a challenging task owing to the inferior strength of the foundation material and excessive total as well as differential settlements. In general, the construction of embankment on such soft deposits as part of an engineered landfill project becomes
critical in terms of assuring the leak-proof lining system, avoiding the risk of failure and associated environmental disasters. The stability of the structure, as well as the integrity of lining system design and engineering needs, is to be critically analyzed prior to the implementing the project. The present study illustrates the design details and insights of expansion of a lined slurry pond dyke height from existing average height of 10 m to 12 m. The perimeter of the dyke is approximately 1600 running meters. Out of
this, space for outward (d/s) expansion of the dyke height was available for 1200 m, the rest of the dyke portion where its toe is situated at property boundary and hence there is no possibility of increasing the dyke height in outward fashion. This scenario imposed a unique challenge in terms of the design and planning for the construction activities including material transport.
In view of the above constraints, it is proposed to expand the 400 m stretch of the dyke in inward (u/s) fashion. This demands a need for enhancing the strength of the underlying sludge so as to withstand the loads from the proposed expansion. For this purpose, special construction techniques were deployed to enhance the strength of the underlying sludge. Prior to the expansion of the dyke in inward fashion, the sludge subgrade was strengthened by using slag which in turn is reinforced with a geogrid layer. On top of the stabilized slag an aggregate layer reinforced with a geocell was placed to form a firm foundation for expansion of the dyke height. This enabled the construction of dyke of 2 m above the sludge slurry. To control the lateral spreading of the deposited sludge upon loading, a vinyl sheet pile of height 4.5 m was installed. The sheet pile was installed parallel to the proposed dyke section at 1 m from its toe line. The inner dyke was lined with GCL and HDPE geomembrane which were connected with the respective liners at the intersections of the outer dyke and thus ensuring the integrity of the lining system. The body of the dyke was also further strengthened by introducing geogrid reinforcement layers to ensure the long-term performance of the dyke.