Challenges and Strategies in Treatment of Terrace Material Encountered in Place of Rock at Foundation of Spillway

Abstract

The Koldam HPP(4 × 200 MW) located on river Satluj in Bilaspur district of Himachal Pradesh, utilizes a drop of about 140 m by constructing a 167 m high rock and gravel fill Dam. The spillway of Koldam Project has 6 bays, each of 17.1 m width with crest at El 625 m, separated by 6 m wide piers. Based on the initial investigations carried out by various agencies during design, the arrangement of spillway was finalized on the left flank considering rock foundation. However, during excavation of spillway foundations in chute area, terrace material was encountered. This was not suitable for founding the Spillway Chute slab. Mid-course correction was necessitated to ensure the structure meets its intended purpose for the designed life. The present paper deliberates the challenges faced and strategies adopted in treatment of terrace material encountered in place of rock at foundation of Spillway. The alternatives were explored for treating above area as either to remove the terrace material and backfill with concrete or to do the grouting of the whole terrace area below spillway chute. NTPC decided to found the spillway chute on the grouted terrace material with design modification required for the terrace material portion. In spite of the various challenges encountered during execution of the Spillway, spillway chute was cast successfully on the treated area with additional measures like grouting of terrace material and structure has efficiently performed during operation.