Grouted Cofferdam for an Intake Structure in Mixed Rock and Gravel Environment

The Ilijan Power Plant is located in the island of Luzon in Philippines. Washington Group is the Engineering, Procure, and Construct (EPC) Contractor with Daelim as the Civil Sub-contractor. The cofferdam for the intake structure had dimensions of 36m x 30m and extended 9.0m below the ground water level. The subsurface conditions at the site are very complex and highly variable. Three alternating layers of dense cemented conglomerate and highly permeable open structured gravel/cobbles were encountered at the intake structure location. An order of magnitude permeability of 1 x 10^–1 cm/sec was estimated by simple pump tests from a large test pit at the site. The intake structure is about fifty meters (50m) from the Pacific Ocean thus excluding de-watering as a solution for construction. The paper describes details of subsurface conditions, the intake structure and various alternates considered for construction of a cofferdam. Schedule, reliability of the proposed solution and cost were the driving factors in the selection process with the first two being the higher priority. The selected solution had redundancy and recovery schemes planned to address unanticipated subsurface conditions and to assure timely completion of the intake structure as planned. The selected solution utilized steel sheet piles, jet grouted columns, soil anchors, and cement- sodium silicate grout mixture as a bottom seal plug placed at a significant distance below the bottom of excavation. On completion of these activities, the intake structure was built on schedule in a relatively dry condition with only two small sump pumps to handle minor seepage.