Concrete is often placed beneath a water surface to seal cofferdams and caissons, to weight objects such as precast tunnel sections, and to construct numerous types of subaqueous foundations. Methods of placement on such projects include lowering the concrete in burlap or tarpaulin sacks, using special buckets to transfer the mix to the underwater floor, laying aggregate and then grouting it, and pumping concrete directly into place. These techniques have proved successful in meeting certain job requirements, but the most common and expeditious way to place large volumes of concrete to considerable thickness under water is the tremie method. Tremie concrete refers to placement by gravity feed from a hopper through a vertical pipe extending for above the surface to the underwater floor. As concrete flows from the bottom of the pipe, more is added to the hopper so that the tremie pipe is continuously charged with fresh mix. The major aim in underwater concreting is to place the mix in its final position with as little disturbance as possible. A mix with sufficient slump to flow easily into place is needed. As concrete flows from the tremie pipe the end of the pipe is buried in the mass. Gradually the mix flows out toward the edges to fill the forms and as the concrete builds up, the pipe is raised sufficiently to keep its delivery end buried about 3 feet. The concrete around the end of the pipe seals it from the water and prevents aggregate segregation and washing away of cement. An important task with tremie concrete is to establish a seal before the initial pour. One of the more effective ways is to place a wooden disk at the bottom of the pipe. The disk is placed against the bottom and water pressure secures it while the pipe is lowered. The weight of the concrete introduced once the pipe is in place overcomes the water pressure and pushes the disk aside.