Instead of bentonite mortar used a lot of filling materials, the authors considered the use of anti-washout underwater concrete satisfying the quality requirements for filling 50-cm thick confined spaces in water between crushed stone foundations and the bottoms of immersed tunnel elements. The concrete is required to have high fluidity to permit filling of thin and extensive spaces, adequate segregation resistance and excellent pumpability over a long distance. The authors investigated the performance of anti-washout underwater concretes by adjusting the mixture proportions with the aim of reducing the dosage of viscosity agent. The placeability and filling capability of the selected concrete was then confirmed by pumping tests and mock-up tests. The concrete was proven to have stable properties by the quality tests in actual production, retaining its placeability throughout the transportation and placing processes.
