Treatment of bentonite fluid for excavation into Chalk

Abstract

This paper is concerned with the treatment of a bentonite support fluid during excavations extending into Chalk. Chalk can disintegrate during the excavation process and can, like any other soil that may disperse into the slurry, significantly alter the properties of the fluid if it is not appropriately managed. A case history in Woolwich, East London is presented to highlight the necessity for treatment of the bentonite slurry used to support grounds that release fines. It was found that, with use of the solids-separation equipment, the slurry could be maintained within specification over the course of the project. Desanders, desilters and a centrifuge were necessary for slurry management during the works. At the end of the works, the centrifuge was used to separate the unwanted residual slurry into a paste and a centrate of near-clear water. Chemical analyses conducted on the centrate confirmed that it was comfortably within specification for disposal to a foul sewer and physical tests on the dewatered bentonite sludge confirmed that it was sufficiently stiff that it could be disposed as a solid waste along with other soil arisings. The centrifuge treatment thus eliminated the high costs of transport and disposal of unwanted bentonite slurry.