The Effect of Quartz and Feldspar Minerals on Formation of Bentonite Filter Cake over Gneissic Bedrocks in Sri Lanka

Bentonite is used as a drilling agent for deep foundation engineering works. Bentonite slurry gives stability to pile walls during the drilling operation by creating a layer called ‘filter cake’ or ‘mud cake’. However, there is evidence that Bentonite filter cake reduces the shear strength capacity of soil-concrete and rock-concrete interfaces since it prevents direct contact of soil and concrete, or rock and concrete. Several studies have been carried out both empirically and experimentally in order to find the characteristics of filter cake formation over soil surfaces. According to the studies, formation of Bentonite filter cake has been studied on rocks such as sandstones and mudstones but not on metamorphic rocks. Thus, the current study investigated the formation of Bentonite filter cake on metamorphic rock samples using a specially designed pressure chamber. This paper presents the influence of major mineral constituents of a few selected Gneissic rock types on the formation of Bentonite filter cake. During the initial investigations of this study, it was found that a Bentonite filter cake of about 4mm thick was formed under a constant slurry pressure of 3 bar during a 12-hour exposure time. It was also found that there is no influence on Bentonite filter cake formation by major mineral constituents: Feldspar and Quartz.