Formation of Bentonite Filter Cake over Crystalline Metamorphic Rocks by Means of a Pressure Chamber: A Laboratory Study

Bentonite is used as the drilling fluid for most of the cast-in-situ bored (CIB) pile constructions. It has been found that bentonite slurry significantly reduces the shaft resistance capacity because of the formation of filter cake in piles driven into sedimentary bed rocks. This effect is incorporated into pile designs by increasing safety factors. Even though crystalline metamorphic rocks do not support the formation of bentonite filter cake to the same extent as sedimentary rocks, conservative pile designs are being used mainly because of a lack of reliable experimental data. Some studies carried out in Sri Lanka on piles driven into metamorphic rocks indicate that the bentonite filter cake formations have a relatively insignificant effect on the shaft resistance capacity. This paper presents the results of experiments carried out on the formation of bentonite filter cake over several types of metamorphic rocks using a specifically built pressure chamber. The results indicate that bentonite filter cake formed over unfractured metamorphic rock is much less than those formed on sedimentary rocks and that the weathering grade of the rock has an influence on the thickness of the filter cake. These findings could lead to design practices that could avoid unnecessary conservatism in pile design.

Automated summary

Bentonite is commonly used as the drilling fluid for cast-in-situ bored pile constructions. It has been observed that the formation of bentonite filter cake significantly reduces the shaft resistance capacity of piles in sedimentary bed rocks. However, its impact on crystalline metamorphic rocks is relatively insignificant. The lack of reliable experimental data has resulted in conservative pile designs being used, but recent studies conducted in Sri Lanka suggest that unnecessary conservatism can be avoided.