Established practice for boring large diameter piles in poor ground conditions is to fill the shaft with bentonite mud. The bentonite forms an impermeable ‘filter cake’ around the shaft annulus and base, which prevents loss of fluid into the ground. This allows the relatively dense bentonite mud level in the pile to be maintained at a higher level than the groundwater level, which stabilises the shaft.
The downside of bentonite is that the filter cake may create a soft layer at the critical pile-ground interface. As a result, bentonite-bored piles may have to be longer – and are therefore more expensive – than piles that can be bored in the dry or using CFA techniques.
Additionally, bentonite processing and recycling equipment is not only costly, it is also extremely bulky, an important issue on urban sites. Sites with bentonite bored piles also tend to become messy, because the bentonite softens the spoil to a slurry.
More recently a fourth factor has come into play – disposal costs. Bentonite mud is now classed as a hazardous waste and only a handful of facilities in the UK will take it. The cost of bentonite disposal to special tips is high and rising.
It is not surprising then, that the foundation industry is looking for other options for boring large piles in poor ground. Top of the list are polymer-based drilling fluids – technology borrowed from the US oil industry. Several UK contractors have dabbled in these high viscosity polymers in the past 10 years, but it is only in the last three years that the piling industry has really started to look closely at their application.