Choose Bentonite or Synthetic Slurry as the Job Demands
Synthetic Polymers Still Relatively New to Drilled Shafts, Foundations
In comparison to mineral (bentonite) slurry, synthetic (polymer) slurry is still relatively new to many in the drilled shaft/foundation drilling industry, and is yet to be approved or accepted by a few states in the U.S. for DOT work. While these two types of slurry work in somewhat similar fashion and the same testing procedures (viscosity, density, pH and sand content) are applicable, there are some very distinct differences and advantages to utilizing synthetic slurry.
The same basic principles for stabilizing a bore hole apply in both mineral and synthetic slurries. Hydrostatic pressure from slurry in the shaft (a minimum of 6-feet hydrostatic head above the water table) is applied against a barrier/membrane created by the slurry to stabilize coarse unconsolidated soils. In a mineral slurry, microscopic bentonite platelets create viscosity and gel strength, and shingle off to form a filter cake against the walls of the borehole providing a barrier for which hydrostatic pressure can be applied. With synthetic slurry, very long molecular chains make water viscous while providing little to no gel strength, and create gel membranes in and near borehole wall pores. Segments of the long molecular chains absorb on solids surfaces and bridge across soil grains to form a chemical net. A bentonite platelet filter cake constantly builds up with time and can become a concern regarding the skin friction and the load bearing capacity of a shaft, while the gel membrane of synthetic slurry is quickly broken down when it comes in contact with cement’s high alkalinity and calcium content.
While gel strength is a necessity for rotary mud drilling to convey drill cuttings to the surface, the lack of gel strength in synthetic slurry is a huge advantage for the construction of drilled shafts. When using mineral slurry for the construction of drilled shafts, drill cuttings/soil particles remain in suspension and the slurry must be circulated through a mechanical solids removal system or be replaced by fresh clean slurry before displacing with concrete. Synthetic slurry designed specifically for drilled shafts will allow cuttings to quickly settle to the bottom of the shaft where they can be removed by the cutting tool (auger bit or bucket). As synthetic slurry is displaced in a shaft by cement, and the slurry can be pumped into a holding tank where it can be tested and adjusted for reuse.
Along with the advantage of being self-cleaning, synthetic slurry is also much easier to mix and can even be mixed in the shaft. Synthetic slurry is also much cleaner to work with and does not leave a residual muddy mess around the spoils pile like mineral slurry. Disposal is much easier because synthetic slurry can simply be broken down with an oxidizer such as calcium hypochlorite. Synthetic slurry is also a great clay inhibitor, preventing the swelling and sticking of reactive clays.
Although the initial purchase price for synthetic slurry products may be higher than mineral slurry products, this price differential is quickly offset when one factors in the cost of time delays waiting to get a mineral slurry to pass density and sand content specifications before pouring cement, mechanical mud cleaning equipment cost, and disposal cost. Another big savings to factor in when considering synthetic slurry is the cost of shipping product to the job site. Mineral slurry requires around 60 pounds of 90-yield bentonite per 100 gallons, while synthetic slurry can do the same job at 3.4 to 8 pounds per 1,000 gallons (depending on soil conditions). For example, a project requiring 50,000 gallons of slurry would require 30,000 pounds (600 50-pound bags) of 90-yield bentonite for mineral slurry, while a synthetic slurry would only require 350 pounds at 7 pounds per 1,000 gallons (7 50-pound bags).
There are countless types and blends of polymer slurry/drilling fluid products on the market today. Therefore, when selecting a synthetic slurry product, make sure it is designed specifically for drilled shaft applications and has a proven track record in the industry. Some polymer products used for viscosifiers and clay inhibitors in rotary mud drilling may not be capable of maintaining borehole stability in coarse unconsolidated soil conditions. When making the switch from mineral slurry to synthetic slurry, it is important to get technical support from the manufacturer to ensure that the products are used correctly. This will prevent costly mistakes and ensure that you can take full advantage of all the benefits that good synthetic slurry has to offer.