Selection of Drilling Slurry for Pile Foundation

Drilling slurry selection is very important in a drilling project which is principally depends on ground condition. Different types of slurries may be required for a project to drill pile through diverse ground formation. Thus it is clear that different types of slurries may be required to drill pile on one contract because of variation in ground condition within a borehole or borehole to borehole.Some factors that dominate on making decision on selection of types of drilling slurry or slurries are listed below:
• Economy 

• Groundwater and ground contamination with disposal

• Air temperature

• Ground temperature 

• Type of formation through which drilling is conducting.

Behavior of drilling slurry is also affected by ground condition. The factors that are prominent:

• Alkalinity or acidity of ground water

• Grain size distribution of the soil

• Flow velocity of groundwater through ground formation

• Cohesion and cementation of soil

• Presence of clay structures or rock within ground formation

Considering these conditions, physical properties of drilling slurries can be modified or adjusted or suitable chemical additive can be added. As most of the drilling slurries cannot be disposed to ground or water without modification, they are reused frequently. These may reduce expense or difficulty of disposal to environment.Drilling slurry, occasionally, is required for another pile followed by the completion of previous pile. It was reported that drilling slurry was successfully reused for another project. But such reuse of slurries should be part of plan submitted to engineer and as approved by him.

Contract specification usually doesn’t prohibit drilling slurry to be reused provided that it must confirm requirements of physical properties as described in specifications. If any contractor proposed to reuse particular or all drilling slurries used in projects conducted under different contract, they should provide test results for physical properties of drilling slurry (described in contract specification) as desired by the engineer before used in drilled hole.

It should keep in mind that drilling slurries degrade with time which is generally evaluated in months. Any drilling slurries must be cleaned properly before it is reused. In case of synthetic slurries, cleaning is performed by allowing adequate time to settle out contaminants. For mineral slurries, cleaning involve application of desanding units and use of chemical additives.

California department of transportation permits three types of drilling slurries, these are:

• Water
• Mineral slurries
• Synthetic slurries


Water is selected as drilling slurry only when ground formation encountered while drilling is good in terms of enough strong not to be subjected to significant deformation during drilling.

Mineral drilling slurries:

They are processed from different clay formations. The most popular clay formations are:

• Bentonite
• Attapulgite

Mineral slurries are suitable for almost all types of ground formations. The best performance is found in sands (cohesionless soils) and open graded gavels. While using in drilling cohesive formation like clays, caution should be taken, as these clays may become a part of mineral slurry which may rapidly alter physical properties of mineral slurry. Moreover, these clay particles may reduce filtration which may prevent formation of filter cakes.


Synthetic slurry:

Polymer slurry can be used to stabilize bore during excavation which is considered alternative to mineral slurries. Unlike mineral slurry, bentonite, they clog the pores exist in soil resulting a barrier. Polymer having different molecules are joined together and have capability to penetrate in silty or sandy soils up to considered depth normal to the direction of drilling. They are biodegradable and so they do not need special measures for disposal. 

The advantages of polymer slurries are as follows:

• Hydration is rapid 

• Storage requirement is less
• Less problem in disposal
• No formation of filter cakes 
• Don’t react with cement particles 
• Simple logistic support is required 

Disadvantages are:

• Polymers are often difficult to mix

• May become wet and may take paste or creamy form
• Shearing action within polymer must be high enough to dispose polymers but should not be too high to break down polymers.


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