Prairie Dog tech tip – sand

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Previously I’ve discussed how to combat particularly difficult cohesive soil conditions where tight reactive clay doesn’t break apart into a good slurry or adding water to it can make the bore hole swell up and constrict the drill string.

The opposite of cohesive soils are granular ones.  Sand is the main culprit and what I will discuss in this tech tip.  Sand consists of large uneven particles.  They don’t stick together very well because they are individually heavy and have voids which allows the absorption of water and reduces the surface area for the weight to size ratio to allow them to stick together.  Think gravel but on a smaller scale.  When water does enter the voids, it weighs the mass down and in boring situations the hole will collapse, not from the weight of the sand alone but from the sand being weighed down by water.  When the hole collapses and you have a drill string strung out in the hole all that sand is laying on top of the drill string, weighing it down and making it hard for the drill string to rotate or move laterally.  Rods can get stuck and eventually must be pulled out of a collapsed hole rendering the bore hole useless.

If you’ve ever built a sand castle on the beach you know that a little bit of water helps the sand stick together.  What happens, though, when the tide comes in?  The extra water weighs down the sand and allows it to collapse.  If only there were a way to coat the outside of the sand castle to block it from absorbing more water than it needs to stay together.  There is.  It’s called bentonite clay.

Bentonite is a type of clay that has been used for years in vertical and horizontal drilling to prevent hole collapse.  Its properties allow it to seal off a hole to prevent the ground from absorbing more water thus preventing sand from collapsing from the weight of the water.  The shield that bentonite creates is known as a filter cake.  Watch this YouTube demonstration for more information.

As you can see in the demonstration the bentonite mixture is not excessively thick or viscous.  The bentonite platelets just need to be suspended enough in the water to coat the hole sand in order to block absorption.  A proper bentonite mixture can be easily pumped through a Prairie Dog boring machine.  You just need a small centrifugal water pump plumbed to a tank to hold the amount of water you’ll need.  (our customers typically use 250 – 300 gallon tanks but I’ve seen 55 gallon drums used as well).  Plumb the pump with a couple of valves so you can circulate the water and bentonite to mix it then divert the mixture to the machine.  Bentonite is a very inexpensive fix when sand is an issue and will save time and headaches when used properly.

In the vast majority of bores, perhaps 90% or more, done with a Prairie Dog machine no drilling fluids are necessary.  In certain cases, though, an additive must be used to keep the drill string moving or the bore hole open.  This will be the case with any wet bore machine.  Some of the many advantages to wet bore machines like the Prairie Dog is that they will work in a much wider range of soil conditions, they will be much more accurate and usually as fast or faster than air missiles, they don’t require as much depth as a mole or missile does to prevent surface upheaval because they are displacing the soil instead of compressing it and they allow for the use of drilling fluids.

Whether you are boring in difficult clay and needing an inhibitor or in sand and needing bentonite both are an inexpensive solution.  To find these products you can contact a local drilling mud supplier, visit Baroid IDP or contact us at Prairie Dog Boring Equipment 866-631-3786 so we can help you find a local supplier.