Cohesive Soils

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Cohesive soils are ones that tend to stick together such as clay.  They do this because the individual particles are small.

Non-cohesive or granular soils aren’t as apt to stick together.  This is because the grains are larger.

When combined with water the surface tension of any soil increases.  Meaning it is more likely to stick together under the effects of that tension.  Because clay is made up of small particles the surface tension between each particle is greater than that of say sand.  Therefor clay will stick together much tighter than sand with an equal volume of water.  The properties of clay also allows for clay to absorb a larger volume of water and hold it than sand can.  On the other hand the weight and mass of sand overcomes the relatively small surface area and surface tension and does not stick together as easily.  It then becomes weighed down with the water it can store which causes it to collapse.

So how does that affect boring or drilling?

When boring a hole with a Prairie Dog Machine th bit is turning at a speed that is expected to break up most soils into the smallest particles to form a slurry that will wash out of the hole.

In particularly sticky clays, sometimes the soil doesn’t want to break up because the surface tension between the particles is strong enough to overcome the particle’s own weight and size combined with the disturbance of the bit actually stirring the clay.  Sometimes it will break up only to stick back together in a different form.

This can cause two easily overcome problems when boring.

1st The particles don’t break up enough to form a slurry with water to easily run out of the hole.  They can build up somewhere along the way out of the hole and start binding against the drill stem.

2nd The remaining clay comprising the area of the pilot hole will absorb water.  Just as a sponge will swell as it absorbs water so will the clay.  This will cause the bore hole to start swelling to a smaller diameter and may eventually close up on the drill stem.  The surface tension of the hole contracting against the pipe combined with the sticky nature of the material itself will begin to cause the drill stem to come under tension and turn slower and slower as the tension builds up.  As the bit turns slower its ability to break up any soil tension becomes significantly reduced and so the problem is compounded.  The longer the drill stem stays in the hole the more the soil will swell up around it.  I’ve seen drill stem left in a hole overnight that had to be left there because it was like it had been set in concrete.  Sometimes a crew will drill a pilot hole easily enough then break for lunch only to come back to find the drill stem is locked up tight.

These problems are not new to the drilling and boring industry.  Companies such as Baroid spend a lot of money learning how to reduce surface tension in clays and make them more conducive to boring and drilling

Fortunately the explanation is fairly simple but a little technical.


Detergents are hydrophobic, and thus spread out across the surface of the water. Hydrophobic molecules don’t participate in hydrogen bonding and therefore have no surface tension.

I’ve seen contractors successfully use something as simple as liquid or automatic dishwashing detergent to relieve the problem of clays not breaking up.  Why automatic dishwashing detergent?  It doesn’t foam!  It can be an effective strategy.

Water absorption and swelling can be a little more complicated but a relatively inexpensive solution still exists.  Clay inhibitors.  The science of clay inhibitors is more complicated than detergents but essentially what they do is block the absorption of water(hydration) and resultant swelling.  By preventing absorption of water they also effectively help break up the soil and prevent it from sticking back together.

The following video shows an example of the use of clay inhibitors.

This video may seem at first to be contrary to what we are trying to achieve which is turning the clay into a good slurring that will transport the cuttings from the bore hole.  What is not happening in the video is a mixing action of a bit.  The clay that is used in the demonstration is not particularly reactive either so it doesn’t stick back together.  What should be taken from the video is how the product prevents the clay from absorbing any water allowing it to break up into a slurry and be transported out of the bore hole.

In an ideal situation these additives would be mixed in a tank and introduced through the water system however they can still be effective by adding a shot of detergent or diluted solution to each drill stem as it is added during the boring process.

From time to time I hear the operator of a Prairie Dog machine tell me that the machine doesn’t have enough power.  At first they seem to think that there isn’t enough horsepower to turn the bit or backreamer but if you think about it the surface contact of a pilot bit or even a backreamer is relatively small.  Even on larger backreamers the contact area is not enough to overcome the horsepower of a machine rated to carry it regardless of soil conditions.

Invariably, a few questions will lead me to a couple simple determinations.  Either the operator is not using enough water, or advancing the pilot bit before turning the water on thus clogging the water holes in the bit.  The operator should always make sure that there is sufficient water running back out of the pilot hole before advancing the bit.

He should then pay attention to what is coming out of the hole.  Boring machines work by displacing the soil.  That soil has to go somewhere.  It doesn’t just magically disappear.  If soil is not coming out of the hole there is a problem and the operator needs to stop immediately to determine what it is.  Continuing on is only going to compound the problem.

Ideally, what you want to see coming out of the hole is a slurry consistent with that of pancake batter.  This means the soil is breaking up and you are introducing a sufficient amount of water.  The proper slurry will also help lubricate the hole for the drill stem to turn and make it easier to install your pipe once you are done.

An experienced operators will likely know the area he is drilling in and thus the soil types or can even look at the soil coming from the bore pit and determine if an additive is needed.  He may even take samples, hold them, add water, squeeze and manipulate the sample to determine reactivity to the water and get a feel for how it will behave during the boring process.  Having an experienced operator or giving him this information along with a little time to learn will go a long way in preventing headaches.

It is always important for the operator to keep in mind that even though he is using additives the boring machine is still doing the work.  Additives work in concert with the mechanical part of the operation and the operator should never try to advance the bore path faster than he can clean it.

For additional information contact Prairie Dog Boring Equipment.

Bill Anderson


Prairie Dog Boring Equipment, Inc.