Tech tip on cohesive soils

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Prairie Dog Tech tip

Soil Cohesion.

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.  This is why even sand will stay somewhat in a ball when wet but not when it is dry.  Because clay is made up of small particles the surface tension between each particle is greater than that of say sand.  Consequently 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.

So how does that affect boring or drilling?

When boring a pilot hole the pilot 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 it’s 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.  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 and resultant swelling.  By preventing absorption of water they also effectively help break up the soil and prevent it from sticking back together.   Again, this happens on a molecular level.  Aquatreat Clay Fix works particularly well in most cases for our customers.

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.

If during the boring process there is clean water and a few small clay chips running out the chances are pretty good that larger pieces of clay are not breaking up sufficiently and will eventually bind against the drill rod slowing it down.  This is when you need either a clay inhibitor or a detergent.  You don’t want to be trying to determine this once you have several drill stems in the hole so it is important to pay attention early on in the pilot hole drilling process to see if you need any additives.

Experienced operators will likely know the area you are 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.

The following video demonstrates the use of clay inhibitors.

For additional information contact Prairie Dog Boring Equipment.

Bill Anderson


Prairie Dog Boring Equipment, Inc.