Portland Cement or Bentonite?
Portland cement is more expensive and labor intensive to install, however, we feel it provides a more reliable casing seal. In all cases, our choice is to use cement grout which is Portland cement mix with water. The mix gives the grout the consistency of a thin slurry which allows it to find its way to the bottom of the casing through the limited opening around the casing. It is poured into the well in most cases and its consistency allows it to flow freely past any obstructions it sometimes falls into the hole while the casing is being installed. It can find its way past most obstructions so that it ends up around the bottom of the casing where it seals the surface casing as it is intended. It makes for a solid permanent seal for the surface casing to insure for years to come that no surface water contamination is introduced into the potable water source below the surface casing.
Bentonite is another form of grout used by some drillers in Missouri. It comes in many forms 3/6 inch chips, 1/4 inch pellets with a slow dissolving coding, and a powder form to be mixed with water which makes a very thick slurry almost solid slurry. In most cases, the 3/8 inch chips are used by pouring the chips out of the bag around the casing in dry form. Bentonite is a clay that is mined from various areas the United States. Its characteristics are that it rapidly absorbs moisture causing it to swell and become sticky. These characteristics make it a poor choice to use when grouting in the limited space around the casing.
Although the casing is only 6 5/8 inches in diameter the couplings located every 20 feet are a little more than 7 inches in diameter. This leaves barely an inch of space around the casing for the chips to fall through and perfect conditions. If the dirt, rocks, and clay swell from hydration or are loosened by the washing effect that happens during the drilling process as happens in most cases when the whole size around the casing can be reduced greatly leaving only a small space for the chips to work there wait past.
Compounding this problem is the fact that a couple of areas have a large squared lip that also makes it difficult for the chips to find their way past. As the chips fall into the hole, they come in contact with the dirt, rocks, and clay that have been hydrated because of the drilling process. This contact with the borehole surface causes the dry bentonite to become hydrated thus making it swell and become sticky. As the bentonite “tumbles” into the hole it bounces back and forth between the outer diameter of the borehole and casing.
The tumbling effect causes the velocity of the following done today to slow. If the diameter decreases enough and the bentonite swells and becomes sticky enough, it has the tendency to become trapped before it reaches the bottom of the borehole. We believe this happens in most cases and if this does happen, then the bentonite is trapped well above the area it needs to be and the surface casing is unable to be sealed allowing surface water and contamination to enter the potable water supply of the well.
The moral of the story here is that easier and cheaper is not always better. For this reason, our choice is to use cement grout instead of bentonite.