Soil Identification is the most important part of the bore. Before you design a drilling fluid mix and purchase your bit or reamer, you must have a good understanding of the ground conditions. The exact soil type or the point where it changes over may never be spot on, but we must be close if we want a successful bore.
At the top of the CETCO Successful Borehole diagram are “planning” and “soil identification.” These two phases are the two most important parts of a successful borehole. Without planning, you will not have the correct materials onsite. I would go further and put soil identification at the very top because ground conditions can affect every part of a plan.
Imagine setting up for a sand hole. You have bentonite, PAC polymer, a suspension aid and a new fluted reamer. Two and a half rods into the pilot, you realize that the ground conditions changed. You are now in reactive clay. Your sand reamer is going to be like a balled-up cork swabber if you don’t adjust before pulling product.
Proper soil identification forms the bedrock for the rest of the process. Drilling fluids, tooling and the volume of drilling fluid required all depend on this step — a step that is not always easy.
“We drilled a block away and hit clay.” You do not have to be an experienced horizontal or vertical driller to know you never want to hear that statement. In some areas, moving 100 feet can make for a completely different hole. Top conditions might be the same due to fill material or topsoil additions, but a rod or two in and you hit the real native soil.
Changing conditions can pose a real challenge. A bore that starts out in unconsolidated sand and gravel is an easy setup, until it changes to clay 40 feet in and back to gravel at 70 feet. The driller needs to be aware of the changing conditions and the locator should be making notes in the tally book. This information will make the pull back that much easier as you will know what is coming. Having two tanks at the ready, consolidated and unconsolidated mixes, will make mixed condition drilling more controllable.
Check soil conditions at a few different places. At the entry and exit pits are self-explanatory, but make sure you look at the soil at depth and not the fill. The side of a road, driveway or another area prone to added fill can lead to false identifications.
Daylighting, potholing, or spotlighting utilities on the bore path can be another great place to examine the soils. While this practice creates a manmade frack point, it is necessary as we cross utilities and other buried components. If you’re opening them up, check your soil conditions and see if the conditions changed or stayed the same. You can examine the exact soil in the bore path.
Beware of hidden clays. While drilling in the very sandy soils of Martha’s Vineyard, we encountered sand that had elements of reactive clay between the grains. While dry sand appeared beach like, smearing it across the wet palm of your hand allowed the clay to present itself. Doing a swell test in our fluid, we were able to replicate the issues downhole. We used a cup of our current fluid to test solutions prior to mixing in the tank. With a small addition to our mix, we got off and running without a hitch.
Crews identified the above scenario in the pilot hole. By using the same fluid on the way out, you get the chance to evaluate your soil condition identification. This will allow you to make changes, if needed, prior to pulling product back. Let the drill spoils flowing out serve as a final check on how well your mix is doing. Let the exit pit be your success gauge.