The soil underfoot isn't just soil (hint: it comes in 12 types and six general taxonomic groups (based on tactile responses), usually scrambled together with rocks of various kinds. The problem is that, in a perfect world, the soil would present in discrete, horizontal layers, each layer a different soil type. In this world; however, drillers, designers and drilling engineers face a world of mixed soils and rock in each project. These variations in soil and rock affect choices in design path, bit and bottom hole assembly, machinery and mud selection.

Let's Get Dirty

The gatekeepers of soil science in North America include the US Department of Agriculture (USDA) and the Soil Sciences Society of America (SSSA). The USDA is a government organization that acts as a clearinghouse for soil information; the SSSA is more hands-on, getting "down and dirty" with their work on soil types and other information.

Soil and Drilling

The USDA and the SSSA recognize 15 types of soil. Formidable when hiding behind their Latin monikers -- but not when those names are translated to plain English - they are:

  • Frozen
  • Organic and wet
  • Sandy and acidic
  • Volcanic ash
  • Very weathered
  • Shrink and swell
  • Very dry
  • Weathered
  • Deep, fertile
  • Moderately weathered
  • Slightly developed
  • Newly Formed, or young

Soil is also described by six general and mostly tactile qualities: clay, sandy, silty, peaty, chalk and loamy. If that were all there is to it, each of the six is relatively easy to drill through. However, things (including drilling or augering) are never that simple.

Soil is often a loose amalgam of dirt and gravel. This loose mixture may best be served by a bit called a fly cutter, possibly with a high-pressure water or mud jet capabilities. In compacted alluvial soils, the high-pressure features may not be necessary, but the fly cutter must have a bi-directional reaming capability that allows it to bore its way back to the starting location, or a milled tooth bore opener bit is the optimum choice.

Rocks: Born in Fire or Water

Igneous Rock

Igneous rock is a less-consolidated metamorphic rock that might be a leftover from the last Ice Age, or it might have worked its way to the surface over millions of years. Created in the volcanic fire or from the high temperatures that come from high pressure, igneous rock includes rock such as basalt, granite, marble, and rhyolite.

Sedimentary Rock

Sedimentary rock's compressive strength is 7,000 psi or less. The two most familiar rocks within the group are sandstone and shale, it began as mud on the bottom of ancient seas. Because it's a soft rock, a hole opener with a milled tooth profile is often appropriate.

Consolidated Crystalline Rock

Consolidated crystalline rock structures include harder granites, basalt, and taconite, among others. They have the same origin as softer igneous rocks, but the greater pressures to which they were submitted over their formation result in compressive strengths that range between 7,000 and 18,000 psi.. Creating a bore through these very hard rock formations requires a tungsten or carbide conical tooth bit.

The Trenchless Game Plan

Geotechnical information based on bores, earth cores or holes dug into Mother Earth with a shovel is bound to miss details. These investigations may even miss a vertical wall of granite -- the final remains of a once-massive subterranean pluton -- in their coring and potholing. This means that the best trenchless game plan often relies on the most common sense idea. There's no need to overload yourself with every type of bit: start the job with the bit that will do the most damage to the hardest rock or soil you're likely to encounter.