If you haven’t encountered air rotary casing hammer (ARCH) drilling before, you’re most likely not alone. The ARCH method hasn’t gotten as much attention as some of its flashier friends like sonic, but for drilling in alluvial formations it’s a solid option every environmental consultant should know.
As implied by the name, the ARCH method combines direct air rotary drilling with simultaneous operation of a casing hammer. With true ARCH, an oversized casing is driven into an undersized bore using air as a circulation medium.
ARCH is best suited for alluvial formations. It really is not built to drill through hard rock, but in some cases can be used initially to cut through overburden and then be seated on bedrock. This allows drillers to use another method for the bedrock, such as direct air rotary with a downhole hammer, or perhaps even switch to mud rotary.
There are many applications for ARCH, such as environmental monitoring wells, extraction and injection wells, soil vapor gas wells, and even mining. It is especially suited to projects where one might want to separate aquifers and run telescoping strings of drive casing.
ARCH offers several major strengths. First, because it is an air rotary drilling method, there is no secondary waste stream generated like one would see with mud rotary or other methods. You typically only excavate the material for the diameter of the bore you’re drilling.
Second, ARCH allows you to set a surface casing without dedicating permanent materials like steel and grout. This saves time on installation, money spent on materials, and allows for easier mobilization.
Third, drilling with air in alluvial provides an almost instantaneous cuttings recovery you can see in the cyclone or containment vessel. This makes it easy to identify which formation you’re in.
Fourth, like hollow stem auger or sonic drilling, the hole is cased so the well can be constructed in a controlled atmosphere. The casing is displaced with annular material as it's extracted from the bore, so the well is never actually exposed to the formation. This prevents borehole degradation that can happen with open hole drilling methods.
Fifth, it’s a very fast method. Depending on the formation, it’s not uncommon to drill 300 feet in four to six hours. That’s typically faster than sonic drilling.
As with any air rotary method, the biggest drawback may be aquifer-related drilling. In scenarios in which you get into a prolific aquifer, the air circulation generates a lot of water. Depending on the project, that may be an untenable situation and you would need to switch to a different drilling method.
In shallower depths, hollow stem auger would be an option. The augers convey the cuttings to the surface, instead of relying on a circulation medium. For deeper drilling, sonic and potentially direct air or dual tube reverse could be options, depending on how the formation stands up. A lot of times, a formation will not stand up to the velocities of air or the ability of water to erode the borehole.
The best way to determine if ARCH is right for your project is to discuss it with a trusted and experienced drilling partner that has no allegiance to any particular drilling method. For example, my team recently bid a project, and I brought up the option of ARCH drilling to understand if it would perhaps be a good fit. However, it turned out they needed more detail about the lithology to fill data gaps for the project, which sonic could do more cost effectively and expeditiously.
In another example, a different client awarded two phases of a project to two different drilling contractors. We received the contract for installing 4” monitoring wells with slotted casings, and another company received the contract for 8” extraction wells and 6” injection wells with stainless steel wire wrap screens. They drilled the wells using mud rotary, which caused damage to the borehole walls and aquifer. Meanwhile, the wells we drilled with ARCH provided productivity that was magnitudes higher than the other wells could produce. ARCH would’ve been a better option for both types of well installation.
ARCH is not the perfect solution for every drilling project, but in alluvial formations it is definitely one worth considering. If you’re interested in learning more about ARCH, email me at [email protected].