Horizontal Environmental Drilling – Not Just for Well Installation Anymore or You Want a Soil Sample from Where?
Horizontal directional drilling methods have been used for monitoring and remediation well installations since the late 1980’s. The technology has been utilized to construct hundreds of wells under obstructions and to enhance remediation processes by placing the entire screen length in the contaminant plume. But did you know that soil samples can be collected using horizontal directional drilling methods?
In the past, when a consultant needed to obtain a soil sample under an obstruction he had two options; angled drilling from
outside the footprint of the impediment or vertical drilling in/through the obstruction. However both of those choices have significant challenges. If the sample point is shallow, say 6′ deep, located 40′ from the edge of an occupied apartment building, angle drilling with an auger rig will not get you to the sample point. The other option, drilling through the obstruction, (living room floor of the apartment) is also a non-starter. So you’re thinking who would want a sample under an occupied residence? Keep reading…
Houses aside, let’s think about a couple of other soil sampling challenges. How about getting a soil sample under a landfill? Do you want to drill through the liner? Most likely not. What about sampling under a river, stream or pier along a waterway?
All of the above soil sampling issues have been addressed by using horizontal directional drilling methods to access a sample point inaccessible to traditional vertical or angle drilling operations.
But first things first. What’s the process to get a sample in a horizontal hole? Isn’t it impossible? Let’s remember a couple of earlier newsletters when we discussed bit guidance and locating. The tools we use to determine the location of the bit in the subsurface are “locked” inside of the drill pipe. We can’t simply steer/drill to a location, remove the locating tools and run a sampler inside of the drill pipe similar to sampling inside of hollow stem augers. So we have to drill/steer to our sample point, remove the entire drill string and then run the sampler on rods or drill pipe through the fluid-filled borehole to the sample point. That process creates two complications:
How do we keep the sampler from “scooping up” formation from the bottom of the borehole as we push the tool into the open bore? DTD has modified a piston sampler such that a spring loaded “bullet nose” end cap is held in place at the end of the sampler. This prevents formation on the bottom of the borehole from entering the sample tube as it is pushed into the hole. When the tool reaches the end of the borehole, the force of the formation causes the end cap to retract and the sample then enters the tube.
How do we know if the samples is taken from the correct location? What if the sampling tool leaves the borehole? Our experience, in most cases, has been that the tooling stays in the borehole through the curve section out to the end of the hole. We are able to determine this by the force required to push the sampler to the end of the hole. However, if the exact location of the sample is required or we are drilling in very soft formations, we can place the sampling tool directly in front of the locating/steering tool and track the tool’s precise location throughout the insertion and sampling process.
Now for the reality check – have we actually accomplished soil sampling using this method?
Case study 1 – Superfund site in Western United States. The objective of the project was to obtain soil samples immediately below occupied housing units in a residential area without impacting the residents. Subsurface conditions at the site consisted of fill and dredge spoils. Most of the samples were obtained about 10′ from the building edge at a depth below ground surface of 3′ – 4′. The total bore lengths averaged about 50′. Due to the “soft” and unconsolidated nature of the formation, a bentonite based drilling fluid was used to keep the borehole open during the sampling episodes. This was a high profile project which included multiple pre-construction meetings to determine and finalize the project sampling and waste stream protocol. A total of 12 samples were successfully
obtained with no impact to the residences above the bore path/sample location.
HDD Soil Sampling
Case study 2 – Superfund site in Washington. The intent of the project was to obtain soil samples under a capped and closed landfill. Several horizontal bores were initially planned to be drilled up to 300′ from either end of the elongated landfill footprint, collecting multiple samples from each bore at a 30′ spacing. By the conclusion of the project, over 91,000′ of rods had been tripped into and out of the bore-the equivalent of installing more than 75 wells, 600′ in length. But as things go in drilling, one borehole encountered what appeared to be landfill debris at an unexpected depth. Working with the client to revise the profile for this bore, the DTD team was able to pull back several rods and redirect the bore to a new elevation-as a result, samples were taken from two depths
at the same X-Y location, in a single directional bore. DTD was able to redirect this particular bore twice, while collecting viable samples in all three forks of the boring.
Bore Profile & Sample Locations
So there you have it. Horizontal directional drilling methods can be utilized to obtain soil samples under obstructions. Is this something that we do on every site or you as a consultant need on every project? Absolutely not. However, the next time you are stymied by surface obstacles, you now know about other options. Stay tuned, we will have more details on soil sampling projects in the future.