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Discrete Soil Sampling Using HDD

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DTD Crews recently completed a unique site characterization project. Field work involved the use of horizontal directional drilling (HDD) for discrete soil sampling in the Western US. Project objectives included characterizing the extent of petroleum contamination within the active right of way of the regional railroad. Normally, this wouldn’t sound like anything particularly complicated; however, there was just one problem. Impacted areas might extend underneath 35-60ft diameter grain silos, which were full of grain.

The crew performed discrete soil sampling under three different silos. They collected three samples per silo, which makes for a total of nine samples collected. Targets included 5ft in from the silos’ edges (2ea) and from directly under the center (1ea). All samples targeted a total depth of ~10ft BGS.

Discrete Soil Sampling with HDD Involves Multiple Trips

piston tip discrete soil sampling tooling

A set screw provides just enough frictional resistance to prevent sloughing when discrete soil sampling using HDD

Discrete soil sampling with HDD can be a cumbersome process. After drilling out the pilot bore to the target interval, the driller must trip out the entire bit assembly. Then he runs the sample barrel back down to the distal end of the boring. Using the hydraulic force of the rig, the driller pushes the split spoon sampler into virgin soil to collect the soil core, and then pulls the whole assembly back to the rig to retrieve the sample. Next the bit assembly is run back down the hole, and drilling recommences until reaching the next target, whereupon the process is repeated.

In HDD, gravity is working against us. Pushing the sampler along the bottom of the bore means slough can enter an open sample shoe. So some mechanism must close off the shoe while advancing the sample barrel in the open hole. DTD utilizes in-house piston sampling tooling to address this issue. Frictional resistance from a set screw holds the piston tip in place while pushing through the open bore, but releases

Discrete soil sampling in sub-freezing temperatures

Precautions were necessary to prevent drilling fluids from freezing over

when the barrel is pushed into virgin soil for sample collection.

Cold Weather Adds Challenging Site Conditions to Project’s Complexity

Adding to these technical challenges, crews mobilized in February and found themselves dealing with sub-freezing temperatures. The guys are tough and Carhartts are warm, but it took some ingenuity to prevent the drilling mud from freezing in the tank or the pit overnight. Mitigation efforts included storing the mud system in a box truck with heaters and placing a work tent over the rig and pit area.

The three bores ranged in total drilled lengths between 85-115ft, and crews tremie grouted upon completion using Portland cement. The field duration lasted 6 days on site including setup activities and site-specific safety training with the railroad.

Although HDD sampling can be cumbersome, sometimes surface obstructions completely preclude any kind of vertical or even high-angled drilling equipment from accessing the areas needed to perform discrete soil sampling. In these cases, HDD presents a welcome solution.

Contact DTD to learn more about the use of HDD for soil sampling and other environmental applications, including remediation and monitoring well installation.

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