Case History: Landfill Sampling in Eastern Washington State

Case HistoryD uring the spring of 2011, DTD accomplished what is likely to be the most extensive sam-pling program yet attempted using directional drilling. A closed landfill in eastern Washington, now a Superfund site, required additional characterization to support remediation or mitigation measures that were being designed and implemented. The landfill is capped with a geosynthetic membrane and soil, and contains several layers of stacked drums known to contain haz-ardous waste. To perform additional sampling with ver-tical drilling would require that the cap be breached, and would also encounter drums of waste, with the risk of puncturing them and dragging the waste downhole—complicating the collection of representative sam-ples. DTD was contacted nearly a year in advance of the project, and begin planning the extensive drilling endeavor. Several bores were initially planned to be drilled up to 300 feet from either end of the elongat-ed landfill footprint, collecting samples at 30 foot spacings. Surface casing was set to a distance of 70 feet, to prevent cross contamination by drilling fluid from the potentially contaminated zone into “clean” soil outside the landfill footprint. A traditional walkover navigation/steering system was selected to keep costs in check. Soil sampling with a directional rig is a prolonged opera-tion. For each sample, the bore must be advanced to the targeted interval, the bit must be tripped out, and then the sampler is tripped in, the sample collected, and then re-trieved. Finally, the bit is tripped in again to drill to the next interval. The use of a walkover navigation system with a battery powered downhole sonde eliminates the need to connect a wireline through the rods, which would add more time and complexity to an already lengthy pro-cess. By the conclusion of the project, over 91,000 feet of rods had been tripped into and out of the bore—the equiv-alent of installing more than 75 wells, 600 feet in length. Drilling was accomplished in several bores and at two depths—at approximately 15 and 25 feet beneath the stacked drums. The drilling itself was relatively easy, in a silt formation and a deeper, coarse sand formation. Due to the relative ease of advancement, the initial design to drill from both ends of the land-fill was revised, and single bores were advanced to over 600 feet in length from one end, with more than a dozen samples taken in each boring. As the bores increased in length, the amount of time spent tripping rods increased significantly — at 600 feet it could take nearly four hours to collect a sample. The drilling schedule was optimized with the consultant to assure that the collected samples would reach the laborato-ry without exceeding hold times. The primary challenge faced during the project was steering and location. DTD used the most advanced walkover system available from Digital Control, the F5, equipped with the DCI high powered downhole sonde. In the deep bore, DTD used a custom, $10,000 sonde, built to our specifications by DCI, to achieve higher signal strength to punch through the passive interference creat-ed by the stacks of steel drums. Even with this advanced equipment, steering and navigation required a high level of skill and deductive reasoning to deter-Directional Sampling Under Landfill Superfund Site – Eastern Washington mine the course to steer and to provide location information. The high level of interference resulted in final reference locations for the samples that described a “zone of probability,” generally 8-10 feet wide, with a high level of confidence for the actual X and Y sample location within that zone. Since the bores were drilled on pitch data to achieve the vertical profile, the actual elevation of the samples was always accurate within a couple of feet. This uncertainty should be contrasted with vertical drilling—in an unguided auger boring of 60-70 feet depth, the drill string can easily deviate more than 20 feet from the surface entry, with no way to tell where the deviation occurs unless the bore is surveyed; this is seldom done. In one boring, the drilling 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 redi-rect this particular bore twice, while collecting viable samples in all three forks of the boring. Another challenge surmounted by the DTD team was the actual process of collecting sam-ples. DTD initially used a sampler obtained from Charles Machine Works (Ditch Witch). This sampler, no longer in production, uses a sliding sampler body, which contains a piston to close off the open end. The sampler is pushed into the soil and retracted to expose the open bore, then pushed again to collect the sample. DTD had Ditch Witch custom-machine a spare sampler of the same design for this project. In practice, the piston sampler worked as designed in silty soils, but did not operate effectively in the coarse sands encountered. The DTD senior geologist and crew redesigned and modified the sampler in the field to enhance the collection of the deeper samples, with excellent results. The results of this field experimentation is being applied to the execution of an entirely new sampler design, which DTD expects to patent and manu-facture shortly. In the end, DTD and the client surmounted a variety of difficult challenges to complete this sampling project. From equipment selection to a liberal application of field savvy and experience, the project resulted in the collection of nearly three dozen samples, which will enable the consultant to plan additional controls at the site. Client Name and Address Pending, with approval of PRP group Client Technical Point of Contact Pending, with approval of PRP group Directed Technologies Drilling, Inc. 8700 State Highway 3 SW Port Orchard, WA 98637 800-239-5950 info@horizontaldrill.com © 2010 Directed Technologies Drilling Inc. All rights reserved Case History—Directional Sampling, Washington State