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GEOPROBE OPERATION . A • GEO 4.12 March 1998 shoe end. Insert the piston tip and piston rod into the sample tube by feeding the piston rod through the hole in the drive head and attach the cutting shoe to the bottom of the sample tube. Ensure that the piston tip is allowed to slide downward until it is seated in the cutting shoe. Tight- en the piston stop pin to complete the assembly. Note that the piston stop pin has a left-handed thread. The piston stop pin must be tight, or vibra- tions may cause the soil sampler to open at the wrong depth. • Attach the 1-foot-long adapter rod, and use this assembly as the lead rod. • Drive rods in a normal manner to the depth from where the sample is to be collected. • When the specified sample depth is reached, raise the hammer assembly to the top of the derrick and remove the drive cap. • Three-foot-long, threaded, 0.25-inch-diameter extension rods are used to loosen the piston stop pin. Insert the extension rods into the open drive rods, and attach each rod with a 0.25-inch-ID coupling nut. Connect as many rods as necessary to reach the piston stop pin. Attach an extension rod handle to the top of the last extension rod. Turn the extension rod handle in a clockwise direction two to three revolutions. Attempt to lift the threaded rods; the rods cannot be lifted if they have started into the piston stop pin. Continue turning the extension rod handle clockwise until the threaded rods have been completely seated into the piston stop pin; the piston stop pin is now ready to be removed. Remove the threaded extension rods one at a time until all have been removed from the drive rods. The last threaded extension rod should have the piston stop pin attached. • Reattach the drive cap and lower the hammer assembly. • Advance the drive rods 2 feet. • Extract all drive rods and remove the soil sampler. • Repeat these steps as necessary to collect additional subsurface soil samples. 12