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followed by 10 gallon(3,8 L)of water through a tremie tube until a 2-foot(0,6 m)bentonite layer is created Use the following procedure 1 i Pour 0 8 liters of granular bentomte into the annulus This volume of bentonite will fill approxi- mately 6 inches(15 cm)of annular space ii Check for bridging inside the annulus Measure the riser depth to the bottom of the annulus The depth should equal the riser length minus the 2-foot sand pack and the added bentomte if the measured depth is significantly less than expected, the bentonite has more than likely bridged ' somewhere inside the rod string A procedure similar to that identified for bridged sand(Section 5 4, Steps 5 and 6)may be used to dislodge the granular bentonite in Hydrate the bentonite by adding I gallon(3,8 L)of water to the annulus through a tremie tube Do not pour water directly into the annulus A tremie tube will help prevent bridging by keeping the rod string dry iv Repeat this procedure an additional three times or until the 2-foot(0,6 m)thick bentonite layer is completed ' 2 Unstable formation A grout machine is required to install the bentonite seal if the formation col- lapsed when the rods were retracted or the sand bridged when installing the grout barrier The grout machine can pump a high-solids bentonite slurry under sufficient pressure to displace collapsing soil Void spaces often develop when poured (gravity installed) granular bentonite is used under these conditions,resulting in an inadequate annular seal Wet rods will often lead to bridging problems as well Use the following procedure to install a bentonite seal with a grout pump a) Mix 1 5 gallons (5,7 L)of high-solids bentonite(20 to 25 percent by dry weight)and place in the hopper of the grout machine ' b) Insert flexible tubing to the bottom of the annulus between the probe rods and well riser Leaving at least 25 feet(8 m)extending from the top of the rod string,connect the tubing to the grout machine This extra length will give needed slack for rod extraction (completed later in the procedure) tNOTE The side-port tremie method is recommended to prevent intrusion of grout into the sand barrier To accomplish side-port discharge of grout,cut a notch approximately one-inch(25,4 mm)up ' from the leading end of the tubing and then seal the leading end with a threaded plug of suitable size c) Reposition the probe unit and attach the 3 25-inch Rod Grip Puller d) Activate the pump and fill the tremie tube with bentonite Begin slowly pulling the rod string approximately 3 feet(1,0 m)while operating the pump (Fig 5 11) This will place bentonite in the void left by the retracted rods before it is filled by the collapsing formation Continue to watch that the PVC riser does not come up with the rod string NOTE. When removing the retracted probe rod,slide the rod over the tremie tube and place it on the ' ground next to the grout machine This eliminates cutting and reattaching the tubing for each rod removed from the string Take care not to"kink"the tremie tube during this process as it will create a weak spot which may cause the tubing to burst when pressure is applied e) Measure the annulus depth to ensure that at least 2 feet(0,6 m)of bentonite was delivered Pump additional bentonite slurry if needed Standard Operating Procedure Page 18 1 0-in x 2 5 in OD Prepack Screen Monitoring Well