Lost Circulation in a Vertical Ground-Loop Borehole
A driller is constructing a vertical closed-loop ground heat exchanger for a new school. The design calls for eight boreholes, each 280 ft deep, on 15-ft centers. The boreholes are 25 ft from the property line, 40 ft from a septic tank, and 60 ft from a municipal water main. The driller plans 5-inch boreholes with 1.25-inch HDPE U-loops and a 20% bentonite slurry grout placed from the bottom up. On borehole 3, drilling encounters a 6-ft void at 120 ft and loses all drilling fluid; the driller wants to dump 20 bags of bentonite chips into the hole and continue. The site is underlain by sandstone and shale, and the static water level is 45 ft. The local building department asks for the grout volume and verification before backfilling. The driller has not calculated grout volume and says the supplier will 'eyeball it.'
1. Evaluate the proposed response to the lost-circulation zone in borehole 3. What should be done before continuing?
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Dumping 20 bags of bentonite chips is not an adequate response to a 6-ft void with total fluid loss. Chips can bridge at the void and leave a permeable channel, and the driller cannot confirm the hole is stable. The zone may connect aquifers or provide a pathway for grout to migrate away. Before continuing, the driller should stop, measure the void and fluid level, and consider temporary casing through the void or a bentonite-cement or high-solids grout designed for lost circulation. If the void is sealed with chips only, the loop may not be properly grouted. The driller should also notify the designer and building department because the borehole may need a revised seal design. Verification should include grout volume versus theoretical volume and returns. Continuing without stabilization risks a failed loop and a groundwater-protection problem.
2. Calculate the approximate grout volume for one 280-ft borehole and explain whether the grout placement method is acceptable.
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For a 5-inch borehole and two 1.25-inch pipes, annulus area = π(2.5²) − 2π(0.625²) = 19.63 − 2.45 = 17.18 in² = 0.119 ft². At 280 ft, volume = 0.119 × 280 = 33.4 ft³, or about 250 gal. Add 10–20% for the void and normal losses, so plan at least 275–300 gal for that hole. A tremie placed from the bottom up is the right method, but a 20% bentonite slurry may be too light in a void or standing water; a higher-solids grout may be needed. The driller must measure the actual volume used and compare it with theoretical; a large overrun suggests lost circulation. The supplier cannot 'eyeball' it, because grout volume is the main verification that the annulus is filled.
3. Are the separation distances a problem? Explain how you would decide.
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The 25-ft property-line, 40-ft septic, and 60-ft water-main distances are not automatically disqualifying, but they cannot be approved by rule of thumb. The driller should check the state well construction rules and local building or zoning setbacks for ground heat exchangers, and get written approval if the local code has specific distances. Functionally, 15-ft borehole spacing is common, but a 25-ft property-line distance may be too close for future excavation or utility easements; a septic tank at 40 ft could be a contamination pathway if the borehole grout is incomplete. The water main at 60 ft is likely outside immediate damage range but still needs utility clearance. The driller should document the review and not rely on the absence of an inspector.