Piedmont Bedrock Well: Casing Seat and Grout Method
An 8-inch borehole is being drilled for a 4-inch SDR-17 PVC casing on a Piedmont site. The permit targets a 250-ft well. Weathered rock is logged at 38 ft and competent bedrock at 46 ft. The driller proposes to set casing at 40 ft - two feet into weathered rock - and to grout the annulus from 40 ft to land surface by pouring bentonite chips from the top. Static water is 70 ft; a 4-gpm yield was estimated during drilling. The homeowner wants the well finished today because the health department permit expires in 10 days. The driller argues that because the casing is PVC and the upper 40 ft is tight clay, full-length tremie grout is unnecessary. The borehole below 46 ft stands open in fractured granite, and the driller has not run any temporary surface casing.
1. Evaluate the proposed casing seat. What minimum casing depth should the driller use, and why?
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The 40-ft seat is not acceptable. NC private well construction rules require the casing to be seated into competent bedrock, not weathered rock, and to extend a minimum distance into that competent rock so the annular seal is anchored below the zone of secondary permeability. With competent bedrock logged at 46 ft, the casing should be set at least 10 ft into competent rock, or about 56 ft. The casing must also extend at least 20 ft below land surface and be grouted full length. A 40-ft seat stops 2 ft into weathered material; that leaves a pathway for surface water and shallow groundwater to move down the annulus into the fractured granite. The driller should ream the hole if needed, set the casing to at least 56 ft, and grout the full annular space from the casing shoe to land surface before completing the well.
2. Is the bentonite-chip grout plan acceptable? Describe the method and volume check for grouting an 8-inch borehole with 4-inch casing to 56 ft.
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No. Pouring bentonite chips from land surface into a narrow annulus commonly bridges, and it does not reliably form a continuous, low-permeability seal. The rule requires grout placed from the bottom of the casing upward, typically by tremie pipe and positive-displacement pump, so the grout displaces water and fills the annulus without voids. For an 8-inch hole and 4-inch casing, the annular area is about 0.262 ft2: π/4 × (0.6672 − 0.3332). Over 56 ft, that is about 14.7 ft3, or 0.54 yd3 of neat cement or bentonite slurry. A prudent order would add 10 to 20 percent for loss and hole irregularities, roughly 16 to 18 ft3. The driller should verify the tremie reaches the bottom, pump until grout returns at land surface, and not stop merely because the calculated volume has been pumped.
3. If the borehole caves at 45 ft before casing is set, what should the driller do?
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The driller should not force the casing through caved material or set it above the caved zone just to save time. Caving above competent rock means the borehole cannot be cleaned and the grout seal cannot be verified. Options are to ream and clean the hole, use temporary surface casing through the caved interval, or grout the lower hole and redrill through the seal. If temporary casing is used, it must be removed while the grout is still fluid or the annulus must be grouted in a way that leaves no void. The driller should also notify the health department if the change affects the permitted construction, and record the cave depth and remedy on the well record. The objective is a continuous grouted annulus from the casing shoe to land surface, not merely a casing that reaches the proposed depth.