Casing and Annular Seal in a Sand-and-Gravel Domestic Well
A client proposes an 8-inch borehole for a domestic well on a half-acre lot. The driller's field log shows 0–25 ft sandy loam, 25–70 ft loose sand and gravel, 70–120 ft stiff clay, and 120–220 ft fractured basalt. Static water is 55 ft below land surface. The client wants 6-inch SDR-17 PVC casing stopped at 60 ft, no annular grout, and a gravel pack from 120 to 220 ft to 'make more water.' A neighbor's 40-year-old well 100 ft away produces turbid water after storms. The proposed well is 70 ft from the property line and 85 ft from the septic tank drain field. The client argues that because the upper sand is only 45 ft thick, a shallow casing is adequate and grout is unnecessary. The driller must decide what to install, how deep to seat the casing, and what to tell the client before drilling.
1. Evaluate the proposed 60-ft casing and no-grout design. What should be installed and why?
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Reject the 60-ft/no-grout design. The 25–70 ft interval is unconsolidated sand and gravel; stopping casing at 60 ft leaves the well open to that material and makes a competent annular seal impossible. Seat the 6-inch casing into the 70–120 ft clay, preferably to about 125 ft, and cement-grout the annulus from the casing shoe back to land surface. The clay is the confining unit that will isolate the shallow sand and the septic area. Do not gravel-pack the fractured basalt interval unless a screen is actually installed; a gravel pack placed in an open-hole volcanic section does not 'make water' and can create a vertical pathway. If the client insists on a screen, set it only in the producing zone and seal above it. Explain that the extra casing and grout are the cost of avoiding sand entry, turbidity, and cross-contamination, especially given the old turbid well nearby.
2. How should the driller verify the annular seal and respond if the well still produces sand after development?
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Verify the seal by calculating the annular volume from the casing diameter and borehole diameter, measuring the grout volume placed, and recording the return at surface. A positive surface return plus the calculated volume is the minimum field check; a pressure or tag test may be used if the seal is doubtful. If the well still makes sand after proper development, do not simply screen it off. First check whether sand is entering through the open hole, a failed seal, or overpumping. If the seal failed, ream and regrout or install a liner and re-seal. If the sand is from the producing zone, reduce the pumping rate or install a properly sized screen and gravel pack. Document the correction and do not report the well as complete until the turbidity source is understood.
3. The well is 85 ft from a septic drain field and 70 ft from the property line. How should the driller handle these siting issues before drilling?
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Treat the septic drain field and property-line distances as siting issues that must be resolved before drilling, not as details to waive verbally. Check the current Nevada well-construction setbacks and any county health requirements; if the proposed location does not meet the applicable isolation distance, relocate the well or obtain the required approval. Also confirm the well permit and any basin restrictions. A drilling contract does not replace the permit. Put the setback determination in writing to the owner, and do not begin drilling on the assumption that 85 ft or 70 ft will automatically be acceptable.