Vertical Loop Sizing in a Tight Wet-Shale Lot
A 2,800 ft² house has a calculated heating load of 48,000 Btu/h and cooling load of 34,000 Btu/h. The lot is 0.25 acre, with a 15 ft rear setback and utility easements that leave a 30 ft by 40 ft drilling area. The contractor proposes two 250 ft vertical boreholes, 1.25 in SDR-11 HDPE U-tubes, spaced 15 ft apart, instead of the three 200 ft boreholes shown on the preliminary plan. Static water is encountered at 12 ft, and fractured shale begins at 140 ft. The contractor says grouting is unnecessary above the water level because the standing water will seal the borehole, and proposes to skip a thermal conductivity test to hold the schedule. The homeowner wants the smallest possible loop field and asks whether the two-borehole plan will meet the load.
1. Evaluate the proposed two-borehole loop field against the preliminary three-borehole design. What information and calculations should drive the decision?
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The preliminary plan is 3 × 200 ft = 600 ft of borehole for 48,000 Btu/h ÷ 12,000 = 4 tons, or 150 ft/ton. The proposed 2 × 250 = 500 ft is 125 ft/ton, 100 ft short of the preliminary basis and without a thermal conductivity test. That is not a minor footprint change: it raises the required extraction rate per foot and lowers entering water temperature in heating. Before approval, obtain a thermal conductivity test or use a formation-specific design; check borehole thermal resistance, grout conductivity, spacing (15 ft is workable but confirm no thermal interference), and whether the cooling load controls. If the 150 ft/ton basis is retained, the two-borehole plan should be rejected or a third borehole added.
2. The driller insists that standing water at 12 ft eliminates the need for grout. Is that acceptable in fractured shale? Describe the grouting requirement and a correct placement method.
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No. Standing water does not provide a reliable annular seal. In fractured shale, grout must seal the entire annulus to prevent vertical migration of surface water and cross-connection between water-bearing zones, and to improve thermal contact. Use a tremie pipe to place high-solids bentonite or cement-based grout from the bottom up; do not free-fall from the top. If circulation is lost in fractures, stage-grout or use casing and verify fill. Grout after the U-tube is installed and pressure-tested, and record grout volume versus theoretical annulus volume.