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(c) Preparation of test hole. The bottom <br /> and sides of the hole shall be care- <br /> fully scratched with a knife blade or <br /> sharp pointed instrument to remove <br /> any smeared soil surfaces and to <br /> provide a natural soil interface into <br /> which water may percolate. All loose <br /> material shall be removed from the <br /> hole and two (2) inches of coarse sand <br /> or fine gravel shall be added to protect <br /> the bottom from scouring. <br /> (d) Saturation and swelling of the soil. <br /> The hole shall be carefully filled with <br /> clear water to a minimum depth of <br /> twelve (12) inches over the gravel. <br /> Water shall be kept in the hole for <br /> at least four (4) hours , and preferably <br /> overnight, by refilling if necessary, <br /> or by supplying a surplus reservoir <br /> of water, such as in an automatic <br /> siphon. In sandy soils containing <br /> little or no clay, the swelling pro- <br /> cedure shall not be required and the <br /> test may be made as described under <br /> Subd. D. (4) (e) (3) after the water <br /> from one filling of the hole has <br /> completely seeped away. <br /> (e) Percolation rate measurement. With the <br /> exception of sandy soils, percolation <br /> rate measurements shall be made on the <br /> day following the procedure described <br /> under Subd. D. (4) (d) . <br /> (1) If water remains in the test hole <br /> after the overnight swelling period, <br /> the depth shall be adjusted to <br /> approximately six (6) inches over the <br /> gravel. From a fixed reference point <br /> the drop in water level shall be <br /> measured at thirty (30) minute intervals <br /> over a two (2) hour period. This drop <br /> shall be used to calculate the percol- <br /> ation rate. <br /> -11- <br />