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2 . 2 .9 Filter Pack Specifications <br /> After the casing has been installed for the full depth of the well <br /> and the screen has been placed in its proper position, an envelope <br /> of siliceous sand and/or gravel shall be placed in the annular <br /> space between the well bore and the casing screen. The filter pack <br /> material shall extend 30 feet above screen sections. <br /> Materials used for this purpose shall be clean, washed, and graded. <br /> The filter pack size shall be carefully selected based on the <br /> character of the water-bearing formation. Gradation analysis and <br /> selection of the filter pack grain size distribution that has a <br /> uniformity coefficient of 2 . 5 or less shall be the Contractor' s <br /> responsibility subject to the supervising geologist' s approval. <br /> Samples of filter pack material, and a sieve analysis of the <br /> material shall be submitted to the supervising geologist for <br /> approval in advance of delivery and placement. The filter pack <br /> material should be delivered to the site only after approval. A <br /> suitable storage area for the filter pack material shall be <br /> provided by the Contractor. The filter pack material may be <br /> delivered, bagged and stacked at the site or it may be delivered in <br /> bulk. If delivered in bulk, it shall be placed on a protective <br /> sheet preventing contact between it and the ground. It shall be <br /> protected from the elements by a suitable covering. The filter <br /> pack material must be at or near the location so there will be no <br /> waiting on gravel once the casing has been installed. <br /> The Contractor will obtain two grab samples from each load of <br /> filter pack material delivered and perform or have performed an <br /> approved mechanical analysis of these samples to determine that the <br /> filter material complies with the grain size and uniformity <br /> criteria previously approved by the supervising geologist. <br /> 2 . 2 . 10 Filter Pack Placement <br /> When the assembled casing and screen are centered in the borehole, <br /> tubing or drill pipe with one close-fitting swab located near the <br /> bottom of the screen will be inserted. Clear water shall be <br /> circulated while installing the sand pack. The fluid properties of <br /> the well bore fluids will be approved by the supervising geologist <br /> before the introduction of filter pack. The filter pack will be <br /> emplaced by pumping through a tremie pipe extending to the bottom <br /> of the casing-hole annulus. The tremie pipe will gradually be <br /> withdrawn as the filter pack is placed. <br /> After the filter pack has been placed, circulating and swabbing <br /> will continue in stages across the entire screen section until the <br /> filter pack is consolidated and cleaned. As the filter pack <br /> settles, more shall be added. <br /> A careful record will be kept of the amount of filter pack added <br /> during placement and consolidation. <br /> 2 - 5 <br />