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Mr. David Kline February 29, 1996 <br />The lower timber wall should be constructed with two rows of Huesker <br />Fortrac 35/20-20 geogrid extending 4 feet from the back of the wall <br />into the backfill (total grid length 4.5 feet). The first grid layer <br />should be placed a distance of one-quarter of the wall height up from <br />the exterior grade elevation, and the second grid layer should be <br />placed a distance of one-quarter of the wall height down from the top <br />of the wall. Therefore, for the 4 foot wall height, the first grid <br />will be placed at a height of 1 foot above exterior grade (1.5 feet <br />above base of wall) and the second grid will be placed at a distance of <br />1 foot from the top of the wall. The geogrid should be secured to the <br />timber with galvanized steel nails. The grid should be pulled taut and <br />again staked into the backfill so that the grid remains taut during <br />backfill placement. It is extremely important that the grid remain <br />taut during backfill placement in order to reduce future creep movement <br />of the wall. The clay backfill should be placed in loose lifts not <br />exceeding 6 inches in thickness and should compacted to at least 95% of <br />the Maximum Standard Proctor dry density at a moisture content within <br />2% of optimum value as determined by the Standard Proctor test. The <br />backfill should be compacted using manually-operated vibrating plate <br />compactors. <br />The upper wall should be reinforced either by geogrid or by earth <br />anchors. The upper wall should be embedded at least one timber course <br />(6 inches) below final grade in front of the wall. We recommend that <br />the timber wall be constructed with a slight batter of 5 to 10 degrees <br />back into the retained embankment. This could be accomplished by <br />either off-setting the timber to provide the necessary setback, or <br />placing the timbers flush but leaning the entire wall back into the <br />backfill. <br />Within 1 foot of the back of the wall we recommend placing free- <br />draining granular fill such as Mn/DOT 31149.24. The first timber <br />course above grade should be gapped about 3 inches every approximately <br />20 feet to allow drainage of the water out of the retained granular <br />drainage course. We recommend stapling a piece of geotextile filter <br />fabric over the opening on the backside of the wall between the wall <br />and the drainage course to prevent migration of rock through the <br />opening. <br />If geogrid is used to reinforce the upper wall, we recommend that the <br />wall be constructed with two rows of Huesker Fortrac 35/20-20 geogrid <br />extending 4 feet from the back of the wall into the backfill (total <br />grid length 4.5 feet). The first grid layer should be placed a <br />distance of one-quarter of the wall height up from the exterior grade <br />elevation, and the second grid layer should be placed a distance of <br />one-quarter of the wall height down from the top of the wall, as <br />previously described for the lower wall construction. The geogrid <br />should be secured to the timber with galvanized steel nails. The grid <br />should be pulled taut and again staked into the backfill so that the <br />grid remains taut during backfill placement. The clay backfill should <br />be placed and compacted as previously described. <br />GMi CONSULTANTS. INC.