03-25-1996 Council Packet - Laserfiche WebLink

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Mr. David Kline February 29, 1996 provide alternate recommendations for tying the upper wall back with geogrid. Our recommendations are limited to determining the required geogrid lengths and elevations for both the upper and lower walls, and also recommending the number, location, and spacing of the upper wall earth anchors. We will also provide general backfill and construction recommendations. Deaian ons Soils information is not available, therefore, we have had to make certain assumptions for our design. We assume that the retained soil will consist of sandy clay till, having a moist unit weight of 125 pounds per cubic foot, a drained friction angle of 27® and a remolded undrained shear strength of 250 pounds per square foot. We have performed a preliminary global stability analysis using our assumed soil parameters. The results are attached. The soil parameters must be confirmed through an appropriate test program in order to perform a final, more detailed, stability analysis. We recommend that the wall be constructed of properly pressure-treated, sound timbers. The units should consist of 6 inch by 6 inch timbers 8 feet long. Adjacent timbers should be fastened together with steel spikes, or nails, inserted into predrilled holes to minimize the potential for splitting of the wood. The lower wall should be embedded at least one timber course (6 inches) below final grade in front of the wall. We recommend that the lowest timber bear on a compacted gravel base, having minimum dimensions of 6 inches deep by 1 foot wide. The base soil should be compacted to at least 95% of the Maximum Standard Proctor dry density, ASTM: D 698. Prior to constructing the gravel base, the exposed clay subgrade should be observed for soft or loose zones. These zones would require additional subcutting and replacement with compacted granular fill. We recommend that the timber wall be constructed with a slight batter of 5 to 10 degrees back into the retained embankment. This could be accomplished by either off-setting each row of the timbers from the row below to provide the necessary setback, or placing the timbers flush but leaning the entire wall back into the backfill. The wall backfill could consist of on-site compacted s.andy clay. However, we recommend that within 1 foot of the back of the wall the fill consist of free-draining granular fill such as Mn/DOT 3149.24. The first timber course above grade should be gapped about 3 inches every approximately 20 feet to allow drainage of the water out of the retained granular drainage course. We recommend stapling a piece of geotextile filter fabric over the opening on the backside of the wall between the wall and the drainage course to prevent migration of rock through the opening. CME C0N8ULTAMTS. IMC.