Landscape Contractor / Design Build Maintain

AUG 2018

LC/DBM provides landscape contractors with Educational, Imaginative and Practical information about their business, their employees, their machines and their projects.

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technical report on soil and groundwater conditions should also be obtained. Other issues to consider are future snow and/or storage loads and freeze/thaw concerns. Digging Down After a base trench is excavated, it should be compacted to the level specified in the geotechnical report and inspected by the onsite soils engineer prior to any base material being placed. The typical depth of the trench is calculated based on a minimum 6 inches deep of compacted wall rock base, and buried block depth equal to 1 inch per foot of wall height, with a standard minimum of 6 inches for commercial projects. The width of the trench is typically 24 inches wide but deeper facing units of course require more width. And, a larger trench, and therefore a larger base, may be needed if poor foundation soils exist. Building Up If deemed necessary, a layer of geogrid is installed 3 to 6 inches above the bottom of the base, with a minimum length of 60% of total wall height measured from the top of the base to the top of the wall. Additional layers are then placed after every other course of blocks. Standard base material used is consistent with wall rock; well-graded compactible aggregate, 0.25 to 1.5 in in size. It is recommended that facing units have an average (front to back) depth of at least 10 inches because units of this size or greater have a proven performance re - cord. And it is recognized that as the unit depth decreases, so does the stability of the facing. After placing the base course, a hand op - erated plate compactor should be used on the ground behind this course within the 3-foot- wide consolidation zone with a minimum of two passes. A proper testing protocol should be es - tablished to consistently achieve the minimum compaction require - ments set by the design requirements. Compaction of the other courses will be - gin by running the plate compactor directly on top of the block facing, and then compacting in parallel paths until the entire consolidation zone has been compacted. The wall rock column depth behind the wall facing is typically 12 inches deep. But this depth will vary with wall height, if select/structural fill is not utilized in the reinforced mass. For walls from 10 to 20 feet tall, the bottom half is suggested to be increased to 24 inches deep and the top half to be the typical depth of 12 inches deep. For walls over 20 feet tall, the bottom half is suggested to be increased to 36 inches deep and the top half is divided in two; with a 24-inch-deep column and a 12-inch-deep column at the top. It is common to place a horizon - tal layer of landscape fabric over the wall rock column at the top of the wall to protect the wall rock from be - ing infiltrated by the topsoil placed to finish the wall. Caps should be secured in place using a high quality flexible exterior grade ma - sonry sealant. This, of course, is not an exhaustive list of best practices. Many more are available at https://ncma.org , https://store.transporta - tion.org and http://allanblock.com LC DBM 26 LC DBM Left This diagram shows best practices for water man- agement of retaining wall installations according to the Allan Block SRW best practices manual. Those practices include creating a swale on the backslope behind the wall so that water does not flow over the top of it, and making sure all drainpipes exit to daylight or to an underground drainage system. Above The lower wall at the Wausau, Wisconsin, site creates an even grade halfway up the slope. The upper wall was built a full story high to match the highest point of the site, and it leveled out the property to accommodate a parking lot on the front side of the building.

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