Landscape Contractor / Design Build Maintain

JAN 2016

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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Page 41 of 71

42 LC DBM analysis is needed, and if so, coordinate with a geotechnical engineer. Finally, determine if seismic design is necessary, and if so, coordinate with the geotechnical engineer. If the wall is to be less than 4' in height, follow the engi- neering instructions recommended by the wall manufac- turer. If the wall is greater than 4', NCMA recommends that the design be reviewed or prepared by a registered professional. When constructing a tiered retaining wall, if the combined height is less than 4', and the distance be- tween walls is at least twice the height of the lower wall, and no surcharges are imposed, then follow manufactur- ers' recommendations. In all other cases, the design should be reviewed or prepared by a professional. When determining height, follow manufacturers' rec- ommendations for walls without reinforcement. Walls with reinforcement should have their maximum height de- termined from a crest toppling design evaluation, factor- ing in an allowance for safety. There should be a minimum soil cover in front of any retaining wall, as well as angular or well-graded gravel aggregate placed in the cores of the wall units if applicable, between the units, and at least 12" behind the units. The NCMA standards provide a solid start for retaining wall construction; however, local codes trump their rec- ommended guidelines. While some locations follow the NCMA guides nearly to the letter, many deviate from them or expand upon them, sometimes in dramatic ways. Local Standards San Diego standards measure walls from the top of the footing to the top of the wall. Residential areas are not al- lowed to have retaining walls higher than 3' in visibility areas such as a front yard; side yards and rear yards can have two walls up to 6' in height when there is an appropri- ate distance between the walls. Inspections are required at three different points during wall construction, as well as one final inspection when work has been completed. Prince William County, Va., requires a permit for any retaining wall with exposed height greater than 2'. If the area has surcharge or problem soil, it must be engineered. The requirements vary from there, and depend on the ma- terial used to construct the wall. Top: If a retaining wall is to be built within 100' of a wetland or within 200' of a river in the state of Massachusetts, the plans must be reviewed under the local Department of Environmental Protection. In Maine, a permit is required for waterfront properties when constructing or replacing a retaining wall within 100' of a water resource. Middle: For tiered retaining walls, most localities require, and NCMA standards recommend, that the distance between the tiers is at least the height of the lower tier. In the township of Kennett, Pa., tiered walls where the tiers are closer than two times the lower wall height require a permit. PHOTO: TRYON CREEK LANDSCAPING Bottom: Any retaining wall system that is over 20' in height in Buncombe County, N.C., must have plantings selected from a list of pre-approved plant material, which is available on the county website. The requirements for how many plants and of what variety depend upon the height of the wall; walls over 30' in height require more plants be added. PHOTO: FURBISH COMPANY

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