Getting the drainage right for the landscape is the most important part of landscaping. Proper grading ensures that water will drain away from structures and prevent erosion. Compactible material should be used for preparing the subgrade. Finish grading material includes gravel, sand and topsoil, depending on what landscape features are being installed.
The subgrade is the existing ground, cleared of organic material. The subgrade is prepared to channel surface water away from the house foundation, without blocking existing drainage patterns or affecting the drainage of adjacent properties. A minimum 2 percent slope is needed for water flow outdoors; keep unpaved slopes to a maximum 33 percent. Stormwater is directed through swales and berms. Swales are subtle depressions, and berms are gentle mounds that can channel stormwater to the swale.
Some existing soils are fine for grading if they drain and compact well. Other soils may have to be excavated and replaced, if only in pockets. Remove any soil with so much organic material that it will create soft spots. Replace it with compactible material that has little organic matter, some gravel or rock, and silt. Stockpile any topsoil for the lawn and garden. Check with building authorities in your area if there are special environmental conditions that impact grading practices and material.
Cut and Fill
Some cut and fill is usually necessary to achieve the desired drainage. Soil is removed from a high area and, if it's suitable material, moved to fill a low spot. Ideally, cut and fill are balanced. Fill material is not expensive, but equipment and labor are costly. Subsurface drainage pipes are used only if the site can't be amended with grading. Avoid the expense of retaining walls if the slopes can be managed through shaping.
Once the drainage pattern is set, finish material for the site elements is needed. For paving, the material is compactible gravel. For lawns and gardens, topsoil is spread and prepared for planting. If any soil will be exposed for a period of time, protect it with straw or shredded wood mulch. For steep slopes, silt fences will keep sediment from eroding and washing into waterways. Erosion and sediment control is an important consideration following subgrade preparation.
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