Sustainable Building Technical Manual - Etn-presco.net

Soil fertility not only supports plant life but also disperses and filters water, and neutralizesor binds many air and water pollutants. The source of all these benefits is the topsoil,a gossamer-thin blanket over the earth; its depth is a millionth or less of the earth’sradius (0.5 inch to 48 inches is typical of most North American soils). 2 Topsoil consists oftwo parts: a mineral element (sand, silt, and clay in varying proportions, weathered fromsubsoil) and organic materials (decaying plant and animal remains known as humus).These organic materials are digested and churned by micro- and macroscopic soil organismswhose health is essential to fertile soil.Soils vary in water-holding ability, nutrient content, pH and salinity, and humuscontent. They may also be contaminated or, in rare cases, naturally toxic. Eachaspect can dramatically affect plant, animal, and human life on the site, as well asconstruction projects.Soil’s mechanical properties affect what can be built on a site. Depth and strength ofbedrock affects structures, excavations, and costs. Percolation affects septic systems andflooding. Erodible soils are vulnerable to foot or vehicle traffic and to changes in vegetativecover. Erosion can damage a watershed’s ability to distribute and retain water, contributeto flooding, and contaminate water sources. Performance and review ofsite-specific soil analysis reveal how these factors affect a building project.. ☛ SUGGESTED PRACTICES AND CHECKLISTI❑ Involve a qualified site-design professional on the design team early in the project.❑ Obtain and evaluate the chemical and physical characteristics of site soils.A general picture of soil characteristics is available for most U.S. counties from the U.S.Soil Conservation Service’s Soil Survey, and is essential for early planning and designwork. Before proceeding with detailed design, contract with a reputable soil-testinglaboratory. Be sure the service includes a written evaluation of soil suitability for theproposed use, and recommendations for soil remediation and amendment. Use soilanalysis to rule out contamination before the site is purchased.❑ Amend the soil in planting areas according to professional advice.Involve a qualified site-design professional on the team early in the project. Develop aplan to leave as much of the native soil undisturbed as possible. Amendments mayinclude sand or gravel for improved drainage, lime or other pH modifiers, organicmanures, and chemicals to improve nutrient availability. Humus is used to increasewater-holding capacity, as are proprietary superabsorbent materials. In some cases,amendment may involve specific plowing or irrigation activities—for example, tobreak a hard-pan or to leach out excessive salts.❑ Protect the soil during construction.Soil compacted or contaminated by construction activity may become lifeless.– Design for minimal grading. Where grading is unavoidable, carefully remove andstockpile existing topsoil, replacing it after rough grading. Depending on soil-testfindings, the top four to six inches of soil are usually stockpiled.– Plan construction sequences that minimize heavy-equipment movement over thesoil; restrict all equipment, including private vehicles, generators, etc., to areas thatwill be paved or built over.– If soil compaction is unavoidable (as with a construction-access road) remediate bytillage and amendments before completing work.(See Chapter 19, “Environmental Construction Guidelines.”)❑ Carefully design for grading and excavation.In siting facilities, work with the existing topography to save both construction andmaintenance costs. Avoid disrupting existing drainage patterns; equalize cut-and-fill;and in general minimize grade changes where possible. Grading for stormwater control