The following is good information on setting up tree protection zones during construction activities to preserve desired trees.
Before construction or site work begins, tree protection zones must be established. A tree protection zone is a designated area around the trees to be saved in which no construction activity or traffic is allowed. Remember soil compaction begins with the first pass of a vehicle. To set up a tree protection zone: 1. Measure the diameter of the tree trunk in inches at 4.5 feet from the ground. This is called the diameter breast height or DBH. Multiply this value by 2.5. This result is the diameter of the root protection zone in feet. This is also considered the critical rooting distance. For example if an oak has a DBH of 20 inches the tree protection zone is 50 feet in diameter (20 x 2.5). Another way to think about it is to protect an area extending 25 feet in all directions from the trunk. Once the size of the area is determined, consider fencing materials. Orange tree save fencing or black silt fencing are commonly used. These materials are easy to install but they often get knocked down or removed when it is inconvenient to go around the tree save area. In some cases more permanent materials, such as chain link fencing, may be required. Whatever fencing material is used, it must be maintained throughout the construction process.
Tree protection zones are extremely important because they prevent harm from construction activities like soil cuts, soil fills, soil compaction, and the effects of chemicals from washing of equipment and disposal of wash waters. Most construction jobs start with rough grading of the property and removal of undesired vegetation. Keep in mind that trees grow in communities and often share rooting areas and wind loads, so grading and tree thinning may make the remaining trees prone to breakage from wind. The removal of soil is called a soil cut. The addition of soil is called a soil fill. The effects of soil cuts and soil fills are greatly influenced by soil texture. Simply adding one inch of clay soil over the root system will affect the health of a tree while three inches of clay will cause massive root damage. Likewise, sandy-textured soils used as fill initiate root damage at a depth of 8 inches, massive root damage at 24 inches. Soil texture influences soil porosity and structure. The finer the soil texture, the smaller the pore size. As pore size decreases, drainage and oxygen levels become more of a problem.
Information from "Tree Protection During Construction and Landscaping Activities" by Todd Hurt & Bob Westerfield, University of Georgia in the Georgia Certified Landscape Professional training materials http://apps.caes.uga.edu/urbanag/GCLP/Resources.cfm