PRELIMINARY GRADING DESIGN

There are a number of places on the residential site where special effort should be made to correctly drain surface water. For positive drainage, it is recommended that lawn surfaces slope at 2 percent or 2 feet fall in every 100 horizontal feet (Figure 11—5). A planting bed that is steeper than this is susceptible to erosion unless protected by ground cover. Exposed aggregate concrete, brick, stone, or other rough pavement materials should have a minimum slope of 1.5 percent (a 1-1/2-foot vertical change for every 100 horizontal feet). A slope of 10 percent rises or falls at a rate of10 feet for every 100 horizontal feet or 1 foot for every 10 horizontal feet. Two general purposes for grading on a residential site are necessity and aesthet­ics. It should be noted that in some locations, such as arid regions or areas with problems of too much runoff from urban development, it may actually be desirable to temporarily hold water on a site during and after a storm. When earth is added to an area, it is called fill. Generally, there is an at­tempt to balance the quantity of cut and fill on a given project to eliminate the need for transporting earth to or from the site. Figure 11-2
The ground should slope away from the house foundation within these suggested limits. Surface water should be drained away from the house and other structures on the site to reduce problems. There are several general existing slope conditions the designer may encounter on residential sites (Figure 11—1). Each specific purpose for grading is discussed more in the following sections. In the second situation, where the house is located on level ground, the surface must be regraded to slope gradually away from the house. Both types of grading should be undertaken together so that all grading is both functional and appealing to the eye. Pools of standing water on any paved surface reduce its safety and usefulness. The third situation for existing slope conditions is where the house is located on a sloped site (bottom of Figure 11—1). Thus, the elevation of the ground has a direct influence on the function and appearance of other ele­ments. 2. Water should be drained as quickly as possible from paved walks and drive­ways so they can be used safely during and immediately after a rainstorm. And finally, the ground plane is the surface on which we walk, run, sit, drive, and so on. When earth is removed or excavated from an area, it is called cut. Drainage
One utilitarian purpose for grading is to provide proper drainage across the ground’s surface. Therefore, its three-dimensional composition is critical. Water should be properly drained from lawn surfaces to prevent standing water or soggy, wet areas. For necessity, grading is undertaken to properly drain surface water and to accom­modate circulation or other uses on the site. have a minimum slope of 1 percent. Therefore, a slope of 1 percent rises or falls 1 foot for every 100 horizontal feet (1 divided by 100 = 0.01 or 1 percent). It is also desirable to prevent water from accumulating during the winter season in northern climates because wet areas are apt to become covered with ice, causing a real safety problem. It is usually recommended that the ground’s surface be sloped away from the house or other structures at a rate of 1 percent to 10 percent (Figure 11—2). Above this maximum, it becomes dangerous to operate a lawn mower. There are several reasons for this. Here it will be necessary to create a swale or shallow valleylike landform on the uphill side to collect surface drainage and direct it around the house. A second reason is that the ground plane is the foundation for every other design element, such as plant materials, pavement, walls, fences, and overhead structures. A paved surface that is steeper than 3 percent is per­ceived as having a definite slope and gives a space an uncomfortable or unstable feel­ing. The term grading is commonly used to refer to the manipulation of the ground’s third dimension and is defined as shaping or molding the ground’s surface for both functional and aesthetic purposes. At the other extreme, paved surfaces in outdoor spaces where people stand or sit for any length of time should not exceed a maximum of 3 percent. First, the three-dimensional design of the ground can and should be done in close association with form composition. Spatial composition in residential site design should start with the ground plane. Grading for necessity is a utilitarian and engineering process, whereas grading for enhancement is an aesthetic and artistic endeavor. A slope of 1 percent is approximately equal to 1/8-inch vertical elevation change for every 1 horizontal foot across the surface (Figure 11—3).