There are several aesthetic purposes for grading on a residential site. First, grading can define edges between spaces and partially enclose space in the vertical plane. The first and simplest method is to provide an elevation change between two adjoining spaces (Figure 11-14). A small difference in elevation between one space and another makes each seem like a distinct place. The greater the change in elevation between spaces, the greater the feeling of spatial separation.
Grading can also be used to provide vertical planes around the outside of a space for implied enclosure. The existing ground can be excavated, built on with earth
Maximum gradient for a handicap ramp.
mounds to provide spatial enclosure (Figure 11—15), or both excavated and filled (Figure 11-16).
In all these situations, the higher the surrounding ground, the greater the sense of spatial enclosure. The greatest feeling of enclosure is gained when the ground fills a 45-degree cone of vision or extends above eye level (Figure 11-17). Whatever height is
created, plant materials can be added to the surrounding slopes or walls of a space to accentuate the ground’s height, thus giving the space an even more pronounced sense of enclosure (Figure 11-18).
Full enclosure with surrounding ground is most appropriate where a sense of privacy is desired, such as in a small sitting area or private outdoor lounging area.
Often, a space requires enclosure on only one side with a more open feeling provided on another (Figure 11—19). The height of the surrounding ground can be varied to give different feelings of enclosure.
In all situations, the height of the surrounding ground should be limited by several guidelines. For slopes, the incline should not exceed a rise of 1 foot vertical change for every 2 horizontal feet—referred to as a 2:1 or 50 percent slope (Figure 11-20). Slopes steeper than this are subject to slippage and erosion.
When enclosing space in the vertical plane, the designer should use slopes or low retaining walls to reinforce the style or design theme established by the form composition. For example, a curvilinear design theme should be enhanced with soft, sweeping slopes or mounds (left side of Figure 11-21). The slopes should move around the outer edge of the curves to reinforce their form in the third dimension. A rectangular design theme can be reinforced with retaining walls or rigid slopes (right
side of Figure 11—21). The designer might also use a combination of slopes and retaining walls (Figure 11—22).
The character of the base plane or floor of spaces created with ground should also reinforce the intended design theme. For curvilinear design themes, the base plane might be gently sloped and contoured with gradual transitions from one space to another (left side of Figure 11—23). For rectangular or other structured themes, the floor of spaces could be kept relatively level (though still providing for proper drainage), with definite grade changes (made with steps or terraces) between one level and the next (right side of Figure 11—23).
Screening and Directing Views
The second aesthetic purpose for grading is to either screen or direct views. Grading of the ground plane can elevate selected areas of the site to block undesirable views
A combination of slopes and retaining walls can be carefully coordinated in terms of form composition.
The slope of the base plane in spaces created with ground should relate to the overall design theme.
(Figure 11—24). Mounds or berms can be placed to screen views of the street, the neighbor’s driveway, the adjoining backyard, and so on. One other suggestion is to make mounds or berms look as if they were part of the existing site. Earth mounds are sometimes graded with abrupt slopes that make the mounds look like unappealing bumps on the site (top half of Figure 11—25). Mounds should gradually flow into each other and the surrounding site (bottom half of Figure 11—25).
The ground plane can also be molded to direct views toward certain points in the landscape. The side slopes of a valley landform can function like blinders to block out all but the intended view (Figure 11—26).