A retaining wall must be strong enough to hold back the pressure of a great weight of soil, yet it must be porous enough to allow for drainage. The most popular types of walls are built of stone. In using stone to build a retaining wall, there are two basic types of construction: the dry-wall, which uses earth as a filler between the stones, and the mortar type, which uses cement as a bonding agent.

First, the base of any retaining wall must be sunk below the frost line. This is about 6-12 inches in the northern half of the United States but may be more in some areas. For a flat wall (one without buttresses or projections), the width of the base should equal one-fourth the height of the wall. The wall can taper to a width of about one-fourth of the width of the base.

For buttressed walls, the base should be approximately one-fourth as wide as the wall is to be high. This refers to the widest points, when buttressing is to be used. In the narrower areas, the base may have slimmer proportions

Drainage pipes should be imbedded in the wall at intervals of about 24 inches, and about 6 inches from the lower ground level of the retaining wall, . In some walls, it is possible to eliminate these drains, if the wall itself is porous enough, but any construction using mortar as a bonding agent, makes drainage pipes essential.

In dry-wall construction it is possible (though not advisable) to start the wall at ground level, and not sink it below the frost line. The most inexpensive way to construct a dry wall is to choose local stone, picking large stones for the main ones and smaller stones for the chinks. The largest stones should be used to form the base of the wall with the smaller ones leading to the top.

The side of the wall facing out should be as level as possible. Any obstructions and edges of outside stones should face inward. This gives the wall a better footing on the soil it retains and insures a good appearance. Stones with round surfaces do not form a good wall and should be discarded.

Stones should be placed in a good bond, which simply means that the edges of stones on one course should overlap spaces in the lower courses. Where a stone on an upper course is crooked or does not fit firmly, earth and small stones should be packed in to improve the bond and no vertical crevices should be left.

The wall itself should slope back against the soil that it is retaining. This gives it greater strength. As a rule of thumb, the width of the base should be one-third of the height. Although this degree of slope is not essential, it is the practice in many areas to slope the wall as much as five or six inches for each vertical foot. Soil should be firmly packed into all pockets in the wall and should be continued back into the earth being retained.

Both the strength and attractiveness of a dry stone wall may be enhanced by using it as a wall garden. It may acquire a mossy and aged appearance simply by green-planting in the crevices. More color can be obtained, however, by planting any of several flowering plants, whose strong roots will serve the added function of holding the wall together.

Plants which may be used to good effect are: flowering types such as phlox, garden pinks, sedum, snowy rock cress, azaleas, alyssum, evergreen candytuft, heather, and creeping veronicas; spreading plants such as moss, phlox, lavender and hardy verbenna; small rosettes as well as little tufts that need sun and room for roots like sempervivium, yarrow, dwarf iris and dwarf pinks; and plants you can grow from seed sown among the rocks such as some ivies, bleeding heart and varieties of poppy and phlox. Semperviviums, prostrate, junipers, azaleas and dwarf azaleas could keep a rock wall green all winter long.

Mortared and Concrete Walls

Mortared walls are simpler than dry walls. The mortar serves as the bond so it is not as essential to make the stones match. For a masonry wall, a cement mixture of one part Portland cement and two parts sand makes for a good bonding agent.

Apply the mortar liberally to form a bed for each stone as it is added. Chinks between stones should be well filled with smaller pebbles or gravel. The mortared wall is more permanent than a dry wall and, in fact, easier to build.

The top of every stone wall, whether dry wall, or masonry, needs protection. This is accomplished by using broad, flat stones as capstones. These can either be slate or other flat stones acquired in the course of collecting the material for the wall.