While retaining walls are built to support vertical, or near vertical earth banks, they must also withstand any loads superimposed on the bank. There are three main types of cast in place concrete retaining walls: (1) gravity wall. These are of mass concrete with very little tensile stress in any part of reinforcing steel; the wall is then classed as "semi-gravity." (2) Cantilever walls. The cross-sectional shape of these is an inverted-T. The wall portion performs structurally as a cantilever from the base slab. (3) Counterfort walls. These are similar to cantilever walls except that both the base slab, and the wall, span horizontally between triangular vertical brackets. The selection of a particular type of retaining wall- keeping cost and efficiency in mind- will depend primarily on soil conditions at the site. If unusual soil conditions prevail, or if complex superimposed loads may occur, the design of a retaining wall should always be checked by a qualified engineer. It is good practice, and good insurance, even with a small, seemingly uncomplicated wall, to use the services of an engineer who has accurate soils data for the area. Proper drainage is essential for all retaining walls. Good drainage will minimize the uncertain load conditions produced by water in the backfill and by frost action. The type of drainage system needed will depend on the nature of the backfill and the amount of water to be removed. In general, these systems are recommended: pervious backfill, weepholes, 6 inches in diameter or larger, at a 5 to 15 foot horizontal spacing through the base of the stem; semi-pervious backfill, just like pervious backfill except a continuous layer of pervious material at least one foot wide should be placed over horizontal drains; fine-grained backfill, longitudinal drains or weepholes together with a continuous blanket or pervious material at least 1 foot thick; expansive clay backfill, a double blanket drain will be necessary.