The walls of the manhole may be built either of brick or of concrete. Brick is more commonly used, as the forms necessary for concrete make the work more expensive unless they can be used a number of times. The walls of the manhole should be at least 8 inches thick. Greater thicknesses are used in treacherous soils and for deep manholes, or to exclude moisture. A rough expression for the thickness of the walls of a brick manhole more than 12 feet deep in ordinary firm material is _t_ = _d_⁄2 + 2, in which _t_ is the thickness in inches and _d_ is the depth in feet. The thickness of brick walls may be changed every 5 to 10 feet or so. Concrete walls may be built thinner than brick walls. [Illustration: FIG. 27.—Baltimore Standard Manhole Details. ] The bottoms of brick manholes are frequently made of concrete as shown in Fig. 27. The floor slopes towards the center and is constructed so that the sewage flows in a half round or U-shaped channel of greater capacity than the tributary sewers. The sides of the channel should be high enough to prevent the overflow of sewage onto the sloping floor, which should have a pitch of about one vertical to 10 or 12 horizontal. In manholes where two or more sewers join at approximately the same level the channels in the bottom should join with smooth easy curves. Where the inlet and outlet pipes are not of the same diameter the tops of the pipes should ordinarily be placed at the same elevation to prevent back flow in the smaller pipes when the larger pipes are flowing full. The dimensions of the manhole should not be less than 3 feet wide by 4 feet long for a height of at least 4 feet, when built in the form of an ellipse, or 4 feet in diameter when built circular. No standard method for the reduction of the diameter of the manhole near the top is observed, the rate being more or less dependent on the depth of the manhole. The use of sloping sides above the frost line is desirable as such a form is more resistant to heaving by frost action. For sewers up to 48 inches in diameter the manhole is usually centered over the intersection of the pipes and has a special foundation. For larger sewers the manhole walls spring from the walls of the sewer as shown in Fig. 28. [Illustration: FIG. 28.—Details of a Manhole and a Well Hole. ]