layer of medium-screened gravel with another layer of fine-screened gravel, untreated paper, or straw of 2 to 3 inches in depth in the trench.

Pipe should be laid with a minimum pitch of 2 inches to a maximum pitch of 4 inches per 100 feet, When open joints are used, they must not be spaced more than 1/2 inch apart. Asphalt-treated paper should be used to cover the joint. The open joint allows for free discharge of solids from the line to the trench. The asphalt-treated paper prevents gravel from entering the pipe.

The layout of the field requires attention to detail to prevent future maintenance and operation troubles. When the field is laid on sloping ground, the flow must be distributed so each lateral gets a fair portion of the flow. Individual lines should be kid nearly parallel to land contours, Leaching fields are commonly laid out either in a herringbone pattern (fig. 10-6) or with the laterals at right angles to the main distribution pipe. Little or no maintenance is required for leaching fields. Preventive measures, such as excluding all vehicle traffic and not planting trees or shrubs in the field area, should ensure trouble-free operation for many years. When a leaching field becomes inoperable, you must replace it with a new system. Tree or shrub roots are a major factor in leaching-field failure. This requires the replacement of field components and complete root removal.

Table 10-17. - Size and Spacing for Disposal Fields