ll work without problems. The added expo- sure and potential for freezing can be compensated for with a thick soil cap over the pipe. CONVENTIONAL SYSTEM MODIFICATIONS Figure 6.4 Elevated drain field on a slope. Design The elevated disposal system, normally a conventional style trench system, is designed in the normal way, except that since it is elevated somewhat, soil must be mounded over the system to pro- vide for freeze protection and prevent accidental exposure to sew- age. This soil cap should be a minimum of 1 ft above the gravel fill and should slope to about 10 ft beyond the excavation. If parallel trenches are used, the whole area where the trenches are located should be mounded. On a slope, trenches should run along the surface contours, SO mounds above these trenches could form dams and prevent rapid runoff of rainfall. This might saturate the soil around the trenches and cause their failure. Therefore, you should fill in the uphill side of the trench SO that there is no depression between the trench cap and the natural soil (Figure 6.4). Elevated drain fields are not recommended for use in areas where slopes exceed about 12%. A very important design consideration for elevated drain fields is that of the system elevation as compared to the septic tank and house sewer. Although many contractors have tried, none have succeeded in making sewage run uphill by itself. It is probable that if the septic tank is buried below ground level, the septic tank efflu- ent will not be able to flow by gravity to the elevated drain field distribution pipe. In this case, a dosing chamber is necessary to lift 104 ONSITE WASTEWATER DISPOSAL the effluent to the drain field. See the section on dosing chambers for further information on system design. Materials One drain field and perhaps a dosing chamber. Construction After excavating the drain field, placing the gravel fill, and laying the distribution pipe as per the instructions for that system, the bottom of the distribution pipe should be no higher at its highest poin