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D-38 To meet code requirements, the corrugated pipe also has to have 1/2- inch holes in the pipe bottom. Typically, manufacturers place the holes at 4 o'clock and 8 o'clock. (This placement is not a code :equirement.) Gravelless pipe systems are allowable, since the rock that traditionally separates trenches provides little or no treatment of the effluent prior to it being dispersed into the soil. Any type of system that holds the soil apart is acceptable as long as it has an established loading rate. The loading rate for gravelless pipe systems was developed through testing in Minnesota. The testing consisted of breaking systems in different soils into small pieces and measuring wastewater flow into the section and out of the section. The resulting loss of effluent was the soil loading factor. By breaking these systems into relatively small pieces, the long-term loading rate was determined quickly and a good design number has been established. The loading rate for a 10-inch gravelless pipe was found to be . equivalent to that of a 3-foot-wide trench with 6 inches of rock below the pipe. The S-inch pipe (inside diameter) was equivalent to a 2-foot-wide trench with 6 inches of rock. These loading rates are key to the design of the system. Potential problems Fine sands have been found to develop a slower long-term acceptance rate, even though they have the same permeability and water flow characteristics as a medium sand. During field reviews, this problem has been noted most often with fine sands. One of the keys to making a gravelless system work is choosing the proper synthetic fabric for wrapping the pipe. O Fabric that is too elastic will collapse against the pipe as dirt is put back around the pipe, not allowing sewage to move around the outside. O Fabric that is too stiff or too thick (or both) will not allow movement of water out of the system. O Appropriate fabric allows sewage to flow around the corrugations so that a biomat can develop around the entire pipe (which is why the sizing factors relate to a wider trench). Some evidence suggests that the fabric also wicks the effluent around the outside of the pipe, using the entire depth of the system. This wicking will also move moisture closer to the surface and allow greater evapotranspiration.