edi- ate size rock or cobbles placed between the large rock and the upper gravel layer SO that the upper gravel will not migrate down into the large rocks. Materials Except for the additional large rocks, if used, the materials are the same as required for the conventional trench. For example, a 6-ft- deep trench would require 4 ft of cobbles and 1 ft of gravel. This is equivalent to 0.44 yd3 of cobbles and 0.11 yd3 of gravel per linear foot of trench if the trench is 3 ft wide. Construction Deep trench construction proceeds pretty much as with the stan- dard trench. The procedure for bottom leveling should be modified if there is any possibility that the trench sides could be unstable. Rather than crawling around in the trench, install "H" shaped sup- ports across the top of the trench, string a level line over the cross- pieces of the "H" along the length of the trench, and measure the distance from the string to the trench bottom with a plumb line (weighted string). Mark the high places and let the backhoe scoop them out. CONVENTIONAL WASTE DISPOSAL SYSTEMS 85 Operation and Maintenance The operation and maintenance requirements are the same for deep trenches as they are for conventional trenches. SEEPAGE PITS Application Seepage pits are also known in various parts of the country as sumps, leaching pits, and dry wells. They are used under the same conditions as deep trenches and for the same reasons. Seepage pits with the same effective depths as deep trenches have no advantage with respect to the absorption area available per square foot of ground surface used, but the area used is less spread out than with deep trenches. Without specialized equipment, seepage pits can require a lot of hand labor and therefore tend to be more expensive than trenches. As with conventional trenches and deep trenches, the bottoms of seepage pits should clear the ground water table and bedrock by an adequate distance. Seepage pits do not work well in soils with slow percolation rates (more than 30 min per in.), accord- ing to the