 Manual of Septic Tank Practice. Process There are some important differences in the way seepage pits work as compared to deep trenches. Contrary to common belief, a proper seepage pit is not just a hole filled with gravel. That is not to say that a hole filled with gravel will not work-that would be sim- ply a circular deep trench. Gravel-fill is normally used when very small diameter pits (2- or 3-ft diameter) must be installed. Such pits filled with small-sized gravel have been know to fail due to the formation of a clogging mat across the top of the gravel. If such a pit is to be installed, use 2-in. or larger diameter gravel or small cobbles. A conventional seepage pit has a hollow center which acts as a reservoir for treated effluent. That effluent seeps through a permea- ble wall of brick or block into a liner of gravel, and from there into the surrounding soil (Figure 5.14). ONSITE WASTEWATER DISPOSAL Figure 5.14 Seepage pit. Design In calculating the absorption area needed for seepage pits, it is necessary to know the percolation rate of the soil near the bottom of the pit and to know the percolation rates of any different soil strata encountered up the pit's sides. One way to collect the necessary percolation rates is to dig several test holes-one or two to the depth of the pit, and one or two dug into any soil type encountered that is different from the soil at the final depth. The Manual of Septic Tank Practice recommends that any strata with percolation rates slower than 30 min per in. not be included as available absorption area. The percolation test holes should be about 10 ft apart, far enough apart that water from one hole will not seep into another hole, and close enough together to be in the same area where the seepage pit will If you are lazy, the soil is fairly uniform in the drain field area, and the pits will be only 6 ft or SO deep, you can probably get by with a couple of percolation tests at the depth of the pits-to-be, if you discount the thickness of any relatively impermeable soil lay