in allowing activities in the drain field area. Vehicles can crush the distribution pipe and compact the soil. A building, even a temporary building, can prevent evapotranspira- tion and may collect toxic and explosive gases. Renovation of failed drain fields is discussed in Chapter 8. ONSITE WASTEWATER DISPOSAL Figure 5.13 Deep trench. DEEP ABSORPTION TRENCHES Application A conventional trench with more than about 12 in. of gravel below the distribution pipe becomes a deep trench (also known as a seep- age trench; Figure 5.13). There are two main reasons for choosing a deep trench over a conventional trench: to get below relatively impermeable soil to use a deeper permeable layer; and to reduce the ground surface area taken up by the disposal field where space is limited. Note that a 6-ft-deep trench which has 1 ft of gravel below the distribution pipe and 5 ft of soil backfill is not a deep trench. This type of trench may be useful to get a level trench bottom where slope contours cannot be followed, but it lacks the increased side- wall absorption area that identifies a deep trench. Process The deep trench depends on sidewall absorption of effluent rather than on bottom area. Although the bottom of the trench initially CONVENTIONAL WASTE DISPOSAL SYSTEMS 83 drains more rapidly than do the sidewalls because of the force of gravity, the sidewalls become more important to its function as the bottom clogs. There are other differences between the processes of conventional and deep trenches, but they are significant only in rare circumstances and should not be discussed except over a beer. Design Because of the increased sidewall absorption area of deep trenches, it is standard practice to allow deep trenches to be shorter than conventional trenches; that is, to have less bottom area. The width of a deep trench can be greater than is normally used in shallow trenches, but because the sidewall is the important surface, extra width simply increases the bottom area and is not worth the cost of the extra gravel that the