Trench systems are the least expensive versions of special septic systems. They are comparable in many ways to a standard pipe-and-gravel bed system. The main difference between a trench system and a bed system is that the drain lines in a trench system are separated by a physical barrier. Bed systems consist of drainpipes situated in a rock bed. All of the pipes are in one large
bed. Trench fields depend on separation to work properly. To expand on this, let me give you some technical information.
A typical trench system is set into trenches that are between one to five feet deep. The width of the trench tends to run from one to three feet. Perforated pipe is placed in these trenches on a six-inch bed of crushed stone. A second layer of stone is placed on top of the drainpipe. This rock is covered with a barrier of some type to protect it from the backfilling process. The type of barrier used will be specified in a septic design.
When a trench system is used, both the sides of the trench and the bottom of the excavation are outlets for liquid. only one pipe is placed in each trench. These two factors are what separate a trench system from a standard bed system. Bed systems have all of the drain pipes in one large excavation. In a bed system, the bottom of the bed is the only significant infiltrative surface. Since trench systems use both the bottoms and sides of trenches as infiltrative surfaces, more absorption is potentially possible.
Neither bed nor trench systems should be used in soils where the percolation rate is either very fast or very slow. For example, if the soil will accept one inch of liquid per minute, it is too fast for a standard absorption system. This can be overcome by lining the infiltrative surface with a thick layer (about two feet or more) of sandy loam soil. Conversely, land that drains at a rate of one inch an hour is too slow for a bed or trench system. This is a situation where a chamber system might be recommended as an alternative.
Because of their design, trench systems require more land area than bed systems do. This can be a problem on small building lots. It can also add to the expense of clearing land for a septic field. However, trench systems are normally considered to be better than bed systems. There are many reasons for this.
Trench systems are said to offer up to five times more side area for infiltration to take place. This is based on a trench system with a bottom area identical to a bed system. The difference is in the depth and separation of the trenches. Experts like trench systems because digging equipment can straddle the trench locations during excavation. This reduces damage to the bottom soil and improves performance. In a bed system, equipment must operate within the bed, compacting soil and reducing efficiency.
If you are faced with hilly land to work with, a trench system is ideal. The trenches can be dug to follow the contour of the land. This gives you maximum utilization of the sloping ground.
Infiltrative surfaces are maintained while excessive excavation is eliminated. The advantages of a trench system are numerous. For example, trenches can be run between trees. This reduces clearing costs and allows trees to remain for shade and aesthetic purposes. However, roots may still be a consideration. Most people agree that a trench system performs better than a bed system. When you combine performance with the many other advantages of a trench system, you may want to consider trenching your next septic system. It costs more to dig individual trenches than it does to create a group bed, but the benefits may outweigh the costs.
