ure that trench bottoms are level should be employed for beds. And the other construction tech- 96 ONSITE WASTEWATER DISPOSAL Figure 5.18 Leaching chamber. niques described for conventional trenches apply to bed construc- tion as well. Operation and Maintenance The operation and maintenance requirements for absorption beds are the same as for conventional trench disposal systems. LEACHING CHAMBERS Application Leaching chambers (also known as infiltration galleries) are basi- cally horizontal seepage pits (Figure 5.18). Since their bottoms are not as deep in the soil as the bottoms of seepage pits, they are not as likely to encroach on limiting zones such as ground water or bedrock. These chambers are often used where old conventional systems have clogged up and must be replaced, but where there is no room to install a completely new conventional system. Some are constructed to standards which allow them to be placed under roads, driveways, and parking lots. Since their manufacture is CONVENTIONAL WASTE DISPOSAL SYSTEMS 97 beyond the scope of the normal home builder, their use is generally limited to areas where precast or preformed units are available. Although the chambers can be expensive, they can be relocated in new trenches as necessary. Process The function of the leaching chamber sections is to create a cavity in the soil, providing a large storage volume for excess sewage, while allowing the sewage to seep out through the excavation bot- tom and out the sides through holes in the sides of the sections. The design is such that no gravel is supposed to be necessary around the chambers. One theory says that this eliminates the "shadow effect" that gravel shows; that is, stones pressed against the sides and bottoms of trenches somewhat compress the soil where they meet, and this area is not then available as an infiltration surface for the effluent. Therefore, these chambers are more efficient than conven- tional trenches. The other theory is that without the gravel, a clog- ging zone can more easily form in 