Agenda - 01-04-1988 - Laserfiche WebLink

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dle renches with a greater depth of rock below the distribution i to increase the sidewall area is more suitable. p pe • Reserve Area: When planning and locating the absorption system, consid- eration should be given to reserving a suitable area for construction of a second system. The second system would be added if the first were to fail or if the system required expansion due to increased wastewater flows. Care must be used in constructing the second system so that the original system is not damaged by the construction equipment. The reserve area should be located to facilitate i t "nating loading of both systems. If the reserve area is used ubecause tthe initial system has failed, the failing system should not be permanently abandoned. With time, the initial system will be naturally rejuvenated and can be used alternately with the reserve system. Reserve areas can be provided very easily with trench systems by reserving sufficient area between the initial trenches as shown in Figure 7-4. D`. Dimensions: The absorption system should be dimensioned to best fit the lot while maintaining separation distances and -avoiding excessive depths :-7 of excavation. Commonly used dimensions are given in Table 7-3. The depth of excavation is determined by the location of the most perme- able soil horizon and flow restricting layers or the high water table elevation. Unless a deep, more permeable horizon exists, the trench or bed bottom elevation should be maintained at about 18 to 24 in. (46 to 61 cm) below the natural ground surface. To prevent freezing in cold climates, 6 to 12 in. (15 to 30 cm) of cover should be backfilled over the aggregate (11). If the water. table or a very slowly #_: ry y permeable layer is too near the ground surface to construct the system at this depth, the system can be .. raised. Very shallow trenches 6 to 12 in. (15 to 30 cm) deep can be in- = ,' stalled and the area backfilled with additional soil (see Figure 7-5). Adequate separation distance must be provided between the trench bottom and the seasonally high groundwater level to prevent groundwater contamination. The length of the trench or bed system depends on the site characteris- tics. The length of the distribution laterals is commonly restricted to 100 ft (30 m). This is based on the fears of root penetration, uneven settling, or pipe breakage which could disrupt the flow down the pipe to render the remaining downstream length useless. However, these fears are unwarranted because the aggregate transmits the wastewater (4) (13) -(17). To assure adequate transmission and distribution of the 219 r'"