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On-Site Wastewater Treatment and Disposal Systems - Forced ...

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4" Inspection Pip<br />

FIGURE 7-9<br />

SEEPAGE PIT CROSS SECTION<br />

te Cover<br />

xtendedto Solid Earth<br />

Brick, Block, Ring, or<br />

Precast Chamber<br />

with Open Joints<br />

4' min. Unsaturated Soil.<br />

Water Table<br />

ImperviousLayer<br />

1 -7-e-m<br />

7.2.3.3 Design<br />

a. Sizing the Infiltrative Surface<br />

Since the dominant infiltration surface of a seepage pit is the side-<br />

wall, the depth <strong>and</strong> diameter of the pit is determined from the perco-<br />

lation rate <strong>and</strong> thickness of each soil layer exposed by the excavation.<br />

A weighted average of the percolation test results (sum of thickness<br />

times percolation rate of each layer divided by the total thickness) is<br />

used. Soil lay&s with percolation rates slower than 30 min/in. (12<br />

min/cm) are excluded from this computation (3).<br />

The weighted percolation rate is used to determine the required sidewall<br />

area. Infiltration rates presented previously in Table 7-2 are used<br />

with the estimated daily wastewater flow to compute the necessary<br />

sidewall area.<br />

236

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