The leaching area required shall be determined in accordance with the provisions of Article III. The leaching area shall be considered as the sidewall area of the excavation below the invert of the inlet. Impervious area of the sidewall below the inlet shall not be considered as available leaching area.
Leaching trenches shall not be constructed in
areas where maximum groundwater elevation is less than four feet below
the bottom of the trench.
When more than one leaching trench is installed,
the distance between excavation sidewalls shall be no less than twice
the effective width or twice the effective depth of the trench, whichever
is greater. In no case shall the distance between the excavation sidewalls
be less than four feet.
The minimum width of the leaching trench bottom
shall be 24 inches.
The maximum length of each leaching trench shall
be 100 feet.
Distribution lines shall have a minimum diameter
of four inches and shall be laid true to line and grade. The distribution
pipe shall consist of vitrified clay pipe laid with an adequate number
of open joints. All distribution pipes from the distribution box to
the leaching trench shall be unperforated and shall be laid with tight
joints. The depth to the crown of the pipe forming the distribution
lines shall be not less than 12 inches from finished grade. The distribution
pipe shall have a minimum slope of 0.005.
The stone shall consist of washed stone, ranging
from 3/4 inches to 1Â 1/2 inches in size, and shall be free of
iron, fines and dust in place. It shall extend the full width of the
trench and shall not be less than 12 inches minimum and 30 inches
maximum beneath the bottom of the distribution pipes. The stone shall
be covered with at least a two-inch layer of washed stone, ranging
from 1/8 to 1/2 inch in size, and shall be free of iron, fines and
dust in place. All stone must have less than 0.2% material finer than
a No. 200 sieve, as determined by the AASHTO Test Method T-11 and
T-27 (latest edition).
A reserve area sufficient to replace the capacity
of the original leaching area must be provided. The area between leaching
trenches shall not be considered, for design purposes, as part of
the reserve area.
Excavations into or fill upon impervious material shall not be allowed. Excavations through impervious material may be allowed if at least four feet of naturally occurring pervious material, as demonstrated by a percolation test, remains beneath the lowest point of excavation. All construction after excavation through impervious material shall be in accordance with § 223-20.
The grade above and adjacent to a leaching trench
shall slope at least 2% to prevent the accumulation of surface water.
The minimum depth of cover material shall be
12 inches. Earth materials used to cover leaching trench systems shall
be free of large stones, frozen clumps of earth, masonry, stumps or
waste construction material. Machinery which may crush or disturb
the alignment of pipe in the disposal system shall not be allowed
on any part of the disposal area.
When trenches are built at different elevations,
construction shall be such as to prevent sewage from upper trenches
from flowing into lower trenches.
Excavation may be made by machinery, provided
that the soil at the bottom of the disposal trench is not compacted.
The bottom of each trench shall be level.
When leaching trench systems are constructed
under areas subject to vehicular traffic, the trenches shall be capable
of withstanding H-20 wheel loads.
No trench shall be constructed in frozen soil.
Trenches may be installed in cases where the construction elevation
is below the depth of the frost line.
Leaching trench systems designed to be dosed
must be vented at the downstream end of the system.
