8.7.4.

(1) The area described in Sentence 8.7.4.2.(1) shall be designed such that the loading rate does not exceed, for soil having a percolation time set out in Column 1 of Table 8.7.4.1.A., the maximum value set out opposite it in Column 2 of Table 8.7.4.1.A.

(1) A leaching bed comprised of absorption trenches may be constructed in leaching bed fill if unsaturated soil or leaching bed fill complying with Clause 8.7.2.1.(1)(b) extends,

(a) to a depth of at least 250 mm over the area covered by the leaching bed fill, and

(b) for at least 15 m beyond the outer distribution pipes in any direction in which the effluent entering the soil or leaching bed fill will move horizontally.

(2) If the unsaturated soil or leaching bed fill described in Sentence (1) has a percolation time greater than 15 minutes, any leaching bed fill added to form the leaching bed shall have a percolation time not less than 75% of the percolation time of the unsaturated soil or leaching bed fill.

(3) Leaching bed fill that does not meet the requirements of Sentence (2) may be used to form the leaching bed if,

(a) the distance from the bottom of the absorption trench to native soil is not less than 900 mm, or

(b) where the distance from the bottom of the absorption trench to native soil is less than 900 mm, the percolation time of the least permeable soil or leaching bed fill within 900 mm from the bottom of the absorption trench is used to calculate the length of the distribution pipe under Article 8.7.3.1.

(4) Sentence (2) does not apply to any leaching bed fill added as backfill above the stone layer in which the distribution pipe is located.

(5) All leaching bed fill added shall be stabilized against erosion.

(6) The site to which the leaching bed fill is added shall be generally clear of vegetation.

(7) The leaching bed fill that is added shall be compacted in layers in such a manner as to avoid uneven settlement of the distribution pipes.

(8) Any distribution boxes, header lines, absorption trenches, or distribution pipes shall be installed only after the leaching bed fill has been compacted in accordance with Sentence (7).

(9) Except as provided in Sentence (10), the sides of the added leaching bed fill shall be sloped to ensure stability, but shall not be steeper than one unit vertically to four units horizontally.

(10) The side slope of the leaching bed fill may be increased up to one unit vertically to three units horizontally if measures are taken to prevent erosion and ensure stability of the leaching bed fill.

(11) The distances as set out in Column 2 of Table 8.2.1.6.B.v1 shall be increased by twice the height that the leaching bed is raised above the original grade.