21. Construction standards: A soil absorption field built with a gravity feed system must comply with the following construction standards:(a) the length of a line of perforated pipes must be not more than 18 m, measured from the point of entry;
(b) the width of each absorption trench must be at least 60 cm;
(c) the distance between the centre lines of the absorption trenches must be at least 1.8 m and allow for the hydraulic barrier separating 2 consecutive absorption trenches to be at least 1.2 m wide;
(d) the depth of the gravel or crushed stone under the perforated piping mentioned in subparagraph h must be at least 15 cm;
(e) the perforated piping mentioned in subparagraph h must be laid in a bed of gravel or crushed stone which is at least 30 cm;
(f) the size of the gravel or crushed stone free from fine particles, must be between 1.5 and 6 cm;
(g) the layer of gravel or crushed stone must be covered with an anti-contaminant material which is permeable to water and air but will retain soil particles, and must be topped with 60 cm of earth backfill permeable to air;
(g.1) infiltration chambers covered with 60 cm of earth backfill permeable to air may be substituted for the layer of gravel and crushed stone provided for in subparagraphs d, e, f and g;
(g.2) where infiltration chambers are used, they must be designed to resist the weight of the backfill and prevent the migration of fine particles from the surrounding soil;
(g.3) a line of infiltration chambers without feed pipes must be not more than 6 m in length, measured from the point of entry;
(g.4) notwithstanding subparagraph b, where the infiltration chambers are not 60 cm in width, the total length of the absorption trenches must be rectified according to the effective infiltration width of the trenches so as to obtain the same absorption area;
(h) perforated piping must have a minimum diameter of 7.5 cm and comply with NQ Standard 3624-050;
(h.1) watertight piping must have a minimum diameter of 7.5 cm and comply with NQ Standard 3624-130;
(i) absorption trenches must comply with the following characteristics:i. they must be level;
ii. they must be completely buried in the soil of the disposal site or, if the ground is sloped, they must be completely buried in the soil of the disposal site at its highest point and not exceed the surface of the ground at its lowest point by more than 15 cm;
iii. in all cases, the bottom of the absorption trenches must be at least 90 cm above bedrock, impermeable soil or low permeability soil or underground water if the effluent is from a primary treatment system, and at least 60 cm if the effluent is from a secondary treatment system.