Minimum grades for full pipe flow are recommended in the Ontario Ministry of Agriculture and Food Publication 29, Drainage Guide for Ontario , but such grades do not make allowance for conditions of excess sediment loads in drains. Drains will not flush out naturally when the depth of sediment exceeds 20 mm. Drains in unstable soil should be designed for the maximum grade available. Drains should be placed at minimum depth. It is essential that construction in these problem soils take place during dry periods, otherwise problems are certain to occur.
Initial installation is a very critical period.
In soils requiring envelope filter protection, and where large main drains are specified in the design some economy may be available if the main drain is of non-perforated corrugated plastic tubing. A 100 mm drain, protected with an envelope filter may be laid parallel to the main drain. It also increased the effective pipe diameter and reduced the velocity of inflow. In Europe, sieved Sphagnum peat moss or peat band is used with success. Straw has been used in Ontario but has only a temporary effect and may seal the water openings.