Trenches for sanitary pipes are typically deeper in comparison to that of water mains and storm drainage pipes. The typical depth range for sanitary sewer pipe trenching from proposed pipe to proposed grades is between 8 and 18 feet, though it can be shallower and much deeper. The width of the trench depends on the size of the pipe being placed and also the depth of the trench. Since most sanitary trenches are deeper than 6’, the use of trench boxes is almost always necessary.
According to current OSHA regulation, trenches 5’ or deeper will require proper benching to establish a safe working environment, however many contractors typically have their own standards in terms of what is safe and reasonable for trench benching. OSHA also required that trench excavations deeper than 20’ have a shoring system designed by a professional engineer. Lastly, OSHA requires that all trenches 4’ or deeper have appropriate means of entrance/egress, i.e. steps, ramps, ladders, etc.
Trenches have something called ‘stand up time’ which refers to how long a trench will last from initial excavation to cave in. The factors influencing stand up time are soil type, water saturation, depth, weather, and construction or traffic activity around the trench.
For sanitary sewer trenches, the very top few feet of the trench is typically 5-7’ wide. Below this area, the trench is typically benched to approximately 4.5’ – 5.5’ wide depending on the size of the trench box and the pipe to be placed. Then one more shelf typically exists below where the side plates of the trench box rests. Below this final shelf is the bedding trench which usually consists of a dimension of approximately 12-24” wider and 12-24” deeper (including above and below) the pipe to be installed. If the soil conditions are non-cohesive (sandy or gravely), wet, or the depth of the trench is beyond 6’, the walls of the trenches can become increasingly less stable and will run the risk of caving in without support.
Dirt caving in from the sides of the trenches is a concern for 2 reasons. Firstly if the contractor is required to backfill with premium material, cave-ins will increase the amount of premium backfill required to be hauled in, which increases the material and trucking costs on the project, not to mention the loading and trucking costs of removing the additional cave-in material. The other concern with respect to cave-ins is safety. Dirt is much heavier than the layman may assume. A large cave-in onto an unsuspecting worker can be lethal. For these reasons, trench excavation for sanitary sewer piping will sometimes require aluminum or steel trench boxes be placed, which shore the earth and all but eliminate the risk of cave-ins.
During any pipe trench process where the excavated material can’t be kept on site, the material is typically hauled off in trucks immediately upon excavation. This keeps the site much cleaner. The spoils removal trucks typically creep right along with the excavator operator as he goes, receiving spoils and trucking them to a predetermined location to be dumped.
With most utility pipe trenches, the proposed pipe must be bedded in granular backfill (sand or crushed gravel). The width of the pipe bedding area is typically less than the width of the trench above. The pipe bedding typically just has to cover the pipe from 6-12” surrounding.
For a sizable project, a typical sanitary sewer trench excavation crew will consist of 2 laborers and 3 operators. If only installing shallow pipes, this crew size can be reduced, and if installing deeper pipes this crew may need to be larger. The equipment utilized will be a backhoe or excavator and a front end loader. The excavator is for the trenching and the loader will be for dumping stone backfill into the trench or moving spoils to be loaded onto trucks for removal. The excavator will typically straddle the proposed trench area and move backwards excavating and loading material onto the trucks, placing bedding, pipe, and backfill material. Depending on the pipe material being used, a larger backhoe or excavator may be utilized to appropriately lift and handle the material. For example ductile iron and concrete pipe is much heavier than pvc storm drainage pipe.