Trenches for storm drainage pipes are typically relatively shallow in comparison to that of sanitary sewer pipes. The typical depth range for small diameter storm drainage pipe is between 3 and 8 feet, but depths can go much deeper depending on the situation and for large diameter sewers. The width of a trench depends primarily on the size of the pipe being placed and also the depth of the trench. According to current OSHA regulation, trenches 5’ or deeper will require proper benching or shoring 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 or shoring. OSHA also requires 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 however the popularity of ladders with safety professionals and enforcers has been on the decline.
Trenches have something called ‘stand up time’ which refers to how long a trench will last from initial excavation to eventual cave in. The factors influencing stand up time are soil type, water saturation, depth, weather, and construction or traffic activity around the trench. If the soil conditions are that of clayey or silty soils, and the soil is in a semi-saturated state, the excavation process is relatively simple. The trench sides will typically hold up well and with depths below 5’, the use of trench boxes may become less critical. If the soil conditions are non-cohesive (sandy or gravely) however, the walls of the trenches can become increasingly less stable as the depth increases and will run the risk of caving in.
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 drainage 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, something that is more important in urban and suburban areas than in rural or non-inhabited areas. 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 and pipe pocket at the bottom of the trench 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 storm drainage trench excavation crew will consist of 2 laborers and 2 operators. If only installing shallow pipes or underdrains, this crew size can be reduced to 3 men. 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.