water main trench excavation
Trenches for water main pipes are typically relatively shallow in comparison to that of sanitary sewer pipes. The typical depth range for water main pipe trenching from proposed pipe to proposed grades between 5 and 7 feet in cold weather climates. Sometimes the trench will be deeper if required for clearance around other underground utilities. The primary concern with the depth of water piping is assuring that the ‘cover’ (distance between top of pipe and finish grade above) is below the frost line for that area, that is the maximum depth the frost will creep below the surface grade during cold months. 5’ depth of cover is relatively standard in terms of watermain installations. The width of the trench depends primarily on the size of the pipe being placed. For water main pipe trenches housing pipe 6 or 8” in diameter at a depth of 5′, and a trench box wall thickness of 6-8″,the typical total trench width will be approximately 42-48″. Some contractors will try to tighten up their trenches even further, but adequate room to work around the pipe and pipe bell (69″ on either side) is needed.
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. 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 6’ and proper benching of excavation is done, the use of trench boxes shouldn’t become required. 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 watermain 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. 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 plastic water main pipe bedding, crushed stone or gravel is common and for ductile iron watermain pipe bedding, sand is relatively common. For a sizable project, a typical water main trench excavation crew will consist of 2 laborer and 2 operators, and a foreman. 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 watermain pipe is much heavier than pvc watermain pipe.