Slope and grade determination skills are minimally functional when installing drainage pipes. Drainage plumbing must be installed so that water flows downhill. The usage of a builder’s level will simplify the assembly process. The volume of water in a storm determines the pipe size needed to transport the water, and you should add about 50% additional capacity for 100-year storms. Instead of using a four-inch tube, if a six-inch one does, go with the larger diameter if possible. For this discussion, a line with a six-inch diameter will do. The catch basin or other collection point outlet height about desired pipe exit or end height must be determined if the pipe is to be started at this location. This is how you use a builder’s level. Place the starting point and the end of the line within view of the level and the tripod.
Select a stationary object in the workspace to use as a height reference. A massive boulder with a convenient indentation for future reference. A fire hydrant’s side lug will also do the trick. Never tighten the top nut; it will shift when the hydrant is turned on. Your assistant will next set a rod or tape measure on the rock and measure upwards from the stone once the level has been leveled. The tape measure’s intersection with the level’s crosshairs is visible via the builder’s group. Say, forty inches in height. The HI, or instrument height, is recorded as 40. Measure once more, this time from the basin’s bottom hole, and read the tape’s new reading. For the sake of argument, let’s say fifty inches. You should also jot down this information, known as your front sight. Ten inches can be obtained by subtracting the HI from the front view. Now, using the tape, measure or look at the place where the pipe emerges, whether it’s into the air or a different basin. Sixty inches is the value here. Taking this number and subtracting it from the HI yields a discount of 20 inches. That means one basin’s opening is higher than the other by 10 inches.
The next step is to measure the two basins or between one inlet and the outlet. Eighty feet will be used as an illustration here. Over eighty feet, a slope of one-eighth of an inch every foot would require a pitch of one full inch every eight feet. Your total height discrepancy of ten inches is acceptable because eighty divided by ten is ten. We can now calculate that the pipe elevation must fall (or rise) by one inch every eight feet. You should probably subtract an inch from each level shot because most operators prefer to dig uphill if possible. It would be best if you took a new picture every eight feet to ensure your operator sloped the trench bottom uphill by one inch at each increment. You saw on the measuring tape that your attempt to enter the bottom basin was sixty inches short. It should read 59.5 inches if you move backward 8 feet from that basin. That’s an increment of one inch up the pipe. Keep going this way until you return to the top bay, where you started. When you reach the upper basin, you’ll have gained ten inches of slope if your measurements are accurate.
Most setups need stonework around and on top of the pipe. Backfilling using a perforated pipe requires this stone and filter fabric to ensure the holes don’t get clogged with silt and the rocks don’t crush or pierce the line. This stone is essential in preventing pipe settlement for other piping systems. Once the pipe is securely in place, you can backfill the rest of the trench, but be careful not to place heavy rocks on top of it. Over time, the pipe might be crushed by large boulders that settle in the soil. Don’t forget to factor in the thickness of the base stone when calculating your shots.
Additional protection, such as steel plates or concrete, may be necessary to prevent pipe crushing in regions where car traffic may cross the pipe trench.
Pete Ackerson has been a building inspector for more than 30 years and has worked in both the public and private sectors. He has experience in both the office of building design and the field of construction in the Eastern United States, having worked on a wide range of projects from schools to treatment plants, individual residences, and condo projects to major residential landscaping projects. Together with two other building inspectors, he established Wagsys LLC in 2006. The company develops and sells software for city departments such as building inspection, planning, and zoning.