Today, I am going to discuss the basics of laying a residential sewer lateral in the ground. Whether the pipe is to be installed inside of a structure or outside, the basic methods are the same. Typically, you will need a starting elevation and an ending elevation. To determine your pitch or fall, measure the difference in elevation between start and ending points and divide the number of horizontal feet of run into the number of inches of fall. This will give you your "drop" per foot of pipe. It is important to note that when installing sewage pipe, your drop should not exceed 1/4" per foot. If your drop exceeds this number the water in the sewage tends to flow faster than the solids. This can result in a plugged sewer line. A quick example; if you find that you have 10 inches of drop between start and ending points and you have a 100' run of pipe to install, you would drop .1 inches per foot. This is a very small drop, particularly for smaller pipe. The recommended pitch for 4" pipe is 1/8" to 1/4" per foot. For drops less than or greater than the recommended range there are other solutions. Contact us for more information.
After the trench is excavated, and remember, safety first when entering an open trench, the pipe layer prepares the trench bottom by removing loose dirt and grading the trench bottom to allow for proper flow of sewage within the pipe. You need to ensure that the material beneath the pipe is solid so that the pipe does not "sag" after it is backfilled. Typically if the "original" ground beneath the new pipe is not disturbed and graded properly, the pipe will be adequately supported. If the original ground is over excavated, however, you will need to install compacted sand or gravel bedding beneath the pipe in order to support it.
Once the ground is prepared, you lay the pipe on the prepared soil. In most cases, you should start on the low end of the run and work your way up to the connecting point. If you use "hub" pipe, that is, pipe with an integral pipe built into the pipe, you should lay your first pipe beginning on the low end of the run with the hub on the uphill side of the pipe. In other words, the sewage flow runs into the hub and continues down the same pipe. If you are using pvc pipe, both ends of the pipe should be primed with purple primer prior to laying it. To connect the two ends of the pipe, you apply glue to the male and to the female ends of the pipe, being careful to not allow dirt to stick to the glued ends of the pipe. Insert the male end into the female end and spin the pipe 1/4 turn. Hold the two pipes in place for ten seconds to ensure that they do not come apart. (A common phenomenon believe it or not).
Once the pipes are connected, you are ready to "bed" the pipe. This means that you should place sand or gravel around and over the pipe - enough to just cover the top of the pipe. Once this is complete, you compact (either with a tool called a "rammer", or by using your feet) the granular material around the pipe. It is important to pay attention to this compaction process so that the pipe does not lift while you are compacting. You should recheck your grade after this process either using a four foot level, a transit, or a construction laser.
Once the bedding process is complete, you are ready to move on the the next pipe. It is important to note that as you install more pipe, you should be backfilling the trench behind you up to a safe level. This accomplishes three things; first is that you will have a safe egress and ingress into the trench (other than your ladder); second is that as you move your trench shoring ahead to allow you to lay more pipe, the banks behind you will be supported by the backfill material; third is that at the end of the day, you do not have such a long stretch of unbackfilled trench to backfill.
For more information, feel free to contact us.