5 minute read
Wastewater Collection System Main Repair
By Michael Beadnell, Instructor, Texas Rural Water Association
Most public works licensed operators don’t have the luxury of sitting in an environment with no risks, and it is up to us to work as safely around the biggest dangers that life throws at us. In my opinion, the field of water and wastewater is by far one of the most dangerous but the most important occupations there is. We are experiencing a change in our lifestyles like we have never seen before. People are asked to stay home to keep from elevating the spread of the Coronavirus. For public works employees throughout the state, it has been a challenge to keep up with the growing needs of a community staying at home. America works for a living. We determine our peak demands when citizens are waking up, having lunch and before going to bed. If the majority of our customers are at home 24/7, this requires recalculating.
As water and wastewater operators, it is our responsibility to provide potable water, water that has a chlorine residual, has been tested and is safe to drink, and also to stabilize unstable organics to protect public health. The biochemical oxygen demands, total suspended solids, bacteriological samples and every other state and federal compliance regulation don’t go away in a pandemic.
On the wastewater side, one of the most difficult emergency call outs is one where there is nothing you can do but to excavate. Customers sometimes flush things that are not compatible to our piping and are not necessarily biodegradable. When excavation is the only option, the first call should be 811. This is the phone number you call before digging to protect yourself and others from unintentionally hitting underground utility lines.
Studies show that excavation work is one of the most hazardous types of work done in the construction industry. Accidents in excavation work occur more frequently than accidents in general construction. The primary type of excavation-related work accident is the cave-in. Though these types of accidents are not the most common, when they do occur the related injuries tend to be more serious. Cave-in accidents are much more likely to result in a fatality than other construction-related accidents and account for the largest percentage of preventable injuries. The majority of cave-in fatalities are a result of excavations that either did not have shoring of the walls or the sloping was inadequate to prevent soil from sliding in. Emergency repairs are focused on returning the system to service as soon as possible and are executed to protect public health, which is the operator’s primary duty. Oftentimes the conditions surrounding emergency repairs are not ideal, such as working in the dark, without adequate resources, with fatigue, stress and—most recently — working in the midst of a pandemic. Obviously, permanent, pre-planned and scheduled repairs are preferred to emergency repairs.
Most collection system repairs require excavating. Before repairing lines, you need to locate the problem. Televising has proven to be the most efficient and easily reveals the location of the problem. Maps of the collection system are also helpful to locate the line. When all the utility companies have given the ok to dig, the proper personal protective equipment is donned
to protect operators from occupational hazards. A skilled operator with a probe and a long-handled sharpshooter probes the designated area that needs to be excavated, looking for any unseen obstacles that a utility may have missed. Next, the backhoe operator excavates to a safe depth. The procedure is repeated until the operator uncovers an object that appears to be pipe.
Here is where what I call the magic numbers come into play: 2,3,4,5,9,10,20,25 and 50. So as not to have dirt and rocks fall on you while repairing, 2 feet is the distance the spoils need to be from the edge of the trench. When using a ladder for entering and exiting the excavation, the latter should extend 3 feet out of the ditch and be secured. If the excavation is 4 feet or deeper there needs to be a ladder. If the excavation is 5 feet or deeper there needs to be some sort of cave-in protection, either shoring, hydraulic timbers or aluminum sloping, cutting the ditch back or benching cutting the ditch back in a stair step effect.
The final cave-in protection is called “shielding,” which are long, thick trench boxes designed to protect operators during an excavation. Nine feet is the distance we strive for between potable water lines and wastewater lines. Ten feet is the distance to be from electric lines. If the excavation is 20 feet, there needs to be a Texas Professional Engineer that designs the cave-in protection. Twenty-five feet is the distance an operator needs to be from a ladder and 50 feet is the maximin spacing of that ladder. If the backhoe operator digs about a foot below on both sides of the pipe, the skilled operator can reveal the pipe.
Once located and revealed, your choices are to either repair the failed pipe or remove it and replace it with new more suitable pipe. Ensure that the joints fit smoothly. Unstable existing bedding should be removed and replaced with new, suitable dry bedding. The replaced piping should be laid on grade on a stable bedded ditch bottom and the connections made in alignment.
Backfill must be as suitable as the bedding and should be compacted appropriately. You may find it advantageous to flood the replaced section with water to help stabilize it. Do not pave over the repair without mechanical compacting or allowing several days for the backfill and bedding to settle properly. Cone off, disinfect, then restore the repaired area to as good of a condition as it was before the emergency.
If you have any questions about collection system repairs, excavation or this article in general, please contact Michael.Beadnell@trwa.org.
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