How a Sewer is Installed
Building and installing a sewer requires planning, and now, there are several trenchless constructions methods that allow the installation of a to be sewer less invasive on the surrounding soil.
Dig a trench, drop the pipe in it, cover it up, right? Uh, no, it's not quite that simple. In fact, installing a sewer definitely is not a do-it-yourself project for a city that serves utility customers, a contractor and a bidding process is the only way to go. And even the most experienced sewer contractor has options, such as conventional trenching or trenchless construction, but still must comply with national standards, local ordinances and the law of gravity.
The Contractor Offers
There are two ways to install a sewer line: the conventional trenching method and via trenchless construction. Get estimates from several professionals, Sewer contractors who bid municipal jobs often will offer both options. They'll also work with a city's planning, services and enforcement departments to ensure compliance with local laws and building codes.
The first step is publishing a request for proposals (RFP) to let contractors know a job is available. Contractors respond with a bid that includes, as a minimum, a description of the way they will do the work and the respective cost to the city. (Read on in "How Bid On That Trenchless City Project.")
As part of the bidding process, you can specify a "two-way bid," which allows a contractor to present a project bid using both conventional trench and trenchless methods and why they would choose that method. In the end, a government's choice of a contractor depends on its belief the chosen contractor will do the job the way it should be done and on time and within budget.
Measure Twice, Dig Once
The project begins with measurements. It helps determine the necessary slope of the pipe. It also guides the contractor in purchasing the pipe joints that will form the new sewer line.
Why must the pipe slope? Because sewers depend on gravity to pull the solid and liquid wastes down the pipe to the sewer main, insofar as possible, mains gravity feed to treatment plants. Ideally, the downward slope of the line as it runs to the main is between 1/8-inch and 1/4-inch per foot of 4-inch diameter pipe and more for larger pipes. The contractor will use a theodolite, surveyor's transit or a laser level to determine the height differences and calculate the final angle at which the pipe must be installed.
To Trench or Not to Trench?
Once the contractor determines the necessary slope and down angle of the soon-to-be-installed sewer, the city will have options. The most glaring is money, followed closely by past experience with each contractor and whether the political liability of an open trench, into which its citizens or visitors might fall, walk, or drive. (See "Studies Claim the Trenchless Construction Cost Comparison is Massive.")
The next step in conventional trenching is digging the trench for the pipe to lay in. The bottom of the trench will be smooth and follow the slope calculated in the initial stages of the project. The bottom of the trench may be "bedded" with sand, to prevent the pipe from sagging in the trench and spoiling the slope.
If the city chooses the trenchless option, the contractor will set up the necessary boring equipment. This may require digging a small access pit so the auger or boring equipment has direct access to the level underground where the pipe is to be installed.
Laying (or Pulling) Pipe
If the city, county or special tax district - a.k.a, a "public-private partnership" - choose the conventional trenching option, the contractor's workers will lay the pipe. Usually, sewer pipe is PVC pipe, which means each joint must be primed with PVC primer and the joints fitted together while they lay in the trench.
To ensure the pipe's slope is correct, the contractor will run a string that is supported every foot, so deviations in the slope are readily recognizable at the correct downslope angle, alongside the pipe.
If the city, county, or special tax district decides to go trenchless, the job will require only enough digging to connect the sewer line. The trenchless contractor will use the boring equipment to create a bore, a small tunnel, barely larger than the pipe, with the correct downslope. Next, the workers will connect the pipe, which is already assembled and tested by the, to the boring machine's bit as it emerges. The contractor's personnel will complete the job by connecting the pipe to lines leading to the treatment facility.
The final steps in conventional trenching include checking the pipe for cracks, leaks or poorly made junctions between pipe joints. If all is in order, the workers then distribute 10-inches of sand or gravel over the pipe and compact it by hand or with a mechanical compactor. If the workers removed the sod at the beginning of the project, they replace it by hand and tamp it into place.
If the trenchless option was used, the contractor's personnel fill in the hole they dug to connect the sewer line and ensure the manhole or other access to the sewer is closed.
Written by Will Carpenter
A retired merchant seafarer, Will Carpenter sailed the world extensively before settling as far from the sea as possible. Now a technical writer, Will lives in the "hills and hollers" of Tennessee with two formerly feral cats.