Installed Joint Testing for Large-Diameter Sewer

With the continued growth of infrastructure in America, the need for trusted pipe solutions in the sewer industry continues to pose challenges to designers and owners alike. The forecast for storm and sanitary sewer pipe is estimated to surpass $8 billion by 2026, equivalent to more than 700 million linear feet of pipe installation (The Freedonia Group, n.d.). Many of these new installations will be tested to ensure joint integrity.

This article focuses on items typical in sanitary sewers in collection systems and larger, which typically is 8-inch diameter or greater. For diameters 30 inches and larger, the ability to test systems becomes more difficult and can make some test methods impractical. Joint testing is dominated on the sanitary side of the industry, but there’s a growing trend to test critical storm sewers, especially in locations with high groundwater or contaminated soils. For the sake of this article, most references will be for sanitary sewers.

Much of the vernacular associated with sewer joints includes the terms “watertight” and “leak resistant.” The American Association of State Highway and Transportation Officials (AASHTO) defines “watertight” as providing zero leakage of water for a specified pressure head for a specified time. The most-used specification to establish “watertight” classification in the gravity sewer pipe industry is ASTM D3212, Standard Specification for Joints for Drain and Sewer Plastic Pipes Using Flexible Elastomeric Seals (e.g., 10.8 psi for 10 minutes). When pipe joints are referred to as “watertight,” they’re typically stating the ability to pass a lab test—such as ASTM D3212—but it doesn’t mean the expectation will be the same for miles of installed pipe. Nearly all regulatory agencies recognize this in the gravity-sewer industry and require leak-resistant systems per a specific leakage rate limitation. Common sanitary sewer leakage rates range from 10 to 200 gallons/inch/mile/day, with 150 likely the most common. For storm sewers, either 200 or 300 gallons/inch/mile/day is commonly referenced.

Methods of Testing Joints

There are three primary ways of verifying installed joint integrity:

1. Low-pressure air

2. Hydrostatic exfiltration

3. Visual inspections

Low-Pressure Air

Low-pressure air testing typically is achieved by isolating a run of sewer pipe between two manholes. The outlet of the upgradient manhole and the inlet of the downgradient manhole are both plugged with inflatable test plugs (e.g., bag, ball, bladder). Lateral pipe connections also must be plugged appropriately to hold the required test pressure. The pipe is pressurized, typically to 2 to 4.5 psi, and is allowed a 1 to 0.5 psi drop within a given time period to pass the test.

Most gravity sewers described as “watertight” have an established leakage rate as described above. The air test’s pressure drop is a way to verify that the required leakage rate isn’t reached. Often there’s no drop in pressure during a test, but most specifications allow for some. Typically, if a run doesn’t pass, a protocol to check for a false “fail” result is performed. Plugs and other items associated with the test are rechecked, and the test may be performed again.

Hydrostatic Exfiltration

Hydrostatic exfiltration (i.e., draw-down) testing is another method. This test method is not as common or frequently used in the industry due to the time and costs associated with the amount of water required to perform the test on long runs and large diameters. The test blocks two manholes and all laterals with inflated plugs and fills the run of pipe between the plugs with water. The water fills the entire run of pipe until the upgradient manhole is filled and a water elevation is established in the manhole riser above the crown of the pipe. The water stabilizes over a given period, and more water is filled to a desired elevation to apply the desired hydrostatic pressure.

Once stable, the test begins for a specified period, typically 15 minutes or longer. The water level in the upgradient manhole is monitored for a drop in elevation, which would indicate a leak. If the water elevation drops during a specified period of time within a given amount per the ASTM, the leakage rate is considered to pass. If the leakage rate exceeds the specified maximum amount, then the system is potentially rendered unacceptable. Typically, if a run doesn’t pass, a protocol to check for a false “fail” result is performed. Plugs and other items associated with the test are rechecked, and the test may be performed again.

Visual Inspection

Another way to verify installed pipe joints is simply via visual inspection. This could be as simple as a walkthrough if the sewer is adequately large enough for safe passage or using more-complex methods that include CCTV robotics. CCTV can be done using various methods of insertion into the sewer, whether snaked, on a sled, on a remote-controlled crawler or even on a remote-controlled flotation device in live sewer flows.

In the sanitary industry, CCTV is very common (as most readers of this article already know) to verify many aspects of quality control for an installed sewer system. One of the limitations of a simple visual inspection is associated with joint integrity. The visual inspection is limited to seeing a visual leak at the moment of the inspection and can’t verify long-term integrity. If the system isn’t subjected to exterior hydrostatic pressure from the groundwater table at elevations equal to the pipe or above that would represent maximum infiltration pressures, then the inspection can’t determine the overall joint integrity of the system.

Visual inspections are much more common in very large sewers where entry is easy and safe, but low-pressure air and hydrostatic tests are either unsafe or simply economically impractical. Based on a recent survey with several-thousand online webinar attendees, a vast majority of participants responded to a poll question that they either required a visual inspection or no test method of sewer joints for diameters of 72 inches or larger.

Other Methods

Larger-diameter sewers in the 48-inch and larger range are increasingly being designed across the country. Using the same joint test methods for this larger-diameter set is more difficult, dangerous and expensive than in the smaller-diameter sets. These challenges have driven alternative ways to verify installed joint integrity. One of the more-common methods is using Joint Isolation Testing Equipment. There are several manufacturers of cylindrical testing gauges that can be moved within the pipe runs and positioned to isolate—or straddle—a specific joint. An exterior bladder is inflated against the inner wall of the pipe, and a small void space is created. The void space is pressurized to a specified pressure and held for a few minutes to verify the leakage rate.

Today, manufacturers have designed ways for the isolation test to be inherent in the pipe’s joint design. By designing a joint with two gaskets, the interstitial space between the gaskets can be injected with low-pressure air to verify the leakage rate of the joint.